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基于Deform模具寿命的预测摘要本文介绍的模具使用寿命基于磨损和在热锻造过程中金属模具的塑性变形的估计方法。模具使用寿命缩短大多由于模具表面层变形材料的热软化，高的热负荷和较长的接触时间引起的。另外，模具使用寿命取决于磨损和金属模具的塑性变形在很大程度上可以通过有限元（FE）分析测定，磨损和热软化试验。这些是一些主要影响模具的精度及模具使用寿命的限制因素，另外形成速度和初始模具温度极大的影响磨损和热锻塑性变形模具。在这项研究中，这两种方法都提出用于估计热锻使用寿命模具通过塑性变形和磨损，而这些应用，根据一个主轴组件两个主要过程变量以预测的产品数量，成型速度和初始模具温度对模具寿命的影响。通过应用中的建议方法，金属模具使用寿命的降低由于局部温度上升导致的热锻模具材料的磨料磨损。 2004爱思唯尔B.V.保留所有权利。关键词：热锻;模具使用寿命;磨损;塑性变形;热软化; 回火参数1.前言热锻是最传统的金属成形过程中所使用的关键部件生产中各行业 1 。实际上，它被广泛应用于汽车，工业机械部件的制造。特别是，该方法可以有效地用于形成材料具有高的流变应力。模具使用寿命大大影响生产成本，生产效率和产品质量。在热锻造的过程中，模具的使用寿命大大缩短了热循环，过度金属流动和减少模具硬度 2 。如今，制造成本取决于模具寿命，如何可以延长产品寿命减少在热锻过程中的任何种类的内部和外部缺陷。分包商和供应商越来越大的压力下，对于降低成本，成为了重要组成部分。这些要求在汽车行业更为关键。因此，提高技术在材料科学和冶金领域，以及在模具设计中是重要的。计算机辅助设计（CAD）和数字模拟的知识也变得非常有帮助的。在锻造业，模具成本可达到约50在组成部分中。因此，很明显，成本的降低，需要的工具的优化，特别是，在性能和使用寿命的提高3。在热锻造过程中，锻造工具不仅受到机械应力，也能引起热循环和连续锻造机械应力。适当地选择模具制造技术和模具材料和确定，在相当大的程度，影响金属模具的使用寿命。模具可能需要被替换为一些原因，如由于磨损的尺寸变化或塑性变形，表面光洁度变差，润滑的击穿，开裂或破损4。在金属成形工艺中许多研究人员已经研究了对模具使用寿命的工艺条件的影响5-7。由于热锻模具的热软化的使模具表面硬度下降。此热软化效应加速模具故障8。模具使用寿命的限制因素，可以在热锻过程中同时或单独出现。由于工艺或产品的不同特性，模具使用寿命降低可以通过磨损，或通过塑性变形9。本研究开发了两种方法来估算模具使用寿命。其一是，可以预测一个管芯的塑性变形的方法，另一种是计算模具磨损量。这些方法已被应用于评估热锻工序对于汽车部件的模具使用寿命，最大的生产量，它描述模具使用寿命将根据初始模具温度和成形速度的变化进行评估。2.估计模具的使用寿命的方法本研究开发了两种方法用于估计模具的使用寿命在热锻过程。其一是，可以预测在模具的塑性变形的方法;另一种是用于计算研磨工具磨损1.1. 模具使用寿命的基于塑性变形 在热锻造过程中，由于金属模具和热变形材料之间的接触使模具的温度升高。温度上升速率可以归因于几个因素，如金属模具和钢坯的初始温度，接触时间和压力，模具材料和表面处理的条件。诱导这种温度上升的热软化逐渐减小模具的硬度，并最终导致模具的塑性变形。8在高温下较长时间的接触引起模具表面硬度减少。为了考虑在估计模具使用寿命抵抗塑性变形的热软化作用，需要引入回火参数，M如式（1），其表示模具硬度变化对接触温度和时间连续锻造循环的效果9：其中，T是回火温度（K），C是具有约为20碳钢，t是回火时间。另外，从开始变形，直到完成锻造部件，在一个周期锻模表面的变化，所以引进等效温度是必需的。等效的温度，可以近似地表示为如图方程 （ 2 ） ：这里，是在一个锻造周期的最高温度和最低温度，分别为。估计模具使用寿命为热软化，回火时间t，塑性变形公式（1）将被替换的硬度保持时间th，其中，th为初始模具硬度由热软化逐渐减少了达到临界硬度的时间，如图式中的时间（ 3 ）：其中，模具硬度的M值等于模具的屈服强度时的硬度。当材料是一个完美的塑料，材料的硬度（HRC）为约3倍的材料的屈服强度10。热加工模具材料，从热软化实验得到的主要回火曲线示H13于图.1。实际工作完成模具淬火在1030，然后它有第一回火为在5503小时，并在600的第二回火3.5小时。52时模具表面的混合物作离子氮化处理14小时。图. 1.主要回火曲线H13的因此，估计模具使用寿命认为硬度保温时间，第一次和第二次淬炼的时间，可以得出如下公式：其中T1，T2是第一和第二回火温度，t1，t2是硬度保持时间，Teq分别是第一和第二Myield值。为了计算硬度保持时间，有效应力和等效的温度可以从刚塑性有限元分析而获得。 Myield值可以从主回火曲线来确定。 t1和t2代入方程（4），得到硬度保持时间。最后，精加工模具的使用寿命除以硬度保持时间由一个锻造周期时间进行计算，因而其模具使用寿命被表示为可能的最大的生产量。用于估计模具使用寿命受塑性变形的方法的概要示于图2。图.2 流程图塑性变形分析。图.3 流程图的磨损分析1.2. 模具使用寿命的基于模具磨损磨损被定义为从一个表面上的故意去除的材料，如工程部件的研磨和抛光，并且当机器组件处于相对运动发生的材料磨损11。在热成型中，模具钢应具有高的热硬性，并应长时间保留这硬度在高温下。在金属接触时，影响模具磨损的因素包括接触中温度和表面的粗糙度，模具材料的硬度，模具表面正常压力，接触金属之间的滑动距离，及润滑的条件下等，模具磨损影响尺寸精度和产品的过程中热锻工艺的表面光洁度12,13。图.4 形状和产物和精加工模的尺寸图.5 主轴产品工艺设计在这项研究中，以预测在金属成形过程中金属模的磨耗轮廓，Archard磨损模型被应用于如图方程（5）14：其中V是磨损深度，k是磨损系数，P是在模具表面上的正常压力，l是滑动距离，h是模具的表面硬度。估计其模具使用寿命基于模具磨损时，硬度变化在模具的高温和接触时间增加时表面层的磨损量相对增加。磨损的公式如（6），通过考虑管芯深度方向上的磨损硬度变化开发的。常压（N），滑动速度（Vs），并在管芯表面上的温度分布是从刚塑性有限元分析来计算，并且从磨损试验得到的磨粒磨损的允许量和表面硬度的临界值和热软化实验。这是由磨损量与临界值进行比较。如果磨损的量超过了允许值的情况，并且其硬度在模具磨损表面仍大于临界值，则磨粒磨损分析将重复，直到模具磨损的累计量达到允许值。最后，磨损量其表达模具使用寿命磨损分析的总数进行测定吗，用于估计模具使用寿命基于磨粒磨损的方法的流程图表示在图3 。3.分析及结果图4显示在热锻基础上分析塑性变形和磨损，一个汽车零部件，主轴的一部分，是生产的三个阶段组成的破坏和两个向前/向后热锻行动，图5显示了热成形主轴工艺设计的一部分结果的。图.6 最终产品的损伤因子图.7 初始模温的温度分布此产品高度320毫米，131毫米最大直径挤压部分。这种离散的部分需要高精度的机加工。不幸的是，磨料磨损或管芯的塑性变形发生在所述台阶式拐角如图所示如在点1，2点图.4，这部分的模具使用寿命取决于这些台阶角热锻造过程中的初始形状和尺寸的的变化。估算模具使用寿命的变化成形的解析条件和过程变量的列于表1和表2中。损坏值从有限元分析获得最终阶段的分布示于图.6，这些值多出现两个台阶式拐角。损伤因子可以用于预测裂缝中形成15,16。图.8 初始模温的节点力和速度分布因此，这些角部的损伤程度可以直接影响模具使用寿命。当初始模头温度低时，它可能会影响产品的质量。当初始模头温度高时，模具的硬度降低。当形成速度变快时，热变形材料和模具之间的接触时间变短，等效温度变低。初始模温控制和变形速度的选择对模具寿命非常重要。图.9 初始模温度对有效压力和磨损深度影响3.1 最初的模具温度的影响在金属成形过程中，塑性变形和摩擦有助于热量产生。在这个过程中可以影响润滑条件，并影响模具寿命，最终产品的性能，生产速率4。首先，当初始模头温度高时，钢坯的内外之间的的温度差变小，并且这个温差小有助于金属流动。另一方面，高的表面温度可能降低模具使用寿命。但模具表面的低温会干扰金属流动，并导致表面缺陷。如可以看到的图.7，在两个上模具阶梯角表面（点1，2）的温度增加是不同的，由于初始模具温度的影响，对于相同的锻造工艺。点1处初始模具温度400，模具温度最初升高，但最高温度低于200或300下。此外，这些结果清楚地表明点2处温度梯度为初始模具温度400有非常大的节点力和速度的分布。