Deep Hardening of Inner Cylindrical Surface by Periodic Deep Rolling - Burnishing Process

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Severe surface plastic deformation is one of the methods to improve the mechanical behavior of metallic components, because such a treatment generates a hardened near-surface layer with compressive residual stresses. As is well known, deep rolling (DR) stands out against a background of the others mechanical techniques owing to maximum values of hardness, residual stress and hardening depth attained. In this work, an accomplished version of DR is proposed, which may be regarded as a superposition of DR and burnishing (B) processes in order to elevate those characteristics up to larger values. In contrast to consecutive application of DR and B techniques, the new approach allows to alter them multiply within one and only technological process with no increasing the working duration and without the need of any additional equipment. The working principles of the device are described. The dependences “depth-hardness” are presented. Numerical assessment of the residual stress is performed. The micro-photographies demonstrate an increase in the structural homogeneity of the near-surface layer material. The results indicate that the newly developed technique attains its main goals.

본문

Modern Applied Science; Vol. 9, No. 9; 2015

ISSN 1913-1844 E-ISSN 1913-1852
Published by Canadian Center of Science and Education

# Deep Hardening of Inner Cylindrical Surface by Periodic Deep Rolling - Burnishing Process

I. M. Gryadunov 1, S. Yu. Radchenko 1, D. O. Dorokhov 1 & P. G. Morrev 1

1 State University, Education-Science-Production Complex, Oryol, Russia

Correspondence: I. M. Gryadunov, State University-Education-Science-Production complex, Orel, Russia.
E-mail: fry14@yandex.ru

Received: October 22, 2014   Accepted: February 3, 2015   Online Published: August 30, 2015

doi:10.5539/mas.v9n9p251     URL: http://dx.doi.org/10.5539/mas.v9n9p251

_The research is financed by Ministry of Education and Science of Russian Federation._

Abstract

Severe surface plastic deformation is one of the methods to improve the mechanical behavior of metallic
components, because such a treatment generates a hardened near-surface layer with compressive residual stresses.
As is well known, deep rolling ( DR ) stands out against a background of the others mechanical techniques owing
to maximum values of hardness, residual stress and hardening depth attained. In this work, an accomplished
version of DR is proposed, which may be regarded as a superposition of DR and burnishing ( B ) processes in
order to elevate those characteristics up to larger values. In contrast to consecutive application of DR and B
techniques, the new approach allows to alter them multiply within one and only technological process with no
increasing the working duration and without the need of any additional equipment. The working principles of the
device are described. The dependences “depth-hardness” are presented. Numerical assessment of the residual
stress is performed. The micro-photographies demonstrate an increase in the structural homogeneity of the
near-surface layer material. The results indicate that the newly developed technique attains its main goals.

Keywords: surface hardening, deep rolling, burnishing, residual stress, microstructure

1. Introduction

Surface affects the impotant characteristics of metallic components: resistance to plastic deformation (surface
hardness), resistance to corrosion, fatigue fracture, fretting fatigue, and wear, because most cases of material
failure originate from the exterior layers of the workpiece [1]. If surface experiences mechanical contact loads,
the role of those characteristics grows dramatically.

Severe surface plastic deformation is an effective way to improve the surface behavior and the three most
popular techniques are: shot peening (SP) [2-5], burnishing (B) [6-9], and deep rolling (DR) [10-14]. All of them
provide similar effects: grain refinement, hardening and compressive residual stress, but there exist essential
distinctions. The main feature of SP is the applicability to workpieces of complex geometry, but essential
problems occur when dealing with inner surfaces of small size. In such case, B and DR are good alternatives.
Burnishing is used as a finishing procedure and provides with high quality treatment in terms of surface
roughness. Problem arises when dealing with surface of complex geometry. DR stands out against a background
of the others (both mechanical and many non-mechanical) techniques owing to large values of hardness, residual
stress and hardening depth attained. Unfortunately, the problem of complex geometry comes on again: whereas
an axial symmetric workpiece can be wrought by means of a turning lathe, in more intricate cases, robot is
selected as the machine platform [15].

