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

www.nature.com/scientificreports OPEN Received: 7 August 2018 Accepted: 26 November 2018 Published: xx xx xxxx Effects of Mo addition on crack tip opening displacement (CTOD) in heat affected zones (HAZs) of highstrength low-alloy (HSLA) steels Seok Gyu Lee1, Bohee Kim1, Woo Gyeom Kim2, Kyung-Keun Um2 & Sunghak Lee1 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. 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 processes1–4. 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, respectively5–7. The fracture toughness of the HAZs is affected by alloying elements8–11. Mo is often added for enhancing strengths in the HSLA steels made by thermo-mechanically-controlled process (TMCP), whose microstructures are various low-temperature-transformed bainitic ones, e.g., bainitic ferrite (BF), granular bainite (GB), acicular ferrite (AF), and quasi-polygonal ferrite (QPF). The HAZ microstructures formed after the welding become more complicated, and their volume fractions significantly affect the fracture toughness. In Mo-containing steels, Mo generally promotes activate the formation of MAs10, and thus its optimization is needed by understanding detailed fracture mechanisms. Particularly in the ICHAZ mixed with QPF and GB microstructures, hard MAs deteriorate the fracture toughness, and their crack susceptibility is varied with their hardness and MA/matrix interfacial features. However, fracture mechanisms related with cracking of MAs or void initiation at MA/matrix interfaces remain to be clarified by correlating the fracture toughness and fracture mechanisms. Crack tip opening displacement (CTOD) used for the fracture toughness evaluation of offshore-application HSLA steel HAZs was measured in this study, and then were correlated with volume fractions of microstructures of BF, GB, AF, QPF, and MA in thermally-simulated HAZ specimens of three HSLA steels (S450~S500 grade, yield strength; 450~500 MPa) whose Mo content was varied. The steels containing 0.002%, 0.194%, and 0.350% Mo are referred to as ‘LM’, ‘MM’, and ‘HM’, respectively, for convenience. Optical and scanning electron microscope 1 Center for Advanced Aerospace Materials Pohang University of Science and Technology, Pohang, 790–784, Republic of Korea. 2Steel Products Research Group 1 Technical Research Laboratories, POSCO, Pohang, 790–785, Republic of Korea. Correspondence and requests for materials should be addressed to S.L. (email: ) Scientific Reports | (2019) 9:229 | DOI:10.1038/s41598-018-36782-6 1 www.nature.com/scientificreports/ Figure 1. Optical micrographs of the simulated CGHAZ of the (a) LM, (b) MM, and (c) HM steels. The CGHAZs basically consist of AF, BF, and GB, and contain coarse packets and prior austenite grain boundaries whose sizes are larger than 40 μm. AF is a needle-shaped fine ferrite, and is grouped by packets having high-density interior dislocations. GB consists of relatively coarse packets, inside which island-shaped MA constituents are present. BF contains coarse packets consisted of parallel low-angle boundary substructures with fine secondary phases at the boundaries. (SEM) observations along with electron back-scatter diffraction (EBSD) analyses were used for identifying and quantizing the complicatedly-mixed microstructures in the CGHAZ and ICHAZ. Interrupted three-point bending tests, nano-indentation, and 3-dim. atom probe (3-d AP) analyses were also conducted for examining characteristics of crack-susceptible MAs and their effects on crack initiation and propagation. Then, effects of Mo addition on microstructural evolution and CTOD of the HAZs were discussed to improve them further. Results Microstructures of coarse-grained HAZ (CGHAZ). Optical micrographs of the CGHAZ of the LM, MM, and HM steels are shown in Fig. 1(a–c). They show coarse packets and prior austenite grain boundaries whose sizes are larger than 40 μm, and basically consist of AF, GB, and BF whose microstructures can be classified by their morphologies and features8,12–23, as marked in Fig. 1(a–c). AF is a needle-shaped fine ferrite, and is grouped by packets having high-density interior dislocations and high-angle boundaries which are effectively resistant to the cleavage crack propagation on {100} planes, thereby showing a good strength-toughness combination26–28. GB consists of relatively coarse packets, inside which island-shaped MA constituents are present to work as crack initiation sites and to deteriorate fracture toughness8,22–25. BF contains coarse packets consisted of parallel low-angle boundary substructures with fine secondary phases at the boundaries8,12,22–25. An EBSD inverse pole figure (IPF) map and misorientation profiles of AF, GB, and BF areas (black arrow marks in Fig. 2(a)) for the CGHAZ of the LM steel are shown in Fig. 2(a,b). Most of boundaries of AF are high-angled ones (misorientations; 50~60 deg), and their spacings are small. Packets of GB are coarse (size; about 25 μm), and packet boundaries are irregular, inside which substructures are well developed. Coarse packets of BF are defined by boundaries of 15-deg misorientations (Fig. 2(b)). SEM micrographs of AF, GB, and BF areas of Fig. 2(a) (black-dashed-circle ma (...truncated)