Corrosion

Introduction

Prince Edward Island soil is famous for its red color. Remember “Bud the Spud from the
Bright Red Mud”? A well-known chemical reaction is actually responsible for this
characteristic color. The actual process is known as corrosion. The spontaneous destructive
oxidation of metals is called corrosion. Corrosion occurs whenever a metal surface is
destroyed by being converted to a metal compound. The reaction is actually more
complicated than most people think.

Elements of the Earth

Roughly 92 chemical elements are known to exist in earth’s crust. Most of these elements
have combined with one or more other elements to form compounds known as minerals.
There are numerous possible combinations of elements and up to 2000 minerals have been
discovered. These minerals exist in mixtures which form the rocks of the earth. Relatively
few of these elements and minerals are of real importance in soils. Approximately 98% of
the earth is composed of only eight chemical elements, most of which is oxygen (O) or
Silicon (Si). The most abundant minerals in soils are light in color. If all soils were
composed of crushed minerals that had undergone little chemical change they would be light
gray. It is obvious that not all soils are gray. The brown, red, and yellow colors of soils are
caused by chemical changes in the elements that make up these minerals. Iron(Fe) is the
element responsible for the chemical changes that occur in Prince Edward Island soil.

Reactions that cause Rusting

To understand what happens in a field or backyard, the events of corrosion must first be
explained. Iron rusts only when there is water and oxygen present. Rust is a complicated
material that contains various types of hydrated iron (III) oxide, Fe O CxH O. Iron begins 2 3 2
to rust at places on its surface where there is an impurity, or where the iron lattice has
imperfections. At these points some of the iron atoms produce iron (II) ions in the solution:

Fe (s) --> Fe(2+) (aq) + 2e-

Here the iron has undergone oxidation. Oxidation is the loss of electrons by ions. As the
iron(II) ions move away they meet hydroxide ions and produce iron (II) hydroxide:

Fe(2+) (aq) + 2OH(-) (aq) --> Fe(OH)2 (s)

Dissolved oxygen will then oxidize the iron (II) hydroxide producing the substance called
rust:

Fe(OH)2 (s) + dissolved oxygen ---> rust (Fe2 O3)

The electrons liberated from the process are taken up by hydrogen ions in the water
producing gas. This is a reduction reaction:

2H(+) (aq) + 2e(-) --> H2 (g)

For a drop of water on an iron surface, rusting will occur near the edges of the drop. This
is because there is more oxygen dissolved from the air near the edges of the drop.

Iron

Iron is the fourth most common element in soil, comprising 5% of the earth’s crust. The iron
in soil is usually found in the soluble cation form (Fe ). This reduced form is more common 2+
because of the lower levels of oxygen in soil. This ion can be readily absorbed by plants.
When high levels of oxygen are present in the air surrounding soil particles, oxidation occurs
and the Fe form of iron prevails. This form of iron is insoluble and therefore not available 3+
to plants. Usually in acid soil sufficient Fe exists in the soil to meet the needs of plants. 2+
However, iron deficiencies are common in alkaline soil. The greater concentration of
hydroxyl causes the oxidation of iron.

Why the Red Soil?

Iron oxides are responsible for the red soil on Prince Edward Island. It is possible to trace
the reactions of iron from the time it is released from rock. Iron olivine is a good example
of a rock which contains iron. This iron can be released due to environmental conditions.
Weathering of iron olivine leads to hydrolysis yielding iron oxide and silicic acid:

Fe2 SiO4 + 2HOH --> 2FeO + H4SiO4

Both of these products are somewhat soluble and can be lost be leaching. However, in the
presence of free oxygen, and when moisture and temperature conditions are favorable for
chemical activity, the iron in the soil minerals is oxidized and hydrated into red and yellow
compounds. The iron oxide (FeO) is oxidized to only slightly soluble iron oxides such as
Fe2O3 or its hydrated counterpart Fe2O3 --> xH2O (the x indicates that the quantity of associated water can vary). This is oxidation reaction:

4FeO + O2 --> 2Fe2O3

Because of the extremely low solubility of these iron oxides, very little of the iron is lost.
This results in a characteristic red color of the soil where the reaction occurs.