So, SP, B, and DR techniques complement one another and their combinations may give an essential benefit. In
this work, a new technique (DRB) is proposed, which may be regarded as a superposition of DR and B. It means
that DR and B alternate multiply; this feature permits to elevate the values of hardening and hardening depth in
comparison to DR. In contrast to B, which is performed with lower pressures and mainly serves to achieve a high
surface quality, the aim of DRB treatment is to induce deep work hardening surface layers with compressive
residual stresses. It should be pointed out that severe plastic deformation may be effective in powder technology
applications [16]. Therefore, DRB has a good perspective in that field also.

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Before to present the working principles, the experimental data, and the numerical simulation results some
remarks should be done. First of all, the surface characteristics under consideration (all of them were aforesaid in
the beginning of the paper) are affected by both B and DR working via process parameters such as: rolling
(burnishing) speed, pressing force, feed rate, number of passes, roller (ball) material and size, lubricant,
temperature, etc.. But, proper dependencies are too complicated because the local deforming process is too
complicated on its own account due to the fact that it may be accompanied not only by high gradient values of
stress and stain tensors but also by essential transformations of the material microstructure. For example, the
hardness profile beneath the surface of hardened cold working die steel was assessed in [17] and it was noticed
that its value increased with applied force, nevertheless, its influence on the depth of the affected zone was
minimal, moreover, the amount of retained austenite decreased after deep rolling (as a result of martensite
transformation) and contributed, together with strain hardening, to the elevation of hardness. One more
interesting example is surface hardening by cryogenic DR [18], where DR working under low temperature
elevated the hardness profile.

It is known that the effect of both B and DR grows as number of passes _n_ increases, but the typical value is
_n_ =1–5. The reason is that n is limited by the crack generation. Thereby, a natural issue comes on: is it possible to
increase number of passes if to combine B and DR processes? The overall aim of the present paper is to resolve
this issue. The paper examines the use of the periodic deep rolling - burnishing process to improve inner
cylindrical surface characteristics such as surface microhardness and compressive residual stress for cast bronze
using turning lathe machine. The effect of two DRB parameters, namely, number of passes and feed rate on the
in-depth hardness profile was studied. The stress tensor hydrostatic component profile was calculated. The
microstructure photographies of the virgin material and the wrought one were obtained.

2. Material Studied and Experimental Technique

The work material is cast bronze (5% Pb, 5%Zn, 5%Sn, 85%Cu), which is commonly used in bush bearing
manufacturing. It should be noted, the material is too hard for cold working and the only way to fulfill such a
treatment is to create sufficiently large compressive stresses in the course of the process. DRB technique permits
to attain the necessary compression value. The blanks were obtained by means of further lathe treatment. The
inner and outer radii, and the length are 50 mm, 70 mm, and 50 mm respectively. Surface hardness
measurements were conducted using microhardness tester Anton Paar MHT-10 (Anton Paar GmbH ).
Microphotography was done by microscope AXIOSCOP 2 MAT (Carl Zeiss). The specimens for
micro-photo-analyses were prepared by grinding and polishing, followed by etching in a water solution of 5%
FeCl3 and 10% HCl.

The idea of DRB working is shown in Figure 1a, where a perfect case of periodic deformation is depicted. In
practice, deformation is not quite periodic and grooves appear on the surface treated – see Section 4.1. The
workpiece is put into container to provide with compressive stresses in the course of treatment and hence to
prevent the phenomenon of rolling out. The DRB tool operates with three equally spaced steel rollers – one
deforming and two burnishing (Figure 1b). The deforming roller has a chamfered fillet of width 1mm to indent
the surface. The device structure provides for equal pressing forces _F_ on all three rollers. They can rotate about
the device axe _O_ and also about their own axes _O_ 1, _O_ 2, _O_ 3. The rollers are conical to give the opportunity of
regulating the indentation depth for the deforming roller. Figure 1c shows DRB tool (axial profile _OO_ 1 in Figure
1b) in details designed and made to perform the experiments. Bolts 4a and 4b regulate the indentation depth by
means of the deforming roller impress size on the wrought surface. All three rollers can roll along conical
supports 8a and 8b, while support 8b can rotate about support 8a. Lock-nuts 5a and 5b fix bolts 4a and 4b in the
position needed. DRB device is mounted on the turning lathe (Figure 2).

