Predictive modeling of multi-track laser hardening of AISI 4140 steel

Laser hardening provides benefits over the conventional hardening processes, including minimum distortion in the parts and the absence of a quenchant. This process is also faster than conventional hardening processes and can be used for selective hardening of specific areas of components. One known problem with laser hardening in steels, however, is back tempering when a large area is hardened by multiple, overlapping passes. This study focused on the development of a numerical model to predict the back tempering in multi-track laser hardening. A tempering model was combined with existing models of thermal behavior and phase change kinetics, which were developed earlier in the authors’ group, to predict three-dimensional hardness profiles after multiple track laser hardening. The combined model was first validated through multi-track laser hardening tests and then used to predict and optimize the laser hardened case depth in multi-track laser hardening of AISI 4140 steel. The predictions and parameters optimized to obtain maximum case depth with the least variation along width of the hardened zone were experimentally verified. Case depths up to 2 mm were obtained with 5 mm overlapping of laser tracks.     

Einfluß einer Laseroberflächenhärtung auf die Dauerlestigkeit von glatten Proben mit kurzem Riß aus dem Stahl C70W2

The influence of laser transformation hardening on fatigue limit of smooth C70W2 steel specimens with small crack Laser transformation hardening as a local heat treatment technology can be used to restore the fatigue strength of crack-damaged components. The investigations have been carried out on smooth steel specimens containing small surface cracks. The obtained 80% fatigue limit improvement of the laser hardened specimens in relation to the crack-damaged initial condition is explained by the fatigue crack growth threshold increase due to martensitic transformation hardening, the action of residual stresses on the crack front and possible crack tip blunting.     

Fatigue strength of high strength dual-phase steel sheet

Fatigue tests have been carried out on lean-alloyed dual-phase steels with tensile strengths ranging from 300–800 MPa. Smooth specimens and specimens with punched holes were tested. The fatigue strength of dual-phase steel was found to be similar to that of other types of steel (eg solution hardened or microalloyed steels) of equal tensile strength. The fatigue strength increases with increasing yield strength. For notched specimens it is also related to the yield ratio. Work and bake hardening increase the fatigue strength of smooth specimens in proportion to the increase in yield strength. For notched specimens this effect is less and is dependent on the yield ratio. Bake hardening of material which was not work hardened also increased the fatigue strength. The notch sensitivity of low yield ratio dual-phase steel is found to be low. The notch sensitivity seems to increase with increasing yield ratio.     

感应淬火对曲轴扭转疲劳性能的影响

目的研究感应淬火对曲轴扭转疲劳性能的影响,为曲轴的设计和制造工艺调整提供技术参考。方法开展淬火曲轴和未淬火曲轴的扭转疲劳强度试验,利用升降法得到疲劳试验结果,从试验数据和微观组织等方面开展分析和讨论。结果未经过淬火的曲轴在99.9%存活率下的扭转疲劳极限为967.6N·m,经过感应淬火的曲轴在99.9%存活率下的扭转疲劳极限为1361.2N·m。感应淬火后曲轴的表面形成深度约3.5 mm的淬火层,平均硬度为HV0.5600,金相组织为细针状马氏体。曲轴的失效情况均为连杆颈油孔处开裂。结论 38MnVS6非调质钢曲轴在感应淬火后的扭转疲劳极限提升了约41%,曲轴油孔内壁的加工缺陷是形成裂纹源的主要原因,对曲轴淬火层区域的油孔内壁进行一定的表面处理,可进一步提高曲轴的扭转疲劳强度。     

Study on Laser Transformation Hardening of HT250 by High Speed Axis Flow CO2 Laser

In this article, laser transformation hardening of HT250 material by high speed axis flow CO2 laser was investigated for first time in China. Appropriate laser hardening parameters, such as laser energy power P(W), laser scanning rate V(m/min), were optimized through a number of experiments. The effect of the mentioned parameters on the hardened zone, including its case depth, microhardness distributions etc., were analyzed. Through the factual experiments, it is proved that axial flow CO2 laser, which commonly outputs low mode laser beam, can also treat materials as long as the treating parameters used are rational. During the experiments, the surface qualities of some specimens treated by some parameters were found to be enhanced, which does not coincide with the former results. Furthermore in the article, the abnormal phenomenon observed in the experiments is discussed. According to the experimental results, the relationship between laser power density q and scanning rate V is shown in a curve and the corresponding formulation, which have been proved to be valuable for choosing the parameters of laser transformation hardening by axial flow CO2 lasers, was also given.     

