Cementite and epsilon carbides in 100Cr6 martensitic bearing steel

Table 1: Chemical composition in wt.% of the 100Cr6 bearing steel.

Table 2: Thermal treatments performed on the 100Cr6 steel before the ageing processes.

Figure 1: 1.) State H + 4 h at 140 C. Dark-field micrograph of epsilon-carbides corresponding the the bold circled spot of the diffraction pattern. The diffraction pattern shows the alpha'-Fe matrix near the [111] orientation (dashed hexagon) and additional spots due to hexagonal e-carbide. 2) State H + 2 h at 240 C. Bright-field image of cementite. The diffraction pattern shows the alpha'-Fe matrix near the [112] orientation (dashed rectangle) and additional spots due to orthorhombic cementite. Scale bars: 0.5 µm.

Carbide name: Cementite, Epsilon carbide
Record No.: 1005
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: TEM
Steel name: 100Cr6
Mat.No. (Wr.Nr.) designation: 1.2067
DIN designation: 100Cr6
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Martensitic bearing steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: The composition of the 100Cr6 steel is given in Table 1. All samples have been austenitized for 15 min at 850 C, quenched in an oil bath and hold in hot water (60 C) for 5 min. The resulting state will be called H in the following. From state H, some samples were held for 1 h at -80 C to reduce the amount of retained austenite. The resulting state will be called HF in the following (see Table 2).
Then, isothermal ageing at temperatures ranging from 110 to 505 C have been performed in oil and salt baths for times ranging from 1 min to 1500 h. In the following, thermal treatments (austenitizing, quenching, rinsing, eventual cold quenching and ageing) will be named by a letter (H or HF) followed by a number (ageing temperature in Celsius). For example, H110 stands for state H followed by ageing at 110 C. It is important to note that at state H, all the carbon (~1 wt:%) does not remain in solid solution (in martensite). A relatively large amount of carbon is (i) segregated to dislocations (concentration hereafter noted Cd and (ii) incorporated into large carbides that do not dissolve during austenitization, hereafter named undissolved carbides.
Note: Tempering of a martensitic 100Cr6 (AISI52100) bearing steel during isothermal treatments is studied using thermoelectric power, transmission electron microscopy (TEM), X-ray diffraction and dimensional analysis. Dimensional changes occurring during tempering are due to: (i) the precipitation of epsilon-carbides, (ii) the decomposition of retained austenite, (iii) the precipitation of cementite and (iv) the recovery of the dislocation structure and coarsening of martensite lathes. Analysis of thermoelectric power evolutions, measured during isothermal ageing treatments, and assisted by TEM characterization, leads to a quantitative estimation of austenite and precipitate volume fraction. These data are used to predict dimensional changes, and these are in very good agreement with dimensional measurements.

Isothermal ageing treatments (tempering) of 100Cr6 samples from state H and HF have been performed at various temperatures. a) Stage A: a sigmoidal-shaped evolution at low ageing temperatures (e.g. 100 min at 110 C) for which the TEP curves from the H and HF states coincide. b) Stage B: another sigmoidal-shaped evolution for higher temperatures (e.g. 20 min at 240 C), for which the TEP of the sample from the H and HF state are no longer similar. c) Stage C: a fairly broad evolution, for the highest investigated ageing temperatures.
To investigate the origin of the first step (stage A), TEM has been performed after H + 4 h at 140 C (Fig. 1-1). Analysis of the diffraction pattern led to the positive identification of epsilon-carbide: a hexagonal structure with six iron atoms per cell a=0.4767 nm and c=0.4354 nmÞ, space group P6322 and cell volume omega=0.0857 nm3 as reported by Hirotsu and Nagakura. This carbide is a superstructure of ‘‘classical” epsilon-carbide (space group P63/mmc, JCPDS#36-1249). This stage is then assumed to be related to the precipitation of epsilon-carbide (first stage of tempering).
As far as the second step (stage B) is concerned, TEM analysis has been performed after H + 2 h at 240 C (Fig. 1-2). Analysis of the diffraction pattern led to the positive identification of cementite (JCPDS#85-1317 [23]): orthorhombic structure with 12 iron atoms per cell a=0.50890 nm; h=0.67433 nm and c=0.45235 nm, space group Pnma and cell volume omega=0.0236 nm3 as reported by Bagaryatskii. Note that no retained austenite has been observed in the TEM after 2 h at 240 C. Moreover, if the TEP evolution of H240 and HF240 are compared, taking into account that the main difference between both states is the initial amount of retained austenite, it can be concluded that this second step is also due to the decomposition of retained austenite. Stage B is then assumed to be related to both the decomposition of retained austenite and the precipitation of cementite (second and third stages of tempering). The same interpretations are also given in the work of Tkalcec, except that no retained austenite remained in their steel.
Finally, the evolution of TEP for high temperature and/ or long ageing times (stage C) is assumed to be connected to the recovery of the dislocation structure and the coarsening of martensite lathes according to Porter and Easterling and Speich and Taylor(fourth stage of tempering).
Links: No data
Reference: Not shown in this demo version.

Copyright © 2018 by Steel Data. All Rights Reserved.