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CHAPTER - 5EN Series Steels

Surface finish and surface hardness of the components play vitalrole in quality of products/components, in general and failureresistance, in particular. One of the finishing process involving surfaceplastic deformation that introduce compressive residual stresses andthereby improve fatigue resistance is Burnishing. Even though theburnishing process is widely employed, its process parameters werenot systematically studied till date and not fully established for variousimportant structural materials. The burnishing process parametersinclude force, speed, feed, and number of tool passes. In the presentstudy, the data obtained from systematically conducted burnishingexperiments are correlated with theoretical design using Taguchimethod in case of EN series steels (EN 8, EN 24 and EN 31). Thesurface characterization employed includes optical microscopy, microhardness and magnitude of residual stress. The study revealed a one-to-one correlation between burnishing depth, increase in averagemicro hardness and magnitude of compressive residual stresses and apeak in all these three at intermittent extent of burnishing (either afterfirst or second pass) in all the three alloy steels.

One of the characterization of materials that was study in thepresent thesis pertained to alloy steels. Alloy steels are defined as asteels alloyed with variety of elements in total amounts ranging from

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1% to 50% by weight to improve their mechanical properties. Theseare classified as low alloy and high alloy steels. The steels with alloycontains lower than 4-5% are considered as low alloy steels whilethose higher than 8% alloying elements are called high alloy steels.The commonly employed elements in these steels include Mn (mostcommon), Ni, Cr, Mo, V, Si and Boron, less commonly used alloyingelements include Al, Co, Cu, Ce, Nb, Ti, W, Sn and Zr. These steelsfind wide range of applications such as turbine blades in jet engines,space crafts and components for nuclear reactors and also findapplications in electrical motors and transformers. Some of thecommonly used alloy steels and their equivalent grades are given inTable 5.1. The standard chemical composition of EN series steels aregiven in table 5.2.Table 5.1: Alloy designations of select Engineering Materials

Equivalent GradesInternalStandard BS DIN IS EN SAE/AISIEN18 530A40 37Cr4 40Cr1 EN18 5140EN24 817M40 34CrNiMo6 40NiCr4Mo3 EN24 4340EN19C 709M40 - 40Cr4Mo3 EN19C 4140, 4142EN19 709M40 42Cr4Mo2 40Cr4Mo3 EN19 4140, 4142EN18D 530A40 37Cr4 40Cr1 EN18D 5140EN18C 530A40 37Cr4 40Cr1 EN18C 5140EN353 815M17 - 15NiCr1Mo12 EN353 -EN18A 530A40 37Cr4 40Cr1 EN18A 5140EN354 820M17 - 15NIVCr1Mo15 EN354 432027C15 - 28Mn6 27C15 - 152720MnCr5 - 20MnCr5 20MnCr1 - -20Mn2 150M28 - 20Mn2 EN14A 152416MnCr5 - 16MnCr5 17Mn1Cr95 - 5120

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15Cr3 523A14 15Cr3 15Cr65 EN206 5015FILESTEEL - - - - -EN18B 530A40 37Cr4 40Cr1 EN18B 5140SCM420 708M20 - - - -SAE8620 805M20 - 20NiCrMo2 EN362 SAE8620

