Long-term Creep Property and Structural Stability of Ni-Based Alloy 740H

Abstract

Long-term creep rupture behavior and structural stability of a nickel-based Alloy 740H were investigated. Creep tests were conducted at 650°C, 700°C, 750°C and 800°C using smooth bar specimens, and the creep deformation and fracture were discussed in relation to the changing microstructure and hardness. It was found that primary and secondary creep stages were considerably shorter than the tertiary stage irrespective of applied stress and temperature. Creep exponent in Norton law significantly decreased with decreasing the stress from 11 at high stress region to about 1 at low stress region. Times to creep rupture were expressed fairly well by using Larson-Miller parameter with the quadratic equation of stress. The size of γ' precipitates continuously increased as aging time increased, and shape of γ' remained spherical at 650°C and 700°C but tended to change to cubic above 750°C. A higher growth rate of γ' precipitates was observed during creep, and it seems to connect in a specific direction in grain. In addition, neither η phase in grain nor G phase at grain boundaries were found. Room temperature hardness on the head portion of specimen increased initially with the aging time but decreased inversely after reaching the maximum value. The hardness on the gage portion of specimen continued to be larger than that on the head portion due to strain hardening during creep.