Quantitative relationship between microstructure/crystallography and strength/toughness in simulated inter-critical heat affected subzone of medium Mn steel

Abstract

The objective of this study is to establish a quantitative relationship between the strengthening effects of different phases and yield strength, and also to gain a new insight into the relationship between crystallographic variants and impact toughness in the simulated inter-critical heat affected zone (ICHAZ) of 690 MPa grade thick-plate medium Mn steel. The results indicated that the heterogeneous microstructure consisted of completely tempered martensite (CTM), partly tempered martensite (PTM), fresh martensite (FM) and retained austenite (RA) was formed in ICHAZ. As the peak temperature increased from 700 to 800 °C, the relative percentage of FM significantly increased from 5.5 to 79.2 vol %, while that of RA, PTM and CTM decreased from 20.3 to 11.3 vol %, from 65.5 to 9.3 vol %, and from 8.7 to 0.2 vol %, respectively. The yield strength correspondingly increased from 800 to 1200 MPa as the peak temperature rose due to the increasing contribution of various strengthening mechanisms of distinct phases. Furthermore, impact toughness decreased paradoxically from 184.9 to 47.8 J with increasing density of high angle boundary (HAB) from 0.806 to 1.47 μm−1 and decreasing size of prior austenite grains (PAGs) from 12.5 to 4.1 μm because of the variable crack propagation path induced by different variant selection mechanisms. In concluding, this research proposed a feasible strategy to improve the welding performance of medium Mn steel.

Keywords

• Medium Mn steel
• ICHAZ
• Microstructure evolution
• Strengthening mechanism
• Variant selection
• Impact toughness