An investigation into roller burnishing

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Abstract

Burnishing, a plastic deformation process, is becoming more popular as a finishing process: thus, how to select the burnishing parameters to reduce the surface roughness and to increase the surface microhardness is especially crucial. This paper reports the results of an experimental program to study the influence of different burnishing conditions on both surface microhardness and roughness: namely, burnishing speed, force, feed, and number of passes. Also, it reports the relationship between residual stress and both burnishing speed and force. The residual stress distribution in the surface region that is orthogonally burnished is determined using a deflection etching technique. Mathematical models are presented for predicting the surface microhardness and roughness of St-37 caused by roller burnishing under lubricated conditions. Variance analysis is conducted to determine the prominent parameters and the adequacy of the models. From an initial roughness of about Ra 4.5 μm, the specimen could be finished to a roughness of 0.5 μm. It is shown that the spindle speed, burnishing force, burnishing feed and number of passes have the most significant effect on both surface microhardness and surface roughness and there are many interactions between these parameters. The maximum residual stress changes from tensile to compressive with an increase in burnishing force from 5 to 25 kgf. With a further increase in burnishing force from 25 to 45 kgf, the maximum residual stress increases in compression.

Introduction

The function performance of a machined component such as fatigue strength, load bearing capacity, friction, etc. depends to a large extent on the surface as topography, hardness, nature of stress and strain induced on the surface region. Nowadays, about 50% of the energy supplied is lost in the friction of elements in relative motion [1]. Roughness values less than 0.1 μm are required for good aesthetic appearance, easy mould release, good corrosion resistance, and high fatigue strength.

During recent years, however, considerable attention has been paid to the post-machining metal finishing operations such as burnishing which improves the surface characteristics by plastic deformation of the surface layers [2]. Burnishing is essential in a cold-forming process, in which the metal near a machined surface is displaced from protrusions to fill the depressions. Beside producing a good surface finish, the burnishing process has additional advantages over other machining processes, such as securing increased hardness, corrosion resistance and fatigue life as a result of producing compressive residual stress. Residual stresses are probably the most important aspect in assessing integrity because of their direct influence on performance in service. Thus, control of the burnishing process (burnishing conditions) in such a way as to produce compressive residual stresses in the surface region could lead to considerable improvement in component life.

A comprehensive classification of burnishing tools and their application has been given by Shneider [3]. A literature survey shows that work on the burnishing process has been conducted by many researchers and the process also improves the properties of the parts, e.g. higher wear resistance [4], [5] increased hardness [6], [7], [8], surface quality [2], [9], [10] and increased maximum residual stress in compression [11]. The parameters affecting the surface finish are: burnishing force, feed rate, ball material, number of passes, workpiece material, and lubrication [2].

This paper examines the use of the roller burnishing process to give a good surface integrity for steel-37. To explore the optimum combination of burnishing parameters in an efficient and quantitative manner, the experiments were designed based on the response surface methodology (RSM) with central composite rotatable design. The effect of four burnishing parameters on the surface finish and surface microhardness were investigated, namely: burnishing speed, force, feed rate, and number of passes. The relationship between the most important burnishing parameters (speed and force) and residual stress were studied.

Section snippets

Experimental work

Specimens were turned and burnished on a center lathe model DIZ 450×1600 (WMW, Germany). The workpiece material was Steel-37 (0.20% C; 0.30% Si; 0.80% Mg; 0.05% P; 0.05% S) of hardness 220 Hv. It was selected because of its importance in industry and its susceptibility to degradation when burnished, through surface and subsurface damage. The work material was received in the form of tube and then machined into two different groups. In the first group, the form of the workpieces were short tubes

Experimental design

In this investigation, experiments were designed on the basis of the experiments design technique that has been proposed by Box and Hunter [12]. A detailed description of this method is presented elsewhere [13]. According to a central composed-second-order rotatable design with four independent variables, the total number, N, of experiments is determined to be 31. The cutting conditions and their coded levels are summarized in Table 1. The experimental design was not used during the study of

Presentation of results

Table 2 shows the results of the first group of the thirty-one experiments carried out in this research. Using the results presented in Table 2 the response surface for the surface roughness Ra, and the surface microhardness Hv, as functions of the fourth parameters used in this work are deduced as the following final equations:Hv=329.3+6.3X1+18.8X2−6.6X3+20.2X4−8.0X211−6.3X222...+7.6X244−10.8X1X3+11.6X1X4−8.8X2X4Ra=1.34+0.22X1+0.43X2+0.524X3+0.186X4+0.194X211+0.418X222...+0.23X244+0.294X1X2

Conclusion

The results of the burnishing process are quite complicated, and many factors affect its results. How to find the optimal burnishing conditions and how to control the results are very important for industry. According to this research, the following conclusions may be drawn:

  • 1.

    A good correlation between the experimental and predicted results derived from the model was exhibited. Thus, using the proposed procedure, the optimum roller burnishing conditions should be obtained to control the surface

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