Research On The Gas Nitriding Characteristic Of20CrMnTi Steel Under Cold Deformation And Pressure Conditions

As a typical representative of low carbon alloy steel20CrMnTi steel is commonlyused for carburizing treatment to improve surface hardness, wear resistance, fatiguestrength and so on. However, as the carburizing temperature is too high (＞900℃), it`seasy to be deformation. While nitriding temperature is much lower(500~580℃), as aresult, the workpiece deformation is small, the hardness and fatigue strength ofnitriding layer is better than carburized layer. However traditional nitriding technoloyexist problems such as time-consuming, the nitride layer is too thin. As a result,nitriding can`t completely replace carburizing.In order to solve all those defects, this dissertation would combine deformationstrengthening and solid solution strengthening which focuse on researching theinfluence of room temperature deformation with pressurized gas nitriding treatment of20CrMnTi steel nitriding layer microstructure, thickness and corrosion resistance. Themain conclusions are as follow:Obtained the nitriding layer thickness of cold-rolled20CrMnTi steel changingwith cold rolling deformation (10~60%) after common gas nitriding treatment for5hin510~570℃. Confirmed the relationship between the thickest nitriding layer withdeformation (the best deformation) under different nitriding temperature. Thethickness of nitriding layer increased first and then dropped with the increasing ofdeformation under different nitriding temperature and there were big differencebetween the best deformation. At510℃,540℃and570℃, the best predeformationwas30~40%,30%and20%. And compared with the nitrided samples not deformation,its thickness increases by about70%,55%and12%respectively.Cold-rolled20CrMnTi steel was nitriding under the condition of510~570℃and0~0.4MPa, studied the microstructure and hardness of nitrided layer determined bythe composite process which combined cold deformation pretreatment and pressurizedgas nitriding. At510℃,540℃and570℃, the surface of cold-rolled20CrMnTi steelwas mainly composed of Fe3N and Fe4N after pressurized gas nitriding, the surface hardness increased with Fe3N phase content increasing, and the hardness gradientdropped slowly with the rise of temperature and pressure.The hardness distribution of nitriding samples from surface to core was obtainedunder the condition of510~570℃and0~0.4MPa, and so were the thickness ofnitriding layer. It could significantly accelerate the nitriding dynamics process whenmedium pressure improved, and compared with traditional nitriding process with colddeformation pretreatment, the composite process could further improve the efficiencyof nitriding. At510℃,540℃and570℃,the nitriding layer of cold-rolled20CrMnTisteel increased with the nitriding pressure improving, the nitriding layer thickness was310μm,345μm and370μm respectively under0.4MPa for5h gas nitriding.The electrochemical corrosion behavior of samples whicn has the colddeformation pretreatment of30%and gas nitriding under the condition of510℃and0~0.4MPa for5h was discussed, mdel of static corrosion weightlessness wasestablished in10%HCl acid environment at room temperature. The corrosionpotential of20CrMnTi steel after composite process increased with the improving ofmedium pressure, while the corrosion current density dropped. Static corrosionweightlessness decreased with the media pressure increasing.