Application Of Surface Thermochemical Treatments To Large Centrifugal Compressor

The fast and deep case plasma nitriding technique is applied to gear of centrifugal compressor.Boronizing,as a thermochemical technology,has been applied to Inconel 718.In order to obtain the optimal combination of strength and ductility,the borided Inconel 718 is subjected to standard heat treatment.This consists of solution treatment at 1050℃for 1h with air cooling to room temperature then a two-step aging treatment consisting of 720℃for 8h with furnace cooling at 50℃/h to 620℃,holding at 620℃for 8h followed by air cooling to room temperature.The nitriding layer and the borided layer are investigated by optical microscopy,X-ray diffraction,electron probe micro analyzer,knoop micro-hardness tester, micro-tribometer and scanning electron microscopy.The thicknesses of the nitriding layers of samples plasma nitrided at 510℃for 10h,15h and 20h are 275.34μm,341.25μm and 502.33μm,respectively.The nitriding layer is composed of the compound layer and the diffusion layer,and the compound layer transforms fromε-Fe_2-3N+γ'-Fe₄N duplex-phase layer toγ'-Fe₄N single-phase layer with the nitriding time increases.Because of the strengthening effect of nitrides in the nitriding layer,the plasma nitrided sample exhibits a higher surface hardness,a lower coefficient of friction and a significant reduction in worn volume.The major wear mechanism of the nitriding layer is adhesive wear.The fast and deep case plasma nitriding technique evidently increases the surface hardness of gear while the interior remains ductile,and enhances its wear resistance too.The borided layer is composed of the compound layer and the boron diffusion zone. Because of the superior hardness of borides(Ni₂B,Ni₄B₃),the borided Inconel 718 exhibits a significant reduction in its wear rate and relatively low coefficient of friction,compared with the unborided Inconel 718.The standard heat treatment efficiently promotes the diffusion of boron into the interior of the material and the generation of new borides(Fe₂B,CrB).The borided layer with standard heat treatment shows much better wear resistance due to the thicker borided layer(0.44mm).