Tempering of steels takes place in four distinct but overlapping stages:
1. First Stage of Tempering:
(Though mild steels as a rule are not hardened and tempered in industry, it is of academic interest to see the changes occurring in such steels during tempering after hardening). The tempering reactions in steels, containing carbon less than 0.2%, differ somewhat from the steels containing more than 0.2% carbon.
In the former, if carbon atoms have not yet segregated (during quenching) to dislocations, these diffuse and segregate around the dislocations and lath boundaries in the first stage of tempering. No e-carbide forms as all the carbon gets locked up to the dislocations (defects).
Martensite in steels with more than 0.2% carbon is highly unstable because of super-saturation, and interstitial diffusion of carbon in BCT martensite can occur. Thus, in the first stage of tempering, the decomposition of martensite into low-tetragonality martensite (containing ~ 0.2% carbon, c/a ~ 1.014) and ε-carbide, Fe2.4C occurs (there are reports of precipitation of eta-carbide, Fe2C and Haggs carbide, Fe2.2C). ε-carbide is a separate phase and is not a preliminary step in the formation of cementite, but it nucleates and grows more rapidly than cementite.
ε-carbide has a hexagonal close-packed structure with:
c = 4.33 A, a = 2.73 A, c/a = 1.58
and forms as small (0.015 – 0.02 µm) platelets, or needles observed under electron microscope. The structure at this stage is referred as tempered-martensite, which is a double phase mixture of low-tetragonal martensite and ε-carbide.
In the first stage of tempering, the volume decreases, because there is a decrease in the specific volume of martensite, due to rejection of carbon from it. In fact, it is possible to study the kinetics of the first stage of tempering by a precision dilatometer, and which has shown that tempering occurs even at room temperature, though, at a very slow rate.
2. Second Stage of Tempering:
The amount of retained austenite in the as-quenched steel depends mainly on the composition of the steel, and the temperature to which steel is quenched. Fig. 7.2 illustrates the effect of carbon on the amount of retained austenite in Fe-C alloys. In the second stage of tempering, retained austenite transforms to lower bainite (the carbide in bainite is e carbide). The matrix in lower bainite is cubic ferrite (c/a = 1), whereas in tempered martensite (obtained in first stage of tempering), the low-tetragonal martensite has c/a ratio of approximately 1.014.
When retained austenite changes to lower bainite, there takes place increase in volume. Here, the kinetics of tempering can be followed by measuring change in length by a dilatometer.
The transformation follows the simple exponential rate law:
where, t is the time required to form a definite amount of bainite, and Q is the empirical activation energy for the reaction.
3. Third Stage of Tempering:
In this stage of tempering, e-carbide dissolves in matrix, and low-tetragonal martensite loses completely its carbon and thus, the tetragonality to become ferrite (the kinetics of this stage can be followed by x-ray method for studying changes in c/a ratio of low-tetragonal martensite). Cementite forms as rods at interfaces of ε-carbide and matrix, twin boundaries, interlath boundaries, or original austenite grain boundaries.
During this stage, volume decreases just as in stage one, due to complete loss of tetragonality. In a 1% carbon steel, the total decrease in length in the first and third stages in around 0.25%.
4. Fourth Stage of Tempering:
Growth and spheroidisation of cementite, as well as recovery and recrystallisation of ferrite occur. Though, the growth of cementite starts above 300°C, its spheroidisation starts above 400°C to 700°C. Spheroidisation takes place due to reduction in interfacial energy of ferrite-cementite interfaces. As-quenched martensite has high concentration of lattice defects. Though their annealing out starts in the third stage of tempering, but the cementite precipitates retard the recovery process.
Substantial recovery process starts occurring only above 400°C. Original lath boundaries are stable up to 600°C, but above this, these are replaced by equiaxed-ferrite grain boundaries—the process, which is best described as ‘recrystallisation’.
In the end, the optical microstructure consists of equiaxed ferrite grains with coarse spheroidal particles of cementite, and then the structure in called globular pearlite, or spheroidised cementite as illustrated in Fig. 7.3 (f). This structure perhaps is the most stable of all ferrite-cementite aggregates, and is the softest with highest ductility with best machinability.
Fig. 7.3 (a to e) illustrates electron micrograph of a steel (C = .56% Mn = 1.36%) tempered for 1 h at 204°C, 315°C, 426°C, 593°C, 675°C depicting stages particularly of spheroidisation. Fig. 7.3 (f) illustrates 0.4%C steel, quenched and tempered at 700°C for 1 hour. It is an optical microstructure showing spheroidised pearlite.