15_chapter 6 residual stresse

8/4/2019 15_chapter 6 Residual Stresse

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CHAPTER 6RESIDUAL STRESSES IN CASE HARDENED MATERIALS

6.1 IntroductionIn many applications, like Automobiles, heavy duty machines, et., where the

machine elements are subjected fatigue loading, Gas carburized and Induction hardenedcomponents are used. F atigue behaviour of case hardened parts depends to a great extenton the type of residual stress developed in the components. In Gas Carburizing andInduction hardening the heating and sudden cooling causes phase transform ations on thesurface layer and beneath the surface of the workpiece. Heat treatment temperature,Quenching Temperature, Type of Quenchant, Quenching period, heat treatment periodare major variables, which influence the phase transformation.

Phase transform ation affects the surface layer characteristics/surface integrity.The concept of surface integrity cannot be defined one d imensionally and does not onlyembrace the surface hardness, surface roughness, case depth or its geometrical shape,but also the characteristics of the surface and the layers directly underneath it. Itcomprehends the mechanical, physical-chemical, metallurgical and technologicalproperties. Surface integrity is defined as the unimpaired surface conditions, which aredeveloped in hardware by using controlled heat treatmen t operations. Two elementscomprise the surface integrity. The first is the topography and the second is themetallurgical alternations produced a t or near the surface. It thus includes dimensionalaccuracy, residual stresses and metallurgical damage of the heat treated component.

Surface integrity assumes importance for the reasons listed below:*> Higher stress levels to w hich the materials are subjected* Reliability demands are stringent* Com ponents with critical sections are becoming more in useThe surface integrity of any part produced depends on* The state of material before the heat treatment starts.*3 The processing variable* The energy levels during the case hardening process and

The type of quenchant and rate of quench ing


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Of all the properties that desc ribe the surface layer characteristics residual stressesregarded as the most representative one as far as mechanical applications are concerned,.In heat treated components, residual stresses developed are due to phase transformationand non-uniform deformation during heating and cooling cycles. In case of phasetransformation, if the transformation is martensite to ferrite or pearlite the volumedecreases hindered by the bulk material produces tensile residual stresses. If the phasetransformation is ferrite to pearlite to martensite the volume increases hindered by thebulk material produces compressive residual stresses. These stresses influence themechanical properties like fatigue strength depending on their nature, magnitude anddistribution across the body. There is basically no m aterial or situation free of thisstresses. Hence, the general interest is the recognition and measurement of these residualstresses.

With the recent improvemen t on machines to measure the residual stress throughXRD, the interest on the know ledge to con trol such stresses has increased. This interesthas its importance due to the fact that the presence of the residual stress interferes withthe fatigue strength of the Materials.6.2 Residual stress in Gas Carbur iz ing

The development of residual stresses, final microstructure and mechanicalproperties in the case and core of the carbunzed components depends on complexinteractions among steels composition, component size and geometry, carburizing andsubsequent austenitizing process parameters., heat transfer associated d u h g quenchingand time and tem perature parameters of tempering.

Com ponent geometry (size and shape) together with heat transfer associated withquenching conditions (i.e., cooling performance of the quenchant, agitation etc.,) affectthe find residual stress state developed in casehardened steels as a result of quenching.Carburized microstructure is alm ost always tempered to transform the unstable and brittlemartensite into stable tempered martensite. Tem pering decreases residual stresses andthis is prom oted 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 withmain focus on Residual stress in Gas Carburizing Process.


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The figure 6.1 shows the measurement of residual stress in the gas carburizedcomponents. On the helix (a), groove (b), knurled region(c) and cylindrical surface (d) ofthe pinion material residual stresses are measured using residual stress analyzer by X-raydiffraction technique and the average surface residual stress is taken for the analysis. Theresidual stress beneath the top surface is measured upto a maximum depth of lmrn in theintervals of 0. lmm.

