Materials

Solid-state welding

For solid phase welding, two clean, solid metal surfaces are brought into sufficiently close contact for a metallic bond to be formed. Solid phase welding can be accomplished at temperatures as low as room temperature.  The bonding process is based either on deformation or on diffusion and limited deformation, so that atomic movement (diffusion) creates new bonds between atoms of two surfaces. Forge welding is a solid-state welding technique known for centuries. Many metals can be forge welded, with the most common being both high and low-carbon steels. 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. 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.

Fusion welding

In fusion welding, two edges or surfaces to be joined are heated to the melting point and, where necessary, molten filler metal is added to fill the joint gap. Due to the high-temperature phase transitions inherent to these processes, a heat-affected zone (HAZ) is created in the material. Fusion welds are created by the coalescence of molten base metals mixed with molten filler metals. Heat for melting is either developed at the intended weld joint or applied to the intended joint from an external source. An example of a means of developing heat at the weld joint is the passing of current through the electrical contact resistance between the contacting surfaces of the materials to be welded. Most fusion welding processes apply heat from an external source to the weld joint to produce the weld bond. Heat is transported from the heat source to the joint by conduction, convection, and radiation. Sources of externally developed heat include electron beams, laser beams, exothermic chemical reactions (used in oxyfuel gas welding and thermite welding), and electric arcs. Electric arcs, the most widely used heat source, are the basis for the various arc welding processes. Fusion welding is used in the manufacture of many everyday items including airplanes, cars, and structures.

In this section we will focus on fusion welding, which is more common than solid-state welding. Fusion welding is used in the manufacture of many everyday items including airplanes, cars, and structures. By using a heat source with sufficient power it is possible to fuse through a complete section of very thick plate. The weld pool produced is difficult to control and the heat affected zone (HAZ) of such welds has a relatively coarse grain, adversely affecting the mechanical properties of the steel. The heat-affected zone (HAZ) is a ring surrounding the weld in which the temperature of the welding process, combined with the stresses of uneven heating and cooling, alter the heat-treatment properties of the alloy. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. In the weld pool, heat is transported by means of convection and conduction.

An understanding of heat transfer is important in the production of welds inasmuch as the properties of a weldment are controlled by its geometry and by the composition and structure of the materials being welded.

See above:
Welding

Welding is one of most common joining processes, two or more metal parts are joined to form a single piece by using high heat to melt the parts together and allowing them to cool causing fusion. Although this concept is easy to describe, it is not simple to effect. Surface roughness, impurities, fitting imperfections, and the varied properties of the materials being joined complicate the joining process. Both similar and dissimilar metals (heterogeneous welding) may be welded. The joining bond is metallurgical (involving some diffusion) rather than just mechanical, as with riveting and bolting. Although welding is considered a relatively new process as practiced today, its origins can be traced to ancient times. Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for millennia to join iron and steel by heating and hammering. Today, a variety of welding methods exist, including arc and gas welding, as well as brazing and soldering. Though portions of this description do not apply to brazing, soldering, and adhesive bonding.

Physics of Welding

The goal of the joining processes is to cause diverse pieces of material to become a unified whole. In the case of two pieces of metal, when the atoms at the edge of one piece come close enough to the atoms at the edge of another piece for interatomic attraction to develop, the two pieces become one.

There are two main categories for welding:

• Fusion welding. In fusion welding, two edges or surfaces to be joined are heated to the melting point and, where necessary, molten filler metal is added to fill the joint gap. Due to the high-temperature phase transitions inherent to these processes, a heat-affected zone (HAZ) is created in the material. Fusion welds are created by the coalescence of molten base metals mixed with molten filler metals. Heat for melting is either developed at the intended weld joint or applied to the intended joint from an external source. An example of a means of developing heat at the weld joint is the passing of current through the electrical contact resistance between the contacting surfaces of the materials to be welded. Most fusion welding processes apply heat from an external source to the weld joint to produce the weld bond. Heat is transported from the heat source to the joint by conduction, convection, and radiation. Sources of externally developed heat include electron beams, laser beams, exothermic chemical reactions (used in oxyfuel gas welding and thermite welding), and electric arcs. Electric arcs, the most widely used heat source, are the basis for the various arc welding processes. Fusion welding is used in the manufacture of many everyday items including airplanes, cars, and structures.
• Solid-state welding. For solid phase welding, two clean, solid metal surfaces are brought into sufficiently close contact for a metallic bond to be formed. Solid phase welding can be accomplished at temperatures as low as room temperature.  The bonding process is based either on deformation or on diffusion and limited deformation, so that atomic movement (diffusion) creates new bonds between atoms of two surfaces. Forge welding is a solid-state welding technique known for centuries. Many metals can be forge welded, with the most common being both high and low-carbon steels. 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. 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.

In this section we will focus on fusion welding, which is more common than solid-state welding. Fusion welding is used in the manufacture of many everyday items including airplanes, cars, and structures. By using a heat source with sufficient power it is possible to fuse through a complete section of very thick plate. The weld pool produced is difficult to control and the heat affected zone (HAZ) of such welds has a relatively coarse grain, adversely affecting the mechanical properties of the steel. The heat-affected zone (HAZ) is a ring surrounding the weld in which the temperature of the welding process, combined with the stresses of uneven heating and cooling, alter the heat-treatment properties of the alloy. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. In the weld pool, heat is transported by means of convection and conduction.

An understanding of heat transfer is important in the production of welds inasmuch as the properties of a weldment are controlled by its geometry and by the composition and structure of the materials being welded.

Types of Welding Processes

The major categories of welding are briefly introduced in the next sections.

