Induction Surface Hardening Research Articles

Use of Eddy-Current Method for Determining the Thickness of Induction-Hardened Layer in Cast Iron

Cast iron with globular graphite is studied after surface induction hardening. Hardness profiles of iron bars are plotted for determining the thickness of the hardened layer. Relations between the thickness of the hardened layer and the output voltage are obtained by the method of dual frequency (induced voltage and normalized impedance) for checking the applicability of the eddy current method. The correlation coefficient and the error of the evaluation of the thickness of the hardened layer are determined with the help of the relations obtained.

Modeling of Spray Cooling during Induction Hardening of Spur Gearwheels Made from 42CrMo4 Hardening and Tempering Steel

The application of water–air spray cooling in the process of induction surface hardening according to the simultaneous dual‐frequency technology represents a hitherto unexploited and equivalent alternative to conventionally employed polymer solutions. The reason for this is that the selection of the optimum parameters is associated with a high experimental outlay and analytical effort for performing the tests and evaluating the results, respectively, and the parameters must be selected for a specific application. In order to reduce this effort, a numerical model was developed to formulate the coupled thermal, microstructural, and mechanical processes during quenching by means of water–air spray cooling of induction heated spur gearwheels made from 42CrMo4 hardening and tempering steel. The model was implemented in the commercial simulation software ANSYS Workbench 13.0 and verified using simulation results for temperature development, hardness distribution, residual stresses, and distortion. A comparison of the simulated and experimental results show that the model introduced here is suitable for predicting the hardening results during quenching using spray cooling subsequent to induction heating.

Measuring in-depth Residual Stress Gradients: The Challenge of Induction Hardened Parts

Highly stressed machine parts such as gears and shafts are often surface treated to increase wear and fatigue resistance at critical locations. For example, induction surface hardening (ISH) is increasingly used in the automotive and aerospace industries thanks to the availability of modern multiple frequencies generators and complex shaped coils that provide a great flexibility in process control. With similar end-results in terms of hardened depths, very different residual stress profiles may be obtained, and optimized by modifying both heating and quenching kinetics. If hardness and microstructures variations are routinely verified, some challenges raise for the measurement of the residual stress gradients within complex geometry parts, in particular for the case of deep hardened layers. The most commonly used technique is X-ray diffraction (XRD). It requires using successive layer removal to get access to in-depth stresses. The measurements must therefore be corrected for the stress redistribution occurring during layer removal. However, industrial geometries are often not covered by traditional correction methods. The present work aims at applying XRD to precisely measure in-depth residual stress profiles in induction hardened thin discs made of martensitic steel. Both issues of microstructural variations and redistribution of stresses during layer removal are tackled. First, X-ray elastic constants were determined experimentally using a miniature custom-made tensile machine with specimens heat treated to simulate different microstructures found in ISH parts. Second, a recently introduced finite elements based layer removal correction method was applied. The proposed methodology is used to show the impact of preheating and core hardness on the residual stresses obtained after induction hardening.

Experimental Determination of Process Parameters and Material Data for Numerical Modeling of Induction Hardening

Induction surface hardening is a widely used manufacturing process to improve the mechanical properties of components. However, better process understanding as well as process development requires numerical modeling. The modeling itself depends on the input data in terms of process parameters and the material behavior. Data acquisition is a rather difficult task due to very short processing times, as seen in contour hardening of gears. The article will give an overview over critical aspects regarding the acquisition of input data. A short presentation of the numerical model used to compare experimental and numerical results shall promote better understanding for improving the modeling or reducing the model complexity necessary for good predictability.

Process Simulation of Single and Dual Frequency Induction Surface Hardening Considering Magnetic Nonlinearity

A 2D computational model of single and simultaneous dual frequency induction hardening has been developed. Process specific aspects such as the nonlinear magnetic material behavior and phase transformation kinetics of quenched and tempered AISI 4140 are considered. Induction surface hardening experiments have been conducted for validation purposes. The measuring methodologies used to extract input data such as the magnetic material behavior, the current, and the effective heating time are presented. Metallurgical characterization and hardness profiles are compared with the results obtained from simulations. The temperature history, hardness depth profile, and hardness distribution are in good agreement.

