Tempering of Martensite

Stage 1
upto – 250 C – precipitation of eta-carbide, partial loss of tetragonality of martensite.
Steels above 0.25 wt% carbon precipitate hexagonal close-packed eta-carbide within the supersaturated martensite until 0.25 wt% carbon level is reached, martensite preserves some tetragonality. The orientation relationship between the laths or rodlets and the cube planes of the matrix was first described by Jack [1].

Stage 2
200-300 C – decomposition of retained austenite.

Stage 3
200-350 C – replacement of eta-iron carbide by cemeneite,; martensite loses tetragonality

Stage 4
350 C – cementite coarsens and speroidizes, recrystallisation of ferrite.

[1] Steels 2nd Ed, Honeycombe and Bhadeshia, Edward Arnold, 1981, p172.

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2 Responses

  1. during stage 2 the retained austenite should become banite, not just decompose into ferrite and cementite,check the TTT diagrams for isothermal transformations of hypoeuectoid steels

  2. I don’t think it’s true that we will always get bainite during stage 2 tempering. The TTT diagram is for transformation from austenite, once transformed to martensite the starting microstructure will have an effect. It might be possible that some of these steels will form bainite, but I myself haven’t seen (but havent looked for!) the experimental evidence.

    Looking for surface relief caused by the displacive transformation might be one thing to do, but this experiment should also probably be done in a dilatometer so that it is possible to distinguish bainite from any martensite that forms upon the cooling from the tempering temperature.

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