The indentation depth was fixed at value 0.2 mm for all experiments. It is known that only optimized rolling
force (depth) can increase the fatigue strength, too low force (depth) has no profound influence on the fatigue
behavior and too high one may even aggravate it by inducing micro-cracks [19, 20]. Feed rate _r_ was equal 0.5,
0.7, and 1.0 [mm/rev]. The point is that at rate _r_ <0.5 [mm/rev] the surface cutting effect does occur, while at
rate _r_ >1 [mm/rev] the deforming roller trace does not overlap. Number of passes _n_ was altered from _n_ =15 up to
_n_ =30. Microphotographies of the wrought material correspond to the following process parameters: _n_ =15,
_r_ =0,5[mm/rev]. Table 1 shows the arrangements and some results of the 6 experiments that were performed.

3. Residual Stress Simulation

To find the residual stress after the course of surface cold working, many investigators applied to numerical
simulations. Both 2-D and 3-D modeling were exploited. The main disadvantage of 2-D modeling is inability to
calculate the tangential component of the residual stress. At the same time, that component is about one third of

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the axial one if to compare the maximum values [19, 21] and one of the goals of this work is to assess the latter
satisfactorily. The results on ball burnishing modeling [22] indicate that the difference between calculated and
measured residual stress values vary by 40% for 2D simulation and by 7% for 3D one. According to that work,
the plane strain assumption for 2D model prohibits the material from flowing along the out-of-plane tangential
direction, and thus, compared to 3D simulations, the plastic strain decreases clearly. Consequently, the maximum
residual stress introduced is affected. As for DRB simulation, the results [22] should be corrected. Indeed, the
contact zone for the deforming roller (see Figure 1b) in DRB process is of the oval form and plastic flowing in
tangential direction is far less than in axial one. Hence, the error of axial stress 2D calculations must be not as
large as 40%. In addition, DRB working permits a large number of passes. Therefore, 3-D calculation is too
time-expensive. Due to all the above, 2-D simulation was preferred. At the same time, the comparison with 3D
numerical model is necessary in further investigations (at least, for small number of passes) to validate the
results presented in Section 4.3.

Table 1. Experimental design matrix and the results of surface topography

Exp. no Feed rate, mm/rev Number of passes Initial out-ofroundness, µm

Final out-ofroundness, µm

Groove's depth,
µm

1 0,5 15 11 19 21

2 0,5 20 10 13 17

3 0,5 25 3 16 12

4 0,5 30 7 12 19

5 0,7 20 3 32 43

6 1,0 20 6 30 76

Figure 1. a Schematic illustration of DRB process, b A

spatial view of the rollers, and c A detailed view of the DRB

tool

4. Results and Discussion

_4.1 Surface Topography_

Figure 2. Experimental setup: (1) Container, (2)

Workpiece, and (3) DRB tool

Optical images of the treated surface under different feed rates are presented in Figure 3. One can see that

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deformation is not quite periodic as it is shown in Figure 1a, but this feature may be an advantage in some cases
because groove appearance on the treated surface means higher capacity for lubricant retention. In principle, one
can regulate groove’s depth by re-distribution of pressing forces on the rollers (Figure 1b), but this is a subject of
further investigations. Figure 3b shows that number of passes _n_ can attain _n_ ≈30, whereupon the workpiece
begins to be damaged. The maximum number of passes is limited by damaging and hence is affected by the
experimental conditions, but in this work, such dependence is not investigated. The experimental data on
out-of-roundness δ and Δ (before and after working respectively) and groove’s depth _g_ are represented in Table 1.
It is interesting to point out that Δ does not depend on δ in the experiment arrangements, but Δ depends on feed
rate _r_ apparently. As for groove’s depth, one can see that at constant feed rate, _g_ diminishes versus _n_ down to 12
μm, whereupon increases up to 19 μm, when the material begins to be damaged. Besides, _g_ depends appreciably
on _r_ .

The main result reported in this Subsection is the maximum number of passes attained in the experiments
exceeds all known data on B and DR techniques.

Figure 3. Effect of number of passes _n_ and feed rate _r_ on surface view:

a 15≤ _n_ ≤25, _r_ =0.5 [mm/rev], b _n_ ≥30, _r_ =0.5 [mm/rev], c _n_ =20, _r_ =0.7 [mm/rev], and d _n_ =20, _r_ =1.0 [mm/rev]

_4.2 Surface Microhardness_

Microhardness was measured in the crosswise section (i.e., normal to _O_ axis in Figure 1b). The in-depth profiles
for treated surfaces are shown in Figure 4, 5.

