Paper ICEM, Seite 1
THE EFFECT OF MACHINING ON THE SURFACE INTEGRITY
A. Javidi, U. Rieger, W. Eichlseder
Chair of Mechanical Engineering
Montanuniversitaet Leoben
8700 Leoben, Austria
1.
Abstract
Steel components often have to be machined after heat treatment in order to obtain the correct shape as well as
the required surface finish. Surface quality impacts many part characteristics such as fatigue strength,
cleanability, assembly tolerances, coefficient of friction, wear rate, corrosion resistance and aesthetics. Hard
turning allows manufacturers to simplify their processes and still achieve the desired surface finish quality. There
are various parameters such as cutting speed, feed rate, and tool nose radius that are known to have a large
impact on surface quality. Therefore it is important to gain better understanding how the finishing process affects
the functional behavior of the machined parts to make it more effective. The goal of this work is to identify a
relationship between surface integrity, turning process parameters and fatigue behavior of components.
2.
Introduction
The fatigue of a machine part depends strongly on its surface layer condition. The experience shows that fatigue
failure begins in most cases at the surface. This is due to the fact that surface layers bear the greatest load and
that they are exposed to environmental effects. The factors acting upon the surface layers that cause crack
initiation are: stress concentration, oxidation, and burning out of alloy elements (at high operational temperatures).
Crack initiation and propagation, in most cases, can be attributed to surface integrity [1]. A part surface has two
important aspects that must be defined and controlled. The first concerns the geometric irregularities of the
surface, and the second concerns the metallurigical alterations of the surface and the surface layer. This second
aspect has been termed surface integrity. Field and Kahles [2] describe surface integrity as the relationship
between surface geometric values and the physical properties such as residual stress, hardness and structure of
the surface layers. Surface integrity influences the quality of the machined surface and subsurface, which both
become extremely significant when manufacturing structural components that have to withstand high static and
dynamic stresses. Tönshoff et al. [3] and several other authors [4–12] have reported how hard turning influences
the surface integrity of the machined part. How turning influences the generation of the residual stress level can
be explained by considering the state of stress produced when the cutting tool slides across the work piece Fig.
2.1 and Fig. 2.2.
Speed direction
Cutting tool
Work piece
1
2
Fig. 2.1: Generation of residual stress by turning
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Mechanical
A
Thermal
B
Yield stress
Cooling
0
Dilatation
0
Temperature
Heat
A
B
Fig. 2.2: The residual stress mechanism
The mechanism of mechanically generated residual stress during cutting (A) can be explained by a plastic
deformation in the surface layer (1) and elastic deformation in the underlying surface layer (2). To achieve force
equilibrium and geometric compatibility after the cutting processes, the elastic dilatation places the surface layer
in residual compressive stress (B). The thermal residual stress mechanism is due to the heat of the cutting
process, which expands the surface layer and produces compressive stress (A). The work piece is then cooled
(B) and contractions in the surface layer (1) produce tension residual stress. The thermal effect decreases further
inside the work piece, thus the main consequence of tension stress is on the surface. Consequently, the
temperature of the cutting edge is very important for reducing tensile residual stress [13]. There are various
parameters such as cutting speed, feed rate, and tool nose radius that are known to have a large impact on
surface quality. Therefore it is important to gain better understanding how the finishing process affects the
functional behavior of the machined parts to make it more effective. The goal of this work is to identify a
relationship between surface integrity, turning process parameters and fatigue behavior of components.
3.
Experimental procedure and results
3.1. Material and mechanical testing
As the test material 0,34%C steel is used for the cutting test and the fatigue life test because steel is very
commonly used as the mechanical parts with balanced strength/weight ratio and the cost. Steel bars of the type
34CrNiMo6 (quenched and tempered) used in this investigation had been heat treated to obtain a tensile strength
of 1100 MPa. Fig. 3.1 shows the microstructure of this material.
Fig. 3.1: Microstructure of 34CrNiMo6
The chemical composition and mechanical properties of this steel are given in Tab. 3.1 and Tab. 3.2, respectively.
Ladle chemical analysis [%]
C Mn Si
P
S
Cr Ni Cu Mo Al
0.36 0.64 0.24 0.008 0.007 1.52 1.44 0.25 0.15 0.024
Tab. 3.1: Chemical composition of 34CrNiMo6
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Mechanical properties
2
Re (N/mm )
1085
2
Rm (N/mm )
1100
A5 (%)
17.0
Tab. 3.2: Mechanical characteristics of 34CrNiMo6
The fatigue life test specimens were machined by turning. The cutting conditions employed for these samples are
shown in Tab. 3.3.
Cutting conditions
Feed rate (mm/rev.)
0.05, 0.1, 0.2, 0.3, 0.4
Nose radius (mm)
0.2, 0.4, 0.8
Depth of cut (mm)
0.5
Cutting speed (m/min)
80
Tab. 3.3: Cutting conditions
Nose radius of the tool and feed rate were varied in this investigation. Then, the surface roughness and residual
stress of the specimens were measured before the fatigue life test. Fig. 3.2 shows the configuration of the fatigue
test specimen for the rotating bending fatigue test.