Arc Welding

Arc welding 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. The intense heat produced by the arc quickly melts a portion of the base metal, resulting in the formation of a weld. This electrical arc is around 3590°C in its center. Filler metal is added in most welding processes to increase the volume and strength of the weld joint. A pool of molten metal, consisting of base and filler metal is formed near the tip of the electrode. As the electrode is moved along the joint, the molten metal solidifies in its wake.

The welding power supply can use either direct current (DC) or alternating current (AC), 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. 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. The voltage supplied by power companies for industrial purposes-120 volts (V), 230 V, 380 V, or 480 V is too high for use in arc welding. Therefore, the first function of an arc welding power source is to reduce the high input or line voltage to a suitable output voltage range, 20 V to 80 V. 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.

Consumable – Inconsumable Electrode

An electrode rod can be either consumable or inconsumable. If the electrode is made of carbon or tungsten rod has the sole purpose of carrying current to sustain the electric arc between its tip and the workpiece. If a nonconsumable electrode is used, and if the joint requires filler metal addition, then that metal must be supplied by a separately applied filler metal rod or wire. For consumable electrode, the arc can be sustained by a electrode, which not only conducts the current for sustaining the arc, but also melts and supplies filler metal to the joint.

Shielded Metal Arc Welding – SMAW

Shielded metal arc welding (SMAW) is one of the most common types of arc welding. It is also known as manual metal arc welding (MMAW) or stick welding. Shielded metal arc welding (SMAW) use an electrode consisting of a filler metal rod coated with flux that protects the weld area from oxidation and contamination by producing carbon dioxide (CO2) gas during the welding process. The filler metal used in the rods must be compatible with  the metal to be welded, the composition is usually close to that of the base metal. Electric current is used to strike an arc between the base material and consumable electrode rod.

The SMAW process is the simplest, in terms of equipment requirements. It is also 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. Most new welders start as “stick welders” and develop the necessary skills through training and 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.

Gas Metal Arc Welding – GMAW

Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is an arc welding process in which the electrode is a consumable bare wire and shielding is accomplished by flooding the arc with a inert gas. Metal inert gas (MIG) welding, differs from the SMAW process in that its electrode is a bare solid wire that is continuously fed to the weld area and becomes the filler metal as it is consumed. In contrast, SMAW electrodes must be discarded when they reach a minimum length. Gas metal arc welding is widely used in semiautomatic, machine, and automated modes. The gas shield must provide full protection, because even a small amount of entrained air can contaminate the weld deposit. Originally, only inert gases such as argon and helium were used for shielding. Today, carbon dioxide is also used and may be mixed with the inert gases. Because GMAW is continuously wire fed, the electrode do not need replacing at regular intervals such as in the case of SMAW, making this process suitable for automated  welding.

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 – TIG Welding

Gas tungsten arc welding, also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode and an inert gas for arc shielding. The GTAW can be implemented with or without filler metal. When filler metal is used, it is added to the weld pool from a separate rod or wire. The typical shielding gases used are argon, helium or a mixture of these gases. TIG welding is 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. Since the GTAW process is a very clean welding process, it can be used to weld reactive metals, such as titanium and zirconium, aluminum, and magnesium.

Submerged Arc Welding

Submerged arc welding (SAW) is a high-quality welding method, which involves submerging the welding arc beneath a mound of granular flux particles (consisting of lime, silica, manganese oxide, calcium fluoride) as the arc is initiated. Additional flux is continually added in front of the electrode as weld travel progresses. The flux protects the arc and molten weld metal from ambient atmosphere, thereby preventing the formation of oxides. Filler metal is obtained primarily from an electrode wire that is continuously fed through the blanket of flux into the arc and pool of molten flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. Melted flux becomes slag, which is waste material and must be removed after welding. During the submerged arc welding process, not all flux turns into slag. Depending on the welding process, 50% to 90% of the flux can be reused.

Submerged arc welding is normally operated in the automatic or mechanized mode. Submerged arc welding is ideally suited for any application involving long, continuous welds. The ability to readily weld thick plates, sometimes with simple joint configurations, makes SAW the method of choice for welding components of large and thick structural assemblies. For example, a reactor pressure vessel is a cylindrical vessel with a hemispherical bottom head and a flanged and gasketed upper head. The bottom head is welded to the cylindrical shell, which consist of seceral rings welded together by submerged arc welding with narrow gap. Narrow gap welding offers two major benefits. It is an economical joint configuration with less weld volume to fill compared to other joint configurations and the automation friendly joint welded with moderate parameter limits weld defects and gives a high quality weld.

Resistance Welding

Resistance welding (ERW) is a welding process which involves the generation of heat from the flow of electrical current through the parts being joined. Small pools of molten metal are formed at the weld area as high current (1000–100,000 A) is passed through the metal. Electric resistance welding is widely used, for example, in manufacture of steel pipe and in assembly of bodies for automobiles. The vehicle manufacturing industry, among others, employs the resistance processes extensively in applications in which the product design specifies gauge thicknesses that are lapped. Fully automatic and robotic systems are used for many of these applications.

Commonly implemented resistance welding processes are:

• resistance spot welding (RSW),
• resistance seam welding (RSEW),
• resistance projection welding (RPW)
• resistance stud welding.

The main process variables associated with these resistance welding processes are welding current, welding time, electrode force or pressure, electrode material, and tip configuration. Resistance welding (RW) was invented in 1886 by Professor Elihu Thomson, and it is one of the simplest and most commonplace fusion welding processes.