Figure 4. Effect of number of passes _n_ on in-depth hardness profile at constant feed rate _r_ =0.5 [mm/rev]

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Figure 5. Effect of feed rate _r_ on in-depth hardness profile at constant number of passes _n=_ 20

Figure 4 depicts the dependence of microhardness profile on number of passes _n_ and Figure 5 – on feed rate _r_ .
The experimental points in both figures (number of points was altered from 12 up to 32 versus the experiment
number in Table 1) are omitted in order to avoid excessively complicated patterns. The work-hardened layer
depth ranges from 2 mm to 5 mm; beyond this layer hardness remains almost unaffected. Such the hardening
depth values exceed corresponding data for all others surface hardening techniques. It is seen in Figure 4 that the
hardening gradient is not affected by _n_, but the surface hardness increasing and the hardening depth are
approximately proportional to _n_ . Figure 5 demonstrates the opportunity to govern the hardness gradient by
means of feed rate _r_ altering.

_4.3 Residual Stress Simulation_

To solve the appropriate contact problem, the approach [23] was used. The irregular finite element mesh consists
of 1922 three-node linear triangles. For in case of 2-D numerical simulation a solution of high accuracy is not
expected, the isotropic material model is accepted as it was done in [22] where the ball burnishing process was
simulated. The material constants are: shear module – 4.4×1010 [n/m2] and Poisson coefficient – 0.3. The
stress-strain curve is depicted in Fig. 6. In this work, the measure of compressive residual stress is a hydrostatic
component of stress tensor. Calculations were performed for r=0.5 [mm/rev] and n=12; the result is presented in
Fig. 7 and agrees with the results on DR [19, 21].

Figure 6. Stress-strain curve of the material Figure 7. Hydrostatic component of the residual

stress tensor

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_4.4 Microstructure_

In cold working hardening processes, two phenomena contribute to the overall effect: strain hardening (due to
generation of barriers preventing plastic deformation carrier motion) and global microstructure transformations
(i.e. phase transition of second order or mixing of phases of which the material consists), while the latter exceeds
the former [24]. The material under investigation possesses a dendrite structure. Hence, one can expect the
dendrite grain crumbling at high extent of deformation and thus, to reveal the predominant mechanism of
hardening.

The microstructure images was obtained in the crosswise section (i.e., normal to _O_ axis in Fig. 1b) for _n_ =15 and
_r_ =0.5 [mm/rev] and are presented in Figure 8.

Figure 8. Microstructure in cross section of workpiece: a, b Virgin material and c, d Wrought material in

near-surface layer

Here, Figures 8 a, b show the structure of the virgin material. One can see dendrite grains (light) into a solid
solution (dark). As was pointed out in Section 2, materials of such a structure are too hard for cold working and
the only way to fulfill the working is to create sufficiently large compressive stresses in the course of the process.
Under working, dendrite begins to crumble interfusing with a solid solution. Figures 8 c, d show the region of
maximum hardening at depth 0.2 mm, where the dendrite grains are splitted into structural elements of size
100-200 nm. This is in a good agreement with one of the results [25], where aluminum alloy 2014 under ball
burnishing process was investigated: the repetition of working passes on the same workpiece leads to an increase
in the structural homogeneity which results in an increase in the surface hardness.

5. Conclusions

- The newly developed deep rolling - burnishing tool can be used for deep hardening of inner cylindrical surfaces
in a time comparable with a time of usual deep rolling at the same number of passes.

- Both out-of-roundness and groove's depth depend appreciably on feed rate.

- Maximum values of hardening and hardened layer depth exceed such values for deep rolling process.

- Hardness gradient is approximately constant and is affected by feed rate; surface hardening value and hardened
layer depth are approximately proportional to number of passes.

- Compressive residual stress is generated in near-surface layer during the course of deep rolling - burnishing
treatment and 2-D numerical modeling allows assessing this effect in terms of hydrostatic component of stress
tensor.

- The microstructure photographies show that deformation causes gradual destruction of dendrite grains, the
mixing of dendrite component and solid solution, and microstructure details of small size (100-200 nm)
formation; an increase in the structural homogeneity results in an increase in the surface hardness.

Acknowledgements

The authors would like to thank the financial support of the research from Ministry of Education and Science of
Russian Federation under Governmental Order “Scientific-Technological Ground Creation for Hardening
Technique by Complex Local Deforming of Assembly Parts in Motor Transport” Number 602.

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