Fig. 3.2: Specimen configuration
3.2. Surface roughness measurements
After the turning operation, the part surface finish was measured using laser scanning microscopy (LEXT,
OLS3000). 3D topographic maps of the machined surfaces were determined using confocal scanning laser
microscope (Fig. 3.3).
Fig. 3.3: 3D topographic map of machined surface
In this investigation, the part surface finish was assessed by means of the surface roughness parameter Rz. A
schematic description of this parameter for an arbitrary machined surface is shown in Fig. 3.4
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Fig. 3.4: A schematic description of surface roughness parameter Rz
Note that Rz (the maximum height of roughness) describes the sum of the maximum peak height Rp and the
maximum valley depth Rv of contour curve at the reference length. The experimental investigation was performed
using three different nose radii (rε) with a feed rate (f) of 0.2 mm/rev.
Fig. 3.5: Effect of nose radius on Rz, feed rate = 0.2 mm/rev.
Fig. 3.5 has been constructed to illustrate the effect of the nose radius on the surface roughness. The results
present that the value of Rz parameter obtained for the samples increases with decreasing the nose radius (rε).
3.3. Residual stress measurements
After the turning operation, the residual stresses were measured using the blind hole drilling method. The basic
hole drilling procedure involves drilling a small hole into the surface of a component, at the center of a special
strain gauge rosette and measuring the relieved strains. The residual stresses originally present at the hole
location are calculated from these strain values. In the hole drilling procedure, a special three-element residual
stress strain gauge rosette (EA-06-062RE-120 of MM series) is bonded to the surface of the specimen. The strain
gauges are then connected to a suitable strain indicator. A hole drilling rig, which is shown in Fig. 3.6, was used
for this purpose.
Fig. 3.6: Hole drilling rig
It uses a small milling tool (about 1.6 mm diameter) mounted on an air turbine reaching a rotational speed of
about 300000 r/min. The drilling device is equipped with an optical microscope to accurately center the hole in
the middle of the rosette. Depth increments of 127 µm were used in the near-surface layer. For each incremental
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step, the strains were acquired until a constant reading was obtained. The holes were drilled for a total depth of
about 1.27 mm The machining of round samples was carried out with different feed rates (f) and nose radii (rε).
Fig. 3.7 shows the distribution of the residual stress of the machined surface measured by the blind hole drilling
method. Measurements were taken along the circumference and axial directions. Residual stress along the axial
direction was expected to affect the rotating bending fatigue life of the specimens.
Fig. 3.7: Residual stress distribution in machined surface
In this study, the integral method is examined as a procedure for determining non-uniform residual stress fields
using strain relaxation data from the hole drilling method. Fig. 3.8 and Fig. 3.9 show the average value of stresses
over a depth range from 0 to 0.127 mm for specimens with different process parameters. All occurring stresses
are compressive.
Fig. 3.8: Axial residual stress σ3 versus feed rate
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Fig. 3.9: Circumferential residual stress σ1 versus feed rate
It can be seen that an increase of feed rate causes an increase of compressive stresses in both directions. On the
other hand, an increase of the nose radius of the chisel causes a decrease of the pressure residual stresses.
4.
Influences on fatigue life
The rotating bending fatigue test that was performed on the test specimens and the cutting conditions that were
employed are shown in Tab. 4.1.
Cutting conditions
Feed rate (mm/rev.)
Nose radius (mm)
Depth of cut (mm)
Cutting speed (m/min)
0.2
0.2, 0.8
0.5
80
Tab. 4.1: Cutting condition employed for fatigue test
Rotation speed was 3800 min-1. At least x specimens were used to verify the fatigue life for one cutting condition.
When the number of rotation exceeded 107, the test was stopped, which means the test specimen will not break
under that condition (test-piece run out). Fig. 4.1 shows the relationship between cutting condition and fatigue life.
Fig. 4.1: Fatigue life of turned specimens with different nose radius
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Although the surface roughness of specimens with rε = 0.2 is higher than turned specimens with
rε = 0.8, the results show that the specimens with a nose radius of rε = 0.2, have a higher fatigue life than
specimens with 0.8 mm nose radius. That means a higher compressive residual stress causes a higher fatigue
life and the effect of residual stress on fatigue life is more than the effect of surface roughness.
5.
Conclusion
This work presents an experimental study on the relationship between surface integrity, turning process
parameters and fatigue behavior of 34CrNiMo6.The following conclusions can be drawn:
•
Surface roughness at the same feed rate becomes higher when a small nose radius is used.
•
The residual stress induced by the turning process tends to become more compressive as the feed rate
increases. On the other hand, an increase of the nose radius of the chisel causes a decrease of the
pressure residual stresses. Thus it is evident that the feed rate and nose radius are the key parameters
that control residual stress in turning.
•
It can be seen that an increase of compressive residual stress causes an increase of fatigue life.
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