Residual Stresses in Case Hardened Materials

Fatigue behavior of case hardened parts depend to a great extent on the type of residual stresses developed in the components. Topography and metallurgical effects were the two elements which contribute much to surface integrity. Micro hardness of the gas carburized (EN 33 and EN 36) and Induction hardened (AISI 1040 and AISI 6150) specimens obtained during experiments, showed that there was gradual decrease of hardness from surface to sub-surface. Results also showed that more the hardness and case depth, the more was the residual stress. The optimum results gave the maximum compressive residual stress in both the gas carburizing and Induction hardening process irrespective of the mechanisms involved in the process. The X-ray diffraction test showed that the distribution of residual stress was uniform both on the surface and beneath the surface. The magnitude and distribution of residual stress obtained from the experiment agreed with the FEM results found in literatures.


INTRODUCTION
In many applications, like Automobiles, heavy duty machines, et., where the machine elements are subjected fatigue loading, Gas carburized and Induction hardened components are used.Fatigue behaviour of case hardened parts depends to a great extent on the type of residual stress developed in the components.In Gas Carburizing and Induction hardening the heating and sudden cooling causes phase transformations on the surface layer and beneath the surface of the workpiece.Heat treatment temperature, Quenching Temperature, Type of Quenchant, Quenching period, heat treatment period are major variables, which influence the phase transformation [1].
Phase transformation affects the surface layer characteristics/surface integrity.The concept of surface integrity cannot be defined one dimensionally and does not only embrace the surface hardness, surface roughness, case depth or its geometrical shape, but also the characteristics of the surface and the layers directly underneath it.It comprehends the mechanical, physical-chemical, metallurgical and technological properties.Surface integrity is defined as the unimpaired surface conditions, which are developed in hardware by using controlled heat treatment operations [2].Two elements comprise the surface integrity.The first is the topography and the second is the metallurgical alternations produced at or near the surface.It thus includes dimensional accuracy, residual stresses and metallurgical damage of the heat treated component.The state of material before the heat treatment starts.

The processing variable
The energy levels during the case hardening process and The type of quenchant and rate of quenching Of all the properties that describe the surface layer characteristics residual stresses regarded as the most representative one as far as mechanical applications are concerned,.In heat treated components, residual stresses developed are due to phase transformation and non-uniform deformation during heating and cooling cycles [3].In case of phase transformation, if the transformation is martensite to ferrite or pearlite the volume decreases hindered by the bulk material produces tensile residual stresses.If the phase transformation is ferrite to pearlite to martensite the volume increases hindered by the bulk material produces compressive residual stresses.These stresses influence the mechanical properties like fatigue strength depending on their nature, magnitude and distribution across the body.There is basically no material or situation free of this stresses.Hence, the general interest is the recognition and measurement of these residual stresses [4].
With the recent improvement on machines to measure the residual stress through XRD, the interest on the knowledge to control such stresses has increased.This interest has its importance due to the fact that the presence of the residual stress interferes with the fatigue strength of the Materials [5].

RESIDUAL STRESS IN GAS CARBURIZING
The development of residual stresses, final microstructure and mechanical properties in the case and core of the carburized components depends on complex interactions among steels composition, component size and geometry, carburizing and subsequent austenitizing process parameters., heat transfer associated during quenching and time and temperature parameters of tempering.
Component geometry (size and shape) together with heat transfer associated with quenching conditions (i.e., cooling performance of the quenchant, agitation etc.,) affect the final residual stress state developed in casehardened steels as a result of quenching.Carburized microstructure is almost always tempered to transform the unstable and brittle martensite into stable tempered martensite [6,7].Tempering decreases residual stresses and this is promoted by increasing the tempering temperature.
With this in mind an experimental investigation is performed using EN33 (AISI 3310) and EN36 (AISI 8620) steel material to study the surface integrity issue with main focus on Residual stress in Gas Carburizing Process.
The Fig. (1) shows the measurement of residual stress in the gas carburized components.On the helix (a), groove (b), knurled region(c) and cylindrical surface (d) of the pinion material residual stresses are measured using residual stress analyzer by X-ray diffraction technique and the average surface residual stress is taken for the analysis [8].The residual stress beneath the top surface is measured upto a maximum depth of 1mm in the intervals of 0.1mm.
Table 1 gives the details on the materials [9,10] subjected for Gas Carburizing (Residual stress analysis).Table 2 shows the operating parameters and their levels adopted in Gas Carburising process.

RESIDUAL STRESS IN INDUCTION HARDENING
In Induction hardening, the components are heated usually for a few seconds only.The hardening temperature varies from 760 -800ºC.The major influencing variables in Induction Hardening are the Power potential, Scan speed and Quench flow rate.The process variables are having a definite relation with hardness and volume fraction of martensite of the hardened components [11].The attainment of correct combination of surface hardness, hardness penetration depth         and high magnitude of compressive residual stress with permitted level of distortion requires the use of proper and optimized process variables.

