Estimation of die service life for a die cooling method in a hot forging process

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. In this paper, die cooling methods are suggested to improve die service life with regards to wear and plastic deformation in a hot forging process. The yield strength of die decreases at higher temperatures and is dependent on hardness. Also, to evaluate die life due to wear, a modified Archard’s wear model has been proposed by considering the thermal softening of die expressed in terms of the main tempering curve. This paper describes the effects of die cooling methods such as cooling hole and direct spray cooling on the life of finisher die during the hot forging of an automobile part. It is shown that the cooling hole method during hot forging is necessary for an effective die service life to be obtained.     

弧齿锥齿轮精锻成形模具磨损特性分析

采用刚塑性有限元法对弧齿锥齿轮精锻成形过程中模具表面磨损进行模拟分析,研究了摩擦因数、坯料温度、模具硬度、锻压速度等对模具磨损的影响.分析结果表明,随着摩擦因数的增大,磨损量增大;随着锻压速度、坯料初始温度及硬度的增大,磨损量减小,并且硬度是影响模具表面磨损量最重要的因素.模拟结果可为弧齿锥齿轮精锻成形工艺方案制定及模...     

A study on life estimation of hot forging die

In this study, the forging die life was investigated using fatigue life and wear quantity, which are generally considered for estimation of die life. The main aim is development of the equations to adapt finite element analysis to life estimation for a hot forging die. The Archard’s model was used to estimate the wear life of the die and Goodman’s and Gerber’s equations, using the stress-life method, were applied to estimation of the fatigue life. These techniques were applied to die life estimation for the hot forging die of a ball joint socket used in an autovehicle system. Rigid-plastic finite element analysis was first carried out for the forging process of the ball joint socket and then the elastic stress analysis was performed for the die in order to obtain fundamental data for the prediction of fatigue life. The die stress analysis requires the deforming loads of the workpiece to be translated into the contact loading of the die. In this case, the size of finite elements of the die and the workpiece is important for good interpolation of the loading. Therefore, the influence of element sizes on the interpolation results of the contact loading was also investigated. The wear volume of the die was measured using a 3-dimensional scanner of probe type. It was found that the measurement had a good agreement with the results of the finite element analysis for the die life estimation.     

Effect of Hot Forging on Microstructure and Abrasion Resistance of Fe-B Alloy

The effect of hot forging on the microstructure and abrasion resistance of Fe-B alloy was studied. The results showed that boride networks are broken down by hot forging. After hot forging, the hardness of Fe-B alloy increased marginally, and the toughness increased considerably.

In the two-body abrasion test, unforged Fe-B alloy exhibited excellent wear resistance and soft abrasive tended to show higher wear resistance. When alloys were tested against very hard abrasives, the wear resistance of forged Fe-B alloy was similar to that of unforged Fe-B alloy, but in the case of soft abrasives the wear resistance of forged Fe-B alloy was lower than that of unforged Fe-B alloy.     

Experimental Study of Wear Behaviour of Hot Forging Tool Steels Under Dry Conditions: 40CrMoV13 Against C35E

The dimensions and quality of forged-steel components are significantly affected by the action of friction and wear. The thermal and mechanical operating conditions of hot forging tools provoke serious degradation, such as oxidative wear, thermal fatigue, plastic damage and mechanical cracking. This slowly causes the tools to lose their original geometry and thus they must be either reformed or discarded [1]. The knowledge and control of this damage is essential and must be taken into consideration, both in die design and in the choice of tool material and the type of surface conditioning. The degradation has direct effects on the lifespan of the tools, and consequently on the cost of the components. This paper deals with the sliding wear behaviour of 40CrMoV13 Steel against C35E in the 700 to 850 °C temperature range under ambient conditions. This steel is used frequently as hot forging die material. This study focuses on the effect of the test temperature and the role of the oxide scales. The purpose of these experiments was to obtain tribological data (friction coefficient, wear rate, etc.), in order to include it in numerical simulations of damage to hot forging tools for the purpose of optimizing the tools' lifespan.     