图.8可以看出，初始模具温度的升高作用在模具表面节点力减小，而模具材料表面随着初始模头温度升高工件附近的速度增加。这样做的原因是，在金属的流动速度随着温度的升高而提高。磨粒磨损和装订模的应力分析的结果示于图.9，当初始模具温度为400，磨损深度（）2点是近似1.898毫米约在200时的4倍。这并不奇怪，因为模具和工件之间的相对速度点2处为初始模具温度400比为200或300更高。此外，作为初始模具温度升高时，钢的模具的表面附近的硬度降低。图.10 温度分布的形成速度模具使用寿命估算根据初始模具温度，塑性变形和磨损结果总结在表3和表4中。初始模具温度升高时，生产量下降。影响最大的生产量的可能是受磨损比塑性变形更高。通常，在较高的温度钢的屈服强度降低，屈服强度也依赖于现有的热处理。高初始模温会导致模具热软化硬度减少。最初的硬度越高，在各种温度下的屈服强度更大。从结果来看，模具寿命从塑性变形引起的模具磨损比从初始模具温度方面磨损更为重要。图.11 节点力和速度分布形成速度3.2. 成形速度的影响当变形速度变快，成形周期时间大大缩短，而模具和工件之间的变形载荷增加。可以看出图.10，在两个上模具的温度阶梯角表面（点1，2）增加不同，由于形成速度的影响，对于相同的锻造工艺。成形速度250毫米/秒，模具温度逐渐增加，但最高温度低于300毫米/秒以上。此外，温度梯度用于成形速度为300mm / s是点2处。图.12 有效的压力和磨损深度形成速度节点力和速度的分布图.11.可以看出，作用在模具表面的节点力在模具材料表面随着初始模具温度升高附近的成形速度会随着增加。这样做的原因是，金属的流动增加成型速度。磨粒磨损和应力分析的结果如图.12，当形成速度为300mm/s时，磨损深度（）点2处大概是1.261毫米，约为在200毫米/秒时的3倍。当形成速度增加时，模具使用寿命由塑性变形变得更长。但它的寿命相对磨料磨损是短的。根据成型速度模具使用寿命推定结果示于表5和6中。当成形速度为200毫米/秒，由于接触时间长引起的局部高温，模具的塑性变形发生在台阶角（点1，2）。当成形速度增大，模具使用寿命基于台阶式拐角塑性变形的提高是通过局部低温和短接触时间。当形成速度增加，模具使用寿命基于磨损减少。从结果来看，模具寿命在磨损造成的比从塑性变形更重要，在形成速度方面上。4.结论在这项研究中，有两种用于估计热锻模具使用寿命方法，通过塑性变形和磨损，且这些适用到预测的产品质量，根据两个主要的过程变量，成型速度和初始模具温度。通过对建议方法的应用，得出以下结论获得。1）模具使用寿命的降低由于热锻造局部温度上升导致, 通过模具的热软化和塑性变形影响磨料磨损。当形成速度增加，磨料磨损减少造成其模具使用寿命增加。2）当初始模具温度升高，塑性变形和磨损降低模具使用寿命增加，尤其是塑性变形似乎是对模具使用寿命起主要限制因素。3）当形成速度增加，模具使用寿命引起塑性变形，提高在台阶拐角的接触时间，另一方面，它的寿命造成磨料磨损减短。致谢这项工作是通过釜山国立大学的ERC/ NSDM和韩国科学技术部支持的国家研究实验室（NRL）进行的。参考1 K. Venkatesan, C. Subramanian, E. Summerville, Three-body abrasion of surface engineered die steel at elevated temperatures, Wear 203204 (1997) 129138.2 K. Lange, Modern metal forming technology for industrial production, J. Mater. Process. Technol. 71 (1997) 213.3 O. Brucelle, G. Bernhart, Methodology for service life increase of hot forging tools, J. Mater. Process. Technol. 87 (1999) 237246.4 T. Altan, S.I. Oh, H.L. Gegel, Metal forming: Fundamentals and Applications, Am. Soc. Metals (1983).5 H. Saiki, Tribology in warm and hot forming, JSTP Int. Sem. Precis. Forg. (1997).6 H. Saiki, Y. Marumo, A. Minami, T. Sonoi, Effect of the surface structure on the resistance to plastic deformation of a hot forging tool, J. Mater. Process. Technol. 113 (2001) 2227.7 T.A. Dean, T.M. Silva, Die temperatures during production drop forging, J. Eng. Ind. 101 (1979) 385390.8 D.J. Jeong, D.J. Kim, J.H. Kim, B.M. Kim, T.A. Dean, Effects of surface treatments and lubricants for warm forging die life, J. Mater. Process. Technol. 113 (2001) 544550.9 K. Yuasa, J. Okamoto, Effects of press slide motion on life of warmforging dies, J. JSTP 22 (241) (1981) 133138.10 K. Serope, Manufacturing Processes for Engineering Materials, third ed., Addison Wesley, 1997.11 D.A. Rigney, Fundamentals of friction and wear of materials, Am. Soc. Metals (1981).12 T.H. Kim, B.M. Kim, J.C. Choi, Prediction of die abrasive wear in the wire drawing process, J. Mater. Process. Technol. 65 (1997) 1117.13 P.H. Hansen, N. Bay, A flexible computer based system for prediction of abrasive wear distribution in forming tools, Adv. Technol. Plastic. 1 (1990) 1926.14 J.F. Archard, Contact and rubbing of flat surfaces, J. Appl. Phys. (1953) 981988.15 S.I. Oh, C.C. Chen, S. Kobayashi, Ductile fracture in axisymmetric extrusion and wire drawing: part 2- workability in extrusion and drawing, ASME, J. Eng. Ind. 101 (1) (1997) 2325.16 D.C. Ko, D.H. Kim, B.M. Kim, Application of artificial neural network and Taguchi method to perform design in metal forming considering workability, Int. J. Machine Tools Manufact. 