The relationship between case depth and bending fatigue strength of gas carburized SAE 8620 steel

The fatigue performance of gas carburized SAE 8620 steel was evaluated as a function of case depth. To vary the case depths, different carburizing times were applied. The typical times were: 45 min, 3 and 5 h at the temperature of 940 °C. To cause failure in the rotating bending fatigue specimens, the applied load was chosen as the equivalent load at 10⁶ cycles for the material when is subjected to bending fatigue. The characterization of the specimens was carried out using X-ray diffraction technique for stress measurement and retained austenite and optical metallographic examination. The fractured surfaces were cleaned in an ultrasonic bath and examined using scanning electron microscope (SEM) equipped with EDX to evaluate the crack initiation and growth characteristics of the materials in the core and carburized case regions of specimens. The results showed that the fatigue limit was associated with the microstructure, the case depth, the distribution of retained austenite, the depth of the internal oxidation and the compressive residual stresses near the surface. The bending fatigue strength of gas carburized specimens was showed to decrease with the increasing case depths caused by the increasing of internal oxidation and nonmartensitic transformation present at the surface.     

Effect of WC/C PVD coating on fatigue behaviour of case carburized SAE8620 steel

Case carburizing steels, such as SAE8620, are generally employed in manufacture of automotive components viz. gears and shafts. These components are often subjected to a combination of wear and fatigue loads. Though a number of coatings are available that can enhance performance under such loadings, but high processing temperatures forbid their application to heat-treated components. On the other hand, the low processing temperature for the WC/C coating permits its application to heat-treated parts. The coating exhibits favourable tribological properties, but its influence on fatigue behaviour is yet unexplored. The present work is aimed at investigating the influence of WC/C coating on the fatigue behaviour of case carburized SAE8620 steel, wherein performance of uncoated and coated specimens is compared by subjecting them to four point rotating bending fatigue tests. The results point out that the coating leads to 7% gain in endurance limit without considerably affecting the hardness and tensile strength.     

Influence of retained austenite on short fatigue crack growth and wear resistance of case carburized steel

The influence of the amount of retained austenite on short fatigue crack growth and wear resistance in carburized SAE 8620 steel was studied in this article. Different amounts of retained austenite in the microstructure of the carburized case were obtained through different heat treatment routes applied after the carburizing process. The wear tests were carried out using pin on disk equipment. After every 200 turns the weight loss was registered. Four point bend fatigue tests were carried out at room temperature, using three different levels of stress and R=0.1. Crack length versus number of cycles and crack growth rate versus mean crack length curves were analyzed. In both tests the results showed that the test pieces with higher levels of retained austenite in the carburized case exhibited longer fatigue life and better wear resistance.     

The effect of retained austenite on contact fatigue in Cr−Ni−W carburized steel

This article describes an experiment to determine the effect of retained austenite on the chipping of the case-hardened layer of Cr−Ni−W carburized steel by examining the same case-hardened layer with varying amounts of retained austenite after different heat treatments. The existence of a critical load level was established; contact fatigue resistance can be improved when the loads are above critical. This is determined by deformation of the austenite structure and substructure, the austenite-martensite transformation under the influence of the load, and the retention of the deformed austenite structure in the martensite. Authors' Note: Alloyed steel compositions are presented in the Soviet standard. The first two numbers give the approximate carbon content in hundredths of a percent. The number after the element shows the approximate percentage of the alloying elements. The number one or no number at all means the percentage is about one or less. Bertold B. Vinokur earned his Doctor of Technical Science in physics of metals and metal science at the Ukrainian Academy of Science, Ukraine, in 1978. He is currently retired. Dr. Vinokur is a member of TMS. Alexandr L. Geller earned his Doctor of Technical Science in metal science and heat treatment at Donetsk Technical University, Ukraine, in 1997. He is currently an assistant professor in the Department of Materials Science and Engineering at Donetsk Technical University, Donetsk, Ukraine.     