Resistance Spot Welding – RSW

Resistance spot welding, or spot welding is a welding process used to join two or more overlapping metal sheets, studs, projections or another closely fitted surfaces in one or more spots. In this  method the joint is produced by the heat generated due to the resistance of work pieces to the flow of current and application of pressure. The weld is limited to the spots on overlapped work pieces and hence not continuous. The pointed copper electrodes conduct the welding current to the work spot and also serve to apply pressure to form the strong joint. These contacting surfaces are heated by a short-timed pulse of low-voltage, high-amperage current to form a fused nugget of weld metal. Small pools of molten metal are formed at the weld area as high current (1000–100,000 A) is passed through the metal. When the flow of current is stopped, the electrode pressure is maintained while the weld metal cools rapidly and solidifies. The electrodes are made of either precipitationed strengthened copper-chromium and/or zirconium alloy and have to be exchanged after a certain amount of welds.

The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. The principal disadvantage is the need to have accessibility to both sides of the workpiece as opposed to being able to make welds from one side only. The most commonly used welding methods for automotive applications include resistance spot welding (RSW). The vehicle manufacturing industry, among others, employs the resistance processes extensively in applications in which the product design specifies gauge thicknesses that are lapped. Fully automatic and robotic systems are used for many of these applications. The conventional steel body of a car, on an average, contains 4500 spot weld joints.

Resistance Seam Welding – RSEW

Resistance seam welding is a process similar to that of spot welding process, but instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. This produces a weld at the faying surfaces of two similar metals. The electrodes are often disc shaped and rotate as the material passes between them. The seam may be a butt joint or an overlap joint and is usually an automated process. The joint is normally gastight or liquidtight. A series of welds is made without retracting the electrode wheels or releasing the electrode force between spots, but the wheels may advance either intermittently or continuously.

Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. Seam welding produces an extremely durable weld because the joint is forged due to the heat and pressure applied. A common use of seam welding is during the manufacture of round or rectangular steel tubing.

Laser Beam Welding

Laser beam welding is one of high-power-density welding processes (on the order of 1 MW/cm²), which utilize a very high rate of heat input. These processes usually require automation and have excellent potential for high-speed production. Laser beam welding uses effects the fusion welding of materials with the heat supplied by a laser beam that impinges on the joint. A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. For weldinf purposes, laser beam is coherent, monochromatic light in the infrared or ultraviolet frequency portion of the electromagnetic radiation spectrum. Therefore, the beam is invisible. Commercial carbon dioxide (CO2) lasers can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot. This emission is in the thermal infrared at 10.6 µm; such lasers are regularly used in industry for cutting and welding. The minimally divergent raw beam is focused into a small spotto obtain greatest power density.

Laser Beam Welding – Advantages and Disadvantages

The main benefits include good flexibility, improved productivity with substantial saving on maintenance and energy cost while producing a strong weld. Metal sheets having thickness in the range 0.2 to 6mm can easily be laser welded. Majority of the automotive industries employ cross flow CO2 laser system in the power range of 3 to 5 kW. A special caution must be taken to personal safety. A safety enclosure is mandatory for protection against scattered radiation. The appropriate protective eyewear and clothing for the given type of laser must be used. Laser equipment is highly sophisticated and expensive, requiring knowledgeable personnel to install it and set parameters. Therefore this process  requires automation and has excellent potential for high-speed production.

Oxyfuel Gas Welding

Gas welding utilizes the heat produced by a gas flame to melt the filler metal, if used, and the base metal, thereby creating a weld. Gas welding 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. A similar process, generally called oxyfuel cutting, is used to cut metals.

The most common gas welding process is oxyacetylene welding. 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. Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material. The temperature at which it burns is a function of the amount of oxygen present in the gas mixture.

Gases for Oxyfuel Welding

A common gases are:

• Acetylene – Oxygen. Compared with other fuel gases, oxyacetylene can produce the hottest and most concentrated flame. The oxyacetylene flame also produces carbon dioxide, which serves as a shielding gas. Oxyacetylene flame burns at about 3,773 K (3,500 °C; 6,332 °F). As a fuel acetylene’s primary disadvantage, in comparison to other fuels, is high cost.
• Stabilized methylacetylene-propadiene (MPS) has the storage and shipping characteristics of LPG and has a heat value a little lower than that of acetylene. MPS is recommended for cutting applications in particular, rather than welding applications.
• Hydrogen – Oxygen. Hydrogen has a clean flame and is good for use on aluminium. It can be used at a higher pressure than acetylene and is therefore useful for underwater welding and cutting. It is a good type of flame to use when heating large amounts of material. Hydrogen is not used for welding steels and other ferrous materials, because it causes hydrogen embrittlement. Oxyhydrogen flame burns at 3,073 K (2,800 °C; 5,072 °F).

Oxydizing – Neutral – Reducing Flames

The temperature at which it burns is a function of the amount of oxygen present in the gas mixture. The figure demonstrates three types of flames that can be produced with oxyacetylene mixtures. Welding is generally carried out using the neutral flame setting which has equal quantities of oxygen and acetylene.

• Reducing flame. The reducing flame is the flame with low oxygen and an excess of acetylene. Flame has a secondary feather extending from the inner cone. This secondary feather is caused by excess acetylene in the flame mixture, which alters the chemical composition of the weld pool by reducing iron oxide (reducing effect) and adding carbon (carburizing effect). It has a yellow or yellowish color due to carbon or hydrocarbons.
• Neutral flame. The neutral flame is the flame in which the amount of oxygen is precisely enough for burning, and neither oxidation nor reduction occurs. The flame is considered neutral because it neither significantly adds to nor subtracts any elements from the weld pool. A flame with a good balance of oxygen is clear blue.
• Oxydizing flame. The oxidizing flame is the flame produced with an excessive amount of oxygen. When the amount of oxygen increases, the flame shortens, its color darkens, and it hisses and roars. Since, as its name suggests, it oxidizes the metal’s surface, this flame has a harmful effect on the properties of ferrous alloys. With some exceptions (e.g., platinum soldering in jewelry), the oxidizing flame is usually undesirable for welding and soldering.