Figure 6.1 Residual stress measurement locations on the Gas carburized componentTable 6.1 gives the details on the materials subjected for Gas Carburizing

(Residual stress analysis). Table 6.2 shows the operating parameters and their levelsadopted in Gas Carburising process. Table 6.3 gives the experimental design matrix andTable 6.4, shows the test results.

Table 6.1 Materials used in Gas CarburizingS.No.

01 I S&P each -0.05%1 C-0.18%,Si-O.lO%,Mn-

Table 6.2 Gas Carburizing-operatingconditions

TypeNickel alloy steel Length = 150mm

02

DesignationEN 33

Chromium alloy steel

Level 3940C

120 minutes

250C120 minutes

NoPreheating

Chemical Composition inPercentageC-0.15%,Si-0.35%,Mn-0.60%,Cr-0.30%, Ni-3.5%,

EN 36

Level 2910C

90 minutes

200C100 minutes

1500C

SizeDiameter= 17.3 mm

0.30%, Cr-0.60%,Ni-3.0%S&P each - 0.058~

S. No.12

45

NotationABCDE

VariablesFurnace temperatureQuenching TimeTemperingTemperatureTempering TimePreheating

Level 1870C

60 minutes

150C80 minutes

NoPreheating


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The experiments have been conducted based on L27 orthogonal array systemproposed in Taguchis' Mixed level series DOE with interactions as given below:

i) Furnace Temperature Vs Quenching Time (AxB)ii) Furnace Ternperature Vs Tempering Ternperature (AxC)

Table 6.3 Experimental design matrix- --


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Table 6.4 Gas carburizing Test resultsMaterials: EN 33 and EN 36


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Furnace Temperature in degree Celsius( 4

Quenching time la minutes@)


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Tempering temperatureIn degree Celsius0

Tempering time in minutes(d)


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Figure 6.2 (a) to (e) Process variables Vs Residual stressTable 6.5 Residual stress and m icro hardness values of the Gas Carburizedcomponent for the selected set of trialsMaterial:EN 33

Depth beneath thesurface in mm


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Table 6.6 Residual stress and micro hardness values of the Gas Carburizedcomponent for the selected set of trialsMaterial: EN 36Depth beneath thesurface in mm

250

;+EN33-Tr ia lS *EN33-Tr ia l23 1100

.E 2-50E5:-

.;:'t-200

-350

Depth bene~~thhe eurface in mm

Figure 6.3 Depth beneath the surface Vs Residual stress(EN 33 - Gas Carburizing Process)


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--I""

Depth beneath the surface innun

Figure 6.4 Depth beneath the surface Vs Residual stress(EN 36 - Gas Carburizing Process)

6.3 Residual stress in Induction Hardening

In Induction hardening, the components are heated usually for a few seconds only.The hardening temperature varies from 760 - 800 "C. The major influencing variables inInduction Hardening are the Power potential, Scan speed and Quench flow rate. Theprocess variables are having a definte relation with hardness and volume fraction ofmartensite of the hardened components. The attainment of correct combination of surfacehardness, hardness penetration depth and high magnitude of compressive residual stresswith permitted level of distortion requires the use of proper and optimized processvariables.


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The surface residual stress and the sub-surface residual stress are of greatimportance on the fatigue resistance of the materials. Number of researchers report thatif those stresses are of compressive natures improve the resistance to fatigue whereas ifthose stresses are of tensile nature depending on their magnitude they contribute to adecline in the fatigue resistance. In order to verify the behaviour of the residual stress inthe surface and sub-surface of the Induction hardened components, experiments havebeen conducted.

The figure (6.5) shows the details on the measurement of residual stress in theInduction hardened components. The measurement was taken at three places, namelyteeth (a), groove (b) and cylindrical surface (c). Residual stresses are measured usingresidual stress analyzer by X-ray diffraction technique and the average surface residualstress is taken for the analysis. The residual stress beneath the top surface is measuredupto a maximum depth of lmm n the intervals of 0.lmm.

