Effect of Carbon Content on the Creep Rupture Properties and Microstructure of 316H Weld Metals

Two types of 316 butt welds with carbon contents of 0.016% and 0.062% have been produced using the gas tungsten arc welding process. The δ-ferrite content decreased from 7.2 to 2.8% in volume as the carbon content increased. The creep-rupture strength and creep ductility of the two types of weld metals have been measured at 550 ℃ over the stress range of 290–316 MPa and at 600 ℃ over 230–265 MPa. The microstructure change and precipitation behavior of the weld metals were observed and related to the creep rupture properties. The creep rupture strength of the C2 (0.062% C) weld metal was higher than that of the C1 (0.016% C) weld metal at both 550 ℃ and 600 ℃. At 550 ℃, as the decrease in the applied stress, the difference of the creep-rupture life between the two weld metals diminished due to the higher depletion rate of carbon by precipitation of M23C6 in the C2 weld metal, while at 600 ℃, the difference enlarged due to the massive precipitation of σ phase and extensive crack formation and propagation along σ/austenite boundaries in the C1 weld metal. For both the C1 and C2 weld metal, the decrease in ductility was adverse with the transformation percentage and related to products of the δ-ferrite transformation.