Stress–relief cracking of 9 wt-%Cr steels containing 0–2 wt-%Mo

The stress-relief cracking susceptibility of simulated heat affected zone (HAZ) microstructures in 9 wt-%Cr steel alloys has been assessed using a four-point bend test procedure. Five alloys were examined, in which the levels of molybdenum, phosphorus, and sulphur were deliberately varied. Stress–relief cracking was apparent only in HAZ microstructures heated to ≥1320°C during simulation. In this condition, alloys containing no molybdenum were most susceptible, exhibiting a low-ductility intergranular mode of fracture during stress-relief heating between 550 and 650–C, and intergranular microvoid coalescence at higher temperatures up to the hold level of 735°C. Phosphorus was found to exacerbate cracking by the low-ductility mechanism, and was observed to segregate to grain boundaries together with a local enrichment of sulphur at crack tips. The presence of molybdenum in 9Cr–1Mo and 9Cr–2Mo alloys greatly reduced susceptibility to stress-relief cracking. The low-ductility fracture mode was completely eliminated in these alloys, despite high levels of phosphorus and sulphur, qnd the tendency to form intergranular cavities was reduced. These effects were associated with the formation of δ-ferrite at grain boundaries during HAZ simulation, promoted by molybdenum additions. A range of low–alloy steels has previously been assessed using the same experimental procedure, and in comparison, the 9Cr–1 Mo and 9Cr–2Mo alloys were highly resistant to stress–relief cracking.

MST/317