Efeito do carbono e do nitrogênio na microestrutura e dureza do aço AISI 420

Abstract

The partial replacement of carbon by nitrogen on stainless steels has been the subject of a lot of scientific works, the majority of these works shows that there is a synergistic effect on the hardness and corrosion resistance. Those effects may are dependents of the relative ratios of carbon and nitrogen content apart from the influences of other alloy elements, though. On this work the microstructure of AISI 420 steel (0,33 %C wt. and 12 %Cr wt.) has been studied after decarburizing cycles on muffle furnace, nitrogen enrichment by SHTPH treatment, and also after quenching and tempering (at 200°C) treatment. Optical microscope, Vickers microhardness test, XRD and MEV were employed to investigate the microstructural transformations. The results showed the obtaining of martensitic and martensiticferritic substrates and carbon and nitrogen rich martensitic layer. After SHTPN treatment the highest hardness were between 637 and 750HV0,025 and the subsequent quenching and tempering treatment lead to a decrease in hardness, 582 and 693HV0,025. The addition of nitrogen in solid solution at high temperatures as 1100 and 1050°C didn’t showed no influence on the complete dissolution primary carbides (formed on annealing treatment). The simultaneously enrichment of nitrogen and the austenitic recrystallization due to the allotropic transformation (α’ > ɣ) has leaded to coarse grains at nitrided layer, more than the substrate grains, that was more pronounce to lowest carbon content sample. XRD results showed the presence of retained austenite for the highest carbon content sample. Despite of the results, it’s possible to conclude that there’s not a hardness improvement after tempering at 200°C for AISI 420 with carbon levels lower than 0,33 % Cp. and nitrogen contents higher as possible by SHTPN treatment.