Effect of heat treatment on the structure and creep resistance of austenitic Fe–Ni alloy

Purpose: The paper addresses the problem of determining the dependence between the initial heat treatment of an austenitic Fe–Ni alloy and its structure, and its creep resistance. Specimens of Fe–Ni alloy were subjected to tests after two variants of heat treatment, i.e. solution heat treatment followed by typical single-stage ageing, and solution heat treatment followed by novel two-stage ageing. Design/methodology/approach: For the investigated Fe–Ni alloy after solution heat treatment in the conditions: 980°C/2h/water, two variants of specimen ageing were applied for a comparison: single-stage ageing (715°C/16h/air) and two-stage ageing (720°C/8h + cooling in the furnace up to the temperature of 650°C + 650°C/8h/air). The thermally treated specimens were then subjected to a static tensile test at room and elevated temperatures, and to a creep test in a temperature range of 650-750°C, at stresses from 70 to 340 MPa. Findings: It was found that both, at the room and elevated temperatures, the specimens of Fe–Ni alloy after 2-stage ageing were distinguished by higher strength properties (Y.S, T.S) with a little lower plastic properties (EL., R.A). As regards extrapolated results of creep tests, it was found that at a longer exposure time of ca. 10.000 h, specimens after single-stage ageing were characterized with higher creep resistance. Lower creep resistance of the Fe–Ni alloy after two-stage ageing can be explained by increased brittleness of the material in boundary areas. Practical implications: The obtained test results may be used to optimise heat treatment and forecast the operation conditions of products made out of Fe–Ni alloy at an elevated temperature. Originality/value: The study shows a significant effect of the applied ageing variants on mechanical properties and creep resistance of the tested austenitic Fe–Ni alloy.