89
CHAPTER - 5
EN Series Steels
Surface finish and surface hardness of the components play vital
role in quality of products/components, in general and failure
resistance, in particular. One of the finishing process involving surface
plastic deformation that introduce compressive residual stresses and
thereby improve fatigue resistance is “Burnishing”. Even though the
burnishing process is widely employed, its process parameters were
not systematically studied till date and not fully established for various
important structural materials. The burnishing process parameters
include force, speed, feed, and number of tool passes. In the present
study, the data obtained from systematically conducted burnishing
experiments are correlated with theoretical design using Taguchi
method in case of EN series steels (EN 8, EN 24 and EN 31). The
surface characterization employed includes optical microscopy, micro
hardness and magnitude of residual stress. The study revealed a oneto-one correlation between burnishing depth, increase in average
micro hardness and magnitude of compressive residual stresses and a
peak in all these three at intermittent extent of burnishing (either after
first or second pass) in all the three alloy steels.
One of the characterization of materials that was study in the
present thesis pertained to alloy steels. Alloy steels are defined as a
steels alloyed with variety of elements in total amounts ranging from
90
1% to 50% by weight to improve their mechanical properties. These
are classified as low alloy and high alloy steels. The steels with alloy
contains lower than 4-5% are considered as low alloy steels while
those higher than 8% alloying elements are called high alloy steels.
The commonly employed elements in these steels include Mn (most
common), Ni, Cr, Mo, V, Si and Boron, less commonly used alloying
elements include Al, Co, Cu, Ce, Nb, Ti, W, Sn and Zr. These steels
find wide range of applications such as turbine blades in jet engines,
space crafts and components for nuclear reactors and also find
applications in electrical motors and transformers. Some of the
commonly used alloy steels and their equivalent grades are given in
Table 5.1. The standard chemical composition of EN series steels are
given in table 5.2.
Table 5.1: Alloy designations of select Engineering Materials
Equivalent Grades
Internal
Standard
BS
DIN
IS
EN
SAE/AISI
EN18
530A40
37Cr4
40Cr1
EN18
5140
EN24
817M40
34CrNiMo6
40NiCr4Mo3
EN24
4340
EN19C
709M40
-
40Cr4Mo3
EN19C
4140, 4142
EN19
709M40
42Cr4Mo2
40Cr4Mo3
EN19
4140, 4142
EN18D
530A40
37Cr4
40Cr1
EN18D
5140
EN18C
530A40
37Cr4
40Cr1
EN18C
5140
EN353
815M17
-
15NiCr1Mo12
EN353
-
EN18A
530A40
37Cr4
40Cr1
EN18A
5140
EN354
820M17
-
15NIVCr1Mo15
EN354
4320
27C15
-
28Mn6
27C15
-
1527
20MnCr5
-
20MnCr5
20MnCr1
-
-
150M28
-
20Mn2
EN14A
1524
-
16MnCr5
17Mn1Cr95
-
5120
20Mn2
16MnCr5
91
15Cr3
523A14
15Cr3
15Cr65
EN206
5015
-
-
-
-
-
EN18B
530A40
37Cr4
40Cr1
EN18B
5140
SCM420
708M20
-
-
-
-
SAE8620
805M20
-
20NiCrMo2
EN362
SAE8620
FILESTEEL
Table 5.2: Chemical composition of EN series steels
C
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 - 0.3
EN 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 - 0.4
EN 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 - 0.4
EN 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 - 0.7
EN 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 - 3.75
EN 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 - 0.25
EN 41B
0.35 - 0.45
0.6 max
0.10 - .45
0.04
0.04
1.5 - 1.8
0.40 max
0.10 - 0.25
EN 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 - 0.15
EN 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 Details
In order to establish the clear picture of burnishing process, a
series of experiments were conducted on metals which find wide range
of industrial applications, such as EN 8, EN 24 and EN 31 alloy steels.
In these experiments, the work pieces were burnished after turning on
lathe, keeping the roller burnishing tool fixed in the lathe tool
dynamometer. The dynamometer is employed to measure three force
92
components, along x, y and z directions (force in z direction is taken
as burnishing force).