Friction Welding

Friction welding is a form of solid state welding where the heat is obtained from the mechanically induced sliding motion between the parts to be welded. In solid state welding the joint is produced by the application o f pressure without significant melting any of the work parts. Because no melting occurs, friction welding is not a fusion welding process in the traditional sense. The weld parts are held together under pressure. Generally, the frictional heat is generated by rotating one part against the other. When certain temperature is reached, the rotational motion is seized and the pressure applied welds the parts together. The bonding process is based either on deformation or on diffusion and limited deformation, so that atomic movement (diffusion) creates new bonds between atoms of two surfaces. The time required to generate friction welds is measured in seconds.

Friction Welding – Advantages and Disadvantages

The combination of fast joining times (on the order of a few seconds) and direct heat input at the weld interface yields relatively small heat-affected zones. If automatic loading and unloading devices are installed, the machines are completely automatic. Shielding gas, flux, and filler metal need not be used. This process successfully joins a wide range of similar materials as well as a number of dissimilar metals, including aluminum to steel. This is particularly useful in aerospace, where it is used to join lightweight aluminum stock to high-strength steels.

On the other hand, the work piece has its dimensional limitations. FRW is restricted mostly for round bars with similar cross-section, pieces of other forms are still possible to wield but it is much harder. Friction welding usually uses specific rotary machine, which needs higher capital costs.

Explosion Welding

Explosion welding involves the joining of materials by pushing them together under extremely high pressure, which is generated by a controlled detonation. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. High-ductility metals that have a facecentered cubic arrangement of atoms and do not work harden quickly are best suited to the process. These include aluminum and copper, stainless steel, gold, silver, and platinum. Typical geometries produced include plates, tubing and tube sheets. The process is commonly used for welding dissimilar materials, including bonding aluminum to carbon steel in ship hulls and stainless steel or titanium to carbon steel in petrochemical pressure vessels. A disadvantage of this method is that extensive knowledge of explosives is needed before the procedure may be attempted safely.

See above:
Heat Treatment

The goal of the joining processes is to cause diverse pieces of material to become a unified whole. In the case of two pieces of metal, when the atoms at the edge of one piece come close enough to the atoms at the edge of another piece for interatomic attraction to develop, the two pieces become one.

There are two main categories for welding:

• Fusion welding. In fusion welding, two edges or surfaces to be joined are heated to the melting point and, where necessary, molten filler metal is added to fill the joint gap. Due to the high-temperature phase transitions inherent to these processes, a heat-affected zone (HAZ) is created in the material. Fusion welds are created by the coalescence of molten base metals mixed with molten filler metals. Heat for melting is either developed at the intended weld joint or applied to the intended joint from an external source. An example of a means of developing heat at the weld joint is the passing of current through the electrical contact resistance between the contacting surfaces of the materials to be welded. Most fusion welding processes apply heat from an external source to the weld joint to produce the weld bond. Heat is transported from the heat source to the joint by conduction, convection, and radiation. Sources of externally developed heat include electron beams, laser beams, exothermic chemical reactions (used in oxyfuel gas welding and thermite welding), and electric arcs. Electric arcs, the most widely used heat source, are the basis for the various arc welding processes. Fusion welding is used in the manufacture of many everyday items including airplanes, cars, and structures.
• Solid-state welding. For solid phase welding, two clean, solid metal surfaces are brought into sufficiently close contact for a metallic bond to be formed. Solid phase welding can be accomplished at temperatures as low as room temperature.  The bonding process is based either on deformation or on diffusion and limited deformation, so that atomic movement (diffusion) creates new bonds between atoms of two surfaces. Forge welding is a solid-state welding technique known for centuries. Many metals can be forge welded, with the most common being both high and low-carbon steels. 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. 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.

In this section we will focus on fusion welding, which is more common than solid-state welding. Fusion welding is used in the manufacture of many everyday items including airplanes, cars, and structures. By using a heat source with sufficient power it is possible to fuse through a complete section of very thick plate. The weld pool produced is difficult to control and the heat affected zone (HAZ) of such welds has a relatively coarse grain, adversely affecting the mechanical properties of the steel. The heat-affected zone (HAZ) is a ring surrounding the weld in which the temperature of the welding process, combined with the stresses of uneven heating and cooling, alter the heat-treatment properties of the alloy. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. In the weld pool, heat is transported by means of convection and conduction.

An understanding of heat transfer is important in the production of welds inasmuch as the properties of a weldment are controlled by its geometry and by the composition and structure of the materials being welded.

See above:
Welding

Quenching

The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In ferrous alloys, quenching is most commonly used to harden steel by introducing martensite, while non-ferrous alloys will usually become softer than normal. Above this critical temperature, a metal is partially or fully austenitized, the cooling rate of the steel has to be rapid so as to let the austenite transform into metastable bainite or martensite.

The selection of a quenchant medium depends on the hardenability of the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired micro-structure.

Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C). Therefore, it is a product of diffusionless transformation. Any diffusion whatsoever results in the formation of ferrite and cementite phases. It is named after the German metallurgist Adolf Martens (1850–1914).

The microstructure of martensite in steels has different morphologies and may appear as either lath martensite or plate martensite. For steel 0–0.6% carbon the martensite has the appearance of lath, and is called lath martensite. For steel greater than 1% carbon it will form a plate like structure called plate martensite. Plate martensite, as the name indicates, forms as lenticular (lens-shaped) crystals with a zigzag pattern of smaller plates. Between those two percentages, the physical appearance of the grains is a mix of the two. The strength of the martensite is reduced as the amount of retained austenite grows.

