Universality of quenching-partitioning-tempering local equilibrium model

doi:10.1016/j.jmst.2021.12.021

Abstract

Abstract:

The design of metastable retained austenite (RA) and its stability have always played a vital role in developing advanced high strength steels (AHSSs). The quenching-partitioning-tempering local equilibrium (QPT-LE) thermo-kinetic model was established by us, with consideration of carbide precipitation and interface migration. As a tool for designing AHSSs, the above model can well predict the microstructural amount and the carbon content in RA (C_γ) in a high-C quenching-partitioning-tempering (Q-P-T) steel. In this work, QPT-LE model predicates the microstructural amount and C_γ in low-C and medium-C Q-P-T steels more accurately than constrained carbon equilibrium (CCE) model without consideration of carbide precipitation and interface migration, and more accurately than quenching and partitioning-local equilibrium (QP-LE) model without consideration of carbide precipitation, which verifies the universality of QPT-LE model. The advantages of QPT-LE model are also discussed.

Key words: Quenching-partitioning-tempering steel, Carbide precipitation, Interface migration, QPT-LE thermo-kinetic model

J.Z. Zhang, L.Y. Zeng, X.W. Zuo, J.F. Wan, Y.H. Rong, N. Min, J. Lu, N.L. Chen. Universality of quenching-partitioning-tempering local equilibrium model[J]. J. Mater. Sci. Technol., 2022, 124: 116-120.

Figures/Tables 5

Fig. 1. Sketch of the Q-P-T processes for (a) low-C samples and (b) medium-C samples. RT: room temperature.

Table 1. Detailed Q-P-T process parameters. TA: austenitization temperature, Tq: quenching temperature, Tp: partitioning/tempering temperature.

Steel	T_A/time	T_q	T_p/time	Sample name
Low carbon	930 °C/360 s	290 °C	400 °C/60 s	Q-P-T (L290-400-60s)
	930 °C/360 s	320 °C	400 °C/60 s	Q-P-T (L320-400-60s)
	930 °C/360 s	350 °C	400 °C/60 s	Q-P-T (L350-400-60s)
Medium carbon	850 °C/360 s	200 °C	400 °C/120 s	Q-P-T (M200-400-120s)
	850 °C/360 s	230 °C	400 °C/120 s	Q-P-T (M230-400-120s)
	850 °C/360 s	260 °C	400 °C/120 s	Q-P-T (M260-400-120s)

Fig. 2. TEM images of Q-P-T samples: (a) Bright field (BF) image indicating η-carbides precipitated from dislocation-type martensite with inserted selective area electron diffraction (SAED) pattern for Q-P-T (L320-400-60s) sample; (b) BF image of RA and martensitic laths and (c) Dark field (DF) image of RA with inserted SAED pattern of the Q-P-T (L320-400-60s) specimen; (d) BF image of η-carbides with inserted SAED pattern for Q-P-T (M230-400-120s) sample; (e) BF image of RA and martensitic laths and (f) DF image of RA with inserted SAED pattern of the Q-P-T (M230-400-120s) specimen. Sketch of the QPT-LE model with dual interfaces migration: (g) initial state and (h) finishing state during partitioning/tempering. η: transition carbide; γ: austenite; α' in (g) refers to primary martensite, α' in (h) refers to primary martensite or bainitic ferrite formed during partitioning/tempering process. Cγ: C content in γ; Cα' : C content in α'; C partitioning from α' to γ results in reduced Cα' and increased Cγ at the finishing state. TKD images of the Q-P-T samples: (i) Q-P-T (L320-400-60s); (j) Q-P-T (M230-400-120s) (red: face-centered cubic (FCC) phase, green: body-centered cubic (BCC) phase).

Fig. 3. Variation of austenite volume fractions during partitioning/tempering process of (a) Q-P-T (L320-400-60s) sample and (b) Q-P-T (M230-400-120s) sample. Variation of C content at the α'/γ interface in austenite during partitioning/tempering process of (c) Q-P-T (L320-400-60s) sample and (d) Q-P-T (M230-400-120s) sample. Volume fraction of carbide and mole fraction of C trapped in carbide as a function of partitioning/tempering time for (e) Q-P-T (L320-400-60s) sample and (f) Q-P-T (M230-400-120s) sample. Variation of austenite volume fractions predicted by the QPT-LE model for (g) low-C Q-P-T samples and (h) medium-C Q-P-T samples quenched to different temperatures.

Table 2. VRA and Cγ of Q-P-T samples measured by experiments and calculated by different models.

Samples	V_RA (%)				C_γ (wt.%)
Samples	XRD	QPT-LE	QP-LE	CCE	XRD	QPT-LE	QP-LE	CCE
Q-P-T (L290-400-60s)	6.5 ± 0.8	6.4	8.7	3.6	1.26±0.03	1.35	1.35	0.63
Q-P-T (L320-400-60s)	6.9 ± 0.6	7.1	9.2	2.6	1.24±0.04	1.35	1.35	0.45
Q-P-T (L350-400-60s)	7.2 ± 0.9	7.9	9.9	2.2	1.21±0.03	1.35	1.35	0.32
Q-P-T (M200-400-120s)	13.6 ± 0.8	14.6	22.6	18.2	1.28±0.05	1.40	1.40	1.23
Q-P-T (M230-400-120s)	14.8 ± 0.9	15.2	23.5	11.8	1.25±0.04	1.40	1.40	0.88
Q-P-T (M260-400-120s)	15.2 ± 1.1	16.1	24.2	7.5	1.23±0.04	1.40	1.40	0.63

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