Hardenable Carbon Steels

Carbon steels are produced in greater tonnage and have wider use than any other metal because of their versatility and low cost. There were several reasons why carbon steels proved satisfactory on reappraisal:

1. their hardenability, though less than that of alloy steels, was adequate for many parts, and for some parts shallower hardening was actually an advantage because of minimized quench cracking;
2. refinements in heat treating methods, such as induction hardening, flame hardening, and "shell quenching", made it possible to obtain higher properties from carbon steels than previously; and
3. new compositions were added to the carbon steel group, permitting more discriminating selection.

There are now almost 50 grades available in the nonresulfurized series 1000 carbon steels and nearly 30 grades in the resulfurized series 1100 and 1200. The versatility of the carbon steel group has also been extended by availability of the various grades with lead additions.

Carbon steels can be divided into three arbitrary classifications based on carbon content.

Steels with 0.10 to 0.25% C. Three principal types of heat treatment are used for this group of steels:

1. conditioning treatments, such as process annealing, that prepare the steel for certain fabricating operations,
2. case hardening treatments, and
3. quenching and tempering to improve mechanical properties.

The improvement in mechanical properties that can be gained by straight quenching and tempering of the low-carbon steels is usually not worth the cost.

An example of process annealing is in the treatment of low-carbon cold-headed bolts made from cold drawn wire. Sometimes the strains introduced by cold working weaken the heads so much that they break through the most severely worked portion under slight additional strain. Process annealing overcomes this condition. Since the temperatures used are close to the lower transformation temperature, this treatment results in considerable reduction of the normal mechanical properties of the shank of the cold headed bolt.

A more suitable treatment is stress relieving at about 1000^oF (540^oC). This treatment is used in order to retain much of the strength acquired in cold working and to provide ample toughness. A common practice is to combine a stress-relieving treatment with a quench from the upper transformation temperature, or slightly above, producing mechanical properties that approach those of cold drawn stock. A common quenching medium is a water solution of soluble oil, the use of which produces two desirable results:

1. the surface of the parts acquires a pleasing black color accepted as a commercial finish, and
2. the speed of the quench is slowed to the point where fully quenched hardness is not produced, so it is not necessary to temper the parts.

Heat treatments are frequently employed to improve machinability. The generally poor machinability of the low-carbon steels, except those containing sulfur or other special alloying elements, results principally from the fact that the proportion of free ferrite to carbide is high. This situation cannot be changed fundamentally, but the machinability can be improved by putting the carbide in its most voluminous form, pearlite, and dispersing this pearlite evenly throughout the ferrite mass. Normalizing is commonly used with success, but best results are obtained by quenching the steel in oil from 1500 to 1600^oF (815-870^oC). With the exception of steels 1024 and 1025, no martensite is formed, and the parts do not require tempering.

Steels with 0.25 to 0.55% C. Because of their higher carbon content, these steels are usually used in the hardened and tempered condition. By selection of quenching medium and tempering temperature a wide range of mechanical properties can be produced. They are the most versatile of the three groups of carbon steels and are most commonly used for crankshafts, couplings, tie rods and many other machinery parts where the required hardness values are within the range from 229 to 447 HB. This group of steels shows a continuous change from water-hardening to oil-hardening types. The hardenability is very sensitive to changes in chemical composition, particularly to the content of manganese, silicon and residual elements, and to grain size; the steels are sensitive to section changes.

The rate of heating parts for quenching has a marked effect on hardenability under certain conditions. If the structure is non-uniform, as a result of severe banding or lack of proper normalizing or annealing, extremely rapid heating such as may be obtained in liquid baths, will not allow sufficient time for diffusion of carbon and other elements in the austenite. As a result, non-uniform or low hardness will be produced unless the duration of heating is extended. In heating steels that contain free carbide (for example, spheroidized material), sufficient time must be allowed for the solution of the carbides; otherwise the austenite at the time of quenching will have a lower carbon content than is represented by the chemical composition of the steel, and disappointing results may be obtained.

These medium-carbon steels should usually be either normalized or annealed before hardening, in order to obtain the best mechanical properties after hardening and tempering. Parts made from bar stock are frequently given no treatment prior to hardening, but it is common practice to normalize or anneal forgings. Most of bar stocks, both, hot finished and cold finished, are machined as received, except the higher-carbon grades and small sizes, which require annealing to reduce the as-received hardness. Forgings are usually normalized, since this treatment avoids the extreme softening and consequent reduction of machinability that result from annealing.

In some instances a "cycle treatment" is used. In this practice the parts are heated as for normalizing, and are then cooled rapidly in the furnace to a temperature somewhat above the nose of the S-curve - that is, within the transformation range that produces pearlite. Then the parts are held at temperature or cooled slowly until the desired amount of transformation has taken place; thereafter they are cooled in any convenient manner. Specially arranged furnaces are usually required. Details of the treatments vary widely and are frequently determined by the furnace equipment available.

Cold headed products are commonly made from these steels, especially from the ones containing less than 0.40% C. Process treating before cold working is usually necessary because the higher carbon decreases the workability. For certain uses, these steels are normalized or annealed above the upper transformation temperature, but more frequently a spheroidizing treatment is used. The degree of spheroidization required depends on the application. After shaping operations are finished, the parts are heat treated by quenching and tempering.

These medium-carbon steels are widely used for machinery parts for moderate duty. When such parts are to be machined after heat treatment, the maximum hardness is usually held to 321HB, and is frequently much lower.

Salt solutions are often successfully used. Salt solutions are not dangerous to operators but their corrosive action on iron or steel parts of equipment is very serious.

When the section is light or the properties required after heat treatment are not high, oil quenching is often used. This nearly always eliminates the breakage problem and is very effective in reducing distortion.

A wide range in austenitizing temperatures is made necessary in order to meet required conditions. Lower temperatures should be used for the higher-manganese steels, light sections, coarse-grained material and water quenching; higher temperatures are required for lower manganese, heavy sections, fine grain and oil quenching.

From these steels are made many common hand tools, such as pliers, open-end wrenches, screwdrivers, and a few edged tools - for example, tin snips and brush knives. The cutting tools are necessarily quenched locally on the cutting edges, in water, brine or caustic, and are subsequently given suitable tempering treatments. In some instances the edge is time quenched; then the remainder of the tool is oil quenched for partial strengthening. When made of these grades of steel, pliers, wrenches and screwdrivers are usually quenched in water, either locally or completely, and are then suitably tempered.

Steels with 0.55 to 1.00% C. Carbon steels with these higher carbon contents are more restricted in application than the 0.25 to 0.55% C steels since they are more costly to fabricate, because of decreased machinability, poor formability and poor weldability. They are also more brittle in the heat treated condition.

Higher-carbon steels such as 1070 to 1095 are especially suitable for springs where resistance to fatigue and permanent set are required. They are also used in the nearly fully hardened condition (Rockwell C 55 and higher) for applications where abrasion resistance is the primary requirement, as for agricultural tillage tools such as plowshares, and knives for cutting hay or grain.

Forged parts should be annealed because refinement of the forging structure is important in producing a high-quality hardened product, and because the parts come from the hammer too hard for cold trimming of the flash or for economical machining. Ordinary annealing practice, followed by furnace cooling to 1100^oF (590^oC), is satisfactory for most parts.

Most of the parts made from steels in this group are hardened by conventional quenching. However, special technique is necessary sometimes. Both oil and water quenching are used - water, for heavy sections of the lower-carbon steels and for cutting edges and oil, for general use. Austempering and martempering are often successfully applied; the principal advantages from such treatments are considerably reduced distortion, elimination of breakage, in many instances, and greater toughness at high hardness.

For heavy machinery parts, such as shafts, collars and the like, steels 1055 and 1061 may be used, either normalized and tempered for low strength, or quenched and tempered for moderate strength. Other steels in the list may be used, but the combination of carbon and manganese in the two mentioned makes them particularly well adapted for such applications.

It must be remembered that even with all hardenability factors favorable, including the use of a drastic quench, these steels are essentially shallow hardening, as compared with alloy steels, because carbon alone, or in combination with manganese in the amounts involved here, does not promote deep hardening to any significant extent. Therefore, the sections for which such steels are suited will be definitely limited. In spite of this limitation the danger of breakage is real and must be carefully guarded against when such parts are being treated, especially whenever changes in section are involved.