39 (1999) 771785.Prediction of service life of mold based onDeformAbstractThis paper describes the estimation method of die service life based on wear and the plastic deformation of dies in hot forging processes. Die service life is considerably shortened due to the thermal softening of surface layer, caused by the high thermal load and long contact time between the dies and the deforming material. Also, the die service life depended on wear and the plastic deformation of dies can be to a large extent determined by finite element (FE) analysis, wear and thermal softening tests. These are some of the major limiting factors affects die accuracy and die service life, and forming velocity and initial die temperatures influence greatly wear and the plastic deformation of hot forging dies. In this study, two methods are suggested for estimating the service life of hot forging dies by plastic deformation and abrasive wear, and these applied to predict the product quantity according to two main process variables, forming velocity and initial die temperature for a spindle component. Through the applications of the suggested methods, the thermal softening of dies due to the local temperature rise led to the reduction of the service life of hot forging dies by plastic deformation more than by abrasive wear. 2004 Elsevier B.V. All rights reserved. Keywords: Hot forging; Die service life; Wear; Plastic deformation; Thermal softening; Tempering parameter1. IntroductionHot forging is one of the most conventional metal-forming processes used in the production of critical parts in various industries 1. Actually, it is widely used in the manufacturing of automobiles and industrial machine components. In particular, this process can be effectively used to form materials with the high flow stress. Die service life greatly influences manufacturing costs, productivity and product quality. During hot forging process, die service life is dramatically shortened by thermal cycle, excessive metal flow and a decrease in die hardness 2. Nowadays, manufacturing costs depend on how die service life can be extended for sound products without any kinds of internal and external defects during hot forging process. Subcontractors and suppliers are increasingly under pressure with regard to cost reduction and responsibility for the development of new components. These requirements are more critical in the automotive industry. Therefore, it is important to improve the technical skills in the areas of material science and metallurgy as well as in the area of tool design.The knowledge of computer aided design (CAD) and numerical simulation also becomes very helpful. In the forging industry, tooling costs can reach up to about 50% of a component cost. Therefore, it is obvious that the reduction of component costs requires an optimization of tools, in particular, an improvement in performance and service life 3. During hot forging process, forging tools are not only subjected to mechanical stresses, but also to thermo mechanical stresses induced by the thermal cycling and successive forging operations.Proper selection of the die material and of the die manufacturing technique determines, to a large extent, the useful life of forming dies. Dies may have to be replaced for a number of reasons, such as changes in dimensions due to wear or plastic deformation, deterioration of the surface finish, breakdown of lubrication, and cracking or breakage 4. Many researchers have been investigated the influences of process conditions on die service life during metal forming process 57. The surface hardness of a die decreases owing to the thermal softening of hot forging dies. This thermal softening effect accelerates tool failures 8. The limiting factors of die service life can occur simultaneously or separately during hot forging process. Due to the different characteristics