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.