Hand tools made from these steels include open-end wrenches, Stillson wrenches, hammers, mauls, pliers and screw drivers and cutting tools, such as hatchets, axes, mower knives and band knives. The combination of carbon and manganese in the steels used may vary widely for the same type of tool, depending partly on the equipment available for manufacture and partly on personal experience with, or preference for, certain combinations. A manganese content lower than standard will be used in some tools. This is justified when it makes a particular carbon range easier to handle, but it should be understood that for many applications, a combination of lower carbon and higher manganese would serve just as well.

Tool Steels

Steels used for making tools, punches, and dies are perhaps the hardest, the strongest, and toughest steels used in industry. It is obvious that tools used for working steels and other metals must be stronger and harder than the steels or material they cut or form.

The metallurgical characteristics of various compositions of tool steels are extremely complex. There are hundreds of different makes and types of tool steels available and each may have a specific composition and end use.

In the United States, the Society of Automotive Engineers, in cooperation with the American Iron and Steel Institute, has established a classification system which relates to the use of the material and its composition or type of heat treatment. This classification system divides the tool and die steels into separate categories that are shown in Table

AISI-SAE Types	Classification of Tools Steels	COMPOSITION %
		C	Cr	V	W	Mo	Other
W1	Water hardening	0.60	-	-	-	-	-
W2		0.60	-	0.25	-	-	-
S1		0.50	1.50	-	2.50	-	-
S5	Shock resisting	0.55	-	-	-	0.40	0.80 Mn 2.00 Si
S7		0.50	3.25	-	-	1.40	-
O1	Oil hardening	0.90	0.50	-	0.50	-	-
O6		1.45	-	-	-	0.25	1.00 Si
A2	Cold work	1.00	5.00	-	-	1.00	-
A4	Medium alloy air hardening	1.00	1.00	-	-	1.00	2.00 Mn
D2	Cold work High carbon High chromium	1.50	12.00	-	-	1.00	-
M1	Cold work	0.80	4.00	1.00	1.50	8.00	-
M2	Molybdenum	0.85	4.00	2.00	6.00	5.00	-
M10		0.90	4.00	2.00	-	8.00	-
H11	Hot work	0.35	5.00	0.40	-	1.50	-
H12	Chromium	0.35	5.00	0.40	1.50	1.50	-
H13		0.35	5.00	1.00	-	1.50	-
P20	Die casting mold	0.35	1.25	-	-	0.40	-

Table 1.

In general, tool steels are basically medium- to high-carbon steels with specific elements included in different amounts to provide special characteristics. The carbon in the tool steel is provided to help harden the steel to greater hardness for cutting and wear resistance. Other elements are added to provide greater toughness or strength.

In some cases elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness so that the tool retains its hardness and strength when it becomes extremely hot. Various alloying elements in addition to carbon are chromium (Cr), cobalt (Co), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), and vanadium (V).

The addition of elements produces different effects on the resultant composition as follows:

• Chromium produces deeper hardness penetration in heat treatment and contributes wear resistance and toughness.
• Cobalt is used in high-speed steels and increases the red hardness so that they can be used at higher operating temperatures.
• Manganese in small amounts is used to aid in making steel sound and further additions help steel to harden deeper and more quickly in heat treatment. It also helps to lower the quenching temperature necessary to harden steels. Larger amounts of manganese in the 1.20-1.60% range allow steels to be oil quenched rather than water quenched.
• Molybdenum increases the hardness penetration in heat treatment and reduces quenching temperatures. It also helps increase red hardness and wear resistance.
• Nickel adds toughness and wear resistance to steel and is used in conjunction with hardening elements.
• Tungsten added to the steel increases its wear resistance and provides red hardness characteristics. Approximately 1.5% increases wear resistance and about 4% in combination with high carbon will greatly increase wear resistance. Tungsten in large quantities with chromium provides for red hardness.
• Vanadium in small quantities increases the toughening effect and reduces grain size. Vanadium in amounts over 1% provides extreme wear resistance especially to high-speed steels. Smaller amounts of vanadium in conjunction with chromium, and tungsten, aid in increasing red hardness properties.

The tool or die steels are designed for special purposes that are dependent upon composition. Certain tool steels are made for producing die blocks; some are made for producing molds, others are made for hot working, and still others for high-speed cutting applications.

The other way for classifying tool steels is according to the type of quench required to harden the steel. The most severe quench after heating is the water quench (water-hardening steels). A less severe quench is the oil quench obtained by cooling the tool steel in oil baths (oil-hardening steels). The least drastic quench is cooling in air (air-hardening steels).

Tool steels and dies can also be classified according to the work that is to be done by the tool. This is based on class numbers.

• Class I steels are used to make tools that work by a shearing or cutting action, such as cutoff dies, shearing dies, blanking dies, trimming dies, etc.
• Class II steels are used to make tools that produce the desired shape of the part by causing the material being worked, either hot or cold, to flow under tension. This includes drawing dies, forming dies, reducing dies, forging dies, etc. This class also includes plastic molds and die cast molding dies.
• The Class III steels are used to make tools that act upon the material being worked by partially or wholly reforming it without changing the actual dimensions. This includes bending dies, folding dies, twisting dies, etc.
• Class IV steels are used to make dies that work under heavy pressure and that produce a flow of metal or other material compressing it into the desired form. This includes crimping dies, embossing dies, heading dies, extrusion dies, staking dies, etc. It is important to understand and have sufficient information concerning the composition of the tool or die, the type of heat treatment that it has received, and the type of work that it performs.

Classification of Carbon Steels and Low-Alloy Steels

Steels can be classified by a variety of different systems depending on:

• The composition, such as carbon, low-alloy or stainless steel.
• The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods.
• The finishing method, such as hot rolling or cold rolling
• The product form, such as bar plate, sheet, strip, tubing or structural shape
• The deoxidation practice, such as killed, semi-killed, capped or rimmed steel
• The microstructure, such as ferritic, pearlitic and martensitic
• The required strength level, as specified in ASTM standards
• The heat treatment, such as annealing, quenching and tempering, and thermomechanical processing
• Quality descriptors, such as forging quality and commercial quality.

Carbon Steels

The American Iron and Steel Institute (AISI) defines carbon steel as follows:

Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Carbon steel can be classified, according to various deoxidation practices, as rimmed, capped, semi-killed, or killed steel. Deoxidation practice and the steelmaking process will have an effect on the properties of the steel. However, variations in carbon have the greatest effect on mechanical properties, with increasing carbon content leading to increased hardness and strength. As such, carbon steels are generally categorized according to their carbon content. Generally speaking, carbon steels contain up to 2% total alloying elements and can be subdivided into low-carbon steels, medium-carbon steels, high-carbon steels, and ultrahigh-carbon steels; each of these designations is discussed below.

As a group, carbon steels are by far the most frequently used steels. More than 85% of the steel produced and shipped in the United States is carbon steel.

Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.

For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.

Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.

High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon steels are used for spring materials and high-strength wires.

Ultrahigh-carbon steels are experimental alloys containing 1.25 to 2.0% C. These steels are thermomechanically processed to produce microstructures that consist of ultrafine, equiaxed grains of spherical, discontinuous proeutectoid carbide particles.

High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition.

The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:

• Weathering steels, designated to exhibit superior atmospheric corrosion resistance
• Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on cooling
• Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation hardening but with low carbon content and therefore little or no pearlite in the microstructure
• Microalloyed steels, with very small additions of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardening
• Acicular ferrite steel, very low carbon steels with sufficient hardenability to transform on cooling to a very fine high-strength acicular ferrite structure rather than the usual polygonal ferrite structure
• Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, thus providing a high-strength steel of superior formability.

The various types of HSLA steels may also have small additions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

Low-alloy Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

As with steels in general, low-alloy steels can be classified according to:

• Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels
• Heat treatment, such as quenched and tempered, normalized and tempered, annealed.

Because of the wide variety of chemical compositions possible and the fact that some steels are used in more than one heat-treated, condition, some overlap exists among the alloy steel classifications. In this article, four major groups of alloy steels are addressed: (1) low-carbon quenched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) heat-resistant chromium-molybdenum steels.

Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are used primarily as plate. Some of these steels, as well as other, similar steels, are produced as forgings or castings.

Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.

Bearing steels used for ball and roller bearing applications are comprised of low carbon (0.10 to 0.20% C) case-hardened steels and high carbon (-1.0% C) through-hardened steels. Many of these steels are covered by SAE/AISI designations.

Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.

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Classification of Stainless Steels

Stainless steels are iron-based alloys containing at least 10.5% Cr. Few stainless steels contain more than 30% Cr or less than 50% Fe. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film. This oxide forms itself in the presence of oxygen.

Other elements added to improve characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulfur, and selenium. Carbon is normally present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades.

The selection of stainless steels may be based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific temperature ranges and product cost. However, corrosion resistance and mechanical properties are usually the most important factors in selecting a grade for a given application.

Stainless steels are commonly divided into five groups: martensitic stainless steels, ferritic stainless steels, austenitic stainless steels, duplex (ferritic-austenitic) stainless steels, and precipitation-hardening stainless steels.

The development of precipitation-hardenable stainless steels was spearheaded by the successful production of Stainless W by U.S. Steel in 1945. The problem of obtaining raw materials has been a real one, particularly in regard to nickel during 1950s when civil wars raged in Africa and Asia, prime sources of nickel, and Cold War politics played a role because Eastern-bloc nations were also prime sources of the element. This led to the development of a series of alloys (AISI 200 type) in which manganese and nitrogen are partially substituted for nickel. These stainless steels are still produced today.

Over the years, stainless steels have become firmly established as materials for cooking utensils, fasteners, cutlery, flatware, decorative architectural hardware, and equipment for use in chemical plants, dairy and food-processing plants, health and sanitation applications, petroleum and petrochemical plants, textile plants, and the pharmaceutical and transportation industries. Some of these applications involve exposure to either elevated or cryogenic temperatures; austenitic stainless steels are well suited to either type of service.

Modifications in composition are sometimes made to facilitate production. For instance, basic compositions are altered to make it easier to produce stainless steel tubing and casting. Similar modifications are made for the manufacture of stainless steel welding electrodes; here combinations of electrode coating and wire composition are used to produce desired compositions deposited weld metal.

Martensitic stainless steels are essentially alloys of chromium and carbon that possess a distorted body-centered cubic (bcc) crystal structure (martensitic) in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are generally resistant to corrosion only to relatively mild environments. Chromium content is generally in the range of 10.5 to 18%, and carbon content may exceed 1.2%. The chromium and carbon contents are balanced to ensure a martensitic structure after hardening.

General corrosion is often much less serious than localized forms such as stress corrosion cracking, crevice corrosion in tight spaces or under deposits, pitting attack, and intergranular attack in sensitized material such as weld heat-affected zones (HAZ). Such localized corrosion can cause unexpected and sometimes catastrophic failure while most of the structure remains unaffected, and therefore must be considered carefully in the design and selection of the proper grade of stainless steel.

Corrosive attack can also be increased dramatically by seemingly minor impurities in the medium that may be difficult to anticipate but that can have major effects, even when present in only part-per-million concentrations; by heat transfer through the steel to or from the corrosive medium; by contact trimmed only on the ends.

Stainless steels are available in the form of plate, sheet, strip, foil, bar, wire, semi-finished products, pipes, tubes, and tubing.

Sheet

Sheet is a flat-rolled product in coils or cut lengths at least 610 mm wide and less than 4.76 mm thick. Stainless steel sheet is produced in nearly all types except the free machining and certain martensitic grades. Sheet from the conventional grades is almost exclusively produced on continuous mills. Hand mill production is usually confined to alloys that cannot be produced economically on continuous mills, such as certain high-temperature alloys.

The steel is cast in ingots, and the ingots are rolled on a slabbing mill or a blooming mill into slabs or sheet bars. The slabs or sheet bars are then conditioned prior to being hot rolled on a finishing mill. Alternatively, the steel may be continuous cast directly into slabs that are ready for hot rolling on a finishing mill. The current trend worldwide is toward greater production from continuous cast slabs.

Sheet produced from slabs on continuous rolling mills is coiled directly off the mill. After they are descaled, these hot bands are cold rolled to the required thickness and coils off the cold mill are either annealed and descaled or bright annealed. Belt grinding to remove surface defects is frequently required at hot bands or at an intermediate stage of processing. Full coils or lengths cut from coils may then be lightly cold rolled on either dull or bright rolls to produce the required finish. Sheet may be shipped in coils, or cut sheets may be produced by shearing lengths from a coil and flattening them by roller leveling or stretcher leveling.

Strip

Strip is a flat-rolled product, in coils or cut lengths, less than 610 mm wide and 0.13 to 4.76 mm thick. Cold finished material 0.13 mm thick and less than 610 mm wide fits the definitions of both strip and foil and may be referred to by either term.

Cold-rolled stainless steel strip is manufactured from hot-rolled, annealed, and pickled strip (or from slit sheet) by rolling between polished rolls. Depending on the desired thickness, various numbers of cold rolling passes through the mill are required for effecting the necessary reduction and securing the desired surface characteristics and mechanical properties.

Hot-rolled stainless steel strip is a semi-finished product obtained by hot-rolling slabs or billets and is produced for conversion to finished strip by cold rolling.

Heat Treatment. Strip of all types of stainless steel is usually either annealed or annealed and skin passed, depending on requirements. When severe forming, bending, and drawing operations are involved, it is recommended that such requirements be indicated so that the producer will have all the information necessary to ensure that he supplies the proper type and condition. When stretcher strains are objectionable in ferritic stainless steels such as type 430, they can be minimized by specifying a No 2 finish. Cold-rolled strip in types 410, 414, 416, 420, 431, 440A, 440B, and 440C can be produced in the hardened and tempered condition.

Experience in the use of stainless steels indicates that many factors can affect their corrosion resistance. Some of the more prominent factors are:

• Chemical composition of the corrosive medium including impurities
• Physical state of the medium-liquid, gaseous, solid, or combinations thereof
• Temperature
• Temperature variations
• Aeration of the medium
• Oxygen content of the medium
• Bacteria content of the medium
• Ionization of the medium
• Repeated formation and collapse of bubbles in the medium
• Relative motion of the medium with respect to the steel
• Chemical composition of the metal
• Nature and distribution of microstruc-tural constituents etc.

Surface Finish. Other characteristics in the stainless steel selection checklist are vital for some specialized applications but of little concern for many applications. Among these characteristics, surface finish is important more often than any other except corrosion resistance. Stainless steels are sometimes selected because they are available in a variety of attractive finishes. Surface finish selection may be made on the basis of appearance, frictional characteristics, or sanitation.

Plate

Plate is a flat-rolled or forged product more than 250 mm (10 in.) in width and at least 4.76 mm (0.1875 in.) in thickness. Exceptions include highly alloyed ferritic stainless steels, some of the martensitic stainless steels, and a few of the free-machining grades. Plate is usually produced by hot rolling from slabs that have been directly cast or rolled from ingots and that usually have been conditioned to improve plat surface. Some plate may be produced by direct rolling from ingot.

For strip, edge condition is often more important than it usually is for sheet. Strip can be furnished with various edge specifications:

• Mill edge (as produced, condition unspecified)
• No.1 edge (edge rolled, rounded, or square)
• No.3 edge (as slit)
• No.5 edge (square edge produced by rolling or filing after slitting)

Foil

Foil is a flat-rolled product, in coil form, up to 0.13 mm thick and less than 610 mm wide. Foil is produced in slit widths with edge conditions corresponding to No.3 and No.5 edge conditions for strip. Foil is made from types 201, 202, 301, 302, 304, 304L, 305, 316, 316L, 321, 347, 430, and 442, as well as from certain proprietary alloys.

The finishes, tolerances, and mechanical properties of foil differ from those of strip because of limitations associated with the way in which foil is manufactured. Nomenclature for finishes, and for width and thickness tolerances, vary among producers.

Mechanical Properties. In general, mechanical properties of foil vary with thickness. Tensile strength is increased somewhat, and ductility is lowered, by a decrease in thickness.

Bar

Bar is a product supplied in straight lengths; it is either hot or cold finished and is available in various shapes, sizes, and surface finishes. This category includes small shapes whose dimensions do not exceed 75 mm and, second, hot-rolled flat stock at least 3.2 mm thick and up to 250 mm wide.

Hot-finished bar is commonly produced by hot rolling, forging, or pressing ingots to blooms or billets of intermediate size, which are subsequently hot rolled, forged, or extruded to final dimensions.