Suitability of CAE neural networks and FEM for prediction of wear on die radii in hot forging

Prediction of tool wear in hot die forging along the entire arbor radius by wear models known so far is a very difficult task. On these parts of tools significant changes of contact pressure and sliding lengths occur along the die curvature during the plastic flow of material formed. A new approach presented in the paper combines the use of a conditional average estimator neural network (CAE NN) with the exploitation of results obtained by the finite element method (FEM) and also data from other sources. Consequently new parameters as well as the results of experimental work can be taken into account. In this paper a brief overview of models for prediction of tool (die) wear are discussed. The theoretical background of CAE NN, as well as its application to the modeling of the tool wear phenomenon, is presented. Some results of FEM analysis of the hot forging process that serve as input parameters in the CAE NN model are also briefly discussed. Two relevant practical applications are shown. In the first example, tool wear was modeled at a higher number of strokes (blows), by knowing wear at a lower number of strokes. In the second example, the number of strokes was the output parameter—the number of strokes causing predetermined wear at any point of the tool engraving curvature (arbor radius) was predicted. A comparison between the measured and predicted values of wear demonstrated good agreement that was assessed by a corresponding coefficient of determination.     

Gear hot forging process robust design based on finite element method

During the hot forging process, the shaping property and forging quality will fluctuate because of die wear, manufacturing tolerance, dimensional variation caused by temperature and the different friction conditions, etc. In order to control this variation in performance and to optimize the process parameters, a robust design method is proposed in this paper, based on the finite element method for the hot forging process. During the robust design process, the Taguchi method is the basic robust theory. The finite element analysis is incorporated in order to simulate the hot forging process. In addition, in order to calculate the objective function value, an orthogonal design method is selected to arrange experiments and collect sample points. The ANOVA method is employed to analyze the relationships of the design parameters and design objectives and to find the best parameters. Finally, a case study for the gear hot forging process is conducted. With the objective to reduce the forging force and its variation, the robust design mathematical model is established. The optimal design parameters obtained from this study indicate that the forging force has been reduced and its variation has been controlled.     

Factors affecting die wear

Simple upsetting tests were used to assess the wear of hot forging dies. The influence of forging variables such as die-billet contact time and lubrication is described. A “white” layer is formed at the surface of lubricated dies when die-billet contact times exceed 5 ms. This layer was identified as martensite of increased wear resistance. Lubrication affects the frictional condition at the die-billet interface thus facilitating the relative movement of the billet material over the die surface resulting in the increased wear of flat dies.     

Brush plated cobalt-molybdenum and cobalt-tungsten alloys for wear resistant applications

Brush plating solutions have been developed that enable sound cobalt alloy coatings containing up to 12% molybdenum or tungsten to be deposited at thicknesses up to 13 μm. The alloy deposits had a very fine grain size but their crystallographic structure could not be determined. The cobalt deposit alone had a mixed cph and fcc structure at room temperature. Evaluation of the wear resistant properties of the cobalt alloys was carried out in the laboratory using a pin and disc technique and a simulated hot forging test. Both tests showed the alloys to have relatively good wear characteristics. Extensive industrial trials have been undertaken to assess the value of these cobalt alloy coatings to enhance the performance of hot forging dies. The results have indicated that die life can be increased by up to 100%     

Hot shape forging of gas turbine disk using microstructure prediction and finite element analysis

The Waspaloy, a Ni-based superalloy, has been widely used for forging material of gas turbine disk since it requires the high tensile strength at high temperature and good resistance to low cycle fatigue. The purpose of this study is to develop a forging process of turbine disk that satisfies the hot deformation characteristics of Waspaloy. Generally, the hot forging of superalloy has been subjected to isothermal forging since the available temperature range of forging is narrow. However, the non-isothermal forging was used to make a turbine disk in this study. Therefore, the analyses of temperature variation and deformation behavior of the material were important to obtain the sound forging products. The hot compression test was carried out to know formability at high temperature and microstructure evolution during hot deformation. In order to define the optimum forging conditions including material temperature, strain rate, strain, microstructure evolution and forging load, the commercial finite element analysis code was used to simulate the forging procedure of turbine disk. The hot forged turbine disk was heat-treated for obtaining the high temperature properties. The cut-off tests on the heat-treated forged disks were carried out. Experimental results were compared with the simulation results by FE analysis. Test results were in good agreement with the simulations. This study shows that the superalloy turbine disk can be manufactured by the semi-closed die forging.     

