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.
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. 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 levels.
Temperature is much more important for tempering than the time at temperature. The exact temperature determines the amount of hardness removed and depends on both the specific composition of the alloy and the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, between 150 and 200°, maintain much of the hardness and strength of the quenched martensite and provide a small improvement in ductility and toughness. In contrast, 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. 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 it 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 forming either pearlite or martensite. Austempering can produce upper or lower bainite depending on the holding temperature.
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 pearlite contains alternating layers of ferrite and cementite, whereas 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, make this ferrite harder than it normally would be. Bainitic steels are generally stronger and harder than pearlitic steels, yet they exhibit a superior impact 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 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.
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 pearlitic steel is 43 HRC, whereas martensite can achieve 72 HRC. Fresh martensite is very brittle if the 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 martensitic steel to a temperature below the eutectoid for a specified 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)
The single-phase BCT martensite, 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 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.