Protection from Rusting

There are a few basic methods for protecting metals from corrosion:
One is to slow down the process. Slowing down the corrosion process is done with
protective coatings such as paint or tar. These help to keep out oxygen, water, and
electrolyte salts. The presence of small salt crystals in the air is the major reason why metal
corrodes more rapidly at seacoasts.
Cathodic protection from corrosion occurs when a metal to be protected is coupled with a
metal more easily oxidized than itself. Metal fences, sheets, and nails made of iron can be
protected by galvanizing them. These materials are coated with zinc and said to be
galvanized. The galvanized metal will not corrode until after the zinc coating does because
zinc corrodes more readily than iron. Instead of Fe ions going into solution, Zn ions are 2+ 2+
lost from the zinc. The iron remains unaffected. Tin is also very good at protecting iron and
steel. This is especially evident with tin cans. It is more difficult to plate steel with a thin
layer of zinc than tin. Also, tin is less reactive than zinc and is less likely to dissolve in the
liquids stored in cans. However, tin is not as effective in protection and it will rust if it is
holed.
In anodic protection, the metal to be protected is briefly made positive to form a stable oxide
film on its surface. The stable oxide film then protects the underlying metal from corrosion.
Stainless steels form a protective film of nickel/chromium oxides since they have a high
content of these metals

Invention of Stainless Steel

Stainless steel, the name doesnt sound like a puzzle of alien world. Everyone must be familiar of this metallic alloy and its uses in our daily life. Main property which makes it so popular and useful is its excellent corrosion resistance, which is attibuted to the presence of metal Chromium in this alloy system. Infact, stainless steel is defined as ferrous alloy containing more than 10.5 % Chromium (by weight). Other main alloying elements are Molybdenum and Nickel.

Just like other major inventions, stainless steel was also an accidental invention. This is accredited to Harry Brearly from Sheffield, UK in 1913. Interesting fact is that he left school at the age of 12 and then by private studies and night school he became an expert in steel analysis. In 1912 Brearley was asked to help in the problems being encountered by a small arms manufacturer, whereby the internal diameter of rifle barrels was eroding away too quickly because of the action of heating and discharge gases. Brearley was therefore looking for a steel with better resistance to erosion, not corrosion. As a line of investigation he decided to experiment with steels containing chromium, as these were known to have a higher melting point than ordinary steels.

He made a number of different melts of 6 to 15% chromium with varying carbon contents. The first true stainless steel was melted on the 13th August 1913. It contained 0.24% carbon and 12.8% chromium. In order to examine the grain structure of the steel he needed to etch (attack with acid) samples before examining them under the microscope. The etching reagents were based on nitric acid, and he found that this new steel strongly resisted chemical attack. He then exposed samples to vinegar and other food acids such as lemon juice and found the same result. Thus, a corrosion resistant variety of steel was born, which was later named as Stainless Steel.

6 DIFFERENT FORGING TECHNIQUES

Cold Work

Cold Working (FIA) — Permanent plastic deformation of a metal at a temperature below its
recrystallization point—low enough to produce strain hardening. Usually, but not necessarily, conducted
at room temperature. Also referred to as cold forming or cold forging. Contrast with hot working.
Cold Stamp (CDA): To restrike a forging cold in order to hold to closer tolerance, sharpen corners or
outlines, reduce section thickness, flatten some particular surface, increase hardness, or add lettering.

Cross-Grain Forge

Cross Forging (FIA) — Preliminary working of forging stock in alternate planes, usually on flat dies, to
develop mechanical properties, particularly in the center portions of heavy sections.

Hot Forge

Hot Forging (FIA) — Same as hot working—plastically deforming an alloy at a temperature above its
recrystallization point, i.e., high enough to avoid strain hardening.
Hot Press Forging (CDA): A method of forming parts by pressing a heated slug, cut from wrought
material, in a closed–impression die.

Upset & Cross-Grain Forge

This method of forging is a combination of Cross-Grain forging in that the work is done on alternate
plains, but also as in Upset Forging it is used to increase the cross-sectional area of a portion or all of the
stock.

Upset Forge

Upset Forging (FIA) — (1) A forging made by upsetting an appropriate length of bar, billet or bloom. (2)
Working metal to increase the cross-sectional area of a portion or all of the stock. (3) A forging formed by
heading or gathering the material by pressure upon hot or cold metal between dies operated in a
horizontal plane. This method is not used as frequently for copper alloys as it is for steels for example
due to the ability of copper to be easily extruded.