Table 5.2: Chemical composition of EN series steelsC Mn Si S P Cr Ni Mo

EN 8 0.35 - 0.45 0.60 -1.0 0.10 0.35 0.05 max 0.05 max - - -EN 8D 0.40 -0.45 0.7 - 0.9 0.05 - .35 0.06 max 0.06 max - - -EN 9 0.50 - 0.60 0.5 - 0.8 0.05 - .35 0.04 max 0.04 max - - -EN 15 0.30 - 0.40 1.3 - 1.7 0.10 - .35 0.04 max 0.04 max - - -EN 16 0.30 - 0.40 1.3 - 1.8 0.10 - .35 0.04 0.04 - - 0.2 - 0.3EN 18 0.35 - 0.45 0.6 - 0.95 0.10 - .35 0.04 0.04 0.85 - 1.15 - -EN 19 0.35 - 0.45 0.5 - 0.8 0.10 - .35 0.04 0.04 0.90 - 1.4 - 0.2 - 0.4EN 24 0.35 - 0.45 0.45 - 0.7 0.10 - .35 0.04 0.04 0.90 - 1.4 1.30 - 1.8 0.2 - 0.4EN 25 0.27 - 0.35 0.5 - 0.7 0.10 - .35 0.04 0.04 0.50 - 0.80 2.3 - 2.8 0.4 - 0.7EN 31 0.90 - 1.2 0.3 - 0.75 0.10 - .35 0.04 0.04 1.0 - 1.6 - -EN 36B 0.12 - 0.18 0.30 - 0.60 0.10 - .35 0.04 0.04 0.60 - 1.1 3.0 - 3.75EN 36C 0.12 - 0.18 0.3 - 0.6 0.10 - .35 0.04 0.04 0.60 - 1.1 3.0 - 3.75 0.10 - 0.25EN 41B 0.35 - 0.45 0.6 max 0.10 - .45 0.04 0.04 1.5 - 1.8 0.40 max 0.10 - 0.25EN 42 0.70 - 0.85 0.55 - 0.75 0.10 - .40 0.04 0.04 - - -EN 45A 0.55 - 0.65 0.7 - 1.0 1.70 - 2.0 0.04 0.04 - - -EN 47 0.45 - 0.55 0.5 - 0.8 0.50 max 0.04 0.04 0.80 - 1.2 - -EN 48A 0.50 - 0.60 0.6 - 0.9 1.35 - 1.65 0.04 0.04 0.55 - 0.85 - -EN 353 0.20 max 0.5 - 1.0 0.35 max 0.04 0.04 0.75 - 1.25 1.0 - 1.5 0.08 - 0.15EN 354 0.20 max 0.5 - 1.0 0.35 max 0.04 0.04 0.75 - 1.25 1.5 - 2.0 0.1 -0 .2

5.1. Experimental DetailsIn order to establish the clear picture of burnishing process, a

series of experiments were conducted on metals which find wide rangeof industrial applications, such as EN 8, EN 24 and EN 31 alloy steels.In these experiments, the work pieces were burnished after turning onlathe, keeping the roller burnishing tool fixed in the lathe tooldynamometer. The dynamometer is employed to measure three force

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components, along x, y and z directions (force in z direction is takenas burnishing force).5.1.1. Materials

The work piece materials are EN 8, EN 24 and EN 31 (alloysteels) and the nominal composition of the experimental materials isgiven in Table 5.3. All the three alloy steels are in quenched(hardened) and tempered condition.

Table 5.3: Chemical composition of the experimental materials

MaterialComposition, in Wt. %

C Si Mn Cr Ni S PEN 8 0.41 0.204 0.70 - - 0.02 0.026EN 24 0.37 0.265 0.64 1.1 0.225 0.023 0.025EN 31 1.01 0.30 0.78 0.76 - 0.024 0.028

5.2. Results and Discussion5.2.1. Surface roughness

The values of surface finish, a direct measurement of surfaceroughness before and after burnishing as a function of burnishingspeed and burnishing feed are given in Table 5.4 and 5.5, respectively.The optimal forces for EN 8, EN 24 and EN 31 are 210N, 170N and200N respectively. The feed for all materials is taken as 0.032 mm/revFrom these data (data in Tables 5.4 and 5.5 and Figs. 5.1 and 5.2)optimal speed and feed values which result in highest increase insurface finish are determined and the same are given in Table 5.6. The

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variation in the extent of improvement in the surface finish for ENseries steels obtained in the present investigation (for that matter, forany other material) depends upon microstructural features and thelevels of hardness and/or strength (in the present case microhardnessvalues).

Table 5.4: Comparison of surface finish values before and afterburnishing for a 30 mm diameter work piece of alloy steels as afunction of burnishing speed.

