Microstructural Characterization and Mechanical Property Evaluation of High Nitrogen Martensitic Stainless Steel Subjected to Heat Treatment

The High Nitrogen Martensitic Stainless Steel (HNMS) was subjected to three different austenitizing cycles of 1050$^\circ$C, 1075$^\circ$C and 1100$^\circ$C followed by subzero treatment at -70$^\circ$C. The fraction of retained austenite has been reduced after sub-zero treatment as revealed by microstructural evolution. The material was subsequently tempered at different temperatures ranging from 180$^\circ$C to 650$^\circ$C and the change in micro-structure, hardness, tensile strength and toughness were investigated after each heat treatment cycle. Optical microscopy, electron microscopy with EDS and X-Ray diffraction techniques were used to characterize the material. This has showed the constituents of microstructure were lath martensite, precipitated metal carbides of type $M_{23}C_6$, $M_7C_6$ and carbo-nitrides. Hardness, tensile testing and Charpy impact testing were carried to evaluate mechanical properties after the heat treatment which has showed the better mechanical properties for the samples solutionised at 1075$^\circ$C. Secondary hardening has been observed on tempering above 450$^\circ$C which can be attributed to the precipitation of secondary phase inter-metallic compounds. Hardness attains a peak value at peculiar temperature range after which it decreases on further tempering which is most likely because of the loss of coherency of the precipitates with the metal matrix. This has been further confirmed by the XRD of the specimens before and after tempering. The study stablishes the structure-property correlation of HNMS for different heat treatment cycles. The results indicate that a good combination of hardness and strength can be achieved after solutionizing at 1075$^\circ$C followed by double tempering at 525$^\circ$C.