Mechanical properties of modified 9Cr–1Mo steel welds with notches

Notched specimens of a modified 9Cr–1Mo steel and its laser welds were extensively evaluated by various methods. Impact, notched tensile and fatigue crack growth tests were employed on specimens tempered at different temperatures. Notched tensile specimens were also tested in gaseous hydrogen, in addition to laboratory air. The results indicated that the deterioration in impact toughness of the weld metal was pronounced for tempering below 680 °C, in contrast to a narrow tempering range around 540 °C for the base metal. The coarse-grained weld metal was more susceptible to hydrogen embrittlement (HE) than the fine-grained base metal for specimens tempered at 250 and 540 °C as verified by the loss of notched tensile strength (NTS) in hydrogen. For specimens with low impact toughness and high NTS losses, the unstable crack growth which was characterized by quasi-cleavage on fracture surfaces and abrupt change in FCGRs was observed for ΔK beyond certain values. In general, the Paris gradient of the weld metal was steeper than that of the base metal with the same tempering treatment, but the discrepancy became insignificant for specimens tempered at 750 °C. For specimens tempered at 750 °C, not only the impact energy but also the HE resistance could be significantly increased. It was suggested that modified 9Cr–1Mo welds should be tempered in the neighborhood of 750 °C for improved mechanical properties.