Warm Work

Warm Forging (FIA) — Deformation at elevated temperatures below the recrystallization temperature.
The flow stress and rate of strain hardening are reduced with increasing temperature; thus, lower forces
are required than in cold working. For steel, the temperatures range from about 1000° F to just below the
normal hot working range of 1900 to 2300° F. See also Cold Working and Hot Working.

Solid-state welding

Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.

Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, such as the welding of aluminum with steel in ship hulls or compound plates. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, friction welding (including friction stir welding), high frequency welding, hot pressure welding, induction welding, and roll welding.

Energy beam welding

Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties.

Resistance welding

Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1000–100,000 ) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.

Spot Welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel. Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.

Gas welding

The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals.^[22] Other gas welding methods, such as air acetylene welding, oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used. A water torch is sometimes used for precision welding of items such as jewelry. Gas welding is also used in plastic welding, though the heated substance is air, and the temperatures are much lower.

Arc welding

(DC) or These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either directalternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.

Power supplies

To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.

The type of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds.Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds.Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.

Processes

One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO₂ gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.

The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. An operator can become reasonably proficient with a modest amount of training and can achieve mastery with experience. Weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though speciality electrodes have made possible the welding of cast iron, nickel, aluminium, copper, and other metals. Inexperienced operators may find it difficult to make good out-of-position welds with this process.

Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. As with SMAW, reasonable operator proficiency can be achieved with modest training. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. Also, the smaller arc size compared to the shielded metal arc welding process makes it easier to make out-of-position welds (e.g., overhead joints, as would be welded underneath a structure).

The equipment required to perform the GMAW process is more complex and expensive than that required for SMAW, and requires a more complex setup procedure. Therefore, GMAW is less portable and versatile, and due to the use of a separate shielding gas, is not particularly suitable for outdoor work. However, owing to the higher average rate at which welds can be completed, GMAW is well suited to production welding. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.

A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration.

Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding (also sometimes erroneously referred to as heliarc welding), is a manual welding process that uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds.

GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process, and furthermore, it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.

Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessels. Other arc welding processes include atomic hydrogen welding, carbon arc welding, electroslag welding, electrogas welding, and stud arc welding.

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Welding

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater and in space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, poisonous fumes, and overexposure to ultraviolet light.

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.

Work hardening

Work hardening, or strain hardening, is an increase in mechanical strength due to plastic deformation. In metallic solids, permanent change of shape is usually carried out on a microscopic scale by defects called dislocations which are created by stress and rearrange the material by moving through it. At low temperature, these defects do not anneal out of the material (within a sufficient amount of time, since the relaxation of these defects is a very thermodynamically spontaneous albeit slow process), but build up as the material is worked, interfering with one another's motion; strength is increased thereby, and ductility decreased by considerable amount.

Any material with a reasonably high melting point can be strengthened in this fashion. It is often exploited to harden alloys that are not amenable to heat treatment, including low-carbon steel. Conversely, since the low melting point of indium makes it immune to work hardening at room temperature, it can be used as a gasket material in high-vacuum systems.

Often, work hardening is carried out by the same process that shapes the metal into its final form, including cold rolling (contrast hot rolling) and cold drawing. Techniques have also been designed to maintain the general shape of the workpiece during work hardening, including shot peening and constant channel angular pressing. A material's work hardenability can be predicted by analyzing a stress-strain curve, or studied in context by performing a hardness test before and after the proposed cold work process.

Cold forming is a type of cold work that involves forging operations, such as extrusion, drawing or coining, that are performed at low temperatures. Cold work may also refer to the process through which a material is given this quality. Such deformation increases the concentration of dislocations which may subsequently form low-angle grain boundaries surrounding sub-grains. Cold work generally results in a higher yield strength as a result of the increased number of dislocations and the Hall-Petch effect of the sub-grains. However, there is a simultaneous decrease in the ductility. The effects of cold working may be removed by annealing the material at high temperatures where recovery and recrystallization reduce the dislocation density.

Dislocation

Dislocation

In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of real materials. The theory was originally developed by Vito Volterra in 1905.

Some types of dislocations can be visualised as being caused by the termination of a plane of atoms in the middle of a crystal. In such a case, the surrounding planes are not straight, but instead bend around the edge of the terminating plane so that the crystal structure is perfectly ordered on either side. The analogy with a stack of paper is apt: if a half a piece of paper is inserted in a stack of paper, the defect in the stack is only noticeable at the edge of the half sheet.

