Plasma Nitriding Behavior on Surface Hardening of Copper-Contained High Tensile Steel(Physics, Processes, Instruments & Measurements)

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Plasma nitriding effects on high tensile steel containing 0.006%C, 0.29%Si, 1.34%Mn, 1.18%Cu, 1.46%Ni, 0.62%Cr, and 0.44%Mo in mass% have been investigated at different nitriding times and temperatures from 500℃ to 640℃ and up to 4 hours. Hardness profiles on cross sections of nitrided layers were measured to determine the case hardening depth along with the surface hardness. Optical micrograph and X-ray diffraction analyses revealed two nitrided layers. The first layer is a compound layer comprised of two nitrides, γ'-Fe_4N and ε-Fe_<2-3>N and the second layer is a diffusion layer below it. The surface hardness of the nitrided layer reached 700 to 800HV due to the nitride compound layer. The hardness of the diffusion layer reached about 650HV just below the compound layer and decreased gradually to substrate hardness inwards. The hardening of the diffusion layer is believed to be due to the precipitation hardening of nitrides. The thickness of the nitrided layer, that is the hardened layer, increased with increasing temperature up to about 500μm including a thin compound layer less than 10μm. Growth kinetics was also discussed.