High-temperature creep resistance and effects on the austenite reversion and precipitation of 18 Ni (300) maraging steel

In this paper, the high-temperature creep resistance and effects on the austenite reversion and the dynamic evolution of precipitates of maraging 300 steel were investigated. The main strengthening mechanism in a solution treated and aged material is the fine needle shaped Ni3(Ti,Mo) precipitates densely dispersed in a single martensitic phase. The specimens were submitted to creep tests at temperatures of 550, 600 and 650 °C and stress conditions of 200, 300 and 500 MPa. Stress exponent (n) varied from 6.0 to 7.2 and activation energy for creep (Qc) from 364 to 448 kJ/mol, associated to the tangled and cells arrangements of the dislocations, show that the dominant creep mechanism is controlled by dislocations climb and slip. The experimentally determined threshold stresses are about 25 MPa at 550 °C and close to 4 MPa at 600 and 650 °C. Due to high-temperature creep exposure, part of martensite was reverted to austenite in a range of 17.2% to 48.5%, depending upon the time, temperature and applied stress. At the same time, the Ni3(Ti,Mo) precipitates were coarsened and Fe2Mo precipitated, leading to undesirable alloy's strength reduction. Volume fraction of reverted austenite showed strong negative correlation with hardness. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45° shear lip.