Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness, or resistance to corrosion. Many phenomena 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. The use of heat treatments on commercial alloys is an exceedingly common practice. Common heat treatment processes include annealing, precipitation hardening, quenching, and tempering.
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 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 microstructure.
Martensite is a very hard metastable structure with a body-centered tetragonal (BCT) crystal structure. Martensite is formed in steels when austenite’s cooling rate is so high 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, and 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 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.
Transformation hardening, also known as martensitic transformation hardening, is one of the most common hardening methods primarily used for steels (i.e., carbon steels and stainless steels). However, the martensitic transformation is not unique to iron-carbon alloys. And it is found in other systems and is characterized, in part, by diffusionless transformation.
Martensitic steels use predominantly higher levels of C and Mn and heat treatment to increase strength. The finished product will have a duplex microstructure of ferrite with varying levels of degenerate martensite, allowing for varying levels of strength. In metallurgy, quenching is 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 due to 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 the case of normal cooling rates, as the austenite cools, the carbon diffuses out of the austenite, 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 enough time to diffuse, transforming into a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. All the carbon atoms remain interstitial impurities in martensite. The cooling rate determines the relative proportions of martensite, ferrite, and cementite. It, therefore, determines the mechanical properties of the resulting steel, such as hardness, tensile strength, and toughness.
Surface Hardening based on Quenching
Case hardening or surface hardening is the process in which the hardness of an object’s surface (case) is enhanced while the inner core of the object remains elastic and tough. Case hardening by surface treatment can be further classified as diffusion or localized heating treatments. Localized heating methods for case hardening include:
- Flame hardening. Flame hardening is a surface hardening technique that 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 a similar technique to induction hardening. The 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 that 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. The 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 that 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.
- Annealing. The term annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended 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 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 used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening to reduce some of the excess hardness. It is done by heating the metal to some temperature below the critical point for a certain period, then allowing it to cool in still air. Tempering makes the metal less hard while enabling it 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, 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 increases the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, steel, and stainless steel. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.