5.1.1. Materials
The work piece materials are EN 8, EN 24 and EN 31 (alloy
steels) and the nominal composition of the experimental materials is
given 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
Material
Composition, in Wt. %
C
Si
Mn
Cr
Ni
S
P
EN 8
0.41
0.204
0.70
-
-
0.02
0.026
EN 24
0.37
0.265
0.64
1.1
0.225
0.023
0.025
EN 31
1.01
0.30
0.78
0.76
-
0.024
0.028
5.2. Results and Discussion
5.2.1. Surface roughness
The values of surface finish, a direct measurement of surface
roughness before and after burnishing as a function of burnishing
speed 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 and
200N respectively. The feed for all materials is taken as 0.032 mm/rev
From 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 in
surface finish are determined and the same are given in Table 5.6. The
93
variation in the extent of improvement in the surface finish for EN
series steels obtained in the present investigation (for that matter, for
any other material) depends upon microstructural features and the
levels of hardness and/or strength (in the present case microhardness
values).
Table 5.4: Comparison of surface finish values before and after
burnishing for a 30 mm diameter work piece of alloy steels as a
function of burnishing speed.
Burnishing
Material
speed
(m/min)
EN 8
EN 24
EN 31
Surface
finish before
burnishing
Ra (µm)
Surface finish after
% increase in
burnishing Ra (µm)
surface finish
First
Second
Third
First
Second
Third
pass
pass
pass
pass
pass
pass
51
1.32
0.10
0.11
0.17
92.42
91.66
87.121
34
1.62
0.43
0.38
0.23
91.98
76.54
85.80
22
1.39
0.33
0.34
0.19
76.26
75.54
86.34
14
1.31
1.04
0.92
0.35
20.61
29.77
73.28
9
1.32
0.24
0.19
0.22
81.81
85.60
83.33
51
2.00
0.25
0.27
0.56
87.50
86.50
72.00
34
3.88
0.36
0.15
0.26
90.72
96.13
93.30
22
3.92
0.18
0.17
0.27
95.41
95.66
93.11
14
3.48
0.48
0.62
0.90
86.20
82.18
74.14
9
3.71
0.53
0.51
0.92
85.72
86.25
75.20
51
0.99
0.62
0.38
0.92
37.37
61.61
07.07
34
0.81
0.11
0.13
0.18
86.45
84.00
77.77
22
0.98
0.28
0.20
0.12
71.43
79.60
87.75
14
1.18
0.23
0.19
0.21
80.51
83.90
82.20
9
0.77
0.20
0.22
0.70
74.02
71.43
09.09
94
Table 5.5 Comparison of surface finish values before and after
burnishing for a 30 mm diameter work piece of alloy steels as a
function of burnishing feed.
Burnishing
Material
feed
mm/rev
Surface
Surface finish after
% increase in
finish before
burnishing Ra (µm)
surface finish
burnishing
Ra (µm)
22
34
51
22
34
51
m/min
m/min
m/min
m/min
m/min
m/min
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
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
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
EN 8
EN 24
EN 31
95
% increase in surface finish
100
90
80
st
1 pass
nd
2 pass
rd
3 pass
70
60
50
40
30
20
10
(a) EN 8
10
20
30
40
50
Speed, m/min
% increase in surface finish
100
st
1 pass
nd
2 pass
rd
3 pass
90
80
70
(b) EN 24
60
10
20
30
40
50
Speed, m/min
% increase in surface finish
100
st
1 pass
nd
2 pass
rd
3 pass
90
80
70
60
50
40
30
20
10
0
(c) EN 31
10
20
30
40
50
Speed, m/min
Fig. 5.1: Variation of burnishing speed with % increase in surface finish
for different passes in (a) EN 8 (b) EN 24 and (c) EN 31 alloy steels.
