Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Figures & data

Table 1. Chemical composition of steel Fe–0·80C/wt-%

	C	Mn	Si	Ni	Cr	Cu	P	S
Fe–0·80C	0·80	0·61	0·3	0·2	0·3	0·23	0·012	0·04

Table 2. Experimental details and results of experiments with varying cooling rates

Cooling rate between Ms and RT/°C s^−1	ΔT at Ms/°C	TKM from KM fit/°C	αm from KM fit/K^−1	Ms from slope/°C	γR XRD/vol.-%
12·5	20			247	10±2
5	18			248
2	17			245
Natural	2	218	0·0122	230	10±2
0·2	2	218	0·0122	226

Table 3. Experimental details and results of experiments with varying austenitising treatments*

Taus/°C	taus/min	D^γ/μm	Ms/°C	TKM/°C	αm/K^−1	γR/vol.-%
800	1	12±4	221	218	0·0122	11±2
850	2	25±5	225	218	0·0122	…
900	2	40±8	230	218	0·0122	10±2
1000	2	65±15	235	218	0·0122	11±2
1050	12	130±25	238	218	0·0122	11±2

*T_aus, austenitising temperature; t_aus, austenitising time.

1. a measured dilatation as a function of temperature due to martensite formation during cooling of Fe–0·80 samples with different rates. Right-hand y-axis shows measured temperature difference between two thermocouples (ΔT) and b dilatometry data plotted against temperature corrected for thermal gradients

2. a dilatation due to tempering of fully martensitic sample at 180°C as a function of time: length decrease of about 2–2·5 μm in some tens of seconds and b dilatation as a function of temperature showing tempering characteristics during heating of samples which were made martensitic with different cooling rates as shown in Fig. 1

3. a measured dilatation as a function of temperature due to martensite formation during natural cooling (open squares). Solid lines represent thermal expansion of both austenite (fcc) and martensite (bcc); open circles describe dilatation of fully martensitic sample during heating and b experimental volume fraction of martensite as a function of temperature derived from Fig. 3a (open squares) can be described by KM equation (solid line) using T_KM = 218°C, α_m = 0·0122 K⁻¹. Volume fraction of retained austenite at room temperature is approximately 0·10

4. Change in length as a function of temperature due to martensite formation in Fe–0·80C during natural cooling of samples which were given different austenitising treatments. Inset shows magnification of dilatometry curves close to M_s temperature

5. Volume fraction of martensite as a function of temperature derived from measured dilatation data shown in Fig. 4 for T_aus = 800°C (open squares). Solid line represents progress of transformation calculated with KM equation using T_KM = 218°C, α_m = 0·0122 K⁻¹

6. Martensite fraction as a function of temperature corresponding measured dilatation signals shown in inset of Fig. 4. Black line represents transformation kinetics calculated with KM equation using T_KM = 218°C, and α_m = 0·0122 K⁻¹