Tempering

The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties. Upon heating, the carbon atoms diffuse and react in a series of distinct steps that eventually form Fe₃C or an alloy carbide in a ferrite matrix of gradually decreasing stress level.

For tempering, temperature is much more important than time at temperature. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, between 150 and 200 ° and maintain much of the hardness and strength of the quenched martensite and provide a small improvement in ductility and toughness. While springs are tempered at much higher temperatures. Tempering above 425 °C significantly improves ductility and toughness but at the expense of hardness and strength. Under certain conditions, hardness may remain unaffected by tempering or may even be increased as a result of it. Also, those alloy steels that contain one or more of the carbide-forming elements (chromium, molybdenum, vanadium, and tungsten) are capable of secondary hardening: they may become somewhat harder as a result of tempering.

Tempered Martensite

The relative ability of a ferrous alloy to form martensite is called hardenability. Hardenability is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness of 50 HRC, for example, or a specific percentage of martensite in the microstructure. The highest hardness of a pearlitic steel is 43 HRC whereas martensite can achieve 72 HRC. Fresh martensite is very brittle if carbon content is greater than approximately 0.2 to 0.3%. It is so brittle that it cannot be used for most applications. This brittleness can be removed (with some loss of hardness) if the quenched steel is heated slightly in a process known as tempering. Tempering is accomplished by heating a martensitic steel to a temperature below the eutectoid for a specified time period (for example between 250°C and 650°C ).

This tempering heat treatment allows, by diffusional processes, the formation of tempered martensite, according to the reaction:

martensite (BCT, single phase) → tempered martensite (ferrite + Fe₃C phases)

where the single-phase BCT martensite, which is supersaturated with carbon, transforms into the tempered martensite, composed of the stable ferrite and cementite phases. Its microstructure is similar to the microstructure of spheroidite but in this case tempered martensite contains extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. Tempered martensite may be nearly as hard and strong as martensite but with substantially enhanced ductility and toughness.

See above:
Heat Treatment

Tempering

The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties. Upon heating, the carbon atoms diffuse and react in a series of distinct steps that eventually form Fe₃C or an alloy carbide in a ferrite matrix of gradually decreasing stress level.

For tempering, temperature is much more important than time at temperature. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, between 150 and 200 ° and maintain much of the hardness and strength of the quenched martensite and provide a small improvement in ductility and toughness. While springs are tempered at much higher temperatures. Tempering above 425 °C significantly improves ductility and toughness but at the expense of hardness and strength. Under certain conditions, hardness may remain unaffected by tempering or may even be increased as a result of it. Also, those alloy steels that contain one or more of the carbide-forming elements (chromium, molybdenum, vanadium, and tungsten) are capable of secondary hardening: they may become somewhat harder as a result of tempering.

Austempering

Austempering is a heat treatment used to form pure bainite, a transitional microstructure found between pearlite and martensite. Austempering consists of rapidly cooling the metal part from the austenitizing temperature to about 230 to 400°C, holding at a constant temperature to allow isothermal transformation. To avoid the formation of pearlite or martensite, the steel is quenched in a bath of molten metals or salts. The steel is then held at the bainite-forming temperature, beyond the point where the temperature reaches an equilibrium, until the bainite fully forms. The steel is then removed from the bath and allowed to air-cool, without the formation of either pearlite or martensite. Depending on the holding-temperature, austempering can produce either upper or lower bainite.

Bainite is a plate-like microstructure that forms in steels from austenite when cooling rates are not rapid enough to produce martensite but are still fast enough so that carbon does not have enough time to diffuse to form pearlite. The key difference between pearlite and bainite is that the pearlite contains alternating layers of ferrite and cementite whereas the bainite has a plate-like microstructure. A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The large density of dislocations in the ferrite present in bainite, and the fine size of the bainite platelets, makes this ferrite harder than it normally would be. Bainitic steels are generally stronger and harder than pearlitic steels; yet they exhibit a superior imact resistance. The hardness of bainite can be between that of pearlite and untempered martensite in the same steel hardness.

Austempering is applicable to most medium-carbon steels and alloy steels. Low-alloy steels are usually restricted to 9.5 mm or thinner sections, while more hardenable steels can be austempered in sections up to 50 mm thick.

The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In ferrous alloys, quenching is most commonly used to harden steel by introducing martensite, while non-ferrous alloys will usually become softer than normal. Above this critical temperature, a metal is partially or fully austenitized, the cooling rate of the steel has to be rapid so as to let the austenite transform into metastable bainite or martensite.

The selection of a quenchant medium depends on the hardenability of the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired micro-structure.

See above:
Heat Treatment

Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. There is a number of phenomena that occur in metals and alloys at elevated temperatures. For example, recrystallization and the decomposition of austenite. These are effective in altering the mechanical characteristics when appropriate heat  treatments or thermal processes are used. In fact, the use of heat treatments on commercial alloys is an exceedingly common practice. Common heat treatment processes include annealing, precipitation hardening, quenching, and tempering.

Tempering

The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties. Upon heating, the carbon atoms diffuse and react in a series of distinct steps that eventually form Fe₃C or an alloy carbide in a ferrite matrix of gradually decreasing stress level.

For tempering, temperature is much more important than time at temperature. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, between 150 and 200 ° and maintain much of the hardness and strength of the quenched martensite and provide a small improvement in ductility and toughness. While springs are tempered at much higher temperatures. Tempering above 425 °C significantly improves ductility and toughness but at the expense of hardness and strength. Under certain conditions, hardness may remain unaffected by tempering or may even be increased as a result of it. Also, those alloy steels that contain one or more of the carbide-forming elements (chromium, molybdenum, vanadium, and tungsten) are capable of secondary hardening: they may become somewhat harder as a result of tempering.