96
% increase in surface finish
100
22 m/min
34 m/min
51 m/min
90
80
70
60
50
40
30
20
10
0.02
(a) EN 8
0.04
0.06
0.08
0.10
0.12
Feed, mm/rev
% increase in surface finish
100
90
80
22 m/min
34 m/min
51 m/min
70
60
50
(b) EN 24
40
0.02
0.04
0.06
0.08
0.10
0.12
Feed, mm/rev
100
% increase in surface finish
90
80
70
60
50
40
30
20
10
0
0.02
(c) EN 31
0.04
0.06
0.08
22 m/min
34 m/min
51 m/min
0.10
0.12
Feed, mm/rev
Fig. 5.2: Variation of burnishing feed with % increase in surface finish
at different speeds in (a) EN 8 (b) EN 24 and (c) EN 31 alloy steels.
97
Table 5.6 Optimal values of burnishing parameters for the alloy
steels, EN 8, EN 24 and EN 31.
Material
Speed
No of
Force
Feed
Ra
(m/min)
passes
(N)
(mm/rev)
(µm)
EN 8
51
1
210
0.032
0.10
EN 24
34
2
170
0.095
0.15
EN 31
34
1
200
0.032
0.11
5.2.2. Microstructure
Figure 5.3 to 5.5 shows the typical set of
optical micrographs
obtained 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 second
pass, Fig. 5.3d for third pass) conditions. The optical micrographs
(corresponding to surfaces from periphery to inner cross section of the
cylindrical specimens) show similar structure with varied burnished
depths for different burnishing conditions in all the three alloy sheets.
These figures clearly show a distinct variation in the thickness of
burnishing affected zone with each of the burnished pass.
The
variation in depth of these zones is measured from micrographs and
the same are given in Fig. 5.6 and Table 5.7. These data clearly reveal
that highest burnishing depth occurs at 1 st pass in EN 8 and EN 31
while the same occurs at 2nd pass in EN 24 alloy steel. It should be
noted here that the highest depth of burnishing presumably provides
maximum effectiveness in surface modification. The actual values of
burnishing layer thickness are obtained experimentally. The variation
of burnishing layer thickness which is different for different EN series
98
steels is a function of many microstructural and surface condition
dependent properties. The principal reasons for such variation
observed in the present study was not investigated in the present
thesis as this requires detailed microstructural analysis involving
transmission electron microscopy.
(a)
(b)
(c)
(d)
Fig. 5.3: Optical micrographs of EN 8 showing the depth of burnishing
in (a) Unburnished (b) Burnished – 1st pass (c) Burnished – 2nd pass
(d) Burnished – 3rd pass conditions
99
(a)
(b)
(c)
(d)
Fig. 5.4: Optical micrographs of EN 24 showing the depth of
burnishing in (a) Unburnished (b) Burnished – 1st pass (c) Burnished –
2nd pass (d) Burnished – 3rd pass conditions
100
(a)
(b)
(c)
(d)
Fig. 5.5: Optical micrographs of EN 31 showing the depth of
burnishing in (a) Unburnished (b) Burnished – 1st pass (c) Burnished –
2nd pass
(d) burnished – 3rd pass conditions
101
Burnishing layer thickness, m
800
EN 8
EN 24
EN 31
700
600
500
400
300
200
B
B1
B2
B3
No of Passes
Fig. 5.6: Correlation of burnishing layer
thickness with burnishing parameters
Table 5.7: Variation of burnishing
burnishing zone for three alloy steels.
layer thickness
in
the
Burnishing Process
BB
B1
B2
B3
Material
Characteristic
EN 8
Burnishing layer thickness
260.0 475.0
425.0
350.0
EN 24
Burnishing layer thickness
250.0 350.0
450.0
430.0
EN 31
Burnishing layer thickness
400.0 650.0
700.0
675.0
[BB – Before burnishing, B1 –
B3 – Burnished-3rd pass]
Burnished-1st
pass, B2 –
Burnished-2nd
pass and
5.2.3. Micro hardness
The specimens polished to obtain microstructure were further used
to determine the variation in micro hardness as a function of distance
from the surface. The micro hardness values are found to be almost
similar with no systematic variation with the burnishing distance.
Hence,
an
average
value
of
micro
hardness
is
taken
as
a
representative value for each of the experimental condition such as
102
unburnished, burnished-1st pass, burnished-2nd pass and burnished3rd pass. These data are summarized and given in Table 5.8 and are
shown in Fig. 5.7. It is interesting to note that maximum burnished
depth (as obtained from optical micrographs) also results in highest
values of average micro hardness. The micro hardness variation
depends on nature and magnitude of residua stresses that arise due
to different extents of burnishing. It should be noted here that in all
the three EN series steels highest micro hardness were obtained either
at 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 effects
of flaking like microstructural degradation.
