Mechanical surface enhancement (MSE) techniques have been used to modify the surface reliability properties of many materials by generating ultrafine or even nanometer-sized grains in the surface and subsurface region. These fine grained materials created by mechanical surface enhancement techniques usually have higher hardness and frequently exhibit enhanced mechanical properties. Turn-assisted deep cold rolling (TADCR) process is used to improve the surface integrity of AISI 4140 steel which is commonly used in automobile industry.

1. Proceedings of the 2nd
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SURFACE LAYER ALTERATIONS IN AISI 4140 STEEL FROM
TURN-ASSISTED DEEP COLD ROLLING PROCESS
P. R. Prabhu*1
, S. M. Kulkarni2
, S. S. Sharma3
, Jagannath K4
1
Associate Professor, Department of Mechanical & Manufacturing Engineering, MIT Manipal, India
2
Professor, Department of Mechanical Engineering, NITK Surathkal, India
3, 4
Professor, Department of Mechanical & Manufacturing Engineering, MIT Manipal, India
ABSTRACT
Mechanical surface enhancement (MSE) techniques have been used to modify the surface reliability properties
of many materials by generating ultrafine or even nanometer-sized grains in the surface and subsurface region. These fine
grained materials created by mechanical surface enhancement techniques usually have higher hardness and frequently
exhibit enhanced mechanical properties. Turn-assisted deep cold rolling (TADCR) process is used to improve the surface
integrity of AISI 4140 steel which is commonly used in automobile industry. Turn-assisted deep cold rolling is
particularly attractive since it is possible to generate, near the surface, deep residual compressive stresses and work
hardened layers while retaining a relatively smooth surface finish. Microstructure alteration to a depth of around 300µm
was obtained from turn-assisted deep cold rolling process, which reflects an increase in residual compressive stress from
as-turned material. Microhardness measurements indicate that the hardness in the small grained layer created by turn-
assisted deep cold rolling is increased by about 36% related to the bulk value. Current results show that turn-assisted
deep cold rolling could be an effective processing method to modify the surface integrity of AISI 4140 steel.
Keywords: Surface Integrity, Microstructure, Deep Cold Rolling, AISI 4140 Steel, Microhardness.
1. INTRODUCTION
It seems that it is the outer layer, lesser in volume relative to the core, which regulates major functional
properties such as: friction, grindability, corrosiveness, fatigue life, load capacity. Improper physical and stereometrical
properties of the outer layer cause failure damage in approximately 85% of the modern machine units [1]. Latest research
results indicate that the life and the reliability of machine components or parts are affected greatly by the technological
manufacturing and varieties of surface enhancement technologies applied, and also by the sequence and conditions of
their application. Deep cold rolling process is an attractive mechanical surface enhancement technique which improves
the surface characteristics by plastic deformation of the surface layer. The enhancement of surface characteristics mainly
serves in terms of improved fatigue behaviour of work-pieces under dynamic load. Besides producing a good surface
finish, it has additional advantages such as increased hardness and corrosion resistance. In addition, this process
transforms tensile residual stresses, present in the surface zone after turning, into compressive residual stresses [2-4].
Residual stresses are probably the most important aspect in assessing integrity because of their direct influence on
performance in service, compressive residual stresses generally improve component performance and life and inhibit
crack nucleation and propagation. These advantages of deep cold rolling and, further, the efficiency, the simple
construction of tooling, the economy, and the possibilities of using typical machine tools in the process and parts of
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2. Proceedings of the 2nd
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various types made of various materials, make the deep cold rolling process more attractive in comparison with other
mechanical surface enhancement techniques.
The literature review shows that earlier investigations on deep cold rolling process are dealing primarily with
microstructure, residual stress and fatigue life of specific materials like aluminium and titanium alloys. Many researchers
have studied experimentally this process with regard to the effect of ball diameter, rolling force, feed and lubricant [5-7].
In these studies, unique and specialized deep cold rolling set-ups are used and the analysis of resulting surface roughness
and surface hardness are less focused upon.
A. Tolga Bozdana et al. [7] developed a new mechanical surface enhancement technique utilizing ultrasonic
vibrations. They discussed surface roughness, surface micro hardness and compressive residual stresses obtained after
treatments of Ti-6Al-4V specimens. P. Juijerm et al. [8] performed the experiment to study the effects of deep rolling on
the fatigue characteristics of AA5083 in the temperature range of 20-3000
C. Residual stresses and work hardening effects
near the surface of the deep rolled condition before and after fatigue tests were investigated by X-ray diffraction methods.
P. Juijerm and I. Altenberger [9] studied the effect of high temperature deep rolling on cyclic deformation behaviour of
solution heat treated Al-Mg-Si-Cu alloy. Near surface properties like compressive residual stresses, work hardening and
hardness were presented in this paper. N. Tsuji et al. [10] investigated the effect of deep rolling on fatigue strength and
wear resistance of plasma carburized Ti-6Al-4V specimens. They reported that the fatigue properties and wear resistance
of Ti-6Al-4V alloy modified by a combination of low temperature plasma carburizing and deep rolling were significantly
improved in comparison with those of the unmodified Ti-6Al-4V alloy. C. M. Gill et al. [11] measured the shakedown of
the compressive residual stresses produced by deep cold rolling caused by low cycle fatigue of the titanium alloys Ti-
6Al-4V and IMI679 at room temperature and elevated temperature. G. H. Magzoobi [12] studied the effects of deep
rolling and shot peening on fretting fatigue resistance of Aluminium 7075-T6. The results showed that fretting fatigue
reduced the normal fatigue life.
The aim of the present study is to investigate the effect of deep cold rolling process on the surface integrity
changes (roughness, hardness, microstructure and residual compressive stress) of AISI 4140 steel. A further aim of this
study is to establish relationships among deep cold rolling conditions and surface integrity properties of this AISI 4140
steel.
2. EXPERIMENTAL WORK
2.1 WORK MATERIAL
The material used in the present investigation was AISI 4140 steel in the form of round bars with 12mm
diameter as shown in Fig. 1 (ASTM standard E466). The chemical composition of AISI 4140 steel in mass% is as
follows: 0.40C, 0.27Si, 0.66Mn, 0.055P, 0.046S, 1.20Cr, 0.25Mo, 0.16Ni. This steel is especially recommended for the
manufacture of transmission shaft, gear shaft, crank shaft and also for a wide variety of automotive type applications
[13]. The hardness of as-received material is measured to be 225HV in average.
Fig. 1: Workpiece geometry (mm)
2.2 TURN-ASSISTED DEEP COLD ROLLING PROCESS
Turn-assisted deep cold rolling experiments were conducted on a conventional lathe. The specially designed and
fabricated deep cold rolling tool and experimental setup is shown in Fig. 2 and Fig. 3 respectively. The generated forces
were collected by a KISTLER 4-component tool dynamometer. The forces acting only in Y direction were taken into
consideration as these forces are causing the plastic deformation. The process parameters for deep cold rolling process
are shown in Table 1.

3. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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1 – Ball, 2 – Hardened pin, 3 – Collet, 4 – Locking nut, 5 – Shank , 6 – Bearing
Fig. 2: Deep cold rolling tool
Fig. 3: Experimental set-up of TADCR process
Table 1: Process parameters for turn-assisted deep cold rolling process
Parameter Turn-assisted deep cold rolling
Ball material Tungsten carbide
Ball diameter (mm) 6 8 10
Rolling force (N) 250 500 750
Initial roughness of the workpiece (µm) 4.84 6.15 7.46
Lubricant oil Brake oil
Feed (mm/min) 36
Number of tool passes 1 2 3
2.3 MATERIAL CHARACTERIZATION
Measurements of the materials’ roughness, microhardness and microstructure in the surface region were
conducted before and after the processing. Microhardness measurements of the AISI 4140 specimens were made by
using a Vickers indentor on a MATZUAWA Micro Vickers hardness tester with 4.905N applied load. Microstructure
analysis was conducted by using an optical microscope. Surface roughness measurements were made by using a
Surtronic Taylor Hobson Talysurf roughness tester and residual stress measurements were made by using a Rigaku X-ray
diffractometer. At least five readings of surface roughness and surface micro-hardness were taken of each specimen in
order to hinder inaccurate results.

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3. RESULTS AND DISCUSSION
3.1 SURFACE ROUGHNESS
Surface roughness measurements were made before and after mechanical surface treatment and are shown in
Table 2. Turn-assisted deep cold rolling result in significant alteration of the surface topography, led to a marked
decrease (by more than 95%) in the measured surface roughness. Specifically, compared with a surface roughness of
4.84µm in the turned sample, the surface roughness of the turn-assisted deep cold rolled sample was 0.242µm. Such
smoother surfaces associated with turn-assisted deep cold rolling can lead to significant improvements in resistance to
fatigue crack initiation and hence contribute to the beneficial effect that this surface treatment can have in prolonging
fatigue lifetimes.
Table 2: Surface roughness of as-turned and turn-assisted deep cold rolled samples
Sample Average surface roughness (µm)
As turned 4.84
DCR at 250N force 0.69
DCR at 500N force 0.486
DCR at 750N force 0.242
3.2 MICROHARDNESS
Hardness profiles, taken perpendicular to the surface of the as-turned and turn-assisted deep cold rolled
microstructures, revealed the existence of a work hardened layer in the surface treated samples. Table 3 shows the micro-
hardness values measured on as-turned and turn-assisted deep cold rolled surfaces. The subsurface microhardness
obtained at different depths of the sample is plotted in Figure 4. The nature of plot obtained for both 250 N and 750 N
force are similar with higher hardness at the surface and progressively decreases due to the differential amount of cold
work experienced by the material. The average microhardness of the as turned specimen on the surface is about 225 HV.
Highest hardness of about 306 HV is recorded on surface for TADCR with a force of 750 N and the hardness is found to
decrease with depth from the surface and eventually settle at hardness of turned sample at a depth of about 300 µm. From
the same figure it could be observed that, surface microhardness settles at depth of 175 µm and 100 µm for TADCR with
500 N and 250 N force respectively. Based on the well-known Hall-Petch relationship between yield stress and grain size
as well as the close interrelations among hardness, yield stress and residual stresses, high hardness values often indicate
fine grain size and large residual stresses. It is reasonable to state that the variations in microhardness were likely due to
the different residual stresses being generated during processing.
Table 3: Surface micro-hardness of as-turned and turn-assisted deep cold rolled samples
Sample Average surface micro-hardness
(HV)
As turned 225