금형재료용 주철의 다이오드 레이저 표면경화처리에 관한 연구(III) - 경화부의 미세조직 특성 -

금형의 공정 과정에서 금형과 성형품 사이에서 발생하는 마찰로 인해 발생하는 금형의 마모가 공차로 작용하여 성형 품질을 저하시킬 수 있다. 따라서 금형의 내마모성을 향상시키기 위해 질화나 침탄처리, 화염 및 고주파 표면처리 등의 방법들이 적용되어 왔다. 하지만 형상의 제한이나 제품의 변형 등과 같은 문제점을 수반하고 있기 때문에, 이러한 문제점을 해결할 수 있는 표면처리 방법으로써 레이저표면처리 기술이 검토되고 있다. 따라서 본 연구에서는 고출력 다이오드 레이저를 이용, 금형재료용 주철의 표면처리를 시도하였다. 앞서 제1보와 제2보의 논문에서는 금형의 재료 및 형상의 차이에 따른 열처리 특성을 비교하였다면, 본 논문에서는 열처리 후 생성되는 경화부, 경계부위 및 모재의 조직적 차이를 분석하기 위해 광학 현미경 및 전자 현미경을 이용하여 미세조직을 관찰하고, EDS를 통해 조직의 상태를 파악하였다. 미세조직 관찰 결과, 경화부는 침상의 마르텐사이트 조직이 형성되어 있었다.

Simulation of multiaxial fatigue strength of steel component treated by surface induction hardening and comparison with experimental results

This paper proposes a method to assess the high-cycle multiaxial fatigue strength of components treated by surface induction hardening (SIH). Surface quenching following surface induction heating is simulated, taking into account the following features of the process: (i) electromagnetic and thermal fields, (ii) phase transformation, and (iii) the residual stress field resulting from the entire process. The fatigue strength of the specimens was simulated using Crossland and Dang-Van criteria; the field of the residual stresses and the fatigue characteristics of both the untreated material and the treated layer (martensite) are considered. Fatigue tests on smooth specimens were carried out to compare simulated results with experimental data. These tests yield information regarding the influence of the thermal treatment on material behaviour and strength, including microstructure evolution and mechanical characteristics, especially in fatigue. For this purpose, residual stresses were analyzed by X-ray diffraction before and after the fatigue tests. Fatigue crack initiation areas (at the specimen’s surface or below) are predicted depending on the depth of the hardened material layer. The simulation of the fatigue strength at 2 × 10 6 cycles is in good agreement with experimental results.

Induction Hardening of Spur Gearwheels Made from 42CrMo4 Hardening and Tempering Steel by Employing Spray Cooling

As a rule, induction surface hardening is carried out industrially by employing polymer solutions since these ensure a more homogeneous quench than immersion cooling in water. Besides reproducing the quenching process, the intention here is to minimise the hardening defects and the distortions arising from the heat treatment. Polymer solutions also have a few disadvantages which include, among others, poor environmental compatibility and handleability. Quenching by means of spray cooling provides an effective alternative. The purpose of the current investigation is to substitute the polymer solution by a water-air spray in induction hardening equipment for surface hardening spur gearwheels made of 42CrMo4 hardening and tempering steel. The suitability of spray cooling was assessed by means of hardness measurements, residual stress conditions, distortion measurements and by metallographic examinations. Based on the analyses currently carried out, it was possible to show that the two-phase spray cooling represents an alternative quenching method which produces comparable component properties.

Surface Induction Hardening Models Based on a Novel Computational Method of Artificial Change of Material Parameters

The paper deals with a novel method suitable for modelling of surface induction hardening and other tasks, where the problem of discretisation of very thin eddy-currents surface layer is present. The method is based on artificial change of material parameters in the computer model. A methodology is suggested and limitations of its application are thoroughly tested. Odds and merits over other possible methods are also discussed.