Table 1. Materials Used in Gas Carburizing
The surface residual stress and the sub-surface residual stress are of great importance on the fatigue resistance of the materials [12].Number of researchers report that if those stresses are of compressive natures improve the resistance to fatigue whereas if those stresses are of tensile nature depending on their magnitude they contribute to a decline in the fatigue resistance [13].In order to verify the behaviour of the residual stress in the surface and sub-surface of the Induction hardened components, experiments have been conducted.
The Fig. (5) shows the details on the measurement of residual stress in the Induction hardened components.The measurement was taken at three places, namely teeth (a), groove (b) and cylindrical surface (c).Residual stresses are measured using residual stress analyzer by X-ray diffraction technique and the average surface residual stress is taken for the analysis [14].The residual stress beneath the top surface is measured upto a maximum depth of 1mm in the intervals of 0.1mm.
Table 7 gives the details on the materials subjected for Induction Hardening (Residual stress analysis).Table 8 shows the operating variables and their levels adopted in  Induction hardening process.Table 9 shows the Experimental design matrix.Tables 10-12 show the test results.
The experiments have been conducted based on 3 3 full factorial DOE.

RESULTS AND DISCUSSION
Micro hardness of the Gas carburized (EN 33 and EN 36) and Induction hardened (AISI 1040 and AISI 6150) specimens are found by using Vickers microhardness tester and reported in the Tables 5, 6, 12 and 13.The higher hardness resulted from the outer surface is due to the formation of martensite, which is obtained during the diffusion, and phase transformation of surface layers with self-quenching.Micro hardness analysis gives that there is a gradual decrease of hardness from surface to sub-surface.
The surface hardness in HRA, case depth in mm, Residual stress in MPa for different experimental combinations of Gas carburized specimens are shown in Table 4. Study indicates that more the hardness and case  depth more will be the residual stress formed.The residual stresses formed are compressive in nature and so it may improve the fatigue strength of the material [15].
The surface hardness in HRA, Distortion in mm, Case depth below the teeth of the Rack in mm, Case depth back of the Rack in mm and Residual stress in MPa for different experimental combinations of Induction hardened specimens are shown in Tables 10 and 11.The magnitude and the nature of the residual stresses left after heat treatment at different operating conditions have been measured by the X-ray diffraction techniques using the Residual stress analyzer.Residual stress analysis indicates that Induction hardening can give a compressive residual stress of (-) 800MPa for the Low alloyed Medium carbon steels [16].However, Gas carburising can give a compressive residual stress of (-) 400MPa for Low alloyed Low carbon steels [17].It is inferred from the graphs (Figs.(2a-e, 6a-c) that the optimum results gives the maximum compressive residual stress in both the Gas carburizing and Induction hardening process irrespective of the mechanisms involved in the process.Figs.(3,4,7,8) shows the Residual stress beneath the surface of the pinion and Rack materials respectively.The X-ray diffraction test shows that the distribution of residual stress is uniform both on the surface and beneath the surface.The magnitude and distribution of residual stress obtained from the experimental work agrees with the FEM results given by Dong-hui Xu and Zhen-Bhang Kuang (1996) [18].

CONCLUDING REMARKS •
Micro hardness analysis shows that there is a gradual decrease of hardness from surface to sub-surface.

•
Study indicates that more the hardness and case depth more will be the residual stress formed.The residual stresses formed are compressive in nature and it improves the fatigue strength of the material.

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Residual stress analysis indicates that Induction hardening can give a compressive residual stress of (-) 800MPa for the Low alloyed Medium carbon steels.However, Gas carburising can give a compressive residual stress of (-) 400MPa for Low alloyed Low carbon steels.

•
The X-ray diffraction test shows that the distribution of residual stress is uniform both on the surface and beneath the surface.The magnitude and distribution of residual stress obtained from the experimental work agrees with the FEM results given by Donghui Xu and Zhen-Bhang Kuang (1996) [19].
Surface integrity assumes importance for the reasons listed below: Higher stress levels to which the materials are subjected *Address correspondence to this author at the Department of Mechanical Engineering, Pondicherry Engineering College, Pondicherry-605014, India; Tel: 91 413 2655281-287, Ext.252,259; Fax: 91 413 2655101; E-mail: palaniradja72@rediffmail.com Reliability demands are stringent Components with critical sections are becoming more in use The surface integrity of any part produced depends on

Table 6 . Residual Stress and Micro Hardness Values of the Gas Carburized Component for the Selected Set of Trials Material: EN 36 Micro Hardness in VHN Residual Stress in MPa Depth Beneath the Surface in mm
Fig. (4).Depth beneath the surface vs Residual stress (EN 36 -Gas Carburizing Process).

Table 12 . Residual Stress and Micro Hardness Values of the Induction Hardened Component for the Selected Set of Trials Material: AISI 1040 Micro Hardness in VHN Residual Stress in MPa Depth Beneath the Surface in mm Trial 6 Trial 16 Trial 6 Trial 16
(Fig.6) contd…..