Study On Microstructure And Properties Of “Skin Layer” And Application Of Large “Fist-Like” Die

To solve the problems such as severe deformation/abrasion of die surface layer and extremely short life time when applying traditional method to manufacture large forging die on the 800 MN hydraulic press for hard-to-deform materials(ultra-high strength steel,titanium alloy etc),an improved method,“fist-like” structure of forging die manufacturing,based on bimetal-gradient-layer surfacing is put forward.Through surfacing a relatively soft material(as the “skin”)on the available forging die(as the“bone”)made by bimetal-gradient-layer surfacing,the “fist-like” die is obtained.In this paper,the performance index of “skin” layer is put forward through analyzing the temperature field and stress field of the die of titanium alloy connection frame during the process of die forming.And the structure of "fist-like" die is determined: a cobalt-based superalloy welding material,JLCo32 and a nickel-based superalloy welding material,CHN327 were both designed as the skin layers.The skin layer was welded on the cast steel matrix through two layers of ferrous alloys(RMD142 and RMD248),which is the bone layer of "fist-like" die.The thickness of skin layer was determined by forming simulation preliminarily.According to the design method of the die mentioned above,two surfacing samples were maded using gas metal arc welding(GMAW),whose skin layers are cobalt-based superalloy and nickel-based superalloy respectively.The microstructure and mechanical properties of two kinds of surfacing materials before and after heat treatments were studied The results indicated that,after ten cycles of heating and water quenching,the major precipitates of JLCo32 were carbides and there were more ?''phases in CHN327.The mechanical properties of the surfacing layer were investigated by microhardness tests and shear tests.High-temperature tensile tests at different temperatures were performed to examine the high-temperature behaviour of these two kinds of superalloy.Experimental results showed that,compared with CHN327,JLCo32 exhibits higher microhardness and more reliable bonding with ferrous alloy matrix.Furthermore,in comparison to CHN327,JLCo32 own superior yield strength and ultimate tensile strength but inferior ductility.The finite element analysis software MSC.Marc was used to carry out the numerical simulation analysis of the welding process of skin layers.The influence of surfacing thickness on the temperature field and residual stress field of the model after welding was studied.The rules are as follows: with the increase of surfacing thickness,the peak value of transverse residual stress gradually rises from 520 MPa at 16 mm to626MPa at 24 mm,with a total increase of 20%.The size of longitudinal residual tensile stress area is directly proportional to the peak value of stress and the height of surfacing.All of these shows that it is not suitable to improve the surfacing thickness of “skin”layer on the basis of ensuring the "skin layer" performance of "fist-like" forging die.In addition,the effects of two surfacing paths,“layer-by-layer surfacing” and “side-by-side surfacing” on the residual stress field of the welding were also studied.The results show that,regardless of the distribution of the residual stress field or the magnitude of the peak stress,“layer-by-layer surfacing” is better than “side-by-side surfacing”.