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One is a method that can predictthe plastic deformation of a die and the other is to calculatethe amount of die wear. These methods have been applied toevaluating the service life of a finisher die for the hot forgingprocess of an automobile part, and the possible maximumproduction quantity which describes die service life will beevaluated according to the variations of initial die temperatureand forming velocity.2. Methods for estimating die service lifeThis study developed two methods for estimating the ser-vice life of dies in hot forging process. One is a method thatcan predict the plastic deformation of the die; the other is forcalculating abrasive tool wear.2.1. Die service life based on plastic deformationDuring the hot forging process, the temperature of a dieincreases due to the contact between the dies and the hotdeforming material. The rate of temperature rise can be at-tributed to several factors, such as the initial temperature ofdies and billet, the contact time and pressure, the die mate-rial and surface treatment conditions. The thermal softeninginduced by this temperature rise gradually reduces die hard-ness, and finally leads to the plastic deformation of a die[8].The longer contact time at the elevated temperature givesrise to a decrease of the surface hardness of a die. In or-der to consider the thermal softening effect in estimatingdie service life against plastic deformation, it is requiredto introduce the tempering parameter, M, as shown in Eq. (1), which represents the effect of die hardness change onthe contact temperature and time successive forging cycles[9]:M = T × (C + log10t) × 10−3(1)where T is the tempering temperature (K), C is the materialconstant which has about 20 for carbon steel, t is the tem-pering time. Also, from starting to deform until ejecting theforged part, the temperatures of die surface change duringone forging cycle, so the introduction of equivalent temper-ature is required. The equivalent temperature, Teq, can beapproximately expressed as shown in Eq. (2):Teq = 2Tmax + Tmin3(2)where Tmax, and Tmin are the highest and lowest temperaturesduring one forging cycle, respectively.To estimate die service life for the plastic deformation ofa die induced by thermal softening, the tempering time, t,atEq. (1) is replaced with hardness holding time th, where this the time which takes until initial die hardness graduallyreduces to reach the critical hardness by thermal softening,as shown in Eq. (3):th = exp Myield × 1000Teq− C (3)whereMyield is theMvaluewhen initial die hardness is equalsto the corresponding hardness of the yield strength of thedie.When the material is a perfect plastic, the hardness (HrC)of material is about three times of the yield strength of ma-terial [10]. The main tempering curves of this hot work diematerial, H13, obtained from thermal softening experimentsis shown in Fig. 1.An actual working finishing die was quenched at 1030 ◦C,and then it had the first tempering for 3 h at 550 ◦C and thesecond tempering for 3.5 h at 600 ◦C. Die surface was treatedas ion-nitriding process for 14 h at 520 ◦C. Therefore, for hardness holding time for estimating thedie service life considers the first and second tempering time,which can be derived as follows:Myield = Teq{C + log(th + t1 + t2)}× 10−3(4.1)th = exp Myield × 1000Teq− C − t1 − t2 (4.2)where,t1 = 10 exp{T1/Teq × (C + log th) − C}t2 = 10 exp{T2/Teq × (C + log th) − C}where T1, T2 are the first and second tempering temperatures,t1, t2 are the hardness holding times at the first and the secondMyield values for Teq, respectively.In order to calculate the hardness holding time, effec-tive stresses and equivalent temperatures can be obtainedfrom rigid-plastic finite element analysis. Myield value canbe determined from the main tempering curve. t1 and t2are substituted into Eq.
(4) to obtain the hardness holdingtime.Finally, the die service life of the finishing die is calculatedby piding the hardness holding time by one forging cycletime, and the die service life is expressed as the possible maximum production quantity. The outline of a method forestimating die service life affected by plastic deformation isshown in Fig. 22.2. Die service life based on abrasive wearAbrasive wear is defined as the intentional removal ofmaterials from a surface, as in grinding and polishing of en-gineering components, and the unwanted loss ofmaterial thatoccurswhenmachine components are in relativemotion [11].In hot forming, the die steel should have a high hot hardnessand should retain this hardness over extended periods of expo-sure to elevated temperatures. The factors affecting abrasivewear during metal contacts are temperature the roughness ofcontacting surfaces, the hardness of die material, the normalpressure on die surface, the sliding distance between con-tacting metals, and lubrication conditions, etc.