Cr——氟化石墨复合镀层在高温轴承中的应用研究

本文对热轧钢运输链上的轴承进行失效分析，找出了主要失效的原因。采用自润滑Ｃｒ－氟化石墨复合镀层代替轴承中的脂润滑。利用电沉积工艺制备了Ｃｒ－氟化石墨镀层，并在摩擦磨损试验机上对镀层进行摩擦系数和耐磨性的试验研究，认为润滑剂存在时，镀层中氟化石墨的含量不影响摩擦系数。在干摩擦时，镀层中氟化石墨的含量增加，摩擦系数随这降低。镀层的耐磨性主要取决于镀层的硬度。     

Microstructure and properties of surfacing layers of dies manufactured by bimetal-gradient-layer surfacing technology before and after service

The purpose of this article is to investigate the microstructure and mechanical properties of surfacing layers (wear layer and transition layer) of a hot forging die manufactured by the bimetal-gradient-layer surfacing method, which is based on ZG29MnMoNi cast steel before and after forged 5761 parts on a 63MN hot die forging press. The finite element model of a die was established. Subsequently, a simulation was conducted to analyze the temperature field of the die and its cycle features under working conditions. Microstructure and mechanical property were measured. Results indicated that the microstructure of the wear layer mainly consists of temper sorbite, ferrite and carbides. The transition layer before and after service is mainly composed of both temper sobrite, lower bainite, and a small amount of temper martensite. The mechanical properties of wear and transition layers declined significantly after service. The tensile strength, yield strength, reduction of area, and elongation of wear layer declined by 41.6, 32.5, 28.3, and 24.5 %, respectively. With those indexes the transition layer decreased by 36.6, 34, 24.4, and 19.8 %, respectively. Microhardness and impact energy of wear and transition layers have showed a decrease of 17, 6 % and 51.2, 32.6 %, respectively. The impact fracture mode of both wear and transition layers is typically intergranular fracture after service. As a conclusion, it was determined that the service process sufficiently influenced the mechanical properties of the surfacing layers.     

基于钴基合金覆层的多层金属热锻模原型制备与性能

基于功能梯度材料(FGM)的思想制备多层金属热锻模是提高模具寿命的有效方法。采用焊条电弧堆焊制备了多层金属热锻模的原型试样,试样经焊后热处理后,进行了金相组织分析、显微硬度测试、磨损实验和冲击韧性测试等实验。实验结果表明:钴基合金堆焊层与W6Mo5Cr4V2堆焊层界面冶金结合情况良好;截面显微硬度呈梯度分布,表面钴基合金硬度达到492HV;制备的多层金属试样耐磨性是H13钢耐磨性的2.5倍,冲击韧性处于合理范围。     

复合陶瓷强化挤压模具钢抗磨减摩研究

本文对复合陶瓷强化挤压模具钢抗磨减摩进行了研究，利用比拟试验研究了在蓖麻油 硬脂酸钙润滑下，有色金属铜（H62）、钢（20）试样与复合陶瓷强化9CrSi环配副的摩擦磨损特性，并与未采用陶瓷强化的钢（H62）、钢（20）/9CrSi环摩擦副进行比较，分析了其减摩抗磨机理，试验结果表明：复合陶瓷强化挤压模具能显著减少摩擦系数，降低磨损量，适合应用于挤压模。     

镍基合金涂层滑动磨损过程中磨屑的演变机理

在真空熔结镍基合金涂层(成分是0．65％C-24％Cr—3．5％Si-3．5％B-3．0％Mo-6％Fe-Ni余量)与65Mn钢组成的滑动磨损系统中，研究了磨屑的演变机理。磨损过程中磨屑不仅存在氧化、碎化等化学、机械过程，还存在因相互挤压、粘结甚至微区热压烧结等冶金过程，两方面的共同作用，使氧化物磨屑成分均匀化。应用Fe-Ni-O系统相图对高温磨损磨屑进行相分析。     

An Integrated Transient and Steady-State Adhesive Wear Model

The integrated adhesive wear model tokes into account both the transient wear and the steady-state wear in wear testing. The transient wear volume is described by an exponential equation while the steady-state wear by a revised Archard's equation. In this study, the integrated wear model was used to analyze experimental wear data obtained previously for an A6061-T6 alloy and MMC-D, an aluminium alloy matrix composite containing 20% spherical alumina particles. Two loads of 7.5 kgf and 10.0 kgf, and a speed of 4.58ms^?1 were used in conducting the experiments. Both the standard wear coefficient and the net steady-state wear coefficient values for both types of materials were determined. On the average, the average standard steady-state wear coefficient, as compared with the net steady-state wear coefficient, was about 52% higher for MMC-D and 246% higher for A6061-T6. A higher load was found to have the effect of increasing the wear coefficient values.     

