Formation Behavior of δ Ferrite Within LB and GTA Weld Zones of Mod. 9Cr-1Mo Steel in Terms of Thermal History

The toughness and creep strength of Mod.9Cr-1Mo steel, a ferritic heat resistant steel, are closely related to the amount of delta ferrite remaining in the weld.This study compared the amount of retained δ ferrite in the laser beam (LB) weld with low heat input and the gas tungsten arc (GTA) weld with high heat input.A greater amount of retained δ ferrite in both the LB and GTA welds was found in the fusion zone (FZ) interface than in the FZ center and coarse-grain heat affected zone (CGHAZ).LB welds had more retained δ ferrite than GTA welds.Thermal history of the welding process and location in the welds were calculated by finite element analysis.By simulating the thermal history, calculated at the FZ interface for each welding process, it was confirmed that the LB welds had more retained δ ferrite than the GTA welds in the specimens.The amount of retained δ ferrite increased as the time spent in the δ ferrite phase region increased and as the cooling rate increased.The effect of the cooling rate on the amount of retained δ ferrite was greater than the time spent in the δ ferrite phase region upon heating.