MaterialBurnishing

speed(m/min)

Surfacefinish beforeburnishingRa (m)

Surface finish afterburnishing Ra (m)

% increase insurface finish

Firstpass

Secondpass

Thirdpass

Firstpass

Secondpass

Thirdpass

EN 8

51 1.32 0.10 0.11 0.17 92.42 91.66 87.12134 1.62 0.43 0.38 0.23 91.98 76.54 85.8022 1.39 0.33 0.34 0.19 76.26 75.54 86.3414 1.31 1.04 0.92 0.35 20.61 29.77 73.289 1.32 0.24 0.19 0.22 81.81 85.60 83.33

EN 24

51 2.00 0.25 0.27 0.56 87.50 86.50 72.0034 3.88 0.36 0.15 0.26 90.72 96.13 93.3022 3.92 0.18 0.17 0.27 95.41 95.66 93.1114 3.48 0.48 0.62 0.90 86.20 82.18 74.149 3.71 0.53 0.51 0.92 85.72 86.25 75.20

EN 31

51 0.99 0.62 0.38 0.92 37.37 61.61 07.0734 0.81 0.11 0.13 0.18 86.45 84.00 77.7722 0.98 0.28 0.20 0.12 71.43 79.60 87.7514 1.18 0.23 0.19 0.21 80.51 83.90 82.209 0.77 0.20 0.22 0.70 74.02 71.43 09.09

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Table 5.5 Comparison of surface finish values before and afterburnishing for a 30 mm diameter work piece of alloy steels as afunction of burnishing feed.

MaterialBurnishing

feedmm/rev

Surfacefinish beforeburnishingRa (m)

Surface finish afterburnishing Ra (m)

% increase insurface finish

22m/min

34m/min

51m/min

22m/min

34m/min

51m/min

EN 8

0.111 1.32 0.75 1.11 0.67 43.18 15.90 49.24

0.095 1.62 0.33 1.08 0.92 79.63 33.33 43.21

0.063 1.31 0.57 0.77 1.09 56.48 41.22 16.80

0.032 1.32 0.19 0.43 0.10 85.60 67.42 92.42

EN 24

0.111 2.00 0.25 0.37 1.70 87.5 81.50 15.00

0.095 3.88 0.54 0.22 0.97 86.08 94.32 75.00

0.063 3.92 0.42 0.32 2.18 89.28 90.45 44.39

0.032 1.8 0.18 0.36 0.25 90.00 80.00 86.11

EN 31

0.111 0.99 0.33 0.19 0.75 66.66 80.80 24.24

0.095 0.81 0.34 0.13 0.44 58.02 83.95 45.68

0.063 0.98 0.72 0.20 0.51 26.53 79.59 47.95

0.032 1.18 0.28 0.11 0.62 76.27 90.67 47.45

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Fig. 5.1: Variation of burnishing speed with % increase in surface finishfor different passes in (a) EN 8 (b) EN 24 and (c) EN 31 alloy steels.

10 20 30 40 5060

70

80

90

100 1st pass 2nd pass 3rd pass

Speed, m/min

% inc

rease in

surfa

ce fin

ish

(b) EN 24

10 20 30 40 500102030405060708090100

1st pass 2nd pass 3rd pass

Speed, m/min

% inc

rease in

surfa

ce fin

ish

(c) EN 31

10 20 30 40 50102030405060708090100

1st pass 2nd pass 3rd pass

Speed, m/min

% inc

rease in

surfa

ce fin

ish(a) EN 8

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Fig. 5.2: Variation of burnishing feed with % increase in surface finishat different speeds in (a) EN 8 (b) EN 24 and (c) EN 31 alloy steels.

0.02 0.04 0.06 0.08 0.10 0.12102030405060708090100

22 m/min 34 m/min 51 m/min

(a) EN 8

Feed, mm/rev

% inc

rease in

surfa

ce fin

ish

0.02 0.04 0.06 0.08 0.10 0.1240

50

60

70

80

90

100

22 m/min 34 m/min 51 m/min

(b) EN 24

Feed, mm/rev

% inc

rease in

surfa

ce fin

ish

0.02 0.04 0.06 0.08 0.10 0.120102030405060708090100

22 m/min 34 m/min 51 m/min(c) EN 31

Feed, mm/rev

% inc

rease in

surfa

ce fin

ish

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Table 5.6 Optimal values of burnishing parameters for the alloysteels, EN 8, EN 24 and EN 31.