Effects of residual stresses on fatigue resistance in torsion with bending in the presence of stress concentrations

Fatigue-test results are given for torsion with bending in cylindrical steel 45 specimens having small semicircular profile notches and with and without residual stresses. The effects of hardening on the fatigue resistance are evaluated from a residual-stress criterion. The test data for the unhardened and hardened specimens are closely described by the equation of an ellipse, which enables one to express the margin coefficient in torsion with bending with allowance for the residual stresses.Translated from Problemy Prochnosti, No. 12, pp. 37–40, December, 1992.     

Optimization of Process Variables in Gas Carburizing—An Experimental Investigation with AISI 3310 Steel Material

Gas carburizing is the most widely used process for surface hardening of critical components used in automobiles, heavy duty machines, etc. In this process the surface composition of the carbon steel changes by diffusion of carbon and results in a hard outer surface with good wear resistance properties. Fatigue behavior of case hardened parts depends largely on the correct combination of its magnitude and depth of hardness penetration without undue distortion. An industrial survey indicates that there is a rejection of 10-12% of case hardened components due to defects like low hardness and strength after hardening, distortion, and warping. Optimal control of the process parameters is the way to ensure the quality of the surface hardened part. This paper is the result of the investigation carried out using limit design concept called Taguchi's Technique in the optimization of process variables in gas carburizing.     

EFFECT OF LASER BEAM RADIATION ON FATIGUE CRACK PROPAGATION IN AISI 4150 STEEL

The effect of laser beam radiation on fatigue crack growth in AISI 4150 steel was performed on compact-tension (CT) specimens, in which a composite region (CR) comprised of the hardened zones (HZs) on the top and bottom surfaces and the base metal (BM) in the interior, was aligned either along or normal to the crack growth direction. The microstructure of the HZs consisted of martensite, while lower bainite was present in the 300 °C preheated laser-hardened specimens. When the crack propagated along the laser tracks (LTs), the fatigue crack growth rates (FCGRs) of the laser-hardened specimen were lower than those of the base plate, particularly at low ΔK ranges. On the other hand, for a crack propagating normally to the LTs, decelerated FCGRs in the regions preceding the CR and accelerated FCGRs within the CR itself were found. However, enhanced FCGRs in the CR were not found in preheated specimens with a bainite structure in the HZs as the crack grew normal to the LTs. The enhancement of FCGRs in the CR, which became more accentuated at high ΔK values, was closely related to an embrittled microstructure (martensite) in the HZs.     

Analysis of the torsional strength of hardened splined shafts

The current study presents a finite element modeling framework to determine the torsion strength of hardened splined shafts by taking into account the detailed geometry of the involute spline and the material gradation due to the hardness profile. The aim is to select a spline geometry and hardness depth that optimizes the static torsion strength. Six different spline geometries and seven different hardness profiles including non-hardened and through-hardened shafts have been considered. The results reveal that the torque causing yielding of induction hardened splined shafts is strongly dependent on the hardness depth and the geometry of the spline teeth. The results from the model agree well with experimental results found in the literature and reveal that an optimum hardness depth maximizing the torsional strength can be achieved if shafts are hardened to half their radius.     

Increase of the bearing capacity of a shaft by plastic deformation

The problem of estimating the degree of hardening of shafts after their axial plastic deformation is examined on the basis of the Backhouse model of anisotropic hardening. Calculated relations are obtained for determining the increased value of the shear yield strength on the basis of given characteristics of the material and value of axial strain. A goniometer for conducting torsion tests of cylindrical specimens to obtain an experimental torsion diagram is described. A comparison of the calculated and experimental data showed a sufficient accuracy of estimating the hardening effect in torsion of shafts after their plastic deformation in the axial direction.Translated from Problemy Prochnosti, No. 6, pp. 86–92, 1994.     