of processes or products, die service life can be decreased by wear or by the plastic deformation 9.This study developed two methods to estimate die service life in hot forging processes. One is a method that can predict the plastic deformation of a die and the other is to calculate the amount of die wear. These methods have been applied to evaluating the service life of a finisher die for the hot forging process of an automobile part, and the possible maximum production quantity which describes die service life will be evaluated according to the variations of initial die temperature and forming velocity.2. Methods for estimating die service lifeThis study developed two methods for estimating the service life of dies in hot forging process. One is a method that can predict the plastic deformation of the die; the other is for calculating abrasive tool wear.2.1. Die service life based on plastic deformation During the hot forging process, the temperature of a die increases due to the contact between the dies and the hot deforming material. The rate of temperature rise can be attributed to several factors, such as the initial temperature of dies and billet, the contact time and pressure, the die material and surface treatment conditions. The thermal softening induced by this temperature rise gradually reduces die hardness, and finally leads to the plastic deformation of a die 8.The longer contact time at the elevated temperature gives rise to a decrease of the surface hardness of a die. In order to consider the thermal softening effect in estimating die service life against plastic deformation, it is required to introduce the tempering parameter, M, as shown in Eq. (1), which represents the effect of die hardness change on the contact temperature and time successive forging cycles 9:where T is the tempering temperature (K), C is the material constant which has about 20 for carbon steel, t is the tempering time. Also, from starting to deform until ejecting the forged part, the temperatures of die surface change during one forging cycle, so the introduction of equivalent temperature is required. The equivalent temperature, , can be approximately expressed as shown in Eq. (2):Where , and are the highest and lowest temperatures during one forging cycle, respectively. To estimate die service life for the plastic deformation of a die induced by thermal softening, the tempering time, t, at Eq. (1) is replaced with hardness holding time th, where th is the time which takes until initial die hardness gradually reduces to reach the critical hardness by thermal softening, as shown in Eq. (3):where is the M value when initial die hardness is equals to the corresponding hardness of the yield strength of the die. When the material is a perfect plastic, the hardness (HrC) of material is about three times of the yield strength of material 10. The main tempering curves of this hot work die material, H13, obtained from thermal softening experiments is 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 the second tempering for 3.5 h at 600 C. Die surface was treated as ion-nitriding process for 14 h at 520 C.Fig. 1. Main tempering curves of H13.Therefore, for hardness holding time for estimating the die service life considers the first and second tempering time, which can be derived as follows:Where,where T1, T2 are the first and second tempering temperatures, t1, t2 are the hardness holding times at the first and the second Myield values for Teq, respectively.In order to calculate the hardness holding time, effective stresses and equivalent temperatures can be obtained from