Heat Treatment of Steel
Properties of steel can be altered by applying a variety of heat treatments. For example, steel can be hardened or softened by using heat treatment; the response of steel to heat treatment depends upon its alloy composition. Common heat treatments employed for steel include annealing, normalizing, hardening, and tempering.
The basic process is to heat the steel to a specific temperature, hold the temperature for a specified period of time, then cool the material at a specified rate. The temperatures used for each of the treatment types are shown in Figure 1.
Annealing of Steel
The objectives of annealing are to refine the grain, soften the steel, remove internal stresses, remove gases, increase ductility and toughness, and change electrical and magnetic properties. Four types of annealing can be performed, depending on the desired results of the heat treatment:
Full annealing requires heating the steel to about 50°C above the austenitic temperature line and holding the temperature until all the steel transforms into either austenite or austenite–cementite, depending on the carbon content. The steel is then cooled at a rate of about 20°C per hour in a furnace to a temperature of about 680°C, followed by natural convection cooling to room temperature.
Due to the slow cooling rate, the grain structure is a coarse pearlite with ferrite or cementite, depending on the carbon content. The slow cooling rate ensures uniform properties of the treated steel. The steel is soft and ductile.
Process annealing is used to treat work-hardened parts made with low carbon steel (i.e., less than 0.25% carbon). The material is heated to about 700°C and held long enough to allow recrystallization of the ferrite phase. By keeping the temperature below 727°C, there is not a phase shift between ferrite and austenite, as occurs during full annealing. Hence, the only change that occurs is refinement of the size, shape, and distribution of the grain structure.
Stress relief annealing is used to reduce residual stresses in cast, welded, and cold-worked parts and cold-formed parts. The material is heated to 600 to 650°C, held at temperature for about 1 hour, and then slowly cooled in still air.
Spheroidization is an annealing process used to improve the ability of high carbon (i.e., more than 0.6% carbon) steel to be machined or cold worked. It also improves abrasion resistance. The cementite is formed into globules (spheroids) dispersed throughout the ferrite matrix.
Normalizing of Steel
Normalizing is similar to annealing, with a slight difference in the temperature and the rate of cooling. Steel is normalized by heating to about 60°C above the austenite line and then cooling under natural convection. The material is then air cooled. Normalizing produces a uniform, fine-grained microstructure.
However, since the rate of cooling is faster than that used for full annealing, shapes with varying thicknesses results in the normalized parts having less uniformity than could be achieved with annealing. Since structural plate has a uniform thickness, normalizing is an effective process and results in high fracture toughness of the material.
Hardening of Steel
Steel is hardened by heating it to a temperature above the transformation range and holding it until austenite is formed. The steel is then quenched (cooled rapidly) by plunging it into, or spraying it with, water, brine, or oil. The rapid cooling changes the grain structure forming martensite rather than allowing the transformation to the ferrite BCC structure. Martensite has a very hard and brittle structure.
Since the cooling occurs more rapidly at the surface of the material being hardened, the surface of the material is harder and more brittle than the interior of the element, creating nonhomogeneous characteristics. Due to the rapid cooling, hardening puts the steel in a state of strain. This strain sometimes causes steel pieces with sharp angles or grooves to crack immediately after hardening. Thus, hardening must be followed by tempering.
Tempering of Steel
The predominance of martensite in quench-hardened steel results in an undesirable brittleness. Tempering is performed to improve ductility and toughness. Martensite is a somewhat unstable structure.
Heating causes carbon atoms to diffuse from martensite to produce a carbide precipitate and formation of ferrite and cementite. After quenching, the steel is cooled to about 40°C then reheated by immersion in either oil or nitrate salts.
The steel is maintained at the elevated temperature for about 2 hours and then cooled in still air.
Example of Heat Treatment of Steel
In the quest to produce high-strength low-alloy steels economically, the industry has developed specifications for several new steel products, such as A913. This steel is available with yield stresses ranging from 345 to 510 MPa. The superior properties of A913 steel are obtained by a quench self-tempering process.
Following the last hot rolling pass for shaping, for which the temperature is typically 850°C, an intense water-cooling spray is applied to the surface of the beam to quench (rapidly cool) the skin. Cooling is stopped before the core on the material is affected.
The outer layers are then self-tempered as the internal heat of the beam flows to the surface. The self-tempering temperature is 600°C.