There are two primary types: edge dislocations and screw dislocations. Mixed dislocations are intermediate between these.

Mathematically, dislocations are a type of topological defect, sometimes called a soliton. The mathematical theory explains why dislocations behave as stable particles: they can be moved about, but maintain their identity as they move. While two dislocations of opposite orientation, when brought together, can cancel each other (this is the process of annealing), there is no way a single dislocation can "disappear" on its ow

Dislocation geometry

A dislocation can be visualized by imagining cutting a crystal along a plane and slipping one half across the other by a lattice vector. The halves will fit back together without leaving a defect. But if the cut only goes part way though the crystal, the boundary of the cut will leave a defect, distorting the nearby lattice. This boundary is the line of the dislocation; the direction of the slip is the Burgers vector.

Dislocations are labeled by the angle between the dislocation line and the Burgers vector. The special cases of 90° and 0° are known as edge and screw dislocations. The dislocations present in real crystalline solids are generally mixed rather than edge or screw; the actual angles of dislocations depend on the lattice structure.

The Burgers vector for an edge dislocation is marked in black in Figure D. It is perpendicular to the dislocation line (marked in blue in Figure D) in the case of the edge, and parallel to it in the case of the screw. In metallic materials, b is aligned with close-packed crystallographic directions and its magnitude is equivalent to one interatomic spacing.

Edge dislocations

Alternatively, edge dislocations can be visualised as being formed by adding an extra half-plane of atoms to a perfect crystal, so that a defect is created in the regular crystal structure along the line where the extra half-plane ends (Figure 1). Such visualisations can be difficult to interpret. Initially, it can be helpful to follow the process of simplification involved in arriving at such representations. One approach is to begin by considering a 3-d representation of a perfect crystal lattice, with the atoms represented by spheres (Figure A). The viewer may then start to simplify the representation by visualising planes of atoms instead of the atoms themselves (Figures B and C).

Finally a simple schematic diagram of such atomic planes can be used to illustrate lattice defects such as dislocations. (Figure D represents the "extra half-plane" concept of an edge type dislocation).

The stresses caused by an edge dislocation are complex due to its inherent asymmetry. These stresses are described by three equations¹:

where μ is the shear modulus of the material, b is the Burgers vector, ν is Poisson's ratio and x and y are coordinates. These equations suggest a vertically oriented dumbbell of stresses surrounding the dislocation, with compression experienced by the atoms near the "extra" plane, and tension experienced by those atoms near the "missing" plane¹.

Screw dislocations

Screw dislocations are more difficult to visualize, but can be considered as being formed by the insertion of a "parking garage ramp" that extends to the "edges of the garage" into an otherwise perfectly layered structure. Basically it comprises a structure in which a helical path is traced around the linear defect (dislocation line) by the atomic planes in the crystal lattice (Figure E).

Despite the difficulty in visualization, the stresses caused by a screw dislocation are less complex than those of an edge dislocation. These stresses need only one equation, as symmetry allows only one radial coordinate to be used¹:

where μ is the shear modulus of the material, b is the Burgers vector, and r is a radial coordinate. This equation suggests a long cylinder of stress radiating outward from the cylinder and decreasing with distance. Please note, this simple model results in an infinite value for the core of the dislocation at r=0 and so it is only valid for stresses outside of the core of the dislocation.¹

Observation of Dislocations

Transmission Electron Micrograph of Dislocations

When a dislocation line intersects the surface of a metallic material, the associated strain field locally increases the relative susceptibility of the material to acidic etching and an etch pit of regular geometrical format results. If the material is strained (deformed) and repeatedly re-etched, a series of etch pits can be produced which effectively trace the movement of the dislocation in question.

