Unraveling the Effects of Austenitizing Temperature and Austenite Grain Size on the Crystallographic Characteristics and Mechanical Properties of Martensitic Transformation Products in a Low-Alloy Steel

doi:10.1007/s40195-022-01501-8

Abstract

Abstract:

The influence of austenitizing temperature and austenite grain size on the crystallographic characteristics and mechanical properties of transformation products was investigated in a low-alloy steel. Annealing at a lower temperature after austenitization at 1050 °C can reduce the vacancy concentration and enhance the stability of austenite, thereby determining the martensite-start (M_s) temperature and density of twin-related V1/V2 variant pairs. V1/V2 variant pairs are predominately generated by autocatalytic nucleation, which can promote transformation by self-accommodation. Annealing at 800 °C after austenitization at 1050 °C generated the highest content of V1/V2 variant pairs, which contributed to high values of strength, Charpy impact energy, and elongation. In addition to refining the austenite grain size, niobium (Nb) microalloying increases the packet boundaries of the transformation products, mainly because the refined austenite grains provide more nucleation sites for the martensitic transformation. However, the density of block boundaries decreases after austenite refinement by Nb microalloying, owing to insufficient autocatalytic nucleation.

Key words: Steel, Martensite, Transformation, Nucleation, Variant

Binbin Wu, Fangzhong Hu, Zhiquan Wang, Shaopeng Yang, Rui Zhong, Chengjia Shang, Zhigang Yang, Chi Zhang. Unraveling the Effects of Austenitizing Temperature and Austenite Grain Size on the Crystallographic Characteristics and Mechanical Properties of Martensitic Transformation Products in a Low-Alloy Steel[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(4): 694-704.

Figures/Tables 10

Table 1 Chemical composition of the experimental steel (wt%)

Steel	C	Si	Mn	Cr	Ni	Mo	Nb
Nb-free	0.17	0.25	0.51	1.65	1.56	0.24	-
Nb-bearing	0.17	0.24	0.53	1.55	1.62	0.22	0.035

Fig. 1 Schematic of heat treatment profiles

Table 2 Statistics for block, sub-block, and packet boundaries

Steel	Austenitizing temperature (℃)	Block boundaries (μm⁻¹)	Sub-block boundaries (μm⁻¹)	Packet boundaries (μm⁻¹)
Nb-free	1050	0.74	0.54	0.31
	925	0.74	0.63	0.34
	800	1.02	0.40	0.37
Nb-bearing	1050	0.57	0.35	0.40
	925	0.76	0.51	0.50
	800	0.76	0.47	0.39

Fig. 2 SEM micrographs and band contrast (BC) maps depicting boundary distribution of Nb-free-1050Q a, d, Nb-free-925Q b, e, and Nb-free-800Q c, f samples, respectively, g density of inter-variant boundaries and h strain-stress curves with Charpy impact energy. (White line: 15° > θ > 2°, black line: 45° > θ > 15°, yellow line: θ > 45°)

Fig. 3 SEM micrographs a-c and band contrast (BC) maps d-f depicting boundary distribution in Nb-bearing-1050Q a, d, Nb-bearing-925Q b, e, and Nb-bearing-800Q c, f sample, respectively, g density of inter-variant boundaries and h strain-stress curves with Charpy impact energy. (White line: 15° > θ > 2°, black line: 45° > θ > 15°, yellow line: θ > 45°)

Table 3 Maximum undercooling at the completion of transformation for all dilatometer samples

Austenitizing temperature (℃)	Cooling rate (℃/s)
	− 100		− 50		− 20
	Nb-bearing	Nb-free	Nb-bearing	Nb-free	Nb-bearing	Nb-free
1050	− 141	− 191	− 237	− 168	− 228	− 259
925	− 186	− 227	− 254	− 277	− 265	− 258
800	− 176	− 187	− 161	− 236	− 211	− 260

Fig. 4 Transformation kinetics curves of Nb-free samples cooled at rates of a − 100, b − 50, and c − 20 °C/s by dilatometer and d a summary of Ms temperatures

Fig. 5 Transformation kinetics curves of Nb-bearing samples cooled at rates of a − 100, b − 50, and c − 20 °C/s by dilatometer and d a summary of Ms temperatures

Fig. 6 Band contrast (BC) maps depicting boundary distribution of Nb-free samples annealed at 800 °C and then cooled at rate of a − 100, b − 50, and c − 20 °C/s sample; d density of inter-variant boundaries; and e transformation rate curves. (White line: 15° > θ > 2°, black line: 45° > θ > 15°, yellow line: θ > 45°)

Fig. 7 Martensitic transformation rate (− df/dT) against undercooling (ΔT) for the two steels cooled from 1050 °C

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