DCR at 250N force 241
DCR at 500N force 270
DCR at 750N force 305
0 50 100 150 200 250 300 350 400
220
230
240
250
260
270
280
290
300
310
Microhardness(HV)
Depth from surface (µm )
Turned
TADCR 250N
TADCR 750N
Fig. 4: Depth profiles of Vickers hardness for untreated and deep cold rolled samples

5. Proceedings of the 2nd
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17 – 19, July 2014, Mysore, Karnataka, India
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3.3 MICROSTRUCTURE
Microstructure analysis under as-turned and turned assisted with deep cold rolled surfaces was carried out after
polishing and etching (97% water + 3% nitric acid) using optical microscopy. The initial microstructure prior to deep
cold rolling is shown in Fig. 5(a). A representative microstructure beneath the turn-assisted with deep cold rolled surface
is presented in Fig. 5(b), where grain deformations along the rolling direction can be noted. As the depth from the surface
increases, the amount of ultrafine grain decreases. Since the strain induced by deep cold rolling decreases with distance
from the surface to the bulk material, it is expected that the amount of ultrafine grains decreases with increasing depth.
As under the turn-assisted deep cold rolled surface the material is heavily strained, a darker area than the base material
could be seen for approximately 300µm from the top surface due to selective etching. This was in correlation with the
results from the microhardness measurements.
(a) (b)
Fig. 5: Microstructures of (a) Turned and (b) deep cold rolled sample at 750N force
3.4 RESIDUAL COMPRESSIVE STRESS MEASUREMENT BY X-RAY DIFFRACTION (XRD) METHOD
To evaluate the magnitude of the residual stress states and the extent of work hardening induced in the near
surface layers by the mechanical surface treatments. X-ray diffraction measurements were performed on the as-turned
and turn-assisted deep cold rolled specimens. It is observed that turn-assisted deep cold rolling process introduced
significant levels of compressive residual stresses at the specimen surface and in the near surface regions. It is apparent
that after deep cold rolling, maximum compressive residual stresses as high as 569MPa was measured immediately
below the surface. Table 4 shows the results of residual compressive stress of turn-assisted deep cold rolled and as-turned
samples; the residual compressive stress of turn-assisted deep cold rolled samples was much larger than that of the turned
sample. It is reported that the peak broadening is contributed by decreasing grain size and increasing dislocation density,
which is consistent with the microstructure observations.
Table 4: Residual compressive stress results of as-turned and turn-assisted deep cold rolled samples
Sample Residual stress (MPa)
As turned +93.83
TADCR at 250N force -292.93
TADCR at 750N force -568.74
4. CONCLUSION
The AISI 4140 steel sample was turned and subsequently deep cold rolled. The effects of deep cold rolling on
the surface microstructure, surface roughness, micro-hardness and compressive residual stress of AISI 4140 steel were
examined and the following conclusions are obtained. Experimental results showed that the microstructure can be
significantly improved by increasing deep cold rolling force. Grain refinement in the surface region was achieved
through deep cold rolling process. Micro-hardness measurements indicated that the hardness was increased up to 36%
relative to the bulk value. Turn-assisted deep cold rolling result in significant alteration of the surface topography, led to
a decrease by more than 95% in the measured surface roughness. These physical effects lead to improvement of fatigue
strength of components as well as increase in resistance to corrosion and foreign objects. Turn-assisted deep cold rolling
can be done with any conventional or CNC machine tool without requiring expensive and complicated equipment.

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