Effect Of Base Salt And Process On Microstructures And Properties Of Deep QPQ Layer Of Pearlite Ductile Cast Iron

Pearlite ductile cast iron is widely used in the manufacture of heavy-duty,wear-resistant parts due to its good general mechanical properties,and the harsh using environment requires high wear and corrosion resistance.The general surface strengthening treatment can only improve the wear resistance or corrosion resistance unilaterally,and the combination of wear resistance and corrosion resistance can be achieved by the QPQ（salt bath compound treatment）process,However,the general QPQ process has little improvement in the wear resistance and corrosion resistance of the workpiece,and it is difficult to meet the requirements for using under high-speed and heavy-load conditions.Therefore,this experiment used a deep QPQ RE catalyst-seeping process to achieve high wear resistance and high corrosion resistance in high-speed and heavy-load environments.In this paper,the effects of different salt formulations and nitriding parameters on the microstructure,element distribution,microhardness,wear resistance and corrosion resistance of the pearlite ductile cast iron QPQ layer were investigated,using metallographic observation,SEM scanning and energy spectrum analysis,X-ray diffraction analysis,microhardness test,friction and wear test,electrochemical corrosion test and other means.Studying the effect of different salt components on the microstructure of the QPQ layer.The result showed that when the[CNO^-]was equal to 30%and[K⁺]/[Na⁺]was equal to 1.4,the thickness of the white layer in the layer was larger,and a certain thickness of the black diffusion layer structure appeared;The lower the RE content,the greater the thickness of the white layer;with the increase of lithium salt content,the thickness and density of the white layer and the diffusion layer were gradually increased.When the salt contained 1.5%RE and 10.5%lithium salt,the white layer thickness was 18μm and the diffusion layer was 20μm.Studying the effect of different salt components on the microhardness of the QPQ layer.The result showed that when the[CNO^-]was equal to 30%and[K⁺]/[Na⁺]was equal to 1.4,the microhardness of the QPQ layer is higher;the lower the RE content,the higher the microhardness of the layer;when the salt contained 1.5%RE,The microhardness of the layer increased first and then decreased with the increase of the lithium salt content.When the 7.5%lithium salt is contained,the microhardness of the white layer reached 1100.8 HV.The QPQ test was conducted at 600-660°C for 60-150 minutes.The results showed that with the increase of nitriding temperature,the thickness and microhardness of the QPQ layer increased first and then decreased.Almost no diffusion layer structure was observed at 600 to 620°C;The mixed structure of a white layer and a diffusion layer appeared at 640°C.With the extension of the nitriding time,the thickness and microhardness of the QPQ layer gradually increased,and there was a certain degree of synergy.The proportion of white high nitrogen compounds in the diffusion layer increased slightly.After QPQ treatment,the QPQ layer consisted of an oxide layer,an extremely thin loose layer,a dense compound layer,and a diffusion layer from the outside to the inside.O was mainly concentrated in the oxide layer;N was mainly concentrated in the compound layer and the diffusion layer organization;C was mainly concentrated in the oxide layer;S was concentrated in the outer surface of the compound layer;Ce was minimal and mainly distributed in compound layer.The oxide phase was mainly consisted of Fe₃O₄ and Li₂Fe₃O₄;the nitride layer was consisted of Fe₃N and Fe₄N.The QPQ sample and substrate were selected for dry friction and wear test.The results showed that the wear resistance of pearlite ductile cast iron increased first and then decreased with the increase of nitriding temperature,and increased continuously with the extension of holding time.When the sample was nitrided at 640°C for 150min,the wear loss was the lowest,as low as 2.0 mg,and the wear loss was only 2.9%of the untreated sample.The main wear mechanisms of the sample were oxidation wear,adhesive wear and slight abrasive wear.The QPQ sample and substrate were selected for electrochemical corrosion test.The results showed that the corrosion resistance of pearlite ductile cast iron after QPQ treatment was greatly improved.The corrosion resistance was best at 640°C for 90 min in all samples.The corrosion potential of the substrate was increased by 0.2888V and the corrosion current density was increased by two orders of magnitude.In conclusion.After analysis and optimization,the deep QPQ salt formulation of pearlite ductile cast iron was:39%CO（NH₂）₂,13%Na₂CO₃,23%K₂CO₃,7%Li₂CO₃,1%Ce₂（CO₃）₂·x H₂O,1%Na₂SO₃,2%K₂SO₃,6%NaCl,10%KCl.The heat treatment process parameters with high wear resistance and corrosion resistance were nitrided at640°C for 120 min and oxidized at 360°C for 40 min+20 min.18μm white layer and20μm diffusion layer were obtained.The white layer had a microhardness of 1057.8HV.