Austempering

Austempering is a heat treatment used to form pure bainite, a transitional microstructure found between pearlite and martensite. Austempering consists of rapidly cooling the metal part from the austenitizing temperature to about 230 to 400°C, holding at a constant temperature to allow isothermal transformation. To avoid the formation of pearlite or martensite, the steel is quenched in a bath of molten metals or salts. The steel is then held at the bainite-forming temperature, beyond the point where the temperature reaches an equilibrium, until the bainite fully forms. The steel is then removed from the bath and allowed to air-cool, without the formation of either pearlite or martensite. Depending on the holding-temperature, austempering can produce either upper or lower bainite.

Bainite is a plate-like microstructure that forms in steels from austenite when cooling rates are not rapid enough to produce martensite but are still fast enough so that carbon does not have enough time to diffuse to form pearlite. The key difference between pearlite and bainite is that the pearlite contains alternating layers of ferrite and cementite whereas the bainite has a plate-like microstructure. A fine non-lamellar structure, bainite commonly consists of cementite and dislocation-rich ferrite. The large density of dislocations in the ferrite present in bainite, and the fine size of the bainite platelets, makes this ferrite harder than it normally would be. Bainitic steels are generally stronger and harder than pearlitic steels; yet they exhibit a superior imact resistance. The hardness of bainite can be between that of pearlite and untempered martensite in the same steel hardness.

Austempering is applicable to most medium-carbon steels and alloy steels. Low-alloy steels are usually restricted to 9.5 mm or thinner sections, while more hardenable steels can be austempered in sections up to 50 mm thick.

The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In ferrous alloys, quenching is most commonly used to harden steel by introducing martensite, while non-ferrous alloys will usually become softer than normal. Above this critical temperature, a metal is partially or fully austenitized, the cooling rate of the steel has to be rapid so as to let the austenite transform into metastable bainite or martensite.

The selection of a quenchant medium depends on the hardenability of the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired micro-structure.

Tempered Martensite

The relative ability of a ferrous alloy to form martensite is called hardenability. Hardenability is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness of 50 HRC, for example, or a specific percentage of martensite in the microstructure. The highest hardness of a pearlitic steel is 43 HRC whereas martensite can achieve 72 HRC. Fresh martensite is very brittle if carbon content is greater than approximately 0.2 to 0.3%. It is so brittle that it cannot be used for most applications. This brittleness can be removed (with some loss of hardness) if the quenched steel is heated slightly in a process known as tempering. Tempering is accomplished by heating a martensitic steel to a temperature below the eutectoid for a specified time period (for example between 250°C and 650°C ).

This tempering heat treatment allows, by diffusional processes, the formation of tempered martensite, according to the reaction:

martensite (BCT, single phase) → tempered martensite (ferrite + Fe₃C phases)

where the single-phase BCT martensite, which is supersaturated with carbon, transforms into the tempered martensite, composed of the stable ferrite and cementite phases. Its microstructure is similar to the microstructure of spheroidite but in this case tempered martensite contains extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. Tempered martensite may be nearly as hard and strong as martensite but with substantially enhanced ductility and toughness.

Other Processes

• Annealing. The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. In this process, metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking; it is also easier to sand, grind, or cut annealed metal.
• Quenching. The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.
• Tempering. The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.
• Aging. Age hardening, also called precipitation hardening or particle hardening, is a heat treatment technique based on the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix to enhance The strength and hardness of some metal alloys.  Precipitation hardening is used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.

See above:
Heat Treatment

The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In ferrous alloys, quenching is most commonly used to harden steel by introducing martensite, while non-ferrous alloys will usually become softer than normal. Above this critical temperature, a metal is partially or fully austenitized, the cooling rate of the steel has to be rapid so as to let the austenite transform into metastable bainite or martensite.

The selection of a quenchant medium depends on the hardenability of the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired micro-structure.

Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C). Therefore, it is a product of diffusionless transformation. Any diffusion whatsoever results in the formation of ferrite and cementite phases. It is named after the German metallurgist Adolf Martens (1850–1914).

The microstructure of martensite in steels has different morphologies and may appear as either lath martensite or plate martensite. For steel 0–0.6% carbon the martensite has the appearance of lath, and is called lath martensite. For steel greater than 1% carbon it will form a plate like structure called plate martensite. Plate martensite, as the name indicates, forms as lenticular (lens-shaped) crystals with a zigzag pattern of smaller plates. Between those two percentages, the physical appearance of the grains is a mix of the two. The strength of the martensite is reduced as the amount of retained austenite grows.

Martensitic Transformation

Transformation hardening, known also as martensitic transformation hardening, is one of most common methods of hardening, which is primarily used for steels (i.e. carbon steels as well as stainless steels). The martensitic transformation is not, however, unique to iron–carbon alloys. It is found in other systems and is characterized, in part, by the diffusionless transformation.

Martensitic steels use predominantly higher levels of C and Mn along with heat treatment to increase strength. The finished product will have a duplex micro-structure of ferrite with varying levels of degenerate martensite. This allows for varying levels of strength. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.

Martensite is produced from austenite as a result of the quenching, or another form of rapid cooling. Austenite in iron-carbon alloys is generally only present above the critical eutectoid temperature (723°C), and below 1500°C, depending on carbon content. In case of normal cooling rates, as the austenite cools, the carbon diffuses out of the austenite and forms carbon rich iron-carbide (cementite) and leaves behind carbon poor ferrite. Depending on alloy composition, a layering of ferrite and cementite, called pearlite, may form. But in case of rapid cooling, the carbon does not have time enough to diffuse and the transforms to a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. All the carbon atoms remain as interstitial impurities in martensite. The cooling rate determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness, tensile strength and toughness as well.

