Effect of Martensite–Austenite Constituent on Low-Temperature Toughness in YS 500 MPa Grade Steel Welds

In Kim,Daegeun Nam,Hyunbin Nam,Myungjin Lee,Yeongdo Park,Namhyun Kang

doi:10.3390/met8080638

Abstract

The effect of martensite–austenite (M–A) constituents and simulated microstructure on low-temperature toughness was investigated in YS 500 MPa grade structural steel welds. The specimens were fabricated using a direct quenching and tempering process. After simulated weld thermal cycles, the coarse-grained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (IRCGHAZ) were produced using a Gleeble tester and real welded joint to support the simulation results. The largest low-temperature toughness was observed in the fine-grained heat-affected zone (FGHAZ) owing to the fine-ferrite microstructure. However, the toughness decreased in the IRCGHAZ because of the slender morphology of the M–A constituents that formed primarily along the prior austenite grain boundaries in the IRCGHAZ.

Highlights

More offshore plants and ships have been operated in extreme environments such as those commonly found in the polar regions [1]
The deterioration of the fracture toughness of thermomechanically controlled processes (TMCPs) steels after welding is attributed to the formation of local brittle zones in the heat affected zones (HAZs) [5]
base metal (BM) used used in in this this study study was was S500G2

Summary

Introduction

More offshore plants and ships have been operated in extreme environments such as those commonly found in the polar regions [1]. The deterioration of the fracture toughness of TMCP steels after welding is attributed to the formation of local brittle zones in the heat affected zones (HAZs) [5]. For multi-pass welding, the most degraded part in the HAZ is known as the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) [7,8] and is caused by the formation of coarse M–A constituents, which are composed of high carbon martensite and retained austenite [9]. It is generally recognized that cleavage fracturing in welded joints is mainly controlled by the microstructure, such as the M–A constituents. The micro-mechanism of cleavage cracks in simulated HAZs is controlled by the size and volume fraction of the martensite–austenite (M–A) constituent [10,11]. It should be noted that the morphologies of M–A constituents are sensitive to cleavage cracking [12,13]

Objectives

Discussion

Conclusion