Designation of Carbon and Low-Alloy Steels

A designation is the specific identification of each grade, type, or class of steel by a number, letter, symbol, name, or suitable combination. Unique to a particular steel grade, type and class are terms used to classify steel products. Within the steel industry, they have very specific uses: grade is used to denote chemical composition; type is used to indicate deoxidation practice; and class is used to describe some other attribute, such as strength level or surface smoothness.

In ASTM specifications, however, these terms are used somewhat interchangeably. In ASTM A 533, for example, type denotes chemical composition, while class indicates strength level. In ASTM A 515, grade identifies strength level; the maximum carbon content permitted by this specification depends on both plate thickness and strength level. In ASTM A 302 grade denotes requirements for both chemical composition and mechanical properties. ASTM A 514 and A 5117 are specifications for high-strength quenched and tempered plate for structural and pressure vessel applications, respectively, each contains several compositions that can provide the required mechanical properties. However, A 514 type A has the identical composition limits as A 517 grade.

Chemical composition is by far the most widely used basis for classification and/or designation of steels. The most commonly used system of designation in the United States is that of the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI). The Unified Numbering System (UNS) is also being used with increasing frequency.

SAE-AISI Designations

As stated above, the most widely used system for designating carbon and alloy steels is the SAE-AISI system. As a point of technicality, there are two separate systems, but they are nearly identical and have been carefully coordinated by the two groups. It should be noted, however, that AISI has discontinued the practice of designating steels.

The SAE-AISI system is applied to semi-finished forgings, hot-rolled and cold-finished bars, wire rod and seamless tubular goods, structural shapes, plates, sheet, strip, and welded tubing.

Carbon steels contain less than 1.65% Mn, 0.60% Si, and 0.60% Cu; they comprise the lxxx groups in the SAE-AISI system and are subdivided into four distinct series as a result of the difference in certain fundamental properties among them.

Designations for merchant quality steels include the prefix M. A carbon steel designation with the letter B inserted between the second and third digits indicates the steel contains 0.0005 to 0.003% B. Likewise, the letter L inserted between the second and third digits indicates that the steel contains 0.15 to 0.35% Pb for enhanced machinability. Resulfurized carbon steels of the 11xx group and resulfurized and rephosphorized carbon steels of the 12xx group are produced for applications requiring good machinability. Steels that having nominal manganese contents of between 0.9 and 1.5% but no other alloying additions now have 15xx designations in place of the 10xx designations formerly used.

Alloy steels contain manganese, silicon, or copper in quantities greater than those listed for the carbon steels, or they have specified ranges or minimums for one or more of the other alloying elements. In the AISI-SAE system of designations, the major alloying elements are indicated by the first two digits of the designation. The amount of carbon, in hundredths of a percent, is indicated by the last two (or three) digits.

For alloy steels that have specific hardenability requirements, the suffix H is used to distinguish these steels from corresponding grades that have no hardenability requirement. As with carbon steels, the letter B inserted between the second and third digits indicates that the steel contains boron. The prefix E signifies that the steel was produced by the electric furnace process.

HSLA Steels. Several grades of HSLA steel are described in SAE Recommended Practice J410. These steels have been developed as a compromise between the convenient fabrication characteristics and low cost of plain carbon steels and the high strength of heat-treated alloy steels. These steels have excellent strength and ductility as-rolled.

UNS Designations The Unified Numbering System (UNS) has been developed by ASTM and SAE and several other technical societies, trade associations, and United States government agencies.

A UNS number, which is a designation of chemical composition and not a specification, is assigned to each chemical composition of a metallic alloy. The UNS designation of an alloy consists of a letter and five numerals. The letters indicate the broad class of alloys; the numerals define specific alloys within that class. Existing designation system, such as the AISI-SAE system for steels, have been incorporated into UNS designations. UNS is described in greater detail in SAE J1086 and ASTM E 527.

AMS Designation

Aerospace Materials Specifications (AMS), published by SAE, are complete specifications that are generally adequate for procurement purposes. Most of the AMS designations pertain to materials intended for aerospace applications; the specifications may include mechanical property requirements significantly more severe than those for grades of steel having similar compositions but intended for other applications. Processing requirements, such as for consumable electrode remelting, are common in AMS steels.

ASTM (ASME) Specifications The most widely used standard specifications for steel products in the United States are those published by ASTM. These are complete specifications, generally adequate for procurement purposes. Many ASTM specifications apply to specific products, such as A 574 for alloy steel socket head cap screws. These specifications are generally oriented toward performance of the fabricated end product, with considerable latitude in chemical composition of the steel used to make the end product.

ASTM specifications represent a consensus among producers, specifiers, fabricators, and users of steel mill products. In many cases, the dimensions, tolerances, limits, and restrictions in the ASTM specifications are similar to or the same as the corresponding items of the standard practices in the AISI Steel Products Manuals.

Many of the ASTM specifications have been adopted by the American Society of Mechanical Engineers (ASME) with little or no modification; ASME uses the prefix S and the ASTM designation for these specifications. For example, ASME-SA213 and ASTM A 213 are identical.

Steel products can be identified by the number of the ASTM specification to which they are made. The number consists of the letter A (for ferrous materials) and an arbitrary, serially assigned number. Citing the specification number, however, is not always adequate to completely describe a steel product. For example, A 434 is the specification for heat-treated (hardened and tempered) alloy steel bars. To completely describe steel bars indicated by this specification, the grade (SAE-AISI designation in this case) and class (required strength level) must also be indicated. The ASTM specification A 434 also incorporates, by reference, two standards for test methods (A 370 for mechanical testing and E 112 for grain size determination) and A 29, which specifies the general requirements for bar products.

SAE-AISI designations for the compositions of carbon and alloy steels are sometimes incorporated into the ASTM specifications for bars, wires, and billets for forging. Some ASTM specifications for sheet products include SAE-AISI designations for composition. The ASTM specifications for plates and structural shapes generally specify the limits and ranges of chemical composition directly, without the SAE.AISI designations.

General Specifications. Several ASTM specifications, such as A 20 covering steel plate used for pressure vessels, contain the general requirements common to each member of a broad family of steel products. These general specifications are often supplemented by additional specifications describing a different mill form or intermediate fabricated product.

European and Japanese Designation Systems

Below some basics of European and Japanese designation systems are explained. Please refer to articles about corresponding national and international standards for more details.

DIN standards are developed by Deutsches Institut fur Normung in the Federal Republic of Germany. All West German steel specifications are preceded by the uppercase letters DIN followed an alphanumeric or numeric code. The latter method, known as the Werkstoff number, uses numbers only with a decimal point after the first digit.

JIS standards are developed by the Japanese Industrial Standards Committee, which is part of the Ministry of International Trade and Industry in Tokyo. The JIS steel specifications begin with the uppercase letters JIS and are followed by an uppercase letter (G in the case of carbon and low-alloy steels) designating the division (product form) of the standard. This letter is followed by a series of numbers and letters that indicate the specific steel.

British standards (BS) are developed by the British Standards Institute in London, England. Similar to the JIS standards, each British designation includes a product form and an alloy code.

AFNOR standards are developed by the Association Francaise de Normalisation in Paris, France. The correct format for reporting AFNOR standards is as follows. An uppercase NF is placed to the left of the alphanumeric code. This code consists of an uppercase letter followed by a series of digits, which are subsequently followed by an alphanumeric sequence.

UNI standards are developed by the Ente Nazionale Italiano di Unificazione in Milan, Italy. Italian standards are preceded by the uppercase letter UNI followed by a four-digit product form code subsequently followed by an alphanumeric alloy identification.

Swedish standards (SS) are prepared by the Swedish Standards Institution in Stockholm. Designations begin with the letters SS followed by the number 14 (all Swedish carbon and low-alloy steels are covered by SS14). What subsequently follows is a four digit numerical sequence similar to the German Werkstoff number.

Gray Cast Iron

Cast irons are alloys of iron, carbon, and silicon in which more carbon is present than can be retained in solid solution in austenite at the eutectic temperature. In gray cast iron, the carbon that exceeds the solubility in austenite precipitates as flake graphite.

Gray irons usually contain 2.5 to 4% C, 1 to 3% Si, and additions of manganese, depending on the desired microstructure (as low as 0.1% Mn in ferritic gray irons and as high as 1.2% in pearlitics). Sulphur and phosphorus are also present in small amounts as residual impurities.

