Steel Heat Treating
Various heat treatments can be used to manipulate speciﬁc properties of steel, such as hardness and ductility, to improve machinability, to remove internal stresses, or to obtain high strength levels and impact properties. The heat treatments of steel commonly employed—annealing, normalizing, spheroidizing, hardening (quenching), and tempering—are brieﬂy described in the following paragraphs.
Annealing Several types of annealing processes are used on carbon and low-alloy steel. These are generally referred to as full annealing, process annealing, and spheroidizing annealing.
In full annealing, the steel is heated to just above the upper critical (A3) tempera-ture, held for a sufﬁcient length of time to fully austenitize the material structure,and then allowed to cool at a slow, controlled rate in the furnace. The microstructureof fully annealed low-carbon steel consists of ferrite and pearlite. A full anneal provides a relatively soft, ductile material, free of internal stresses.
Process annealing sometimes referred to as stress-relieving, is carried out at temperatures below the lower critical (A1) temperature. This treatment is used to improve the ductility and decrease residual stresses in work-hardened steel.
The usual purpose of spheroidizing is to soften the steel and improve its machinability. Heating steel that possesses a pearlite microstructure for a long time at justbelow the lower critical temperature, followed by very slow cooling, will causes pheroidization. This is an agglomeration of the iron carbide, which eventually assumes a spheroidal shape. The properties of this product normally represent the softest condition that can be achieved in the grade of steel being heat-treated.
The austenitic stainless steels are annealed differently from carbon steels. First, since they posses a fully austenitic structure, the temperature used is not related to a critical transformation temperature. Rather, the intent of the annealing is to remove residual strain in the lattice, recrystallize the metal grains, and to dissolve any iron and chromium carbides that may exist in the matrix material. The temperature selected is usually at or above 1900 deg F (1038 deg C). Second, the cooling rate from the annealing temperature is normally as rapid as possible. This suppresses the reformation of carbides at the austenitic grain boundaries during cooling. Formation of grain boundary carbides results in local depletion of chromium in the matrix in the vicinity of the carbides, rendering this thin band of material susceptable to attack in a number of corrosive media. This susceptible condition is referred to as sensitization, and the resultant corrosion is termed intergranular attack.
The temperature range in which carbides are most apt to form in austenitic stainless steels is between about 850 and 1500 deg F (454 and 816 deg C). Slow cooling through or holding in this zone will sensitize the steel. The degree of sensitization that will occur can be greatly reduced by adding small amounts of elements that possess a stronger tendency to form carbides than the chromium. Two such elements, niobium and titanium, are added to form the so-called stabilized austenitic stainless steels. Alternately, the carbon content can be held as low as possible, thereby resulting in as few carbides as possible. These are termed the L grade stainless steels.
Ferritic and martensitic stainless steels will also be adversely affected by slow cooling from annealing temperatures. When slow cooled, or held in the temperature range of 750 to 950 deg F (400 to 510 deg C), these materials embrittle (see discussion on ‘‘474 deg C’’ embrittlement).
Normalizing This carbon and low-alloy steel heat treatment is similar to the annealing process, except that the steel is allowed to cool in air from temperatures above the upper critical temperature. Normalizing relieves the internal stresses caused by previous working. While it produces sufﬁcient softness and ductility for many purposes, it leaves the steel harder and with higher tensile strength than after annealing. Normalizing is often followed by tempering.
Hardening (Quenching) When steels of the higher-carbon grades are heated to produce austenite and then cooled rapidly (quenched), the austenite transforms into martensite. Martensite is formed at temperatures usually below about 400F (204 deg C), depending on the carboncontent and the type and amount of alloying steel. It is the hardest form of heat-treated steel and has high strength and resistance to abrasion. Martensitic steelshave poor impact strength and are difﬁcult to machine.
Tempering Tempering is a secondary heat treatment performed on some normalized and almost all hardened steel structures. The object of tempering is to remove some of the brittleness by allowing certain solid-state transformations to occur. It involves heating to a predetermined level, always below the lower critical temperature, followed by a controlled rate of cooling. In most cases tempering reduces the hardness of the steel, increases its toughness, and eliminates residual stresses. The higher the tempering temperature used for a given time, the more pronounced is the property change. Some steels may become embrittled on slowly cooling from certain tempering temperatures. Steels so affected are said to be temper-brittle. To overcome this difﬁculty, steels of that type are cooled rapidly from the tempering temperature. #Little_PEng
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