Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr-24Ni-6Mo Super-Austenitic Stainless Steel During Hot Deformation

doi:10.1007/s40195-017-0657-5

Abstract

Abstract:

Hot compression tests were carried out with specimens of 20Cr-24Ni-6Mo super-austenitic stainless steel at strain rate from 0.01 to 10 s^-1 in the temperature range from 950 to 1150 °C, and flow behavior was analyzed. Microstructure analysis indicated that dynamic recrystallization (DRX) behavior was more sensitive to the temperature than strain rate, and full DRX was obtained when the specimen deformed at 1150 °C. When the temperature reduced to 1050 °C, full DRX was completed at the highest strain rate 10 s^-1 rather than at the lowest strain rate 0.01 s^-1 because the adiabatic heating was pronounced at higher strain rate. In addition, flow behavior reflected in flow curves was inconsistent with the actual microstructural evolution during hot deformation, especially at higher strain rates and lower temperatures. Therefore, flow curves were revised in consideration of the effects of adiabatic heating and friction during hot deformation. The results showed that adiabatic heating became greater with the increase of strain level, strain rate and the decrease of temperature, while the frictional effect cannot be neglected at high strain level. Moreover, based on the revised flow curves, strain-dependent constitutive modeling was developed and verified by comparing the predicted data with the experimental data and the modified data. The result suggested that the developed constitutive modeling can more adequately predict the flow behavior reflected by corrected flow curves than that reflected by experimental flow curves, even though some difference existed at 950 °C and 0.01 s^-1. The main reason was that plenty of precipitates generated at this deformation condition and affected the DRX behavior and deformation behavior, eventually resulted in dramatic increase of deformation resistance.

Key words: Super-austenitic stainless steel, Hot compression, Adiabatic heating, Constitutive modeling, Microstructure evolution

Yan-Sen Hao, Wan-Chun Liu, Zhen-Yu Liu. Microstructure Evolution and Strain-Dependent Constitutive Modeling to Predict the Flow Behavior of 20Cr-24Ni-6Mo Super-Austenitic Stainless Steel During Hot Deformation[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(4): 401-414.

Figures/Tables 17

Table 1 Chemical composition of investigated alloy (wt%)

C	P	S	N	Si	Cu	Mn	Ni	Cr	Mo	Fe
≤ 0.03	≤ 0.04	≤ 0.03	0.215	0.354	0.684	0.452	22.4	19.4	6.65	Bal.

Fig. 1 Initial microstructure of 20Cr-24Ni-6Mo SASS steel

Fig. 2 Experimental flow curves under different deformation temperatures with strain rates: a 0.01 s-1; b 0.1 s-1; c 1 s-1; d 10 s-1

Fig. 3 Hot deformation micrographs of specimens under different strain rates at 950 °C: a 0.01 s-1; b 0.1 s-1; c 10 s-1

Fig. 4 Hot deformation micrographs of specimens: a 1050 °C, 0.01 s-1; b 1050 °C, 10 s-1; c 1150 °C, 0.01 s-1; d 1150 °C, 10 s-1

Table 2 Values of specific heat of 20Cr-24Ni-6Mo SASS

Temperature (°C)	950	1000	1050	1100	1150
Specific heat (J kg^-1 K^-1)	645.2	649.9	654.6	659.2	663.9

Fig. 5 Variation of temperature rise with strain for specimens deformed at strain rates of a 10 s-1; b 0.01 s-1

Fig. 6 Schematic diagrams of processing methods for adiabatic heating correction: a linear relationship; b nonlinear relationship between temperature and true stress

Fig. 7 Comparison of original experimental flow curves and adiabatic heating corrected flow curves at strain rates of a 10 s-1; b 0.01 s-1

Fig. 8 Typical appearance of the specimens after deformation

Table 3 Values of barreling coefficient B of investigated specimens

Strain rate (s^-1)	Temperature (°C)
Strain rate (s^-1)	950	1000	1050	1100	1150
0.01	1.205	1.204	1.219	1.205	1.218
0.1	1.256	1.197	1.197	1.191	1.205
1	1.137	1.218	1.205	1.183	1.455
10	1.147	1.160	1.122	1.191	1.218

Fig. 9 Comparison of adiabatic heating corrected and adiabatic heating and friction corrected flow curves at strain rates of a 10 s-1; b 0.01 s-1

Fig. 10 Evaluation of values of a n1 by plotting lnσ versus ln$\dot{\varepsilon}$; b β by plotting σ versus ln$\dot{\varepsilon}$; c n by plotting ln[sinh(ασ)] versus lnε˙; d Q by plotting ln[sinh(ασ)] versus 10,000/T; e lnA by plotting lnZ versus ln[sinh(ασ)] at strain of 0.45

Fig. 11 Relationships between a α ; b n; c Q; d ln A and true strain ε

Table 4 Polynomial coefficients of α, n, Q, and ln A for investigated alloy

α	n	Q	ln A
C ₁ = 141.90078	D ₁ = 403.44852	E ₁ = 22,385.8426	F ₁ = 2006.36665
C ₂ = 1895.0011	D ₂ = 6369.08584	E ₂ = 318,032.5293	F ₂ = 28,783.30076
C ₃ = 13,299.762	D ₃ = 48,827.5076	E ₃ = 2.2817 × 10⁶	F ₃ = 207,530.3504
C ₄ = 54,875.835	D ₄ = 214,080.063	E ₄ = 9.4742 × 10⁶	F ₄ = 864,381.2256
C ₅ = 137,933.421	D ₅ = 562,823.928	E ₅ = 2.3728 × 10⁷	F ₅ = 2.1693 × 10⁶
C ₆ = 207,593.649	D ₆ = 877,220.454	E ₆ = 3.5396 × 10⁷	F ₆ = 3.2409 × 10⁶
C ₇ = 171,871.903	D ₇ = 747,109.433	E ₇ = 2.8998 × 10⁷	F ₇ = 2.6579 × 10⁶
C ₈ = 60,182.7957	D ₈ = 267,789.971	E ₈ = 1.0053 × 10⁷	F ₈ = 922,130.2720

Fig. 12 Comparison between modified data and predicted data at strain rates of a 0.01 s-1; b 0.1 s-1; c 1 s-1; d 10 s-1

Fig. 13 SEM microstructures of specimens deformed at 950 °C with strain rates of a 0.01 s-1; b 0.1 s-1; c 10 s-1; d precipitate morphology by TEM at 950 °C, 0.01 s-1, and corresponding results of e EDX, f SAED (GB grain boundary)

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