Thermal wear and electrical sliding wear behaviors of the polyimide modified polymer-matrix pantograph contact strip

In the present study, the polyimide resin (PI)/cashew-modified resin (YM) polymer-matrix pantograph contact strip (PMPCS) was prepared by using hot repressing, hydro-solidification and dipping treatment processes. The thermal properties of cured resins were studied by thermogravimetry analyzer and differential scanning calorimetry. The thermal wear and electrical sliding wear behaviors of PMPCS against copper were evaluated by a ring block wear tester at elevated temperature under dry sliding conditions and a wear tester which simulated the train motion under laboratory conditions, respectively. Worn surfaces and wear debris of PMPCS were analyzed by scanning electron microscopy and energy dispersive spectrometer, and the wear mechanism was discussed. It has been found that the thermal stability of the PI/YM is superior to that of the YM under the same testing conditions. The results also showed that PI/YM-PMPCS had superior wear resistance than that of YM-PMPCS at elevated temperature and with electrical current. At elevated temperature, the wear mechanism of tribological pair evolved from adhesive wear to oxidative wear with mild delamination wear. Arc erosion wear, oxidative wear, and adhesive wear were the dominant mechanisms of tribological pair during the electrical wearing process.     

Analysis of the friction and wear behavior of hot work tool scale: application to the hot rolling process

In hot forming processes, the interface tool/product is important for the quality of the finished product. In hot rolling, the scale formed on the roll material plays an essential role. As soon as the contact oxide–oxide is established between the roll and the slab, friction allows the process to start. However, the oxide scale will continue to grow under the cyclic action of hot contacts and water cooling and will be subjected to thermo-mechanical stresses. Beyond a certain critical thickness, the oxidized surface layer of the cylinders has not sufficient mechanical strength to withstand the shear stresses.

The wear and friction behavior of the oxides appearing on the surface of the hot working rolls is not well known. The influence of these oxides on the friction and, consequently, the quality of the products of finishing mills, seems very significant. So, in this study, we investigate the evolution of the friction coefficient and the wear, according to the growth, the nature and the thickness of the formed scale. We use a high temperature pin on disc tribometer. The pin consists of material “rolls” while the disc consists of the slab. The pin is instrumented with thermocouples in order to couple the friction coefficient measurements with the thermal gradient in the pin and the surface temperature and the formed oxides. Then, the characterization of the surfaces is done by scanning electronic microscopy (SEM) and EDS analyses. We use the method of sin² Ψ to evaluate residual stresses of oxide and correlate these data with shear stress behavior.     

Design of overlay coated region with hardfacing,transition and damage diminution layers for the reduction of damages of hot forging tools

Hot forging dies are damaged by the mixed failure modes including wear, thermal cracks, impact cracks, corrosion cracks, etc.. In order to extend the service life of the hot forging die, the diminution technology of the mixed failure modes should be developed. The aim of this paper is to design an overlay coated region consisting of a Hardfacing layer (HL), a Transition layer (TL) and a Damage diminution layer (DDL) to reduce damage of hot forging tools. The HL and the DDL are contrived to improve the wear resistance and the ductility of the overlay coated region, respectively. The TL is adopted to prevent thermal fatigue induced by the difference in the thermal expansion coefficients between successive layers. In order to estimate design alternatives of the overlay coated region, the effects of the thickness and the assignment of the DDL on stress-strain distributions in the overlay coated region are investigated via Finite element analyses (FEAs). The influence of the assignment of the DDL on thermal behaviors of the overlay coated region is investigated through thermal fatigue experiments. From the results of the FEAs and the thermal fatigue experiments, an appropriate design of the overlay coated region is determined.     

提高热锻模具寿命的有效途径

提高热锻模具寿命是降低锻件生产成本的重要途径。本文从锻模材料选择、锻模设计、锻模热处理以及模具的使用与维护等方面,分析提出了提高热锻模具寿命的有效途径,对提高锻模寿命和产品质量以及降低生产成本具有指导意义。