The composition of gray iron must be selected in such a way to satisfy three basic structural requirements:

• The required graphite shape and distribution
• The carbide-free (chill-free) structure
• The required matrix

For common cast iron, the main elements of the chemical composition are carbon and silicon. High carbon content increases the amount of graphite or Fe3C. High carbon and silicon contents increase the graphitization potential of the iron as well as its castability.

The combined influence of carbon and silicon on the structure is usually taken into account by the carbon equivalent (CE):

CE = %C + 0.3x(%Si) + 0.33x(%P) - 0.027x(%Mn) + 0.4x(%S)

Although increasing the carbon and silicon contents improves the graphitization potential and therefore decreases the chilling tendency, the strength is adversely affected. This is due to ferrite promotion and the coarsening of pearlite.

The manganese content varies as a function of the desired matrix. Typically, it can be as low as 0.1% for ferritic irons and as high as 1.2% for pearlitic irons, because manganese is a strong pearlite promoter.

The effect of sulfur must be balanced by the effect of manganese. Without manganese in the iron, undesired iron sulfide (FeS) will form at grain boundaries. If the sulfur content is balanced by manganese, manganese sulfide (MnS) will form, which is harmless because it is distributed within the grains. The optimum ratio between manganese and sulfur for a FeS-free structure and maximum amount of ferrite is:

%Mn = 1.7x(%S) + 0.15

Other minor elements, such as aluminum, antimony, arsenic, bismuth, lead, magnesium, cerium, and calcium, can significantly alter both the graphite morphology and the microstructure of the matrix.

In general, alloying elements can be classified into three categories. Silicon and aluminum increase the graphitization potential for both the eutectic and eutectoid transformations and increase the number of graphite particles. They form colloid solutions in the matrix. Because they increase the ferrite/pearlite ratio, they lower strength and hardness.

Nickel, copper, and tin increase the graphitization potential during the eutectic transformation, but decrease it during the eutectoid transformation, thus raising the pearlite/ferrite ratio. This second effect is due to the retardation of carbon diffusion. These elements form solid solution in the matrix. Since they increase the amount of pearlite, they raise strength and hardness.

Chromium, molybdenum, tungsten, and vanadium decrease the graphitization potential at both stages. Thus, they increase the amount of carbides and pearlite. They concentrate in principal in the carbides, forming (FeX)_nC-type carbides, but also alloy the aFe solid solution. As long as carbide formation does not occur, these elements increase strength and hardness. Above a certain level, any of these elements will determine the solidification of a structure with Fe₃C (mottled structure), which will have lower strength but higher hardness.

Generally, it can be assumed that the following properties of gray cast irons increase with increasing tensile strength from class 20 to class 60:

• All strengths, including strength at elevated temperature
• Ability to be machined to a fine finish
• Modulus of elasticity
• Wear resistance.

On the other hand, the following properties decrease with increasing tensile strength, so that low-strength irons often perform better than high-strength irons when these properties are important:

• Machinability
• Resistance to thermal shock
• Damping capacity
• Ability to be cast in thin sections.

Successful production of a gray iron casting depends on the fluidity of the molten metal and on the cooling rate, which is influenced by the minimum section thickness and on section thickness variations.

Casting design is often described in terms of section sensitivity. This is an attempt to correlate properties in critical sections of the casting with the combined effects of composition and cooling rate. All these factors are interrelated and may be condensed into a single term, castability, which for gray iron may be defined as the minimum section thickness that can be produced in a mold, cavity with given volume/area ratio and mechanical properties consistent with the type of iron being poured.

Scrap losses resulting from missruns, cold shuts, and round corners are often attributed to the lack of fluidity of the metal being poured.

Mold conditions, pouring rate, and other process variables being equal, the fluidity of commercial gray irons depends primarily on the amount of superheat above the freezing temperature (liquidus). As the total carbon content decreases, the liquidus temperature increases, and the fluidity at a given pouring temperature therefore decreases. Fluidity is commonly measured as the length of flow into a spiral-type fluidity test mold.

The significance of the relationships between fluidity, carbon content, and pouring temperature becomes apparent when it is realized that the gradation in strength in the ASTM classification of gray iron is due in large part to differences in carbon content (~3.60 to 3.80% for class 20; ~2.70 to 2.95% for class 60). The fluidity of these irons thus resolves into a measure of the practical limits of maximum pouring temperature as opposed to the liquidus of the iron being poured.

The usual microstructure of gray iron is a matrix of pearlite with graphite flakes dispersed throughout. Foundry practice can be varied so that nucleation and growth of graphite flakes occur in a pattern that enhances the desired properties. The amount, size, and distribution of graphite are important. Cooling that is too rapid may produce so-called chilled iron, in which the excess carbon is found in the form of massive carbides. Cooling at intermediate rates can produce mottled iron, in which carbon is present in the form of both primary cementite (iron carbide) and graphite.

Flake graphite is one of seven types (shapes or forms) of graphite established in ASTM A 247. Flake graphite is subdivided into five types (patterns), which are designated by the letters A through E. Graphite size is established by comparison with an ASTM size chart, which shows the typical appearances of flakes of eight different sizes at l00x magnification.

Type A flake graphite (random orientation) is preferred for most applications. In the intermediate flake sizes, type A flake graphite is superior to other types in certain wear applications such as the cylinders of internal combustion engines.

Type B flake graphite (rosette pattern) is typical of fairly rapid cooling, such as is common with moderately thin sections (about 10 mm) and along the surfaces of thicker sections, and sometimes results from poor inoculation.

The large flakes of type C flake graphite are formed in hypereutectic irons. These large flakes enhance resistance to thermal shock by increasing thermal conductivity and decreasing elastic modulus. On the other hand, large flakes are not conducive to good surface finishes on machined parts or to high strength or good impact resistance.

The small, randomly oriented interdendritic flakes in type D flake graphite promote a fine machined finish by minimizing surface pitting, but it is difficult to obtain a pearlitic matrix with this type of graphite. Type D flake graphite may be formed near rapidly cooled surfaces or in thin sections. Frequently, such graphite is surrounded by a ferrite matrix, resulting m soft spots in the casting.

Type E flake graphite is an interdendritic form, which has a preferred rather than a random orientation. Unlike type D graphite, type E graphite can be associated with a pearlitic matrix and thus can produce a casting whose wear properties are as good as those of a casting containing only type A graphite in a pearluic matrix. There are, of course, many applications in which flake type has no significance as long as the mechanical property requirements are met.

Cold Rolled Steels

Cold rolled steels provide excellent press formability, surface finish, and thickness and flatness tolerances. Steel companies manufacture three groups of low- or ultra-low-carbon grades to meet a variety of customer formability requirements: CS Type B, DS Type B, EDDS, and EDDS+. They also produce HSLA steels and structural steel grades for those applications that require specified strength levels.

Cold rolled steels can also be specified as dent resistant or bake hardenable for applications that require dent resistance after forming and painting. Each grade can be processed with several surface finishes depending on customer requirements. Lubricants can be applied to enhance formability and to avoid at-press lubrication.

Cold rolled steels have the following features:

• Excellent Surface Appearance. Cold Rolled Steels have manufacturing controls in place assuring consistent surface quality to satisfy customer requirements.
• Formability. Cold Rolled Steels can be used to produce parts containing simple bends to parts with extreme deep drawing requirements.
• Paintability. Due to stringent surface roughness controls, Cold Rolled Steels are readily paintable using essentially any paint system.
• Weldability. Cold Rolled Steels can be joined using virtually any accepted welding practice.

Standard grades for cold rolled steels are:

• Commercial Steel (CS Type B). May be moderately formed; a specimen cut in any direction can be bent flat on itself without cracking.
• Drawing Steel (DS Type B). DS Type B is made by adding aluminum to the mol steel and may be used in drawing applications.
• Extra Deep Drawing Steel (EDDS). Interstitial Free (I-F) steels are made Drawing Steel by adding titanium and/or niobium to the molten steel after vacuum degassing and offer excellent drawability.
• Extra Deep Drawing Steel Plus (EDDS+). Interstitial Free (I-F) steels are made by adding titanium and/or niobium to the molten steel after vacuum degassing and offer excellent drawability.

Surface Finish

Cold rolled steels are manufactured with a matte finish obtained by rolling with specially roughened rolls on the cold mill and the temper mill. This finish helps to maintain effective lubrication during metal forming and improves the appearance of painted surfaces. Non-standard matte finishes can be provided that optimize the opposing effects of surface roughness on painted part appearance and lubrication during press forming.

