Effect of high temperature ageing on microstructure and mechanical properties of a nickel-free high nitrogen austenitic stainless steel

The effect of ageing and oxidation at 850°C for different durations viz., 1, 10, 100 and 200h on microstructure and mechanical properties of a nickel free high nitrogen steel (Fe-Cr-Mn-N) was investigated. The microstructure of the alloy in as-received condition essentially consists of fine equi-axed austenite grains with annealing twins. However, ageing at 850°C has resulted in the formation of very fine, globular and discrete Cr2N precipitates initially at grain boundaries for an ageing time of 1h and subsequently within the grains with increasing ageing time beyond 1h. The size of these precipitates has progressively increased with further ageing. In addition, coarse intermetallic precipitates of (Fe,Mn)Cr type with body centred tetragonal structure were observed in the samples aged for ≥100h at 850°C. The effect of these precipitates on mechanical properties of the alloy was studied. It was noticed that the mechanical properties deteriorated on ageing, which has been attributed to the depletion of solid solution strengthening elements like nitrogen, chromium and manganese from the matrix to form nitrides and intermetallic precipitates. Moreover the alloy aged in air (i.e., oxidised) exhibits marginally inferior tensile properties as compared to unoxidised alloy.