Transmission electron microscopy can be used to observe dislocations within the microstructure of the material. Thin foils of metallic samples are prepared to render them transparent to the electron beam of the microscope. The electron beam suffers diffraction by the regular crystal lattice planes of the metal atoms and the differing relative angles between the beam and the lattice planes of each grain in the metal's microstructure result in image contrast (between grains of different crystallographic orientation). The less regular atomic structures of the grain boundaries and in the strain fields around dislocation lines have different diffractive

Transmission Electron Micrograph of Dislocations
properties than the regular lattice within the grains, and therefore present different contrast effects in the electron micrographs. (The dislocations are seen as dark lines in the lighter, central region of the micrographs on the right). Transmission electron micrographs of dislocations typically utilize magnifications of 50,000 to 300,000 times (though the equipment itself offers a wider range of magnifications than this). Some microscopes also permit the in-situ heating and/or deformation of samples, thereby permitting the direct observation of dislocation movement and their interactions. Note the charcteristic 'wiggly' contrast of the dislocation lines as they pass through the thickness of the material. Note also that a dislocation cannot end within a crystal; the dislocation lines in these images end at the sample surface. A dislocation can only be contained within a crystal as a complete loop.

Field ion microscopy and atom probe techniques offer methods of producing much higher magnifications (typically 3 million times and above) and permit the observation of dislocations at an atomic level. Where surface relief can be resolved to the level of an atomic step, screw dislocations appear as distinctive spiral features - thus revealing an important mechanism of crystal growth: where there is a surface step, atoms can more easily add to the crystal, and the surface step associated with a screw dislocation is never destroyed no matter how many atoms are added to it.

(By contrast, traditional optical microscopy, which is not appropriate for the observation of dislocations, typically offers magnifications up to a maximum of only around 2000 times).

After chemical etching, small pits are formed where the etching solution preferentially attacks the more highly strained material around the dislocations. Thus, the image features indicate points at which dislocations intercept the sample surface. In this way, dislocations in silicon, for example, can be observed indirectly using an interference microscope. Crystal orientation can be determined by the shape of dislocations - 100 elliptical, 111 - triangular (pyramidal).

Sources of Dislocations

Dislocation density in a material can be increased by plastic deformation by the following relationship: . Since the dislocation density increases with plastic deformation, a mechanism for the creation of dislocations must be activated in the material. Three mechanisms for dislocation formation are formed by homogeneous nucleation, grain boundary initiation, and interfaces the lattice and the surface, precipitates, dispersed phases, or reinforcing fibers.

The creation of a dislocation by homogeneous nucleation is a result of the rupture of the atomic bonds along a line in the lattice. A plane in the lattice is sheared, resulting in 2 oppositely faced half planes or dislocations. These dislocations move away from each other through the lattice. Since homogeneous nucleation forms dislocations from perfect crystals and requires the simultaneous breaking of many bonds, the energy required for homogeneous nucleation is high. For instance the stress required for homogeneous nucleation in copper has been shown to be , where G is the shear modulus of copper (46 GPa). Solving for τ_hom, we see that the required stress is 3.4 GPa, which is very close to the theoretical strength of the crystal. Therefore, in conventional deformation homogeneous nucleation requires a concentrated stress, and is very unlikely. Grain boundary initiation and interface interaction are more common sources of dislocations.

Irregularities at the grain boundaries in materials can produce dislocations which propagate into the grain. The steps and ledges at the grain boundary are an important source of dislocations in the early stages of plastic deformation.

The surface of a crystal can produce dislocations in the crystal. Due to the small steps on the surface of most crystals, stress in certain regions on the surface is much larger than the average stress in the lattice. The dislocations are then propagated into the lattice in the same manner as in grain boundary initiation. In monocrystals, the majority of dislocations are formed at the surface. The dislocation density 200 microns into the surface of a material has been shown to be six times higher than the density in the bulk. However, in polycrystalline materials the surface sources cannot have a major effect because most grains are not in contact with the surface.

The interface between a metal and an oxide can greatly increase the number of dislocations created. The oxide layer puts the surface of the metal in tension because the oxygen atoms squeeze into the lattice, and the oxygen atoms are under compression. This greatly increases the stress on the surface of the metal and consequently the amount of dislocations formed at the surface. The increased amount of stress on the surface steps results in an increase of dislocations^[1].

Dislocations, slip and plasticity

Until the 1930s, one of the enduring challenges of materials science was to explain plasticity in microscopic terms. A naive attempt to calculate the shear stress at which neighbouring atomic planes slip over each other in a perfect crystal suggests that, for a material with shear modulus G, shear strength τ_m is given approximately by:

As shear modulus in metals is typically within the range 20 000 to 150 000 MPa, this is difficult to reconcile with shear stresses in the range 0.5 to 10 MPa observed to produce plastic deformation in experiments.

