Prediction for Flow Stress of 95CrMo Hollow Steel During Hot Compression

doi:10.1007/s40195-016-0498-7

Abstract

Abstract:

The compressive deformation behavior of 95CrMo hypereutectic steel was studied at temperatures ranging from 800 to 1050 °C and strain rates from 0.1 to 3 s^-1 on a Gleeble-3500 thermo-simulation machine. The results showed that, with the decrease in deformation temperature and increase in strain rate, the fragmented retained austenite in finer and distributed more uniformly in the ferrite matrix as a result of the inhibited recovery. The recorded flow stress suggested that the stress level decreases with increasing temperature and decreasing strain rate. Based on the classical stress-dislocation relation, the constitutive equations of flow stress determined by work-hardening and softening mechanisms were established. A comparison between the experimental and calculated values confirmed the reliability of the model, and the predictability of the model was also quantified in terms of correlation coefficients and average absolute relative errors, which were found generally above 0.99 and below 2.50%, respectively. In the whole range of strain rate, the activation energy is 419.84 kJ/mol. By further identification based on Sch?ck’s model and Kocks-Argon-Ashby model, the rate-controlling mechanism is found to be dislocation cross-slip.

Key words: Tempering, 95CrMo hypereutectic steel, Hot deformation behavior, Hot compression, Rate-controlling mechanism, Constitutive model

Bao-Sheng Xie,Qing-Wu Cai,Wei Yu,Li-Xiong Xu,Zhen Ning. Prediction for Flow Stress of 95CrMo Hollow Steel During Hot Compression[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(3): 250-260.

Figures/Tables 16

Table 1 Chemical compositions of experimental steels (wt%)

C	Si	Mn	Cr	Mo	Cu	Fe
0.95	0.25	0.30	0.15-0.40	0.15-0.30	<0.25	Bal.

Fig. 1 Microstructures of the as-received drilling steel a overall microstructure at lower magnification and b detailed microstructure of pearlite. LP lamellar pearlite, CNcementite network, L average pearlite lamellar spacing

Fig. 2 SEM images of the specimens under different hot deformation conditions: a 0.1 s-1, 1050 °C; b 0.1 s-1, 950 °C; c 0.1 s-1, 850 °C; d 1 s-1, 1050 °C; e 1 s-1, 950 °C; f 1 s-1, 850 °C; g 3 s-1, 1050 °C; h 3 s-1, 950 °C; i 3 s-1, 850 °C

Fig. 3 Cooling routines after hot deformation for the specimens compressed at different temperatures

Fig. 4 EBSD analysis of the microstructures of specimens deformed at 3 s-1 and 1050 °C. aCombined image of band-contrast map and color code phase map showing the austenite (face-centered cubic phase) in red and b Combined image of band contrast and inverse pole figure analysis of the austenite

Fig. 5 True stress-strain curves for 95CrMo steel under different deformation temperatures and strain rates: a 0.1 s-1, b 1 s-1, c 3 s-1

Table 2 Yield stress (σ0, MPa) at different strain rates and temperatures

Strain rate (s^-1)	1050 °C	1000 °C	950 °C	900 °C	850 °C	800 °C
0.1	29.02	32.76	38.08	56.73	57.70	63.32
1	39.88	44.27	55.88	73.63	95.24	108.89
3	55.10	57.94	77.00	94.62	111.83	142.82

Fig. 6 Relationship between ln[sinh(ασ)] and ln ε?

Fig. 7 Relationship between ln[sinh(ασ)] and temperature

Fig. 8 Relationship between strain rate, temperature and Ω

Fig. 9 Relationship between Zenner-Hollomon parameter and Ω Ω=34.6768Z-0.0333.Ω=34.6768Z-0.0333.

Fig. 10 Change in strain rate exponent on root mean square error (RMSE), maximum relative error (MXRE) and correlation coefficient (R)

Fig. 11 Comparisons between the experimental flow stress and predicted flow stress of Eq. (16) at hot deformation. The solid lines represent the experimental flow stress, and the dash lines means the predicted flow stress: a 0.1 s-1, b 1 s-1, c 3 s-1

Table 3 AARE (average absolute relative error) and R (correlation coefficient) for the predicted and experimental flow stress

Strain rate (s^-1)	Temperature (°C)	AARE (%)	R
0.1	1050	2.83	0.967
0.1	1000	3.05	0.992
0.1	950	1.90	0.998
0.1	900	2.40	0.999
0.1	850	2.38	0.994
0.1	800	1.27	0.998
1	1050	2.63	0.996
1	1000	2.42	0.996
1	950	2.50	0.994
1	900	1.91	0.996
1	850	1.69	0.993
1	800	2.32	0.995
3	1050	2.26	0.995
3	1000	1.69	0.996
3	950	2.05	0.992
3	900	1.82	0.994
3	850	2.20	0.993
3	800	2.39	0.992

Fig. 12 Relationship between true stress and kTln(ε?/εo?)Gb3Gb3 for flow stress of 1 and 3 s-1

Fig. 13 Relationship between true stress, U(σ) and Q(σ) for flow stress of 1 and 3 s-1

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