Technology Research Of Die By Thermal Diffusion RE-chromvanading Process

With rapid development of mold industry, higher comprehensive performance ofmolds is urgently demanded. Because of simple operation, low cost and goodcomprehensive performance for coating, more attentions are focused on TD (Thermaldiffusion) processing home and abroad. Problems of high temperature, long time andthinner coating layer exist in TD salt bath single vanadizing coating. Single TD salt bathchromizing coating cannot bring excellent performances into full play since its wearresistance is poor. TD salt-bath chromvanading not only overcomes drawbacks of singlelayer but also gets the best comprehensive performance of composite coating. However,some mould performance still cannot meet the demanding service condition. This paperinvestigated the thus combines excellent physical and chemical properties of rare earthwith TD chromvanading salt-bath, therefore, research of TD RE-chromvanading is ofgreat significance.In this paper, TD RE-chromvanading was carried on Cr12and Cr12MoV cold workdie steel. First, under process condition of940℃by4h, the best rare earth contentexperiment formulations was obtained by comparing and analyzing coating hardness andthickness through salt bath RE-chromvanading with five different contents of rare earth.Then influences of temperature and time on coating hardness of TD RE-chromvanadinglayer were investigated. Phase structure and morphology changes of the coating weresystematically analysed by microhardness tester, metallographic microscope, X-raydiffractometer (XRD) and scanning electron microscopy (SEM), energy spectrum analyzer(EDS) and other analytical testing methods, and further analysis of thermodynamics andkinetics for RE-chromvanading layer forming was taken along with influences of rare earthon the coating discussed. Through experimental research and comprehensive analysis,conclusions were obtained as followed:1.Formula with better comprehensive performance of salt bath for TDRE-chromvanading treatment was followed as salt(Na2B4O7), activator(NaF),6%RE, forseveral vanadium and chromium agent. On this condition, when treated at940℃by4h,surface coating thickness of Cr12and Cr12MoV cold work die steel reaches to12.7umand11.9um, and the hardness raises up to2876HV0.05and2799HV0.05respectively.2.Formed in TD RE-chromvanading, the surface layer of these two kind materialswere composed of VC, Cr23C6and Cr7C3, with great characteristics of densification, uniform, smooth, and good metallurgical bonding to the substrate.3.In TD RE-chromvanading treatment process, temperature factor showed greaterimpact than time on coating growth significantly.4.Section line scanning analysis of these two kind coating layers formed in TDRE-chromvanading, showed that rare earth element (Ce) appeared in both layers andchanged with [V] distribution in the coating. Results showed that rare earth had superiorpush effect to [V] than [Cr], since rare earth elements might combine with [V] to rare earthcompounds, or even rare earth compounds showed good intermiscibility with VC.