In 1934, Egon Orowan, Michael Polanyi and G. I. Taylor, roughly simultaneously, realized that plastic deformation could be explained in terms of the theory of dislocations. Dislocations can move if the atoms from one of the surrounding planes break their bonds and rebond with the atoms at the terminating edge. Even a simple model of the force required to move a dislocation shows that shear is possible at much lower stresses than in a perfect crystal. (Hence, the characteristic malleability of metals).

When metals are subjected to "cold working" (deformation at temperatures which are relatively low as compared to the material's absolute melting temperature, T_m, i.e., typically less than 0.3 T_m) the dislocation density increases due to the formation of new dislocations and dislocation multiplication. The consequent increasing overlap between the strain fields of adjacent dislocations gradually increases the resistance to further dislocation motion. This causes a hardening of the metal as deformation progresses. This effect is known as strain hardening (also “work hardening”). Tangles of dislocations are found at the early stage of deformation and appear as non well-defined boundaries; the process of dynamic recovery leads eventually to the formation of a cellular structure containing boundaries with misorientation lower than 15º (low angle grain boundaries).

The effects of strain hardening by accumulation of dislocations and the grain structure formed at high strain can be removed by appropriate heat treatment (annealing) which promotes the recovery and subsequent recrystallisation of the material.

Dislocation Climb

Dislocations can slip in planes containing both the dislocation and the Burgers Vector. For a screw dislocation, the dislocation and the Burgers vector are parallel, so the dislocation may slip in any plane containing the dislocation. For an edge dislocation, the dislocation and the Burgers vector are perpendicular, so there is only one plane in which the dislocation can slip. There is an alternative mechanism of dislocation motion, fundamentally different from slip, that allows an edge dislocation to move out of its slip plane, known as dislocation climb. Dislocation climb allows an edge dislocation to move perpendicular to its slip plane.

The driving force for dislocation climb is the movement of vacancies through a crystal lattice. If a vacancy moves next to the boundary of the extra half plane of atoms that forms an edge dislocation, the atom in the half plane closest to the vacancy can "jump" and fill the vacancy. This atom shift "moves" the vacancy in line with the half plane of atoms, causing a shift, or positive climb, of the dislocation. The process of a vacancy being absorbed at the boundary of a half plane of atoms, rather than created, is known as negative climb. Since dislocation climb results from individual atoms "jumping" into vacancies, climb occurs in single atom diameter increments.

During positive climb, the crystal shrinks in the direction perpendicular to the extra half plane of atoms because atoms are being removed from the half plane. Since negative climb involves an addition of atoms to the half plane, the crystal grows in the direction perpendicular to the half plane. Therefore, compressive stress in the direction perpendicular to the half plane promotes positive climb, while tensile stress promotes negative climb. This is one main difference between slip and climb, since slip is caused by only shear stress.

One additional difference between dislocation slip and climb is the temperature dependence. Climb occurs much more rapidly at high temperatures than low temperatures due to an increase in vacancy motion. Slip, on the other hand, has only a small dependence on temperature.

Scanning Tunneling Microscope - Gallery Image gallery, including a dislocations page, seen at the atomic level of metal surfaces, by the surface physics group at the Faculty of Physics, Vienna University of Technology, Austria.

NDT Inspection of Reformer Tubes

TCR has secured a project to undertake NDT Inspection of 200 Reformer Tubes at a very prestigious client in India. As part of this project, the NDT Services division will depute Engineer / Technicians along with Instruments and Consumables at the site location.

As part of the project, a Sr. Inspection Engineer from TCR will conduct in-depth visual examination and then proceed to take measurement of OD at every 3 meters and at ends in two places perpendicular to each other, i.e. at 8 locations on a 9 meter long tube for each of the 200 tubes. Ultrasonic thickness measurement at 12,3,6 & 9 O’Clock position at both ends and 3m from each end, i.e. at 16 points on each of 200 tubes. Portable hardness measurement at all 200 tubes at 3 locations on each tube. Ferrite measurement at 3 locations on each tube. In-situ metallography at 10 locations selected by the Client’s officers out of 200 tubes.

RIDSDALE-DIETERT HAND OPERATED UNIVERSAL SAND STRENGTH MACHINE (METRIC)

I GENERAL PRINCIPLES

The Universal Sand Strength Machine, together with the appropriate accessories, will determine the compression, shear, tensile, transverse and splitting strengths of moulding and core making materials by means of dead weight loading.