Example: Martensitic Stainless Steel

Martensitic stainless steels are similar to ferritic steels in being based on chromium but have higher carbon levels up as high as 1%. They are sometimes classified as low-carbon and high-carbon martensitic stainless steels. They contain 12 to 14% chromium, 0.2 to 1% molybdenum, and no significant amount of nickel. Higher amounts of carbon allows them to be hardened and tempered much like carbon and low-alloy steels. They have moderate corrosion resistance, but are considered hard, strong, slightly brittle. They are magnetic and they can be nondestructively tested using the magnetic particle inspection method, unlike austenitic stainless steel.  A common martensitic stainless is AISI 440C, which contains 16 to 18% chromium and 0.95 to 1.2% carbon. Grade 440C stainless steel is used in the following applications: gage blocks, cutlery, ball bearings and races, molds and dies, knives.

Tempered Martensite

The relative ability of a ferrous alloy to form martensite is called hardenability. Hardenability is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness of 50 HRC, for example, or a specific percentage of martensite in the microstructure. The highest hardness of a pearlitic steel is 43 HRC whereas martensite can achieve 72 HRC. Fresh martensite is very brittle if carbon content is greater than approximately 0.2 to 0.3%. It is so brittle that it cannot be used for most applications. This brittleness can be removed (with some loss of hardness) if the quenched steel is heated slightly in a process known as tempering. Tempering is accomplished by heating a martensitic steel to a temperature below the eutectoid for a specified time period (for example between 250°C and 650°C ).

This tempering heat treatment allows, by diffusional processes, the formation of tempered martensite, according to the reaction:

martensite (BCT, single phase) → tempered martensite (ferrite + Fe₃C phases)

where the single-phase BCT martensite, which is supersaturated with carbon, transforms into the tempered martensite, composed of the stable ferrite and cementite phases. Its microstructure is similar to the microstructure of spheroidite but in this case tempered martensite contains extremely small and uniformly dispersed cementite particles embedded within a continuous ferrite matrix. Tempered martensite may be nearly as hard and strong as martensite but with substantially enhanced ductility and toughness.

See above:
Heat Treatment

The term thermal annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. This process alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. In this process, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. Metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking. Typically, annealing is carried out to relieve stresses, increase softness, ductility, and toughness; and/or produce a specific microstructure.

Generally, in plain carbon steels, annealing produces a ferrite-pearlite micro-structure.  Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are relatively inexpensive and are produced in large tonnages.

Any annealing cycle consists of three stages:

• heating to the desired temperature,
• holding or “soaking” at that temperature,
• cooling, usually to room temperature.

Time and annealing temperature are important parameters in these procedures. Especially the target temperature defines the annealing thermal cycle.

Reactor Pressure Vessel Annealing

The body of the reactor vessel is constructed of a high-quality low-alloy carbon steel, and all surfaces that come into contact with reactor coolant are clad with a minimum of about 3 to 10 mm of austenitic stainless steel in order to minimize corrosion.

During the operation of a nuclear power plant, the material of the reactor pressure vessel and the material of other reactor internals are exposed to neutron radiation (especially to fast neutrons >0.5MeV), which results in localized embrittlement of the steel and welds in the area of the reactor core. This phenomenon, known as irradiation embrittlment, results in:

• Steadily increase in DBTT. It is not likely that the DBTT will approach the normal operating temperature of the steel. However, there is a possibility that when the reactor is being shut down or during an abnormal cooldown, the temperature may fall below the DBTT value while the internal pressure is still high.
• Drop in the upper shelf fracture energy. Radiation effects are also manifested by a drop in the upper shelf fracture energy and decrease in fracture toughness.

All these effects must be monitored by plant operators. Therefore nuclear regulators require that a reactor vessel material surveillance program be conducted in watercooled power reactors.

Once a material of RPV is degraded by radiation embrittlement (e.g. significant increase in Charpy ductile‐brittle transition temperature or reduction of fracture toughness), thermal annealing of the RPV is the only way to recover the RPV material toughness properties.

According to 10 CFR 50.66 – Requirements for thermal annealing of the reactor pressure vessel:

„For those light water nuclear power reactors where neutron radiation has reduced the fracture toughness of the reactor vessel materials, a thermal annealing may be applied to the reactor vessel to recover the fracture toughness of the material.“

 Thermal annealing („dry“ method) of reactor pressure vessel is a method by which the pressure vessel (with all reactor internals removed) is heated up to some temperature (usually between 420 – 460 °C) by use of an external heat source (electrical heaters, hot air), held for a given period (e.g. 100 – 200 hours) and then slowly cooled. The annealing equipment is usually a ring‐shaped furnace with heating elements on its external surface. The power output of installed heaters may reach up to 1 MWe. It was shown that for the specially fabricated materials the upper shelf recovered 100 % after 24 hours annealing and more rapidly than transition temperature. Annealing for 168  hours recovered 90 % of transition temperature shift.

Wet Annealing

There is also a possibility of the so‐called “wet” annealing method which was applied in USA and Belgium. The annealing at that temperature ~340 °C was reached without external heating, but by increasing the coolant temperature achieved by the energy of the circulating pumps of the primary circuit. This type of annealing provides only partial recovery for the material due to the limitation in maximum temperature.

Special Reference: Annealing and re-embrittlement of reactor pressure vessel materials. AMES report N.19; ISSN 1018-5593. European Communities, 2008.

See above:
Thermal Annealing

Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. There is a number of phenomena that occur in metals and alloys at elevated temperatures. For example, recrystallization and the decomposition of austenite. These are effective in altering the mechanical characteristics when appropriate heat  treatments or thermal processes are used. In fact, the use of heat treatments on commercial alloys is an exceedingly common practice. Common heat treatment processes include annealing, precipitation hardening, quenching, and tempering.