Figure 6.5 Residual stress measurement locations on the Induction hardenedcomponent

Table 6.7 gives the details on the materials subjected for Induction Hardening(Residual stress analysis). Table 6.8 shows the operating variables and their levelsadopted in Induction hardening process. Table 6.9 shows the Experimental designmatrix. Table 6.10,6.11 and 6.12 show the test results.


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Table 6.7 Materials used for Induction H arden ing(Residual stress analysis)

Table 6.8 Induction hardening operating conditions

S.No.0102

The experiments have been conducted based on 33 full factorial DOE.

Table 6.9 Experimental design matr ix

TypeUnalloyed carbonsteelSilicon alloy steel

Designation

AISI 6150

Chemical Composition inPercentageC-0.35%,Si-O.lO%,Mn-0.60%S&P each 0.06%C-O.SO%,Si-0.50%,Mn-0.50%,Cr-0.80%,V-0.15%S&P each -0.05(each)

Size

Diameter= 23 mmLength = 200 mm


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Table 6.10 Induction hardening Test results

Material: AISI 1040


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Table 6.11 Induction hardening Test results

Materials: AISI 6150


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2 4

+ ISI 1040 -) AlS16150

Power potentialin kW1 sq.inch(a)

Scan speed in mlminutes@)


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-748Quench flow rate in litres /minutes

Q

Figure 6.6 (a) to (c) Process variables Vs Residual stressTable 6.12 Residual stress and micro hardness values of theInduction hardened component for the selected set of trials

Material: AISI 1040


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Table 6.13 Residual stress and micro hardness values of theInduction hardened component for the selected set of trials

Material: AISI 6150

Figure 6.7 Depth beneath the surfaceVs Residual stress(AISI 1040 steel material- Induction hardening process)


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-900 IDepth beneath the surface in mm

Figure 6.8 Depth beneath the surface Vs Residual stress(AISI 4140 steel material - Induction hardening process)

6.4 Results and DiscussionMicro 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 testerand reported in the Tables 6.5, 6.6,6 .12 and 6.13. The higher hardness resulted from theouter 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 analysisgives that there is a gradual decrease of hardness from surface to sub-surface.

The surface hardness in HRA, case depth in mm, esidual stress in MPa fo rdifferent experimental combinations of Gas carburized specimens are shown in Table 6.4.Study indicates that more the hardness and case depth more will be the residual stressformed. The residual stresses formed are compressive in nature and so it may improvethe fatigue strength of the material.


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The surface hardness in HRA, Distortion in mm, Case depth below the teeth ofthe Rack in mm, Case depth back of the Rack in mm and Residual stress in MPa fordifferent experimental combinations of Induction hardened specimens are shown inTables 6.10 and 6.1 1.

The magnitude and the nature of the residual stresses left after heat treatment atdifferent operating conditions have been measured by the X-ray diffraction techniquesusing the Residual stress analyzer. Residual stress analysis indicates that Inductionhardening can give a compressive residual stress of (-) 8OOMPa for the Low alloyedMedium carbon steels. However, Gas carburising can give a compressive residual stressof (-) 400M Pa for Low alloyed Low carbon steels.

It is inferred from the graphs (Figures 6.2 (a) to (e), and 6.6 (a) to (c)) that theoptimum results gives the maximum compressive residual stress in both the Gascarburizing and Induction hardening process irrespective of the m echanisms involved inthe process. Figure 6.3, 6.4 6.7and 6.8 shows the Residual stress beneath the surface ofthe pinion and Rack materials respectively. Th e X-ray diffraction test show s that thedistribution of residual stress is uniform both on the surface and beneath the surface. Themagnitude and distribution of residual stress obtained from the experimental workagrees with the FEM results given by Dong-hui Xu and Zhen-Bhan g Kuang (1996).

15_chapter 6 residual stresse

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