MICROSTRUCTURAL STUDY OF CYCLIC STRAIN HARDENING BEHAVIOUR IN BIAXIAL STRESS STATES AT ELEVATED TEMPERATURE

This paper describes the cyclic strain hardening behaviour and dislocation structures of material in biaxial low cycle fatigue at elevated temperatures. In this study, push-pull, reversed torsion and combined push-pull/reversed torsion tests were carried out using a type 304 stainless steel in air. While there was no significant difference between the cyclic stress amplitudes in the push-pull and reversed torsion tests on a von Mises' base, combination tests exhibited a 40% increase in stress amplitude. Most of the dislocations in the first two types of test adopted ladder or maze structures, while in the later case cells were found. Changing the loading mode at a certain cycle, for example, from push-pull to reversed torsion, revealed that stress amplitude depended mainly on the concurrent applied strain mode and furthermore, that the strain mode before the interchange had little or no effect on the stress amplitude after the interchange. Tests were also performed in order to examine how prestrained material hardened in the three different loading modes, with the following results: prestrained material in push-pull or in reversed torsion exhibited an anisotropic stress response, while the material in the combined tests exhibited an isotropic response. These cyclic responses are discussed in connection with the dislocation structure.     

Experimental observation and numerical modeling of formation of local plastic zones in hardened surface layers due to contact overloading

The problem of formation of plastic zones in case-hardened metallic bodies due to contact overloading is studied both experimentally and numerically. Metallic materials exposed to surface hardening demonstrate spatial variation of the material hardness and yield strength with a decreasing profile with depth and belong to the class of so-called plastically graded materials. The presented experimental program employs micro-Vickers hardness tests to map the variation in material hardness and corresponding yield strength for both virgin and loaded case-hardened specimens made of a chromium tool steel. It is shown that, depending on the profile of the yield strength in the near-surface zones and contact parameters, a plastic deformation can originate underneath the hardened layer. The distribution of the effective plastic strain extracted from the micro-hardness increment measurements are found in good agreement with the results of finite element simulations of a plastically graded material subjected to similar loading conditions. Numerical analysis reveals significant perturbations in the stress field distribution within the hardened layer due to formation of a closed-shaped plastic zone in the gradient layers, including development of a tensile stress on the boundary between the elastic and plastic zones as well as an overall increase in the effective stress intensity. It is shown that the hardened layer behaves similar to an elastic beam on a compliant foundation. These stress field perturbations in the hardened layers with low deformation capacity can greatly affect the durability and serviceability of surface treated mechanical parts.     

纯铜表面的连续摩擦压扭处理

提出了一种在纯铜表面得到大面积亚微米细晶组织及晶表面硬化处理的新工艺。在T2紫铜表面，利用连续摩擦压扭过程中产生的剧烈的剪切塑性变形，使材料表面形成了一层厚为0．1－0．2mm，晶粒直径200－300nm的亚微米细晶组织，表面硬度比基材提高了1倍，压扭头行走速度和转速对变形区晶粒细化和硬化效果影响显著。理论分析结果表明，在垂直于行走方向上加热能量和变形程度基本均匀。     

Strength of Cylindrical Specimens Subjected to Laser Hardening along the Generator and a Spiral Path

We have shown that the path of a continuous CO₂-laser beam over the cylindrical surface of specimens in the course of their hardening substantially affects the strength of steels in cyclic axial tension and bending with torsion. Hardening on a spiral path ensures higher mechanical parameters as compared with that along the generator of the cylindrical surface.     

Laser Hardening of Austempered Ductile Cast Iron (ADI)*

Austempered ductile cast iron (ADI) has emerged as a major engineering material in recent years. In addition to high strength and relatively light weight (compared to steel), it has high ductility, good wear resistance and good damping capacity. It has many potential applications such as automotive components (e.g. crank shafts and gear boxes) as well as aircraft components (landing gears).