Quenching

The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In ferrous alloys, quenching is most commonly used to harden steel by introducing martensite, while non-ferrous alloys will usually become softer than normal. Above this critical temperature, a metal is partially or fully austenitized, the cooling rate of the steel has to be rapid so as to let the austenite transform into metastable bainite or martensite.

The selection of a quenchant medium depends on the hardenability of the particular alloy, the section thickness and shape involved, and the cooling rates needed to achieve the desired micro-structure.

Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when the cooling rate from austenite is at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe₃C). Therefore, it is a product of diffusionless transformation. Any diffusion whatsoever results in the formation of ferrite and cementite phases. It is named after the German metallurgist Adolf Martens (1850–1914).

The microstructure of martensite in steels has different morphologies and may appear as either lath martensite or plate martensite. For steel 0–0.6% carbon the martensite has the appearance of lath, and is called lath martensite. For steel greater than 1% carbon it will form a plate like structure called plate martensite. Plate martensite, as the name indicates, forms as lenticular (lens-shaped) crystals with a zigzag pattern of smaller plates. Between those two percentages, the physical appearance of the grains is a mix of the two. The strength of the martensite is reduced as the amount of retained austenite grows.

Martensitic Transformation

Transformation hardening, known also as martensitic transformation hardening, is one of most common methods of hardening, which is primarily used for steels (i.e. carbon steels as well as stainless steels). The martensitic transformation is not, however, unique to iron–carbon alloys. It is found in other systems and is characterized, in part, by the diffusionless transformation.

Martensitic steels use predominantly higher levels of C and Mn along with heat treatment to increase strength. The finished product will have a duplex micro-structure of ferrite with varying levels of degenerate martensite. This allows for varying levels of strength. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.

Martensite is produced from austenite as a result of the quenching, or another form of rapid cooling. Austenite in iron-carbon alloys is generally only present above the critical eutectoid temperature (723°C), and below 1500°C, depending on carbon content. In case of normal cooling rates, as the austenite cools, the carbon diffuses out of the austenite and forms carbon rich iron-carbide (cementite) and leaves behind carbon poor ferrite. Depending on alloy composition, a layering of ferrite and cementite, called pearlite, may form. But in case of rapid cooling, the carbon does not have time enough to diffuse and the transforms to a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. All the carbon atoms remain as interstitial impurities in martensite. The cooling rate determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness, tensile strength and toughness as well.

Surface Hardening based on Quenching

Case hardening or surface hardening is the process in which hardness the surface (case) of an object is enhanced, while the inner core of the object remains elastic and tough.  Case hardening by surface treatment can be classified further as diffusion treatments or localized heating treatments. Localized heating methods for case hardening include:

• Flame hardening. Flame hardening is a surface hardening technique which uses a single torch with a specially designed head to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. It is similar technique as induction hardening. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
• Induction hardening. Induction hardening is a surface hardening technique which uses induction coils to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
• Laser hardening. Laser hardening is a surface hardening technique which uses a laser beam to provide a very rapid means of heating the metal, which is then cooled rapidly (generally by self-quenching). This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough.

Other Processes

• Annealing. The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. In this process, metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking; it is also easier to sand, grind, or cut annealed metal.
• Quenching. The term quenching refers to a heat treatment in which a material is rapidly cooled in water, oil or air to obtain certain material properties, especially hardness. In metallurgy, quenching is most commonly used to harden steel by introducing martensite. There is a balance between hardness and toughness in any steel; the harder the steel, the less tough or impact-resistant it is, and the more impact-resistant it is, the less hard it is.
• Tempering. The term tempering refers to a heat treatment which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. Tempering makes the metal less hard while making it better able to sustain impacts without breaking. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.
• Aging. Age hardening, also called precipitation hardening or particle hardening, is a heat treatment technique based on the formation of extremely small, uniformly dispersed particles of a second phase within the original phase matrix to enhance The strength and hardness of some metal alloys.  Precipitation hardening is used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.

See above:
Heat Treatment

The term thermal annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. This process alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. In this process, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. Metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking. Typically, annealing is carried out to relieve stresses, increase softness, ductility, and toughness; and/or produce a specific microstructure.

Generally, in plain carbon steels, annealing produces a ferrite-pearlite micro-structure.  Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are relatively inexpensive and are produced in large tonnages.

Any annealing cycle consists of three stages:

• heating to the desired temperature,
• holding or “soaking” at that temperature,
• cooling, usually to room temperature.

Time and annealing temperature are important parameters in these procedures. Especially the target temperature defines the annealing thermal cycle.

Full Annealing

Full annealing produces a microstructure that is softer and more amenable to other processing such as forming or machining. The temperatures for full annealing are typically 50 °C above the upper critical temperature (A₃) for hypoeutectic steels and the lower critical temperature (A₁) for hypereutectoid steels. It is referred to as full annealing because it achieves full austenitization of hypoeutectoid steels. The alloy is then furnace cooled. That means, the heat-treating furnace is turned off, and both furnace and steel cool to room temperature at the same rate, which takes several hours. The cooling rate of the steel has to be sufficiently slow so as to not let the austenite transform into bainite or martensite, but rather have it completely transform to pearlite and ferrite or cementite. A full anneal typically results in the second most ductile state a metal can assume for metal alloy. The metal attain relatively low levels of hardness, yield strength and ultimate strength with high plasticity and toughness. Full annealing is often used in low- and medium-carbon steels that will be machined or will experience extensive plastic deformation during a forming operation. Stainless and high-alloy steels may be austenitized (fully annealed) and quenched to minimize the presence of grain boundary carbides or to improve the ferrite distribution.

See above:
Thermal Annealing