rigid-plastic finite element analysis. Myield value can be determined from the main tempering curve. t1 and t2 are substituted into Eq. (4) to obtain the hardness holding time.Finally, the die service life of the finishing die is calculated by dividing the hardness holding time by one forging cycle time, and the die service life is expressed as the possible maximum production quantity. The outline of a method for estimating die service life affected by plastic deformation is shown in Fig. 2Fig. 2. Flow chart for plastic deformation analysis.Fig. 3. Flow chart for abrasive wear analysis.2.2. Die service life based on abrasive wearAbrasive wear is defined as the intentional removal of materials from a surface, as in grinding and polishing of engineering components, and the unwanted loss of material that occurs when machine components are in relative motion 11. In hot forming, the die steel should have a high hot hardness and should retain this hardness over extended periods of exposure to elevated temperatures. The factors affecting abrasive wear during metal contacts are temperature the roughness of contacting surfaces, the hardness of die material, the normal pressure on die surface, the sliding distance between contacting metals, and lubrication conditions, etc. The abrasive wear of dies influences dimensional accuracy and the surface finish of products during hot forging processes 12,13.Fig. 4. Shape and dimensions of a product and finishing die.Fig. 5. Process design of a spindle product.In this study, in order to predict the wear profile of a die in metal forming processes, Archard wear model is applied as shown in Eq. (5) 14:where V is the wear depth, k is the wear coefficient, P is the normal pressure on die surface, l is the sliding distance and h is the surface hardness of the die.To estimate the die service life based on abrasive wear, it is needed to consider the hardness change at high temperature of a die and the wear amount increase of surface layer with regard to the contact time and temperature. A numerical model of abrasive wear as shown in Eq. (6), is developed by considering the hardness change of a die toward the direction of wear depth.The normal pressure (n), the sliding velocity (vs), and the temperature distributions on die surface are calculated from the rigid-plastic FE analysis, and the permitted amount of abrasive wear and the critical value of surface hardness were obtained from wear test and thermal softening experiments.The amount of abrasive wear at each point on the die surface for one forging cycle was calculated through the wear analysis of Eq. (6), and then compared with the permitted value. Also, the hardness at the worn surface that resulted from this amount of abrasive wear was compared with the critical value. If the amount of abrasive wear is smaller than the permitted value, and the hardness at worn die surface is still greater than the critical value, then abrasive wear analysis will repeat until the integrated amount of abrasive wear reaches the permitted value. Finally, the production quantity which expresses die service life was determined from the total number of wear analysis. The flowchart of a method for estimating the die service life based on abrasive wear is shown in Fig. 3.3. Analyses and result Fig. 4 shows a hot forging product to be analyzed based on plastic deformation and abrasive wear. One of automobile components, spindle part, is manufactured in three stages composed of upsetting and two forward/backward hot-forging operations. Fig. 5 shows the process design result for the hot forming of spindle part.Fig. 6. Damage