The structural transformation of quenched carbon steel during tempering is representative for various steels. The tempering process includes four types of reactions: martensite decomposition, carbide precipitation, transformation, aggregation and growth, ferrite recovery and recrystallization, and retained austenite decomposition. The transitions during tempering of low and medium carbon steels are schematically summarized in Figure 1. According to their reaction temperature, it can be described as four stages overlapping each other.
The first stage of tempering (below 250 ℃):
Martensite is unstable at room temperature, and the interstitial carbon atoms can move slowly in the martensite, resulting in a certain degree of carbon segregation. With the increase of tempering temperature, martensite begins to decompose, ε-carbides are precipitated in medium and high carbon steel, and the squareness of martensite decreases. The observed increase in hardness of high carbon steel after tempering at 50 to 100 °C is the result of precipitation hardening of ε-carbides in martensite (see precipitation). ε-Carbide has a close-packed hexagonal structure, in the shape of narrow strips or thin rods, and has a certain orientation relationship with the matrix. The primary ε-carbides are likely to remain coherent with the matrix. After tempering at 250°C, the martensite still retains about 0.25% carbon. Martensite containing less than 0.2% carbon does not cause ε-carbide precipitation when tempered below 200 °C, only carbon segregation, while tempering at higher temperatures directly decomposes cementite.
The second stage tempering (200 ~ 300 ℃):
Transformation of retained austenite. When tempering to a temperature range of 200-300 °C, the retained austenite in the quenched steel that has not been completely transformed will decompose and form a bainite structure. This transformation is more pronounced in medium carbon and high carbon steels. For carbon and low alloy steels containing less than 0.4% carbon, this transformation is essentially negligible due to the small amount of retained austenite.
The third stage tempering (200 ~ 350 ℃):
The decomposition of martensite is completed and the squareness disappears. The ε-carbides are transformed into cementite (Fe3C). This transformation is carried out by the dissolution of ε-carbides and the re-nucleation and growth of cementite. The initially formed cementite and matrix maintain a strict orientation relationship. Cementite tends to nucleate at the interface of ε-carbide and matrix, at the interface of martensite, at twin boundaries in high-carbon martensite sheets, and at the grain boundaries of prior austenite. The formed cementite starts as a film, and then gradually spheroidizes into granular Fe3C.
Fourth stage tempering (350～700℃):
Spheroidization and growth of cementite, recovery and recrystallization of ferrite. The cementite begins to spheroidize from 400℃, and grows in agglomeration after 600℃. As the process proceeds, the smaller cementite particles dissolve in the matrix while the carbon is delivered to the selectively grown larger particles. Carbide grains located at the martensite grain boundaries and prior austenite grain boundaries spheroidize and grow the fastest because diffusion is much easier in these regions.
The recovery process of ferrite occurs at 350-600 °C. At this time, in low carbon and medium carbon steels, the dislocations in the lath martensite and on the lath boundary are merged and rearranged, so that the dislocation density is significantly reduced, and the lath and the original martensitic lath are formed. The bundles are closely associated with elongated ferrite grains. The original martensitic lath boundary can remain stable to 600 °C; in high carbon steel, the ferrite formed by the disappearance of twins in acicular martensite still maintains its acicular morphology. Significant recrystallization occurred in the ferrite at 600～700℃, and equiaxed ferrite grains were formed. After that, the Fe3C particles became thicker and the ferrite grains gradually grew.
