Effect of Constraint on Creep Crack Propagation of Mod. 9Cr-1Mo Steel Weld Joint

In order to clarify the effect of constraint on creep crack propagation of modified 9Cr-1Mo steel, 1) 25.4mm thickness C(T) specimen with side grooves (thick specimen) and 2) 10mm thickness C(T) specimen without side grooves (thin specimen) were machined from base metal (BM) and weld joint (WJ), and subjected to creep crack propagation experiments under 650 °C. The crack in WJ propagated along the fine grain heat affected zone (HAZ), where the multi-axial constraint was high due to the difference in deformation properties among HAZ, WJ, and BM. While the thick specimens hardly allow deforming to the thickness direction and showed less crack tunneling, the thin BM specimen showed remarkable reduction of thickness and crack tunneling. The thin WJ specimen showed some thickness reduction and crack tunneling, but also showed larger crack extension than that of BM. In terms of the relationship between the crack propagation rate (da/dt) and the creep J integral (C* parameter), the data shows a wide scatter band. The specimens with higher geometrical constraint (thick specimen, WJ) locate above the lower constraint (thin specimen, BM) ones. All the specimen showed the reduction in thickness and suggests the change in constraint condition during the experiments. By picking up the data without significant thickness reduction, the C*- da/dt relationship has been gathered in a narrow and straight band regardless of the specimen geometry or difference in material. The picked up data locates at the upper bound of the current and literature data.