Spray Quenching in Induction Hardening Applications

This article discusses the subtleties of spray quenching in induction surface hardening (case hardening) and selective hardening applications utilizing scanning and single-shot heating modes. Computer modeling provides the ability to predict how different factors may impact the transient and final heat-treating conditions of the workpiece and what must be accomplished in the design of the induction heating system to improve the effectiveness of the process and guarantee the desired results. One of the major features of computation of induction hardening processes deals with the specifics of modeling, i.e., both heating and quenching stages that can often be overlapped in time. Water or water-based polymers are the most popular types of quenchants used with induction hardening. Results of computer modeling of different induction hardening applications, using Inductoheat’s proprietary finite element codes, are presented here as well.

Residual stresses in surface induction hardening of steels: Comparison between experiment and simulation

Deep induction hardening has been performed on two batches of smooth cylindrical specimens with a hardening depth respectively around 2 mm and 3 mm. The distributions of axial and circumferential residual stresses are analysed for the two specimen batches by X-ray diffraction technique. The radial normal stress field is estimated through the use of the well known Moore and Evans correction. Finally, the experimental residual stresses are compared with those obtained from a multiphysic finite element modelling of the whole induction treatment process, including electromagnetic, thermal, metallurgical and mechanical phenomena. The simulated residual stress field is in good agreement with X-ray analysis especially at depths lower than one-tenth the specimen diameter. At deeper depths, a correction of the experimental X-ray analysis has been done to obtain realistic values.

Overview of residual stresses after induction surface hardening

The paper provides a short survey of the technical literature on induction surface hardening. A general introduction is followed by a description of metallurgical aspects of induction heating, the discussion being focused on the dependence of the heating rate and the temperature required for homogeneous austenitising from the heating time. Particular attention is thus paid to temperature-time dependence in induction surface heating with reference to power density and heating time at a given high current frequency. This is followed by a description of modes of quenching and the related changes of internal stresses and occurrence of residual stresses in modelling and experimental measurement with both heat-treatment steels and case-hardening steels. Finally, the relationship between fatigue strength for finite life and fatigue strength of different steels is shown in relation to different modes of heat treatment.

Influence of various induction-heating and grinding conditions on residual stress profiles

In induction surface hardening, a maximum compressive residual stress in the surface layer is achieved. The transition from compressive to tensile residual stresses should be as gentle as possible, lessening the effect of stress concentration on loaded components. This contributes to the fact that a machined component is less susceptible to overloads in operation. It has been shown that residual stresses are closely linked to hardness variations and microstructure in the transition zone of the hardened layer and base material, i.e., in the narrow range between the hardened and non-hardened microstructure. Additional grinding of induction hardened surface causes the stress state in the surface layer to deteriorate, since grinding always induces tensile stresses. Through the right selection of machining conditions and grinding wheels, taking into account its properties, the engineer will contribute to lessening tensile residual stresses and will avoid causing the favourable residual stress state to deteriorate after induction surface hardening.

Influence of induction surface heating and quenching on residual stress profiles, followed by grinding

In the case of induction surface hardening, a maximum compressive residual stress in the surface layer is achieved. The transition from compressive into tensile residual stresses should be as gentle as possible, lessening the effect of stress concentration in the loaded components. This contributes to the fact that a machined component is less susceptible to overloads in operation. It has been shown that residual stresses are closely linked to hardness variation and microstructure in the transition zone of the hardened layer and base material, i.e. in the narrow range between the hardened and non-hardened microstructure. Additional grinding of the hardened induction surface deteriorates the stress state in the surface layer, since grinding has always induced tensile stresses. With the correct selection of machining conditions and grinding wheel, the engineer will be able to lessen the tensile residual stresses and will avoid deteriorating the favourable residual stress state after induction surface hardening.

Input and output control of steel intended for induction surface hardening

Automation of production necessitates an ever increasing demand for a greater uniformity of the properties of heat treated steel products. One of the most important properties is hardenability which should be a uniform and constant property of steel regardless of the conditions of treatment in the ironworks. In the experimental part, the starting point was the analysis of steel hardenability according to Jominy. On this basis the conditions for induction hardening were prescribed for a worm gear tooth. The successfulness of the induction hardening was verified by the measurement of microhardness along the centerline of the tooth profile. The experiments on heat treatable carbon steel C35 of different history have shown that by the existing heat treatment conditions it is not possible to achieve a microhardness profile required by the user.

