Effects of Sulfur Content on the Plain Strain Fracture Toughness of Inertia Welds in 4340 Steel

摘要

The plain strain fracture toughness of post-weld, reheat treated inertia welds in two heats of AISI 4340 steel of equivalent tensile properties, but different sulfur concentrations, was determined. The adverse reorientation of elongated sulfide inclusions in both heats, resultant from the forging stage of the welding cycle, caused reductions in ductility and toughness that were not remedied by reheat treatments. The percent elongation of inertia welded joints was found to be no greater than 50% that of the parent metal even at the lowest sulfur concentrations of 0.004%. KIC data was less significantly effected by sulfur concentration and fiber morphology. In addition, the fracture toughness of these welds in 0.014% S material, as determined from sharp notch fatigue cracked specimens, was actually found to be greater than that of welds in 0.004% S material. This was due to the more tortuous fracture path and the resultant greater fracture surface formed during crack propagation. L-C rather than L-R crack plane orientations for KIC specimens produced more reliable mechanical property data due to the more uniform microstructure ahead of the crack front. The magnitude of base metal elongation, especially in the short transverse orientation, is proposed as an index of inertia/friction weldability.