Effect of stresses on the structural changes in high-chromium steel upon creep

Resumo

The effect of stresses on the microstructure and dispersed particles in a heating-performance Fe‒0.12C–0.06Si–0.04Ni–0.2Mn–9.5Cr–3.2Co–0.45Mo–3.1W–0.2V–0.06Nb–0.005B–0.05N (wt %) steel has been studied under long-term strength tests at Т = 650°C under initial applied stresses ranging from 220 to 100 MPa with a step of 20 MPa. Under an applied stress of 160 MPa, which corresponds to a time to fracture of 1703 h, a transfer from short- to long-term creep takes place. It has been shown that alloying with 3% Co and an increase in W content to 3% significantly increase the short-term creep resistance and slightly increase the long-term strength upon tests by more than 10⁴ h. The transfer from short- to the long-term creep is accompanied by substantial changes in the microstructure of the steel. Under long-term creep, the solid solution became depleted of tungsten and of molybdenum down to the thermodynamically equilibrium content of these elements in the solid solution, which leads to the precipitation of a large amount of fine particles of the Laves phase at the boundaries of laths and prior austenitic grains. At a time to fracture of more than 4 × 10³ h, the coalescence of the M₂₃С₆ carbides and Laves-phase particles occurs, which causes the transformation of the structure of fine tempered martensite lath structure into a subgrained structure.