Modeling and reducing edge effects in laser bending

Laser forming of sheet metal offers the advantages of requiring no hard tooling, no spring back and no external force, thus this technology reduces cost, increases flexibility and accuracy. Laser forming is a complicated and transient thermo-mechanical process involving elastic and plastic strain depended on time history. How to control and enhance the accuracy of the laser forming attracts much attention. This paper delivered a deep insight into the mechanism of the edge effects in laser forming, improved an existing analytical model to approximately predict the edge effects, and based on this model a new strategy was put forward to reduce the edge effects. Numerical simulations and experiments were carried out to validate the model and the presented strategy.     

20钢铸铁浴渗碳渗层组织的分析

本文探讨了20钢在铸铁浴中经不同温度及时间渗碳的渗层组织。结果表明，试样在1270℃的铸铁浴中保留30秒，距表面130－150μm处，其渗层组织为渗碳体＋珠光体＋铁素体；试样在1300℃的铸铁浴中保留30秒，距表面0－120μm处，其渗层组织为渗碳体＋珠光体，且在基体上形成棒条状的碳化物。     

Role of the cyclic stability of retained austenite in fatigue performance of carburized 14NiCr11 steel

The role of retained austenite in the fatigue strength of carburized 14NiCr11 steel was studied by considering two gas-carburizing treatments leading to two maximum retained austenite fractions of 20 and 40%. These states led to endurance limit improvements evaluated at 40 and 10%, respectively, compared with the untreated state. These improvements were explained by the evolution of retained austenite during the cyclic loading using the dispersive x-ray diffraction technique. This reveals that the best fatigue strength is attributed to the homogeneous transformation of the retained austenite fraction in the treated layer during the cyclic loading.     

大颗粒碳化物对GCr4Mo4V钢旋转弯曲疲劳的影响

以“真空感应+电渣重熔”工艺制备的退火态热轧直条高温轴承钢GCr4Mo4V为试验材料,经过1120℃保温30 min油淬以及3组530℃保温2 h的回火处理后,开展960、1000、1040、1080 MPa四个应力水平的旋转弯曲疲劳试验,得到S-N曲线并计算出中值疲劳极限为686 MPa,对旋弯疲劳试样断口形貌分析表明,GCr4Mo4V轴承钢旋转弯曲断口由近表面起裂源、裂纹扩展区、应力撕裂区3个特征区域构成,起裂源距试样表面约240μm,中心为粒径范围为16.93～53.94μm的大颗粒碳化物,裂纹在大颗粒碳化物与基体界面处形核,并逐渐向试样中心扩展,最终在扭矩作用下将试样撕扯断裂;数值分析表明,大颗粒碳化物粒径D与疲劳寿命对数lg(N)呈线性关系,经线性拟合后得到的数学关系式为lg(N)=-0.053 77D+7.326 82,由此指明了GCr4Mo4V轴承钢大颗粒碳化物的极限细化是进一步实现轴承钢长寿化的关键举措。     

Tempering of carburized steel in a fluidized bed

Conclusions The decomposition of retained austenite in carburized steel 18Kh2N4VA during tempering at 650°C in a fluidized bed is the same as that with tempering in a lead bath and occurs twice as fast as with tempering in the electric furnace. This can be explained by the greater precipitation of carbides on lattice defects due to rapid heating in the fluidized bed, and the slowed redisribution of nickel in the solid solution.Ural Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 58–61, January, 1973.     

Modeling size effects on fatigue life of a zirconium-based bulk metallic glass under bending

A size effect on the fatigue-life cycles of a Zr₅₀Cu₃₀Al₁₀Ni₁₀ (at.%) bulk metallic glass has been observed in the four-point-bending fatigue experiment. Under the same bending-stress condition, large-sized samples tend to exhibit longer fatigue lives than small-sized samples. This size effect on the fatigue life cannot be satisfactorily explained by the flaw-based Weibull theories. Based on the experimental results, this study explores possible approaches to modeling the size effects on the bending-fatigue life of bulk metallic glasses, and proposes two fatigue-life models based on the Weibull distribution. The first model assumes, empirically, log-linear effects of the sample thickness on the Weibull parameters. The second model incorporates the mechanistic knowledge of the fatigue behavior of metallic glasses, and assumes that the shear-band density, instead of the flaw density, has significant influence on the bending fatigue-life cycles. Promising predictive results provide evidence of the potential validity of the models and their assumptions.