In many structural applications, (e.g. aircraft landing gear) it is often required that the material be hardened at the surface while the interior of the material must remain soft or ductile. The higher hardness at the surface layer imparts excellent wear resistance while the soft inner core provides higher toughness and fracture resistance. The conventional methods of surface hardening such as carburizing and nitriding or shot peening have several limitations, e.g. retained austenite, massive carbide formations and insufficient case depth. In recent years, there has been significant interest in use of laser in surface treating of materials. Surface hardening by means of laser is a very useful technique because of self quenching and minimum of distortion. Laser hardening can also improve significantly the surface properties such as wear and fatigue resistance.     

Dot Matrix Hardening of Steels Using a Fiber Optic Coupled Pulsed Nd:YAG Laser

This paper describes a novel process called “Dot Matrix Hardening” as applied to Ol, D2 and AISI 4340 steels. This process uses a pulsed laser (particularly an Nd:YAG laser) to create a uniform distribution of transformation-hardened spots to cover only a certain percentage of the desired surface. Due to significantly reduced energy input, wear resistance can be imparted to thin and intricate parts without distortion. In addition, with the use of a coupled fiber optic beam delivery system, this process provides greater flexibility compared to conventional CO₂ laser hardening for a number of applications. The use of an Nd:YAG laser also eliminates the need of absorptive coating required for hardening with a CO₂ laser. With optimized processing parameters, a relatively uniform hardened layer is obtained within the hardened spot, with a thickness of about 60 um and hardness values around 800 HV₁₀₀. The sliding wear test results show that the wear resistance of Ol samples with only 20-40% area coverage of laser-hardened spots is similar to the 100% covered laser dot hardened sample as well as the furnace hardened (Re 60) sample.     

Numerical simulation of finish hard turning for AISI H13 die steel

A coupled thermo-mechanical model of plane-strain orthogonal turning of hardened steel was presented. In general, the flow stress models used in computer simulation of machining processes are a function of effective strain, effective strain rate and temperature developed during the cutting process. However, these models do not adequately describe the material behavior in hard machining, where the workpiece material is machined in its hardened condition. This hardness modifies the strength and work hardening characteristics of the material being cut. So, the flow stress of the work-material was taken with literature [H. Yan, J. Hua, R. Shivpuri, Development of flow stress model for hard machining of AISI H13 work tool steel. The Fourth International Conference on Physical and Numerical Simulation of Materials Processing, Shanghui in China, 2004, p. 5] in order to take into account the effect of the large strain, strain-rate, temperature and initial workpiece hardness. Then a series of numerical simulations had been done to investigate the effect of machining parameters on the machinability of hardened steel AISI H13 in finish turning process. The results obtained are helpful for optimizing process parameters and improving the design of cutting inserts in finish turning of hardened steel AISI H13.     

Zum Einfluß einer lokalen Laserhärtung auf die Ermüdungsrißausbreitung im Stahl C70W2

Influence of local laser hardening on the fatigue crack propagation in steel C70W2 Laser transformation hardening as a modern technology for improving the mechanical surface properties is useful to raise the materials resistance against fatigue crack growth. The velocity of a moving crack can be decreased by laser hardened zones at the side surfaces of flat specimens made from normalized steel C70W2. Dependent on the arrangement of zones in relation to the crack plane different improvements in fatigue life were obtained.     

Effect of hich-temperature thermomechanical treatment on mechanical properties of 45KhNMFA steel in torsion

High-temperature thermomechanical treatment involving deformation in torsion ensures a substantially larger increase in the fatigue limit of steel carrying pulse torsion loads than the ordinary quench-hardening treatment. Rapid electrical resistance heating for quenching also has a beneficial effect on the fatigue strength. Oriented strain hardening of specimens subjected to a high-temperature thermomechanical treatment produces an additional increase in their fatigue strength in torsion.     

Einsatzhärten: Untersuchungen zur Änderung der Härteannahme nach isothermischer Umwandlung

Case hardening: Investigations on the change in hardenability after isothermal transformation To increase economic efficiency of case hardening processes great efforts are made in raising carburising temperature. One possibility to remove the coarse grain appearing after high temperature carburising is to integrate an isothermal transformation into the carburising procedure. Apart from a fine martensitic structure two extra effects appear if this technique is used giving reason to more research work. On the one hand there appears a reduction in hardenability of the investigated material. On the other hand bending strength increases towards conventionally case hardened samples.