Table 5.8: Variation of average micro hardness values in the
burnishing zone for three alloy steels.
Burnishing Process
Material
Characteristic
BB
B1
B2
B3
EN 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 and
B3 – Burnished-3rd pass]
103
400
350
B1
Micro hardness
300
250
B2
B3
B
B1
B2 B3
B1 B2
B
B3
B
200
150
100
50
0
EN 8
EN 24
EN 31
Fig. 5.7: Correlation of surface micro-hardness
with burnishing parameters
5.2.4. Residual stress
The 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 a
function of number of passes for the three alloy steels. The data in
Fig. 5.8 show that the residual stresses gradually build up with
burnishing and exhibit a peak in residual stresses at 1 st or 2nd
burnishing pass. Unlike in EN 8 steel the other two alloy steels
namely EN 24 and EN 31 show significant decrease in the magnitude
of compressive residual stresses. The magnitude of compressive
residual stress is also found to be strongly dependent on nature of
alloy steel. The harder is the alloy steel; the highest is the magnitude
of compressive residual stresses.
104
Table 5.9: Compressive residual stresses for EN series steels
Material
EN 8
EN 24
EN 31
Principal
Principal
Direction
Max
Burnishing
Stress
Stress
of
shear
condition
(max)
(min)
Principal
stress
(MPa)
(MPa)
Stress *
(MPa)
BB
-171
-331
14.4
80
286.9
B1
-223
-368
6.6
72.4
323
B2
-203
-369
4.6
83
323.4
B3
-205
-358
2.9
76.6
314.5
BB
-208
-285
5.7
38.5
258.6
B1
-272
-667
11.4
197.4
582.7
B2
-293
-598
2.2
152.5
519.7
B3
-249
-628
10.8
189.4
549.4
BB
-160
-317
8.1
78.8
311.7
B1
-208
-285
5.7
38.5
258.6
B2
-175
-275
6.3
49.8
241.8
B3
-171
-331
14.4
80
286.8
Equivalent
stress
(MPa)
[BB – Before burnishing, B1 – Burnished-1st pass, B2 – Burnished-2nd pass and
B3 – Burnished-3rd pass];
* Angle in degrees from the axial direction of the cylindrical sample
Burnishing depth too revealed a systematic correlation with the
average 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° and
Bragg angle: 156°.
105
Compressive residual stress, MPa
300
EN 8
EN 24
EN 31
250
200
150
B
B1
B2
B3
No of passes
Fig. 5.8: Variation of magnitude of residual compressive
stresses with burnishing pass in case of the three alloy steels.
5.3. Technological Implication
Surface 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 where
compressive mean stress is in vogue. However, in most engineering
applications the rotary parts grossly experience tensile loading
conditions and compressive residual stresses are desirable and they
effectively enhance fatigue resistance. Hence, burnishing is highly
beneficial for most rotating structural components in improving their
service life. Further studies are required to evaluate the effectiveness
106
of compressive residual stresses that result an industrial burnishing
process by extending the present studies to at least high cycle fatigue
loading. Such studies also need to address the progressive relaxation
in the net compressive residual stresses with the extent of high cycle
fatigue damage as occurs with number of such fatigue cycles. Such
studies have not been attempted till date and should be of significant
technological value in case of present low cost EN series alloy steels.
5.4. Conclusions
1. 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
of
compressive residual stresses, is higher in case of softer alloy
steels EN 8 and EN 24 as compared to the relatively harder EN 31
alloy steel.
3. In all the three alloy steels, higher extent of burnishing resulted in
different extents of micro structural modification (as reflected by
the magnitude of compressive residual stresses) and in general,
showed a maximum at intermediate burnishing pass – First in
case of EN 8, EN 31 and second in case of EN 24 steel.
4. The
present
study
revealed
one-to-one
correlation
between
burnishing depth, increase in micro hardness and magnitude of
compressive residual stresses.