Cold Cracks in Fillet Weldments of 600 MPa Tensile Strength Low Carbon Steel and Microstructural Effects on Hydrogen Embrittlement Sensitivity and Hydrogen Diffusion

Hydrogen flowing into a weld causes hydrogen embrittlement (HE), so to prevent cold cracks in the weld, HE must be suppressed. In this study, single-pass fillet weldments were fabricated using two different welding fillers with the same strength level as the American Welding Society standard. The specimen F-HNi, which had a high, reversible hydrogen content, exhibited cold cracking in the fillet weld. The HE sensitivity index (HE index) was calculated using the in-situ slow strain rate test (SSRT). The reversible hydrogen trap concentration (Crev) and hydrogen diffusion coefficient (Deff) were calculated using the permeation test. The formation of low-temperature transformation phases such as bainite and martensite increased the Crev and decreased the Deff, thereby increasing the HE index. In addition, it was determined that reversible hydrogen was most effectively trapped in the low-temperature transformation phase, as confirmed by silver decoration. We concluded that the cold cracks in the F-HNi specimen were associated with the large Crev, small Deff, and large HE index. HE sensitivity can be controlled by optimizing the microstructure, even when welding fillers with the same level of strength are used. To reduce HE sensitivity, it is important to reduce the number of reversible hydrogen trap sites, by reducing the fraction of the low-temperature transformation phase where the reversible hydrogen trapping most occurs.