Experimental Study On Material Mixture Ratio Of Thermal Resistant Layer On Die Impression Of Hot Forging Die

Life expectancy of hot forging die has been under much concern in the industry. The key solution to increase the life is releasing thermal stress amplitude. In this study, the method to mitigate hot forging die surface thermal stress is discussed, in terms of material testing and optimization of hot forging die surface. Principal preparation method includes plasma spraying, plasma remelting and plasma welding.In the first part, failure modes of hot forging die, correlation between life and thermal stress, and parameters that affect the useful life of hot forging die have been brought out. Cutting edge developments of plasma spraying, plasma remelting and plasma welding are also presented.In the second part, testing material and method are described. High Speed Steel W6Mo5Cr4V2 is adopted as substrate, bonding metal and bonding base material is Ni60 powder. Ceramic powder is SiC, Cr3C2 and WC. Ni60 and the above mentioned three type of ceramic powder are mixed by the composition of 70%/30%,80%/20%,90%/10%, thus nine composition of powder mixtures are used in the tests.Design of plasma spraying-remelting and plasma welding experiment is described thereafter. Firstly, spray Ni60-SiC, Ni60-Cr3C2, Ni60-WC on substrate; Secondly, remelting the plasma spraying layer on plasma arc. At the same time, use plasma overlay welding to obtain plasma spray welding layer. finally, comparing the plasma spraying layer, plasma re-melting layer(called two step method) and plasma spray welding layer (called one step method). The test methods include microscopy, micro-hardness and determine thermal constant which including coefficient of thermal expansion, thermo-conductivity and specific conductance.The third part of this article is comparison among the testing results. Each of the three types of Ceramic is mixed by three ratios, and the results of plasma spraying layer, plasma re-melting layer and plasma spray welding layer in each ceramic powder are compared.Firstiy, analyzing on Ni60-SiC sample. At metallomicroscopy, the plasma spraying layer is tytical layer structure; in plasma re-melting layer, and overlying strata bond well, we can see overlying strata is dilutioned by; plasma spray welding layer has homogeneous and dense microstructure. At microhardness, comparing three kind of preparation method, plasma spray welding layer has the highest hardness, plasma spraying layer in the middle, plasma re-melting layer has the lowest hardness. Microhardness of layer increases with SiC content increasing. Plasma spray welding sample has the best thermal constant. Coefficient of thermal expansion of plasma spray welding layer is the closest to, and coefficient of thermal expansion of 30%SiC is the lowest. In conclusion, plasma spray welding layer of 30%SiC is the best one to release heat stress amplitude.Then, analyzing on Ni60-Cr3C2 sample. It's the same as Ni60-SiC sample at metallomicroscopy. Plasma spray welding layer is the best one because it has homogeneous and dense microstructure. At microhardness, plasma spray welding layer has the highest hardness, plasma spraying layer is in the middle, plasma re-melting layer has the lowest hardness. Microhardness of layer decreases with Cr3C2 content increasing. Plasma spray welding sample has the best thermal constant. In conclusion, plasma spray welding layer of 10%Cr3C2 is the best one to release heat stress amplitude.In the end, analyzing on Ni60-WC sample. At metallomicroscopy, plasma spray welding layer is the best one. At microhardness, plasma spray welding layer has the highest hardness, plasma spraying layer is in the middle, plasma re-melting layer has the lowest hardness. Microhardness of layer increases with WC content increasing. Plasma spray welding sample has the best thermal constant. In conclusion,plasma spray welding layer of 30%WC is the best one to release heat stress amplitude.The fourth part of this article compared the sample between 30%SiC,10% Cr3C2 and 30%WC, at the base of part three.At macroscopic feature, plasma spraying layer has roughening surface and crack easily, and thickness of spray has been restricted by internal stress; plasma re-melting layer has a ablation dent, caused by overheat of spray gun; plasma spray welding layer is smooth and dense, the thickness can achieve 2mm, and overlying strata bond well. At microcosmic feature, plasma spraying layer has layer structure; plasma re-melting layer has homogeneous microstructure, overlying strata is diluted by, lacking hard phase in layer; plasma spray welding layer and overlying strata bond well with plasma spray welding layer, has carbon-enriched layer or plane zone in transition region which can enhance bonding strength.At micro-hardness, plasma spraying layer is the same as, but in transition region has a suddenly decent point; plasma re-melting layer is lower than, far away from the target; Plasma spray welding layer of 30%WC is the same as, don't enhance much hardness. Plasma spray welding layers of 30%SiC and 10%Cr3C2 are higher, achieving the effects to improve hardness.In thermal constant analyse, coefficient of thermal expansion is sensitive to comprehensive stress. Plasma spray welding layer of 30%SiC can satisfied heat resistance well.According to best proportion of powder in theory,10%Cr3C2 and 30%WC can't meet the demand, only 30%SiC can match it.In conclusion, one step method is better than two step method; plasma spray welding layer of 30%SiC are more reliable, showing longer life expectancy under operational condition.