Analysis Of Hardness Change Tendencyon Crankshaft Hot Forging Die’s Cavity Surface Layer

According to statistics, wearing leads to70%hot forging die’s failure. Study onhot forging die’s wearing has been a research focus in recent years. More and moreattention has been paid to hardness, which is animportant factor influencing die’swearing. Along with the forging number increases, the die’s surface layer surfing hightemperature and thermal softening occurs. Thus, the surface layer’s hardness decreases.Influencing factors such as forging temperature, material composition, technology forheating processing, forging technology lead to surface layer’s hardness change verycomplicated.The first part of this paper introduces the development status of die overseas anddomestic, pointing out that: stock utilization, energy consumption, die life are lowerthan abroad. Then, the research progress in hot forging die’s high temperature wearingand hardness are analyzed. At last, the research content and purpose are introduced.The second part of this paper introduces the hot forging die’s temperature field,pointing out that the cavity surface and near-surface layer surfer thermal load, whichroots from heat conduction and friction. What’s more, it leads to thermal fatigue, phasetransformation, tempering and plastic deformation. Then surface layer’s thermal load,mechanical load, die material’s high temperature properties (thermal stability, hightemperature hardness and friction, equivalent tempering behavior)are introduced. At last,rigid plastic theory, elastic-plastic theory and numerical simulation software ofDEFORM-3D are introduced in detail.The third part of this paper builds the finite element model according to the actualwork condition of hot forging die, especially temperature field, stress field duringcontinuous forging process and draw the conclusions as following:(1) the highesttemperature on the surface of hot forging dieappears in the plate and front end, while thelowest temperature appears at the bottom of the cavity. The temperature fluctuation’srange is7-10mm straight down from the plate.(2) The surface layer’s temperaturevariation similar to sine wave and the die come into a stable working stage after severalforging time. The effective tempering temperature of the plate and front end’s surfaceare453℃,490℃.(3) Also the metal flow and force condition are analyzed: theeffectivestress on the surface are300MPa,410MPa..The forth part of this paper introduces the die surface layer’s welding material in detail. The research show that: it has good mechanical properties at high temperature.According to the actual working condition, the relative tempering experiment isaccomplished in500℃with different tempering time using the electric pit-type heatingfurnace. Then, the tempering sample’shardness and microstructure are analyzed in detail.At last, the variation of failure die plate’s hardness, tensile property, surface morphologyand element contents are analyzed.The fifth part of this paper builds the hardness variation formula of the plate’ssurface. Then,comparative analysis is used in different hot forging die to find surfacelayer’s hardness variation law. It shows that: the hardness decreases dramaticallyinthesurface layer within2mm. But the surface layer’s hardness variation tendency is hardto predict for it is influenced by many factors.The methods of numerical simulation and experiment have been used to analysisthe temperature field and hardness variation law of crankshaft final forging die’s cavityduring forging process. It provides theoretical foundation and production guidance forhot forging die’s wearing study and life elongation.