Material Speed(m/min)

No ofpasses

Force(N)

Feed(mm/rev)

Ra(m)

EN 8 51 1 210 0.032 0.10EN 24 34 2 170 0.095 0.15EN 31 34 1 200 0.032 0.11

5.2.2. MicrostructureFigure 5.3 to 5.5 shows the typical set of optical micrographsobtained from EN 8, En 24 and EN 31 alloy steels in the unburnished(Fig. 5.3a) and burnished (Fig. 5.3b for first pass, Fig. 5.3c for secondpass, Fig. 5.3d for third pass) conditions. The optical micrographs(corresponding to surfaces from periphery to inner cross section of thecylindrical specimens) show similar structure with varied burnisheddepths for different burnishing conditions in all the three alloy sheets.These figures clearly show a distinct variation in the thickness ofburnishing affected zone with each of the burnished pass. Thevariation in depth of these zones is measured from micrographs andthe same are given in Fig. 5.6 and Table 5.7. These data clearly revealthat highest burnishing depth occurs at 1st pass in EN 8 and EN 31while the same occurs at 2nd pass in EN 24 alloy steel. It should benoted here that the highest depth of burnishing presumably providesmaximum effectiveness in surface modification. The actual values ofburnishing layer thickness are obtained experimentally. The variationof burnishing layer thickness which is different for different EN series

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steels is a function of many microstructural and surface conditiondependent properties. The principal reasons for such variationobserved in the present study was not investigated in the presentthesis as this requires detailed microstructural analysis involvingtransmission electron microscopy.

(a) (b)

(c) (d)Fig. 5.3: Optical micrographs of EN 8 showing the depth of burnishingin (a) Unburnished (b) Burnished 1st pass (c) Burnished 2nd pass(d) Burnished 3rd pass conditions

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(a) (b)

(c) (d)Fig. 5.4: Optical micrographs of EN 24 showing the depth ofburnishing in (a) Unburnished (b) Burnished 1st pass (c) Burnished 2nd pass (d) Burnished 3rd pass conditions

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(a) (b)

(c) (d)

Fig. 5.5: Optical micrographs of EN 31 showing the depth ofburnishing in (a) Unburnished (b) Burnished 1st pass (c) Burnished 2nd pass(d) burnished 3rd pass conditions

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Table 5.7: Variation of burnishing layer thickness in theburnishing zone for three alloy steels.Material Characteristic Burnishing ProcessBB B1 B2 B3EN 8 Burnishing layer thickness 260.0 475.0 425.0 350.0EN 24 Burnishing layer thickness 250.0 350.0 450.0 430.0EN 31 Burnishing layer thickness 400.0 650.0 700.0 675.0

[BB Before burnishing, B1 Burnished-1st pass, B2 Burnished-2nd pass andB3 Burnished-3rd pass]

5.2.3. Micro hardnessThe specimens polished to obtain microstructure were further used

to determine the variation in micro hardness as a function of distancefrom the surface. The micro hardness values are found to be almostsimilar with no systematic variation with the burnishing distance.Hence, an average value of micro hardness is taken as arepresentative value for each of the experimental condition such as

Fig. 5.6: Correlation of burnishing layerthickness with burnishing parameters

B B1 B2 B3200

300

400

500

600

700

800 EN 8 EN 24 EN 31

No of Passes

Burni

shing

layer th

ickne

ss,m

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unburnished, burnished-1st pass, burnished-2nd pass and burnished-3rd pass. These data are summarized and given in Table 5.8 and areshown in Fig. 5.7. It is interesting to note that maximum burnisheddepth (as obtained from optical micrographs) also results in highestvalues of average micro hardness. The micro hardness variationdepends on nature and magnitude of residua stresses that arise dueto different extents of burnishing. It should be noted here that in allthe three EN series steels highest micro hardness were obtained eitherat B1 or B2 (depending upon the extent of burnishing in each stage)and comparatively lower micro hardness values in B and B3, the first(B) for the lack of any surface modification and the later for the effectsof flaking like microstructural degradation.