Surface Protection and Lubrication

To prevent rusting in transit and storage, cold rolled steels can be supplied with a rust protective oil film or press forming lubricants. A pre-applied press forming lubricant provides uniform lubrication and eliminates the housekeeping problems.

A dry film (acrylic/polymer) lubricant can also be supplied by further processing the cold rolled product through a coil coating facility. These specialty organic coatings are easily removed with a mild alkaline cleaner.

Formability and Mechanical Properties

The formability of all steel products is a result of the interaction of many variables, the main ones being the mechanical properties of the steel, the forming system (tooling) used to manufacture parts, and the lubrication used during forming.

Tight control over chemical composition, hot rolling parameters, amount of cold reduction, annealing time and temperature, and the amount of temper rolling allow the production of high-quality cold rolled steel products to meet customers requirements. Commercial Steel (CS Type B) should be used for moderate forming or bending applications. CS Type B products are produced from aluminum-killed continuously cast slabs and, unless otherwise specified, have a carbon content of less than 0.15%.

To prevent the occurrence of fluting or stretcher strains during forming, CS products are tempered as a normal step in the mill processing.

For more severe forming applications, Drawing Steel Type B (DS Type B) should be used. DS Type B has a controlled carbon content (<0.06%)>

Extra Deep Drawing Steel (EDDS) or Extra Deep Drawing Steel plus (EDDS+) should be used for the most demanding forming applications. These steels (also known as Interstitial Free or I-F steels) are produced from a vacuum degassed, titanium stabilized grade. EDDS and EDDS+ have the lowest carbon content available (<0,010%)>

For high strength or structural applications, cold rolled steels are also available in yield strengths up to 50 ksi.

Paintability

Cold rolled steels can be easily painted using a variety of paint systems provided proper care is taken in preparing the material. Prior to painting, the surface should be carefully cleaned with either a solvent or alkaline cleaner.

Cleaning should be followed by a pre-treatment prior to painting. Zinc or iron phosphates give good results on cold rolled steels. Mild abrasion prior to pre-treating may also be used to enhance mechanical bonding of the paint.

Cold rolled steels can be in general supplied as pre-painted or pre-primed.

Alloy Steel Products resist abrasion and impact

Used to fabricate plates, chutes, and hoppers, WELLBRAZE and WELL-CLAD suit high- and severe-wear applications. As solid sheet with manganese and nickel content, WELLBRAZE has 17-20% work hardening factor and increases in toughness with use. WELL-CLAD has impact and wearing, chromium-carbide overlay applied to low-carbon steel (.12 max) substrate that absorbs impact resulting from heavy service. Composite material can be rolled or press-brake formed without separation.

WELLBRAZE and WELL-CLAD are the names of the alloy steel products developed to resist abrasion and impact in tough wear applications such as asphalt/concrete plants, coal mines, sand/gravel/rock crushing plants, power utilities, pulp mills, railroads, steel mills, foundries and refractories. The products are used in fabricating chute liners, hoppers, tumbling barrels, excavating buckets, conveyor parts, impellor fan blades, scrapers, pushers, grader and plowblades.

WELLBRAZE is a solid sheet (homogenous) with high Manganese and Nickel content. The product work-hardens (17-20% work hardening factor). The longer in service, the tougher it gets. WELLBRAZE can be welded with all low hydrogen production welding processes (E7018, E9018, E11018, E12018 and 12018 manual electrodes). It can be drilled with ordinary high speed drills. Material thicknesses up to 1" thick may be brake- formed (with proper bend allowances). Size range of material available is: Thicknness: 1/8" to 6"; Widths:48",72",96" ; Lengths: 144", 240", 288". Spec. sizes are available.

WELL-CLAD is an impact and wearing, chromium-carbide overlay applied to a very ductile, easy-to-weld, easy-to-form, low carbon steel substrate. The mild steel plate (.12 max carbon) was selected to absorb impact resulting from heavy service. The composite material (mild steel backing plate with the hardfacing overlay applied by a proprietary arc welding process) can be rolled or press-brake formed without fear of separation. WELL-CLAD was developed for severe wear applications.

Wellington Alloys is well-known as a leading nationwide supplier of alloy steels for severe duty applications in sheet and plate, "raw material only," as well as completely fabricated chute, hopper and bin formats for all tough industrial manufacturing, contractor, municipal, pit, quarry and mining applications, etc. Auxiliary product lines include wear points, edges, plow blades, shoes, discs, hammers and bolts.

For technical information, please contact:

Steve Sucher

Wellington Alloys

P.O. Box 250298

Franklin, MI 48025

Tel (248) 737-4216

Website: http://www.wellingtonalloys.com

steve@wellingtonalloys.com.

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COPYRIGHT 2006 Gale Group

Metal bonding

The metallic bond accounts for many physical characteristics of metals, such as strength, malleability, ductility, conduction of heat and electricity, and lustre.

Metallic bonding is the electrostatic attraction between delocalized electrons, called conduction electrons, and the metallic ions within metals. Because it involves the sharing of free electrons among a lattice of positively-charged metal ions, metallic bonding may be compared to that within molten salts.

Metallic bonds are non-polar, because in alloys there is little difference among the electronegativities of the atoms participating in the bonding interaction (and in pure elemental metals, none at all), and the electrons involved in the interaction are delocalized throughout the crystalline structure of the metal.

Metal atoms contain few electrons in their valence shells relative to their periods or energy levels. Such electrons can stray easily from the atoms and become delocalized, forming a sea of electrons permeating a giant lattice of positive ions. The freedom of conduction electrons to migrate gives metal atoms, or layers of them, the capacity to slide past each other, giving rise to metals' typical characteristic phenomena of malleability and ductility.

The electrons and positive ions in metals have a strong attractive force between them. Much energy is required to overcome it. Therefore, metals often have high melting and boiling points. The principle is similar to that of ionic bonds.

Because metals' conduction electrons move independently in a sea of negative charge, metals exhibit electrical conductivity, allowing charge to pass quickly through them, manifested as current. A few non-metals conduct electricity, notably graphite (which, like metals, has free electrons) and molten and aqueous ionic compounds, which have free ions.

Heat conduction works on the same principle; free electrons can transfer energy at a faster rate than the fixed electrons of other substances, such as those which are covalently bonded.

Metal atoms have at least one valence electron which they do not share with neighboring atoms, nor do they lose electrons to form ions. Instead the outer energy levels of the metal atoms overlap. They are similar to covalent bonds.

Zinc CathodicSacrificial Protection of Steel (pdf)

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Alloy steels

During the last fifty years engineers have demanded steels with higher and higher tensile strength, together with adequate ductility. This has been particularly so where lightness is desirable, as in the automobile and aircraft industries. An increase in carbon content met this demand in a limited way, but even in the heat-treated condition the maximum strength is about 700 MPa above which value a rapid fall in ductility and impact strength occurs and mass effects limit the permissible section.

Heattreated alloy steels provide high strength, high yield point, combined with appreciable ductility even in large sections. The use of plain carbon steels frequently necessitates water quenching accompanied by the danger of distortion and cracking, and even so only thin sections can be hardened throughout. For resisting corrosion and oxidation at elevated temperatures, alloy steels are essential.

The Alloy Steels Research Committee adopted the following definition: “Carbon steels are regarded as steels containing not more than 0,5% manganese and 0,5% silicon, all other steels being regarded as alloy steels”.

The principal alloying elements added to steel in widely varying amounts either singly or in complex mixtures are nickel, chromium, manganese, molybdenum, vanadium, silicon and cobalt.

The effect of the alloying element in the steel may be one or more of the following:

(1) It may go into solid solution in the iron, enhancing the strength. The general effectiveness is shown in Fig. 1.
(2) Hard carbides associated with Fe,C may be formed.
(3) It may form intermediate compounds with iron, e.g. FeCr (sigma phase), Fe,W,.
(4) It may influence the critical range in one or more of the following ways:

(a) Alter the temperature. For example, 3% nickel lowers the Ac points some 30°C, while 12% chromium raises the Ac1, temperature to about 800°C and also forms a range of 150/200°C above this in which the pearlite changes to austenite. Fig. 2 shows the effect of alloys on the eutectoid temperature.

(b) Alter the carbon content of the eutectoid (Fig. 2). The carbon content of the pearlite in a 12% chromium steel is 0,33%, as compared with 0,87 in an ordinary steel. Nickel also reduces the amount of carbon in the pearlite and consequently increases the volume of this constituent at the expense of the weaker ferrite.