Residual stress and fatigue strength after transformation hardening and various strain hardening

Induction surface hardening of surfaces creates very desirable compressive residual stresses. A major difficulty in induction surface hardening is, however, to ensure a very slight/slow variation in hardness and the existence of compressive residual stresses in transition areas to the hardness of the core material. Additional grinding of induction surface hardened surface deteriorates the stress state in the surface layer, since grinding has always induced tensile residual stresses. Among the strain hardening processes only to heavy shot peening and heavy shot peening followed by light peening processing can be recommended since they provide reproductible results of the residual-stress variation, i.e., fatigue strength. Surface hardening by cold deformation thus turned out very suitable since it provides the desired hardness and residual-stress profiles of the thin surface layer.

Prediction of fatigue limit of induction surface hardened 1.05Cr–0.23Mo steel alloy using extreme value statistics

Fatigue characteristics of the surface hardened steel are different from that of normal steel, so the prediction of the fatigue limit of surface hardened steel is very complicated. In this paper, specimens are tested using rotary bending, and the surface of 1.05Cr–0.23Mo steel alloy is hardened by induction surface hardening. Variation of the distribution of microvickers hardness and residual stress is discussed, and the difference of S-N diagram between surface hardened steel and unhardened steel is examined. The maximum defect size of surface hardened specimen is calculated by the extreme value statistics to predict conservative fatigue limit. Actual shape of defect in the specimen is three dimensional, so a conversion method from 2D to 3D defect size based on examination volume and inclusion size is used to predict statistical maximum defect size. The predicted results can be defined as a lower fatigue limit which may be useful to predict conservative fatigue limit of surface hardened specimen.

Influence of Grinding Conditions on Residual Stress Profiles after Induction Surface Hardening

Induction surface hardening creates very desirable residual stresses in the hardened surface layer. Residual stresses are always of a compressive nature and are usually present to the depth of the induction-hardened layer. By the appropriate selection of grinding wheel and grinding conditions and taking into account the physical and mechanical properties of the workpiece material very favourable compressive residual stresses in the hardened surface layer can be retained. How is it possible to assure a desirable surface and surface layer quality after induction hardening and fine grinding Finding an answer to this question requires a very good knowledge of the process of grinding on the micro-level as well as knowledge of mechanical and heat effects acting on the layer of the workpiece including the type and condition of the grinding wheel. An allinclusive consideration of the numerous influences of the kind and condition of the tool on the changes on the surface and in the surface layer of the workpiece in the given machining conditions is described by the term “surface integrity”.

Overview of residual stress after quenching. Part II: factors affecting quench residual stresses

The paper deals with both the internal stresses occurring during steel quenching in various heat-treatment processes for steels and the occurrence of residual stresses. Residual stresses are caused by plastification of a material on a micro-level, which results in the distortion of machine parts and residual stresses. The paper treats residual stresses in induction surface hardening and case-hardening of the machine parts and gears subjected to dynamic loads during their operation. Special attention is paid to the influences of surface heat treatment, final grinding to size, and residual-stress profiles. Results obtained by numerous researchers investigating influences of different quenching conditions, taking into account different quenching media and different temperatures of quenching media, standing still or in motion, are presented. A comparison of the relevant fatigue strengths obtained after various thermo-chemical processes of hardening combined with polishing or honing, and strain hardening is given. The paper gives a review of investigations conducted in the field of quenching and distortion in terms of quenching agents, quenching issues, and results of quenching such as hardness, residual stresses, volume changes, and distortion.

Computer simulation of induction surface hardening axially symmetric workpieces

To achieve a higher quality process planning and a more flexible execution of induction hardening computer simulation of electromagnetic and thermodynamic processes as well as microstructural transformation is recommended. This paper describes and suggests an own simulation model of the induction hardening process and explains a special simulation program that has been developed for the surface induction hardening of axially symmetric steel workpieces. The simulation program performance has been tested by experiments in some cases of high frequency induction hardening cylindrical specimens made of DIN 42CrMo4 steel grade. The measured values of surface hardness and hardening depths have been compared with the simulated values and a good correspondence between simulated and measured values has been obtained.