Study On Microstructure And Mechanical Properties Of Surface Layer Formed During Low Temperature Plasma Nitriding Of Steel2Cr13

In order to obtain grain refinement of the supersaturated nitrogen solid solutionα′Nphase in the nitrided layer and to improve the surface mechanical properties,2Cr13martensitic stainless steel was nitrided based on theoretical calculations. Thestructural stability and mechanical properties of nitrogen solid solution wereinvestigated by first principles calculations, and the formation condition of α′Nwasalso studied by thermal calculation.Then, a verification test of the pulse plasmanitriding was carried out at low temperatures and low N-H ratio for2Cr13steel. TheXRD was used to study the phase structure of the layer, and the morphology wascharacterized by a series of observation and analysis of OM and TEM.The results of first principles calculations show that the α′NFe₁₄Cr₂N_x（x=0,2,4,6） structures transformed from bcc to bct with great lattice expansion due tosupersoluted N atoms. The binding energies of Fe₁₄Cr₂N_xwere negative. With theincreasing of N contents, the structual stability decreased while the hardness wasimproved.The results of thermodynamic calculations show that at low nitridingtemperatures （400C⁴60C）, the free energy of α′Ndecreased with the increasingof both elements （N, Cr） content and nitriding temperature. The solution of Cratoms can stabilized α′Nwith low nitrogen content.The results of verification tests show that the optimal flux of N-H ratio was0.05:0.4L min-1at low tempretures （400C⁴60C） for2Cr13steel, the grain of α′Nphase was ultrafine in the surface layer. The thickness of nitrided layer increasedwith the temperature increased. With the increasing of nitriding time at460C, thevolume fraction of′-Fe₄N and ε-Fe_2³N in the surface layer increased and theorientation of′-Fe₄N grains changed from random orientation to preferredorientation （200）. While CrN precipitated with small amount at560C and590C.The wear rate of nitriding layer with ultrafine α′Ngrain was lower than theuntreated substrate and the friction coefficient decreased from0.5to0.45. The wearrate of nitrided at400C was1.75×10^-5(mm³·（N·m）^-1), and the wear scar widthbecame narrow. After nitriding at400C, the nano hardness and elastic modulus ofsurface layer can reach11.078GPa and279.736GPa respectively, which had a goodagreement with the theoretical calculations.