Tempering of Martensite

Tempering of Martensite

The tempering of martensite is usually carried out in the range 150–600 C. Extensive studies have been carried out on the tempering behaviour of martensitic steels. For a carbon steel, this is divided into three stages:

1. Precipitation of Epsilon carbide at 70–150 C. This has a hexagonal crystal structure (a = 2.755 A, c = 4.349 A) and a composition Fe2.4C, and forms as narrow plates with a well-defined orientation relationship.

2. Decomposition of retained austenite at 150–280 C, possibly to bainite and cementite.

3. Precipitation of cementite above 200 C. As the cementite grows, Epsilon carbide dissolves. However, the tempering of alloy steel is divided into four stages.

The first three of these are the same as those of carbon steels, but the temperature at which each stage occurs depends on the alloy composition. For example, Si and Cr stabilize Epsilon carbide so that the third stage occurs at a higher temperature (above 300 C). Si and Cr also retard the growth of cementite, and steels containing these elements resist softening up to 500 C. The addition of carbide-forming elements such as Mo, V, and Nb, even in small amounts, gives a pronounced softening resistance. These elements retard the climb of dislocations and keep the dislocation density high even if the steel is tempered above 500 C. This resulting high dislocation density aids the precipitation of alloy carbides in the subsequent fourth stage.
The fourth stage of tempering of alloy steel martensite is the process during which complex alloy carbides precipitate with the complementary dissolution of cementite. These alloy carbides may themselves dissolve at later times of this stage as different, more stable carbides start to precipitate. The nucleation mechanism of alloy carbides is classified into two categories: (1) in-situ transformation and (2) separate nucleation. In the former case, carbides nucleate at the same place as the existing cementite, and the hardening effect is reduced because the distribution of the nucleating carbides is dominated by that of the existing cementite. In the alternative scenario, carbides nucleate independently of the cementite, and may produce a considerable hardening effect if the precipitates are coherent with the matrix. The initial metastable precipitates are those for which nucleation is easiest. At longer times, other more stable phases may form. Although the nucleation of these phases is more difficult, their formation leads to a reduction in the free energy of the system and is, therefore, thermodynamically favourable. The formation of the more stable precipitates is accompanied by the dissolution of the existing metastable precipitates. However, it is also possible for thermodynamically stable phase to precipitate in the early stage of the tempering sequence (e.g. Nb(C,N)). Such phases do not dissolve during subsequent tempering. In the following paragraphs, the characteristics of some carbide phases which precipitate during tempering in alloy steels will be summarized.

Reference: Shingo Yamasaki, Modelling Precipitation of Carbides in Martensitic Steels, University of Cambridge, Darwin College, PhD Thesis, 2004, p. 4.