II DESCRIPTION

This machine consists of three major parts: frame, pendulum weight and pusher arm. The pusher arm is motivated by means of a small handwheel which, through a gear box, rotates a pinion engaged in a rack on the quadrant. The pendulum weight swings on ball bearings and can be moved by the pusher arm, via a test specimen, from a vertical position, through 90°, to a horizontal position, with a consequent increase of load on the test specimen. A magnetic rider is moved up a calibrated scale by the pendulum weight and indicates the point at which specimen collapse occurs. The machine is calibrated in kN/m2 for 50 mm diameter x 50 mm height standard sand specimens. The accessories required for the determination of shear, dry, tensile, transverse and splitting strengths are described separately.

III INSTALLATION

(a) Set the machine on a rigid bench and level by means of the two adjusting screws until the bubble of the spirit level is centred. The front edge of the pusher plate should now coincide with the ‘O’ line on the scale and the pendulum weight should swing freely in the frame, with the pusher plate just clearing the scale.

(b) Place the coil spring in the hole in the gear box cover plate with the small brass wear pad on the protruding end of the spring. Ensure that the felt washer is in position in the recess of the hand-wheel boss and place the hand-wheel on the pinion shaft. Adjust until the felt washer is nipped lightly between the hand-wheel and the gear box cover plate and secure with the set screw in the hand-wheel boss, ensuring that this locates on the flat on the pinion shaft.

IV TEST PROCEDURE

(A) Green Compression Strength

• (a) Place the compression heads in the position shown on the illustration.
• (b) Raise the weight arm slightly and insert a metric standard 50 mm diameter x 50 mm height test specimen between the compression heads so that the face that was uppermost in the ramming operation is facing the right-hand compression head. Care should be taken not to damage the specimen.

• (c) See that the magnetic rider is resting against the pusher plate and that there is at least 6 mm clearance between the rubber bumper and the lug on the weight arm. If this clearance is not sufficient, it means that the specimen is smaller than the permitted tolerance and should be discarded.

• (d) Apply a load to the specimen by turning the hand-wheel at a uniform rate (approximately 25 kN/m2 green compression in 10 seconds)* until the specimen collapses.

• (e) Record the reading shown on the lower edge of the magnetic rider, reading the scale designated “Green Compression Strength”.

• (f) Return the weight to zero by reversing the rotation of the hand-wheel.Remove the sand from the compression heads.

* This loading rate applies to all tests on the machine.

IV TEST PROCEDURE (cont’d)

(B) Green Shear Strength

• (a) Place the shear test heads in the lower position in the machine, with the head having the half round holder attached to it in the pusher arm.
• (b) Raise the weight arm slightly and insert a metric standard 50 mm diameter x 50 mm height specimen between the heads.

• (c) Ensure that the magnetic rider is resting against the pusher arm and that there is 6 mm clearance between the rubber bumper and the lug on theweight arm.
• (d) Apply the load uniformly until the specimen shears.

• (e) Read the lower edge of the magnetic rider on the scale designated “Green Shear”.
• (f) Remove the sheared specimen as under (A) “Green Compression Strength”, section (f).

(C) Dry Compression and Dry Shear Strengths

• (a) Place either the compression heads or the shear heads in the top position of the machine. This position increases the load applied by a factor of 5.
• (b) Prepare metric standard 50 mm diameter x 50 mm height test specimens in the usual way and dry in an oven at 110 °C for 2 hours.

• (c) When cool, place in position between test heads and adjust clearance between rubber bumper and lug on weight arm to approximately 13 mm using the adjusting screw in the pusher arm.
• (d) Apply the load as for “Green Compression” and “Green Shear” until the specimen collapses.

• (e) Read the scale designated “Dry Compression” or “Dry Shear” according to the test heads being used.
• (f) Remove the broken specimen as under (A) “Green Compression Strength”, Section (f).
  