Table 5.8: Variation of average micro hardness values in theburnishing zone for three alloy steels.Material Characteristic Burnishing ProcessBB B1 B2 B3EN 8 Micro Hardness 251.2 303.5 279.4 294.3

EN 24 Micro Hardness 297.2 312.7 339.6 335.1

EN 31 Micro Hardness 196.1 251.6 254.1 223.9[BB Before burnishing, B1 Burnished-1st pass, B2 Burnished-2nd pass andB3 Burnished-3rd pass]

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5.2.4. Residual stressThe residual stresses that are determined by XRD for EN series

steels are given in Table 5.9 and the data are shown in Fig. 5.8 as afunction of number of passes for the three alloy steels. The data inFig. 5.8 show that the residual stresses gradually build up withburnishing and exhibit a peak in residual stresses at 1st or 2nd

burnishing pass. Unlike in EN 8 steel the other two alloy steelsnamely EN 24 and EN 31 show significant decrease in the magnitudeof compressive residual stresses. The magnitude of compressiveresidual stress is also found to be strongly dependent on nature ofalloy steel. The harder is the alloy steel; the highest is the magnitudeof compressive residual stresses.

Fig. 5.7: Correlation of surface micro-hardnesswith burnishing parameters

0

50

100

150

200

250

300

350

400

B3

B3B3

B2

B2

B2B1

B1

B

BB1

B

EN 31EN 24EN 8

Micro

hardn

ess

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Table 5.9: Compressive residual stresses for EN series steels

Material Burnishingcondition

PrincipalStress(max)(MPa)

PrincipalStress(min)(MPa)

Directionof

PrincipalStress *

Maxshearstress(MPa)

Equivalentstress(MPa)

EN 8

BB -171 -331 14.4 80 286.9B1 -223 -368 6.6 72.4 323B2 -203 -369 4.6 83 323.4B3 -205 -358 2.9 76.6 314.5

EN 24

BB -208 -285 5.7 38.5 258.6B1 -272 -667 11.4 197.4 582.7B2 -293 -598 2.2 152.5 519.7B3 -249 -628 10.8 189.4 549.4

EN 31

BB -160 -317 8.1 78.8 311.7B1 -208 -285 5.7 38.5 258.6B2 -175 -275 6.3 49.8 241.8B3 -171 -331 14.4 80 286.8

[BB Before burnishing, B1 Burnished-1st pass, B2 Burnished-2nd pass andB3 Burnished-3rd pass];

* Angle in degrees from the axial direction of the cylindrical sample

Burnishing depth too revealed a systematic correlation with theaverage hardness of the alloy steel. According to the expected lines,softest alloy steel of the three exhibited the highest burnishing depth.Parameters chosen for XRD analysis are wave length: 2.291 A andBragg angle: 156.

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5.3. Technological ImplicationSurface compressive residual stresses have been found to be

beneficial for tensile mean stress controlled fatigue as well as creep.The same would be grossly detrimental to the conditions wherecompressive mean stress is in vogue. However, in most engineeringapplications the rotary parts grossly experience tensile loadingconditions and compressive residual stresses are desirable and theyeffectively enhance fatigue resistance. Hence, burnishing is highlybeneficial for most rotating structural components in improving theirservice life. Further studies are required to evaluate the effectiveness

Fig. 5.8: Variation of magnitude of residual compressivestresses with burnishing pass in case of the three alloy steels.

B B1 B2 B3150

200

250

300 EN 8 EN 24 EN 31

No of passes

Comp

ressiv

e residu

al str

ess, MP

a

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of compressive residual stresses that result an industrial burnishingprocess by extending the present studies to at least high cycle fatigueloading. Such studies also need to address the progressive relaxationin the net compressive residual stresses with the extent of high cyclefatigue damage as occurs with number of such fatigue cycles. Suchstudies have not been attempted till date and should be of significanttechnological value in case of present low cost EN series alloy steels.

5.4. Conclusions1. Burnishing results in significant surface finish depth of

burnishing and increase in micro hardness and residual stresses.2. The present systematic study reveals that the burnishing depth,

increase in micro hardness or increase in magnitude ofcompressive residual stresses, is higher in case of softer alloysteels EN 8 and EN 24 as compared to the relatively harder EN 31alloy steel.

3. In all the three alloy steels, higher extent of burnishing resulted indifferent extents of micro structural modification (as reflected bythe magnitude of compressive residual stresses) and in general,showed a maximum at intermediate burnishing pass First incase of EN 8, EN 31 and second in case of EN 24 steel.

4. The present study revealed one-to-one correlation betweenburnishing depth, increase in micro hardness and magnitude ofcompressive residual stresses.