Synopsis Recently, a warm forging process is remarkable from the points of view in near net shaping and abolishment of additional heat treatment by micro-structural controlling. In generally, a die life in forging decreases accompany with the increase of forging temperature. The die life in hot forging is only about 1/10 1/100 compared with that in cold forging, because of the serious heat damage. The die life in warm forging is normally about same level of that of hot forging in spite that it

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... s estimated to be about 3 10 times longer than that in hot forging. There are many important factors such as cooling, lubrication, tool material with high strength and heat resistance, surface treatment, so it is difficult to find out the optimal combination. To solve the above mention problems, we have developed the new Punch Damage Test using parts former for warm forging by use of FE analysis. In this study, we investigated the effects of surface treatments and the kinds of tool steels on die damage in warm forging by the developed Punch Damage Test. And we tried to clarify the mechanism of the die damage by use of FE analysis. The obtained results are following. 1. The wear depends on the depth of plastic flow in hot forging. In order to decrease the wear, it is important to increase of the yield strength of tool steel at elevated temperature, and to apply the surface treatment with high heat resistance. 2. The length and the number of heat cracks increase by applying hard coating, on the other hand hard coating can decrease the length of crack because of defensible boundary between hard coating and base steel. 3. The developed Punch Damage Test is very effective for the evaluation of hard coatings and tool steels in warm forging.