Nanostructured Carbide-Free Bainite Formation in Low Carbon Steel

doi:10.1007/s40195-020-01091-3

摘要/Abstract

Abstract:

It is an important challenge to reduce the carbon content in nanostructured bainitic steels for commercialization purposes while still being able to gain the desired microstructural characteristics in nanoscale and not to deteriorate the strength-ductility combinations. That is the point at which an appropriate heat treatment procedure design would be an important parameter. This article aims to investigate how to obtain nanostructured bainite in steel with 0.26 wt% carbon content by applying multi-step austempering procedures. One-, two- and three-step austempering processes have been implemented, and proper heat treatment temperatures and approaches were selected based on dilatometry tests. Results indicated that it has become possible to achieve bainitic ferrites and austenite films with overall thicknesses of 164, 145 and 132 nm and 134, 105 and 90 nm at the end of one-, two- and three-step austempering heat treatments, respectively. Meanwhile, microstructural characteristics resulted in enhanced mechanical properties with ultimate tensile strength (UTS) of 1435, 1455 and 1428 MPa in combination with elongation levels of 15.4, 13.6 and 11.4% after implementing those heat treatments. Finally, it has been shown that applying the multi-step austempering heat treatments resulted in enhanced yield strength and impact toughness values due to the microstructural characteristics and proper heat treatment procedure design.

Key words: Bainite, Multi-step austempering, Austenite, Mechanical properties, Impact toughness

. [J]. 金属学报英文版, 2020, 33(12): 1635-1644.

Hamid Mousalou, Sasan Yazdani, Naghi Parvini Ahmadi, Behzad Avishan. Nanostructured Carbide-Free Bainite Formation in Low Carbon Steel[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(12): 1635-1644.

图/表 13

Fig. 1 Dilatometry curves a to measure the AC1 and AC3 temperatures and b to measure the MS1 temperature, c thermal cycle and dilatation curves showing the kinetics of bainitic transformation at 315 °C

Fig. 2 Dilatometry curves a to determine the time at which almost 75% bainite formed at 315 °C and b to measure martensite start temperature for remaining austenite (MS2) being present at the end of partial bainite transformation

Fig. 3 One-, two- and three-step austempering heat treatment cycles as well as anticipated microstructural features during the heat treatment procedures. γb, M, αb1, αb2 and αb3 represent austenite blocks, martensite, bainitic ferrite formed at the first step, bainitic ferrite formed at the second step and bainitic ferrite formed at the third step, respectively

Fig. 4 SEM micrographs of a one-step, b two-step, c three-step austempering samples and d high-magnification image of the selected region in c. GB, γb, γf, M and αb represent prior austenite grain boundary, austenite blocks, austenite films, martensite and bainitic ferrite, respectively

Fig. 5 Nanoscale bainitic ferrites and austenite films formed at the end of the third step of austempering heat treatment

Fig. 6 Mean thicknesses and size distributions of bainitic ferrite plates at the end of different heat treatment conditions

Fig. 7 Mean thicknesses and size distributions of austenite films at the end of different heat treatment conditions

Fig. 8 XRD patterns of samples after one-, two- and three-step austempering heat treatments

Fig. 9 Changes in hardness values and the volume fraction of retained austenite after one-, two- and three-step austempering heat treatments

Fig. 10 Engineering stress-strain curves for heat-treated materials

Table 1 Yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) values at the end of different heat treatment procedures

Heat treatment procedure	YS (MPa)	UTS (MPa)	EL (%)
One-step austempering	895±20	1435±5	15.4±0.5
Two-step austempering	1016±10	1455±10	13.6±1
Three-step austempering	1000±15	1428±10	11.4±1

Fig. 11 Toughness variations after applying different heat treatment procedures

Fig. 12 SEM images of fracture surfaces of impact test samples subjected to a one-step, b two-step, c three-step austempering heat treatment conditions, d high-magnification image from the rectangular region in c

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