Effects of Mo addition on crack tip opening displacement (CTOD) in heat affected zones (HAZs) of high-strength low-alloy (HSLA) steels

Seok Gyu Lee,Sunghak Lee,Bohee Kim,Kyung-Keun Um,Woo Gyeom Kim

doi:10.1038/s41598-018-36782-6

Abstract

Effects of Mo addition on microstructures and crack tip opening displacement (CTOD) in heat affected zones (HAZs) of three high-strength low-alloy (HSLA) steels were investigated in this study, and the correlation between them was explained by fracture mechanisms related with martensite-austenite constituent (MA) characteristics. The coarse-grained HAZ (CGHAZ) consisted of acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), whereas the inter-critically heated HAZ (ICHAZ) consisted of quasi-polygonal ferrite (QPF), GB, and MA. Since Mo promoted the formation of GB, BF, and MA and prevented the formation of AF and QPF, the CTOD decreased in both HAZs with increasing Mo content. According to the interrupted three-point bending test results of the ICHAZ where many MAs were distributed in the QPF or GB matrix, many voids were observed mainly at MA/QPF interfaces, which implied that the void initiation at the interfaces was a major fracture mechanism. The atomic probe data of MAs indicated the segregation of C, Mn, Mo, and P at MA/QPF interfaces, which could result in the easy MA/matrix interfacial debonding to initiate voids. Thus, characteristics of MA/QPF interfaces might affect more importantly the CTOD than the MA volume fraction or size.

Highlights

Fracture toughness of heat affected zones (HAZs) is generally deteriorated in conventional high-strength low alloy (HSLA) steels by the formation of undesirable microstructural parameters induced from complicated heat cycles during welding processes[1,2,3,4]
Effects of Mo addition on microstructures and Crack tip opening displacement (CTOD) in the HAZs of the three high-strength low-alloy (HSLA) steels were investigated in this study, and the correlation between them was explained by fracture mechanisms related with martensite-austenite constituent (MA) characteristics
(1) The coarse-grained HAZ (CGHAZ) of the three HSLA steels were composed of acicular ferrite (AF), granular bainite (GB), and bainitic ferrite (BF), while the inter-critically heated HAZ (ICHAZ) were composed quasi-polygonal ferrite (QPF), GB, and MA

Summary

Introduction

Fracture toughness of heat affected zones (HAZs) is generally deteriorated in conventional high-strength low alloy (HSLA) steels by the formation of undesirable microstructural parameters induced from complicated heat cycles during welding processes[1,2,3,4]. The HAZs formed by single-pass heat cycles are classified into coarse-grained HAZ (CGHAZ), super-critically heated HAZ, inter-critically heated HAZ (ICHAZ), and sub-critically heated HAZ by peak temperatures of welding heat cycles. Among these HAZs, it is known that the CGHAZ and ICHAZ have the low fracture toughness because of large prior austenite grains and hard crack-susceptible martensite-austenite (MA) constituents, respectively[5,6,7]. Effects of Mo addition on microstructural evolution and CTOD of the HAZs were discussed to improve them further

Methods

Results

Discussion

Conclusion