(c) Alter the “critical cooling velocity”, which is the minimum cooling speed which will produce bainite or martensite from austenite. Typical critical speeds obtained by quenching from 950°C are given in Table 1.

Table 1. Effect of alloying on the critical cooling speed of steel

Carbon, %	Alloying Element, %	Cooling Speed to form Martensite, °C per sec (650°C)
0.42	0.55 Mn	550
0.40	1.60 Mn	50
0.42	1.12 Ni	450
0.40	4.80 Ni	85
0.38	2.64 Cr	10

The efficiency of the additions of the various alloy elements in reducing the effect of mass during quenching may be judged by the relative reduction of the critical velocity of the steel. Chromium and manganese respectively are far more effective than nickel.

(5) Combinations of elements can be chosen so that the volume change is reduced and also the risk of quench cracking. It may produce effeets characteristic of the alloying element.

(a) It may render the alloy sluggish to thermal changes, increasing the stability of the hardened condition and so producing tool steels which are capable of being used up to 550°C without softening and in certain cases may exhibit an increase in hardness.

(b) It may have a chemical effect on the impurities. Under suitable slag conditions vanadium, in quite small quantities, "cleans" the steel and renders it free from slag inclusions. Manganese and zirconium form sulphides.

(c) Certain elements such as chromium, Aluminium, silicon and copper tend to produce adherent oxide films on the surface of the steel which increase its resistance to corrosion and oxidation at elevated temperatures.

(d) Creep strength may be increased by the presence of a dispersion of fine carbides, e.g. molybdenum.

Classification of alloying additions

Classification of alloying metals according to their effect in the steel is difficult, because the influence varies so widely with each addition depending on the quantity used and other elements present. A useful grouping, however, is based upon the effect of the element on (a) the stability of the carbides and (b) the stability of the austenite.

(1) Elements which tend to form carbides. Chromium,tungsten,titanium, columbium, vanadium, molybdenum and manganese. The mixture of complex carbides is often referred to as cementite.

(2) Elements which tend to graphitise the carbide. Silicon, cobalt, aluminium and nickel. Only a small proportion of these elements can be added to the steel before graphite forms during processing, with attendant ruin of the properties of the steel, unless elements from group 1 are added to counteract the effect.

(3) Elements which tend to stabilise austenite. Manganese, nickel, cobalt and copper.

These elements alter the critical points of iron in a similar way to carbon by raising the A₄ point and lowering the A₃ point, thus increasing the range in which austenite is stable, and they also tend to retard the separation of carbides. They have a crystal lattice (f.c.c.) similar to that of g-iron in which they are more soluble than in a-iron.

(4) Elements which tend to stabilise ferrite. Chromium, tungsten, molybdenum, vanadium and silicon .

These elements are more soluble in a-iron than in g-iron. They diminish the amount of carbon soluble in the austenite and thus tend to increase the volume of free carbide in the steel for a given carbon content. On the binary equilibrium diagram of these elements with pure iron the A₄ point is lowered and A₃ raised (although it may be lowered initially), until the two points merge to form a “closed gamma loop”.

Thus, with above, a certain amount of each of these elements the austenite phase disappears and ferrite exists from the melting-point down to room temperature. No critical points exist and such steels (e.g. 18% chromium irons) are not amenable to normal heat treatment, except recrystallisation after cold work. This effect, however, can be counteracted by adding elements from group 3. For example, 2% of nickel is added to the 18% chromium stainless steel to enable it to be refined by normal heat-treatment; carbon has the same effect. Aluminium has the reverse effect in 12 % chromium steel.

Principles of Heat Treating of Steels

Steel is usually defined as an alloy of iron and carbon with the carbon content between a few hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels, stainless steels (>10.5%) and heat resisting CrNi steels (>18%). Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which in turn depend on the heat treatment.

The Fe-C Phase Diagram

The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig 1). Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe₃C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different combinations of carbon concentration and temperature.

We distinguish at the low-carbon end ferrite (α-iron),which can at most dissolve 0.028% C, at 727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the carbon-rich side we find cementite (Fe₃C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures.

Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite.

In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names:

• A₁, the so-called eutectoid temperature, which is the minimum temperature for austenite
• A₃, the lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α boundary
• A_cm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe₃C boundary

The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates into cementite particles dispersed with a ferrite matrix after extended holding close to A₁.

Fig. 1. The Fe-Fe3C diagram.

The Fe-C diagram in Fig 1 is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram. This is particularly useful when phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to develop.

If alloying elements are added to the iron-carbon alloy (steel), the position of the A₁, A₃, and A_cm boundaries and the eutectoid composition are changed. It suffices here to mention that

1. all important alloying elements decrease the eutectoid carbon content,
2. the austenite-stabilizing elements manganese and nickel decrease A, and
3. the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A₁.

Transformation Diagrams

The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature. That is when the diffusion of carbon and alloying elements is suppressed or limited to a very short range.

Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite. Martensite, the hardest constituent, forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing.

The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevated-temperature structure of austenite or austenite-carbide is also of importance in the heat treatment of steel.

One can conveniently describe what is happening during transformation with transformation diagrams. Four different types of such diagrams can be distinguished. These include:

• Isothermal transformation diagrams describing the formation of austenite, which will be referred to as ITh diagrams
• Isothermal transformation (IT) diagrams, also referred to as time-temperature-transformation (TTT) diagrams, describing the decomposition of austenite
• Continuous heating transformation (CRT) diagrams
• Continuous cooling transformation (CCT) diagrams

Isothermal Transformation Diagrams

This type of diagram shows what happens when a steel is held at a constant temperature for a prolonged period. The development of the microstructure with time can be followed by holding small specimens in a lead or salt bath and quenching them one at a time after increasing holding times and measuring the amount of phases formed in the microstructure with the aid of a microscope.

ITh Diagrams (Formation of Austenite). During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume decreases with the formation of the dense austenite phase. From the elongation curves, the start and finish times for austenite formation, usually defined as 1% and 99% transformation, respectively, can be derived.

IT Diagrams (Decomposition of Austenite). The procedure starts at a high temperature, normally in the austenitic range after holding there long enough to obtain homogeneous austenite without undissolved carbides, followed by rapid cooling to the desired hold temperature. The cooling was started from 850°C (1560°F). The A₁ and A₃ temperatures are indicated as well as the hardness. Above A₃ no transformation can occur. Between A₁ and A₃ only ferrite can form from austenite.

CRT Diagrams

In practical heat treatment situations, a constant temperature is not required, but rather a continuous changing temperature during either cooling or heating. Therefore, more directly applicable information is obtained if the diagram is constructed from dilatometric data using a continuously increasing or decreasing temperature.

Like the ITh diagrams, the CRT diagrams are useful in predicting the effect of short-time austenitization that occurs in induction and laser hardening. One typical question is how high the maximum surface temperature should be in order to achieve complete austenitization for a given heating rate. To high a temperature may cause unwanted austenite grain growth, which produces a more-brittle martensitic microstructure.

CCT Diagrams

As for heating diagrams, it is important to clearly state what type of cooling curve the transformation diagram was derived from.

Use of a constant cooling rate is very common in experimental practice. However, this regime rarely occurs in a practical situation. One can also find curves for so-called natural cooling rates according to Newton’s law of cooling. These curves simulate the behavior in the interior of a large part such as the cooling rate of a Jominy bar at some distance from the quenched end.

Close to the surface the characteristics of the cooling rare can be very complex. Each CCT diagram contains a family of curves representing the cooling rates at different depths of a cylinder with a 300 mm (12 in.) diameter. The slowest cooling rate represents the center of the cylinder. The more severe the cooling medium, the longer the times to which the C-shaped curves are shifted. The M, temperature is unaffected.

Fig.2. CCT (a) and TTT (b) diagrams.

It should be noted, however, that transformation diagrams can not be used to predict the response to thermal histories that are very much different from the ones used to construct the diagrams. For instance, first cooling rapidly to slightly above M_s and then reheating to a higher temperature will give more rapid transformation than shown in the IT diagram because nucleation is greatly accelerated during the introductory quench. It should also be remembered that the transformation diagrams are sensitive to the exact alloying content within me allowable composition range.

Steel-making processes

Steel is made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes.