V MAINTENANCE

Keep the machine clean, removing surplus sand and pieces of broken specimens with a soft brush after each test. Oil the hand-wheel shaft once a month by means of the spring loaded oiler located at the top of the gear box behind the hand-wheel. Lightly grease the path of the hand-wheel brake pad from time to time to ensure smooth operation whilst loading the specimen.
The mainshaft ball journals and the gear box of the pusher arm are pre-packed with grease and require no attention.
The gear rack should be free from grease to prevent sand sticking to it. It is important that the rubber bumper is in good condition to absorb the shock when the specimen breaks and thus prevent damage to the gears. Replace when it has worn down to 3 mm thickness.

VI RECOMMENDED SPARES

Part No Description

403 Adjusting screw – pusher arm
404 Ball journal – mainshaft - weight
406 Levelling screw – main frame
409 Gear rack – main frame
410 Scale – main frame
411 Magnetic rider – scale – main frame
412 Pusher plate - weight
413 Rubber bumper – pusher arm
414 Handwheel
415 Felt washer - handwheel
417 Drive pinion – gear box – pusher arm
421 Brake spring - handwheel
422 Compression heads – pusher arm - weight
424 Rack pinion – gear box – pusher arm
425 Reduction gear – gear box – pusher arm
426 Pad – brake spring – handwheel.

Welding Certification and Welder Qualification Services from TCR

TCR Engineering Services has expanded its quality assurance and third party inspection services to include a comprehensive welder certification and welding procedure qualification program. As part of this enhanced service offering, TCR will undertake the following:

· Welder Qualification Testing for performance qualification and certification of welders (a welder / welding operator performance qualification - WQT) to ASME, ANSI, AWS, API code

· Preparation of Weld Procedure Qualification as per client or project requirements.

· Coupon Testing as per Weld Procedure Qualification which includes visual examination, mechanical testing, metallographic examination and non destructive testing.

· Documentation of the Procedure Qualification Record as per ASME, ANSI, AWS, API codes

· In depth weld inspection to include review of the applicable qualification e.g. weld procedure specification, welder performance qualification and validity for process materials and consumable items, equipment, set up and other factors, including certificates of calibration and/or conformity governing the work.

· Check safety of set up and operation having due regard for self, welder and other workers in vicinity, particular in respect of ultraviolet radiation from arc during welding.

The welding inspector deployed at a site from TCR will be responsible for monitoring and verifying that the following functions of the work conform in all aspects to the specific requirements of the relevant code, specification and/or standard:

• - Check correct weld procedure(s) employed.
• - Check weld procedure and welder qualifications.
• - Inspect weld profile preparation.
• - Inspect joint fit-up
• - Check filler metals and consumable materials
• - Check correct welding performance parameters observed
• - Perform visual examination upon completion of welding
• - Monitor pre and post weld heat treatment, where specified.

• - Monitor the physical examination including non-destructive test, hydrostatic test, mechanical test etc. If needed, the inspector may choose to send test coupons to the TCR Engineering Services' material testing laboratory.
• - Ensure that all necessary visual inspection is completed and verify that all other necessary non-destructive examinations are executed in the specified manner for the method and coverage by appropriately qualified personnel.

When, and where, required the inspector shall employ the following equipment to aid in the performance of duties:

• a) Inspection mirrors.
• b) Torch or other electrical lighting facilities (permitted by safety codes eg. 24V system etc.)
• c) Physical size measuring instruments such as welding gauge, rule, vernier etc.
• d) Electrical parameter measuring instruments such as ammeter, voltmeter etc.
• e) Temperature measuring instruments (thermometer)/aids (thermo chalk).

All TCR welding inspectors are generally certified in accordance with the requirements of at least one of the following schemes - Certification Scheme for Weld Inspection Personnel (CSWIP), American Welding Society (AWS), BGAS (previously British Gas ERS), and/or ASNT Level II VT. All inspectors have the ability to interpret various standards including ASME B&PV Code, Section IX, API Std. 1104 and ANSI / AWS D1.1.

The Welding Inspection department at TCR is headed by Mr. Nilesh Pathare who has over 11 years of experience in QA /QC inspection in oil and Gas industry, Petrochemical and refineries and is qualified as a CSWIP 3.1 Welding Inspector, AWS-CWI, BGAS- CSWIP Painting Inspector and ASNT LEVEL II UT, MT, PT, RT. He is experienced in pressure vessel fabrication (static equipment) inspection and Third Party Inspection of materials like plates, pipes, forgings, casting at a vendor's location. He also has hands on experience in NDT (UT, MT, PT) and Radiographic Film interpretation and Destructive testing of various materials.