脱碳层对38CrMoAl钢离子渗氮的影响

对表面残存不同厚度脱碳层的38CrMoAl钢塔形试样进行离子渗氮,借助显微硬度计、光学显微镜、扫描电镜等多种手段,对试样渗速、硬度及渗氮前后的微观组织进行研究。结果表明,在残余的脱碳层里存在较严重的魏氏组织;残余脱碳越严重魏氏组织越多;离子渗氮的渗速随着表面脱碳层厚度的增加先增加后降低;魏氏组织中大量的铁素体为氮原子扩散提供了快速渗入通道及形成大量氮化物的场所,在渗层表面形成大量的鱼骨状、网状、脉状氮化物,数量随着脱碳层的增加而增加,并且不断向渗层深处延伸。38CrMoAl钢表面残存脱碳层虽然有加快渗速的作用,但会增加渗层的不良组织,增加渗层剥落的风险。故38CrMoAl钢制工件做普通调质应适当加大加工余量。

Effect of decarbonized layer on plasma nitriding of 38CrMoAl steel

Tower-like specimen of 38CrMoAl steel with different thickness decarbonized layer was plasma nitrided. Infiltration rate, surface hardness and microstructure of the specimen before and after nitriding was studied by microhardness tester, OM and SEM. The results show that the widmannstatten structure is found in residual decarbonized layer and gradually increases with increase of the decarbonized layer. The speed of plasma nitriding increases first and then decreases with the increase of decarbonized layer thickness. The diffusion expressway of nitrogen-atoms and formation of nitride is provided by ferrite in widmannstatten structure. There are many fishbone, net and nervation nitrides on the surface of diffusion layer and it exhibits increase tendency with the increase of number and depth of decarbonized layer. Although the residual decarburized layer on the surface of the 38CrMoAl steel can accelerate the penetration rate, it will increase the bad structure of the carburized layer and increase the risk of exfoliation. Therefore, the machining allowance of the 38CrMoAl steel workpieces should be appropriately increased.