Crucible and high-frequency methods

The Huntsman crucible process has been superseded by the high frequency induction furnace in which the heat is generated in the metal itself by eddy currents induced by a magnetic field set up by an alternating current, which passes round water-cooled coils surrounding the crucible. The eddy currents increase with the square of the frequency, and an input current which alternates from 500 to 2000 hertz is necessary. As the frequency increases, the eddy currents tend to travel nearer and nearer the surface of a charge (i.e. shallow penetration). The heat developed in the charge depends on the cross-sectional area which carries current, and large furnaces use frequencies low enough to get adequate current penetration.

Automatic circulation of the melt in a vertical direction, due to eddy currents, promotes uniformity of analysis. Contamination by furnace gases is obviated and charges from 1 to 5 tonnes can be melted with resultant economy. Consequently, these electric furnaces are being used to produce high quality steels, such as ball bearing, stainless, magnet, die and tool steels.

Figure 1. Furnaces used for making pig iron and steels. RH side of open hearth furnace shows use of oil instead of gas

Acid and basic steels

The remaining methods for making steel do so by removing impurities from pig iron or a mixture of pig iron and steel scrap. The impurities removed, however, depend on whether an acid (siliceous) or basic (limey) slag is used. An acid slag necessitates the use of an acid furnace lining (silica); a basic slag, a basic lining of magnesite or dolomite, with line in the charge. With an acid slag silicon, manganese and carbon only are removed by oxidation, consequently the raw material must not contain phosphorus and sulphur in amounts exceeding those permissible in the finished steel.

In the basic processes, silicon, manganese, carbon, phosphorus and sulphur can be removed from the charge, but normally the raw material contains low silicon and high phosphorus contents. To remove the phosphorus the bath of metal must be oxidised to a greater extent than in the corresponding acid process, and the final quality of the steel depends very largely on the degree of this oxidation, before deoxidisers-ferro-manganese, ferro-silicon, aluminium-remove the soluble iron oxide and form other insoluble oxides, which produce non-metallic inclusions if they are not removed from the melt:

2Al + 3FeO (soluble) « 3Fe + Al2O3 (solid)

In the acid processes, deoxidation can take place in the furnaces, leaving a reasonable time for the inclusions to rise into the slag and so be removed before casting. Whereas in the basic furnaces, deoxidation is rarely carried out in the presence of the slag, otherwise phosphorus would return to the metal. Deoxidation of the metal frequently takes place in the ladle, leaving only a short time for the deoxidation products to be removed. For these reasons acid steel is considered better than basic for certain purposes, such as large forging ingots and ball bearing steel. The introduction of vacuum degassing hastened the decline of the acid processes.

Bessemer steel

In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity (Fig. 1). The oxidation of the impurities raises the charge to a suitable temperature; which is therefore dependent on the composition of the raw material for its heat: 2% silicon in the acid and 1,5-2% phosphorus in the basic process is normally necessary to supply the heat. The "blowing" of the charge, which causes an intense flame at the mouth of the converter, takes about 25 minutes and such a short interval makes exact control of the process a little difficult.

The Acid Bessemer suffered a decline in favour of the Acid Open Hearth steel process, mainly due to economic factors which in turn has been ousted by the basic electric arc furnace coupled with vacuum degassing.

The Basic Bessemer process is used a great deal on the Continent for making, from a very suitable pig iron, a cheap class of steel, e.g. ship plates, structural sections. For making steel castings a modification known as a Tropenas converter is used, in which the air impinges on the surface of the metal from side tuyeres instead of from the bottom. The raw material is usually melted in a cupola and weighed amounts charged into the converter.

Open-hearth processes

In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas. But the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating.

The furnaces have a saucer-like hearth, with a capacity which varies from 600 tonnes for fixed, to 200 tonnes for tilting furnaces (Fig. 1). The raw materials consist essentially of pig iron (cold or molten) and scrap, together with lime in the basic process. To promote the oxidation of the impurities iron ore is charged into the melt although increasing use is being made of oxygen lancing. The time for working a charge varies from about 6 to 14 hours, and control is therefore much easier than in the case of the Bessemer process.

The Basic Open Hearth process was used for the bulk of the cheaper grades of steel, but there is a growing tendency to replace the OH furnace by large arc furnaces using a single slag process especially for melting scrap and coupled with vacuum degassing in some cases.

Electric arc process

The heat required in this process is generated by electric arcs struck between carbon electrodes and the metal bath (Fig. 1). Usually, a charge of graded steel scrap is melted under an oxidising basic slag to remove the phosphorus. The impure slag is removed by tilting the furnace. A second limey slag is used to remove sulphur and to deoxidise the metal in the furnace. This results in a high degree of purification and high quality steel can be made, so long as gas absorption due to excessively high temperatures is avoided. This process is used extensively for making highly alloyed steel such as stainless, heat-resisting and high-speed steels.

Oxygen lancing is often used for removing carbon in the presence of chromium and enables scrap stainless steel to be used. The nitrogen content of steels made by the Bessemer and electric arc processes is about 0,01-0,25% compared with about 0,002-0,008% in open hearth steels.

Oxygen processes

The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air. In Austria the LID process (Linz-Donawitz) converts low phosphorus pig iron into steel by top blowing with an oxygen lance using a basic lined vessel (Fig. 2b). To avoid excessive heat scrap or ore is added. High quality steel is produced with low hydrogen and nitrogen (0,002%). A further modification of the process is to add lime powder to the oxygen jet (OLP process) when higher phosphorus pig is used.

Figure 2.

The Kaldo (Swedish) process uses top blowing with oxygen together with a basic lined rotating (30 rev/min) furnace to get efficient mixing (Fig. 2a). The use of oxygen allows the simultaneous removal of carbon and phosphorus from the (P, 1,85%) pig iron. Lime and ore are added. The German Rotor process uses a rotary furnace with two oxygen nozzles, one in the metal and one above it (Fig. 2c). The use of oxygen with steam (to reduce the temperature) in the traditional basic Bessemer process is also now widely used to produce low nitrogen steel. These new techniques produce steel with low percentages of N, S, P, which are quite competitive with open hearth quality.

Other processes which are developing are the Fuel-oxygen-scrap, FOS process, and spray steelmaking which consists in pouring iron through a ring, the periphery of which is provided with jets through which oxygen and fluxes are blown in such a way as to "atomise" the iron, the large surface to mass ratio provided in this way giving extremely rapid chemical refining and conversion to steel.

Vacuum degassing is also gaining ground for special alloys. Some 14 processes can be grouped as stream, ladle, mould and circulation (e.g. DH and RH) degassing methods, Fig. 3. The vacuum largely removes hydrogen, atmospheric and volatile impurities (Sn, Cu, Pb, Sb), reduces metal oxides by the C – O reaction and eliminates the oxides from normal deoxidisers and allows control of alloy composition to close limits. The clean metal produced is of a consistent high quality, with good properties in the transverse direction of rolled products. Bearing steels have greatly improved fatigue life and stainless steels can be made to lower carbon contents.

Figure 3. Methods of degassing molten steel

Vacuum melting and ESR. The aircraft designer has continually called for new alloy steels of greater uniformity and reproducibility of properties with lower oxygen and sulphur contents. Complex alloy steels have a greater tendency to macro-segregation, and considerable difficulty exists in minimising the non-metallic inclusions and in accurately controlling the analysis of reactive elements such as Ti, Al, B. This problem led to the use of three processes of melting.

(a) Vacuum induction melting within a tank for producing super alloys (Ni and Co base), in some cases for further remelting for investment casting. Pure materials are used and volatile tramp elements can be removed.
(b) Consumable electrode vacuum arc re-melting process (Fig. 4) originally used for titanium, was found to eliminate hydrogen, the A and V segregates and also the large silicate inclusions. This is due to the mode of solidification. The moving parts in aircraft engines are made by this process, due to the need for high strength cleanness, uniformity of properties, toughness and freedom from hydrogen and tramp elements.
(c) Electroslag refining (ESR) This process, which is a larger form of the original welding process, re-melts a preformed electrode of alloy into a water-cooled crucible, utilising the electrical resistance heating in a molten slag pool for the heat source (Fig. 5). The layer of slag around the ingot maintains vertical unidirectional freezing from the base. Tramp elements are not removed and lead may be picked up from the slag.

Figure 4. Typical vacuum arc remelting furnace

Figure 5. Electroslag remelting furnace