factor of a final product.Fig. 7. Temperature distributions for the initial die temperature.This product has the height of 320 mm, maximum diameter of 131mm and a long extruded part. This discrete part requires a minimum machining and high dimensional accuracy. Unfortunately, abrasive wear or plastic deformation of the die occurred at the stepped corners as shown as point 1, 2 in Fig. 4, the die service life of this part depends on the change of the initial shape and dimension of these stepped corners during hot forging. The forming analysis conditions and the variations of process variables for estimating die service life are listed in Tables 1 and 2, respectively. The distributions of damage value at final stage obtained from the FE analysis is shown in Fig. 6, these values appeared highly at two stepped corners. The damage factor can be used to predict fracture in forming operations 15,16.Fig. 8. Nodal force and velocity distributions for the initial die temperature.Therefore, the damage degree of these corners may directly relate to die service life. When the initial die temperature is low, it may influence product quality. When the initial die temperature is high, die hardness decreases. When the forming velocity becomes faster, the contact time between the hot deforming material and the dies is shortened and the equivalent temperatures become low. The initial die temperature control and selection of deformation velocity are very important to the die life.Fig. 9. Effective stress and wear depth for initial die temperature.3.1. Influence of the initial die temperatureIn metal forming process, both plastic deformation and friction contribute to the heat generation. The temperatures developed in the process influence lubrication conditions, tool life, the properties of the final product, and the rate of production 4. Above all, when the initial die temperature is high, the temperature difference between inside and outside of a billet becomes small, and this small temperature difference assists the sound metal flow. On the other hand, a high surface temperature may reduce die service life. But the low temperature of die surface can disturb metal flow and cause the surface defects.As can be seen in Fig. 7, the temperature on die surface at two stepped corners (point 1, 2) increase differently, due to initial die temperature effect, for the same forging process. For the initial die temperature 400 C at point 1, the die temperature is initially higher, but the maximum temperature is lower than for either 200 or 300 C. Also, these results clearly indicate that the temperature gradient for the initial die temperature 400 C is very large at point 2. The distributions of nodal force and velocity are shown in Fig. 8. It can be seen that nodal force acting on die surface decreases as the initial die temperature increases, whereas velocity of the workpiece at the vicinity of the die/material interface increases as the initial die temperature increases. The reason for this is that the metal flow increase with increasing temperature. The results of abrasive wear and stress analysis of finisher die are shown in Fig. 9, when initial die temperature is 400 C, the wear depth () at point 2 is approximate 1.898 mm, and is about four times of that at 200 C. This is not surprising because the relative velocity between die and workpiece at point 2 for initial die temperature 400 C is higher than for either 200 or 300 C. Moreover, as initial die temperature increases, the hardness of the steel near the surface of the die decreases.Fig. 10. Temperature distributions for forming velocity.The