Visualization of microstructural factors resisting the crack propagation in mesosegregated high-strength low-alloy steel

doi:10.1016/j.jmst.2019.05.075

Abstract

Abstract:

While relationship between fracture mechanism and homogeneous microstructures has been fully understood, relationship between fracture mechanism and inhomogeneous microstructures such as the mesosegregation receives less attention as it deserves. Fracture mechanism of the high-strength low-alloy (HSLA) steel considering the mesosegregation was investigated and its corresponding microstructure was characterized in this paper. Mesosegregation refers to the inhomogeneous distribution of alloy elements during casting solidification, and leads to the formation of positive segregation zones (PSZ) and negative segregation zones (NSZ) in ingots. The fracture surface of impact sample exhibits the quasi-cleavage fracture at -21 °C, and is divided into ductile and brittle fracture zone. Meanwhile, the PSZ and NSZ spread across ductile and brittle fracture zone randomly. In ductile fracture zone, micro-voids fracture mechanism covers the PSZ and NSZ, and higher deformation degree is shown in the PSZ. In brittle fracture zone, secondary cleavage cracks are observed in both PSZ and NSZ, but present bigger size and higher quantity in the NSZ. However, some regions of the PSZ still present micro-voids fracture mechanism in brittle fracture zone. It reveals that the microstructures in the PSZ exhibit a higher resistance ability to crack propagation than that in the NSZ. All observations above provide a better visualization of the microstructural factors that resist the crack propagation. It is important to map all information regarding the fracture mechanism and mesosegregation to allow for further acceptance and industrial use.

Key words: High-strength low-alloy steel, Heavy forgings, Mesosegregation, Inhomogeneous microstructures, Fracture mechanism

Shuxia Wang, Chuanwei Li, Lizhan Han, Haozhang Zhong, Jianfeng Gu. Visualization of microstructural factors resisting the crack propagation in mesosegregated high-strength low-alloy steel[J]. J. Mater. Sci. Technol., 2020, 42: 75-84.

Figures/Tables 9

Table 1 Chemical composition (wt%) of the as-received HSLA steel.

C	Mn	Mo	Ni	Cr	Si	Al	Cu	P	Fe
0.20	1.42	0.49	0.86	0.18	0.18	0.014	0.03	0.003	Bal.

Fig. 1. Three-dimensional characterization of mesosegregation in the ring shape heavy forging: (a) image of mesosegregation distribution in the view of three-dimensional; (b) sampling schematic diagram of CVN sample corresponding to (a); (c-e) mesosegregation distributions in the plane of XOZ, XOY and YOZ.

Fig. 2. Microstructural characterization: (a) OM image of mesosegregation and corresponding EPMA surface scanning analysis results of elements; (b, c) OM images in the PSZ and NSZ; (d, e) SEM images in the PSZ and NSZ; (f, g) EBSD analyzed inverse pole figures (IPF) maps in the PSZ and NSZ.

Fig. 3. SEM images showing the fracture surface morphology of CVN impact sample: (a) entire fracture surface morphology in low-magnification; (b) dimples in fibrous region shown in (a); (c) microstructures in radial region shown in (a); (d) river patterns in cleavage facets of crack radial region shown in (c); (e) dimples in rough stripes of crack radial region shown in (c).

Fig. 4. SEM (a, b) and OM (c-f) images showing the fracture surface morphology of sample: (a) schematic showing the method used to cut the CVN impact sample; (b) determination of demarcation line between ductile and brittle fracture zone in fracture surface; (c) distribution characteristic of mesosegregation among ductile and brittle fracture zone in cross-section surface; (d) deformation characteristic of PSZ and NSZ in ductile fracture zone shown in (c); (e) deformation characteristic of PSZ and NSZ in brittle fracture zone shown in (c); (f) second crack in the NSZ distributed in brittle fracture zone shown in (c).

Fig. 5. Deformation analysis results of ductile fracture zone in cross-section surface: (a) OM microstructural image of ductile fracture zone; (b) EBSD analyzed strain contouring map corresponding to (a); (c, d) SEM images of deformed microstructures in the PSZ and NSZ selected in (a); (e, f) EBSD analyzed local misorientation (LM) maps of deformed microstructures in the PSZ and NSZ selected in (b); (g) statistical chart of data extracted from the LM maps in (e) and (f).

Fig. 6. Analysis results of the adiabatic shear bands (ASBs) adjacent to the ductile fracture cracks in cross-section surface: (a) OM image of the ASBs; (b) EBSD analyzed strain contouring map corresponding to (a); (c, d) dimples corresponding to the PSZ and NSZ in fracture surface perpendicular to the cross-section surface shown in (a); (e, f) SEM images of the deformed microstructures in the PSZ and NSZ at cross-section surface near to the edge of fracture surface.

Fig. 7. Deformation analysis results of brittle fracture zone in cross-section surface: (a) OM microstructural image of brittle fracture zone; (b) EBSD analyzed strain contouring map corresponding to (a); (c, f) SEM microstructural images of the deformed PSZ, according to regions (Ⅰ) and (Ⅱ) shown in (a), respectively; (d, g) EBSD analyzed LM maps of the deformed PSZ, according to regions (Ⅰ) and (Ⅱ) shown in (a), respectively; (e, h) EBSD analyzed IPF maps of the deformed PSZ, according to regions (Ⅰ) and (Ⅱ) shown in (a), respectively.

Fig. 8. Secondary cleavage cracks’s analyses in the NSZ of brittle fracture zone: (a) SEM image of secondary cleavage cracks; (b) EBSD analyzed strain contouring map corresponding to (a); (c, f) SEM image of secondary cleavage cracks according to regions (Ⅰ) and (Ⅱ) shown in (a), respectively; (d, g) LM maps of secondary cleavage cracks according to regions (Ⅰ) and (Ⅱ) shown in (a), respectively; (e, h) IPF maps of secondary cleavage cracks, according to regions (Ⅰ) and (Ⅱ) shown in (a), respectively.

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