CHANGE IN MICROSTRUCTURE AND HARDNESS OF ADDITIVELY MANUFACTURED AISI H13 STEEL BY HEAT TREATMENT AND NITRIDING PROCESSES

AISI H13 steel samples were additively manufactured using a laser powder bed fusion (LPBF) system. The effect of annealing tem-perature, quenching & tempering, and nitriding were determined. The microstructure and properties of the samples were investigated using optical microscopy, scanning electron microscopy, electron backscattered diffraction, electron probe micro-analysis, X-ray diffraction, roughness measurement, and a hardness tester. The results show that the as-built AISI H13 steel sample had a roughness on the surface and pores inside. The microstructure consisted of martensite and retained austenite. The average hardness was 460 HV, and the porosity was 0.086 %. The annealing process helped homogenize the microstructure, increase the density, and reduce the porosity and hardness of the LPBF-manufactured sample. The quenching process helped increase the hardness of the steel to the maximum of 787 HV, then the tempering process reduced the hardness to 572 HV. Heat treatment and nitriding processes tended to increase the martensite block size, reduce the retained austenitic content, and precipitate the V-Mo-rich carbide in the LPBF-manufactured AISI H13 steel. After nitriding was conducted, the nitriding case depth was 87 um, and the surface hardness increased up to higher than 1020 HV due to the formation of CrN and Fe3-4N.