results of the die service life estimation according to initial die temperatures for plastic deformation and abrasive wear are summarized in Tables 3 and 4, respectively. As the initial die temperature increases, the production quantity decreases. The possible maximum production quantity affected by abrasive wear is higher than that by the plastic deformation of a die. Generally, the yield strength of steels decrease at higher temperatures and yield strength is also dependent on prior heat treatment. The high initial die temperature causes the reduction of die hardness by thermal softening. The higher the initial hardness, the greater the yield strengths at various temperatures. From the results, die life resulting from plastic deformation of die is more important than from abrasive wear in terms of initial die temperature.Fig. 11. Nodal force and velocity distributions for forming velocity.3.2. Influence of the forming velocityWhen the deformation velocity becomes fast, forming cycle time is shortened, whereas the deformation load between the dies and the workpiece increases. As can be seen in Fig. 10 , the temperature on die surface at two stepped corners (point 1, 2) increase differently, due to forming velocity effect, for the same forging process. For the forming velocity 250 mm/sec, the die temperature increases gradually, but the maximum temperature is higher than for 300 mm/sec. Also, temperature gradient for the forming velocity 300 mm/s is large at point 2.Fig. 12. Effective stress and abrasive wear depth for forming velocity.The distributions of nodal force and velocity are shown in Fig. 11. It can be seen that nodal force acting on die surface decreases as the forming velocity increases, whereas velocity of the workpiece at the vicinity of the die/material interface increases as the initial die temperature increases. The reason for this is that the metal flow increase with increasing forming velocity. The results of abrasive wear and stress analysis of finisher die are shown in Fig. 12, when forming velocity is 300 mm/s, the wear depth () at point 2 is approximate1.261 mm, and is about three times of that at 200 mm/s.When the forming velocity increases, the die service life evaluated by the plastic deformation becomes longer. But its life by abrasive wear is relatively short. The estimation results of die service life according to forming velocity are shown in Tables 5 and 6, respectively. When the forming velocity is 200 mm/s, the plastic deformation of a die occurred early at the stepped corners (point 1, 2) owing to the local high temperature caused by the long contact time. As the forming velocity increases, the die service life based on plastic deformation was improved by the low local temperature through the short contact time at the stepped corners. When the forming velocity increased, the die service life based on abrasive wear decreased. From the results, die life resulting from abrasive wear of die is more important than from plastic deformation in terms of forming velocity.4. ConclusionsIn this study, two methods for estimating the service life of hot forging dies by plastic deformation and abrasive wear are suggested, and these applied to predict the product quantity, according to two main process variables, forming velocity and initial die temperature. Through the applications of the suggested methods, the following conclusions were obtained. 1) The thermal softening of dies due to the local temperature rise led to the reduction of the service life of hot forging dies by plastic deformation more than by abrasive wear. When the forming velo