Fatigue Crack Propagation and Fracture Surface Analysis of GMAW-Welded HY-60 Steel at Room and Low Temperatures

The fatigue crack propagation behavior and fracture surface characteristics of the base metal (BM) and weld metal (WM) of HY-60 high-strength low-alloy steel welded by gas metal arc welding (GMAW) were compared at room temperature (RT) and at a low temperature (-51℃). The fatigue crack propagation resistance of the BM and WM was compared under adjusted loading conditions using da/dN?ΔK curves derived from the a?N data obtained from fatigue crack propagation tests. At RT, both BM_RT and WM_RT exhibited excellent linearity on a log-log scale, indicating stable crack propagation behavior consistent with the Paris law (da/dN = C(ΔK)^m). In the region close to final fracture, unstable fracture behavior (region III), where the data deviated above the fitting line, was observed. At -51°C, both the BM and WM exhibited near-threshold behavior: the fatigue crack growth rate decreased sharply at low ΔK values and subsequently increased before transitioning to the stable Paris region. The unstable fracture behavior near the final fracture was more pronounced in WM_-51°C. As a result of the fracture surface analysis using SEM, river marks and striations were observed in the BM, and dimples, a characteristic of ductile fracture, were confirmed at the final fracture section where the crack reached a critical size, indicating that overload fracture occurred after fatigue crack propagation. In the WM, rough fracture surfaces, stair-step fracture patterns, crack-path deflection, and secondary cracks were observed. These features may be associated with segregation zones formed during cooling and with local microstructural heterogeneity. The above results indicate that the fatigue crack propagation behavior of HY-60 steel welded by GMAW is influenced by not only the test temperature but also the microstructural differences between the BM and WM, the maximum load, and local stress concentration.