Cutting speed influence on surface integrity of hard turned bainite steel

Cutting speed influence on surface integrity of hard turned bainite steel

Michael Jacobson*, Patrik Dahlman, Fredrik GunnbergProduction Engineering, Chalmers University of Technology, Gothenburg, Sweden

Received 2 October 2001; received in revised form 4 July 2002; accepted 4 July 2002

Abstract

The purpose of this paper is to examine how the surface integrity of bainite steels is affected by hard turning. Tests were conducted on a

lathe using different cutting speeds. The surface was evaluated in terms of residual stress, surface roughness and microstructure analysis. Tests

showed that hard turning provides good surface integrity. Effect of cutting speed on residual stress was investigated in order to find how the

residual stress could be altered. It was found that there is a cutting speed where maximum compressive residual stress is generated. The surface

roughness of the hard turned test samples followed the same pattern; an certain cutting speed gives the lowest surface roughness values. The

surface of the test samples showed no or little alteration to the microstructure. Based on the results it can be concluded that hard turning of

bainite steel is a suitable manufacturing process.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Hard turning; Surface integrity; Residual stress; Bainite steel

1. Introduction

The conventional method for producing precision pro-

ducts has been soft machining and heat-treatment, finishing

with rough and fine grinding, and finally honing the heat-

treated component. In order to increase flexibility of pro-

duction and make it more agile, manufacturers have realized

the potential of replacing the grinding operations with hard

turning. The turning lathe can incorporate more operations,

making it easier to reset, and it has fewer type-dependent

tools than a grinding machine. In short, the hard turning of

components is a more suitable technology for the production

of small lots than grinding.

In order to change from grinding to hard turning, three

levels of substitution can be recognised: the first level

substitutes rough grinding; the second level substitutes fine

grinding; and the last level includes substitution of the

honing operation. The more grinding and honing that can

be substituted, the greater are the benefits of hard turning.

In order to accomplish this, it is necessary to have good

knowledge of the surface integrity created by hard turning.

Field and Kahles [1] have defined surface integrity as the

relationship between the physical properties and the func-

tional behaviour of a surface. The surface integrity is built

up by the geometrical values of the surface such as surface

roughness (for example, Ra andRt), and the physical properties

such as residual stresses, hardness and structure of the surface

layers. Residual stress strongly affects the fatigue life of a

component. Brinksmeier et al. [2] give a good overview of

the subject of residual stresses. In their article, a comprehen-

sive description is given of different machining operations

and how they create different residual stress. The difference

in stresses created by different operations also affects the

performance of a machined component. Mittal and Liu [3,4]

present a more focused study on the effects of hard turning

on residual stress. Their work is divided into two parts. In the

first part, they show how different machining parameters in

hard turning affect residual stress. In the second part, they

describe a method to achieve optimal pre-stress in a com-

ponent to maximize fatigue life in a rolling contact. Their

work is groundbreaking in the sense that it suggests machin-

ing parameters should be optimised towards enhancing

fatigue life of the machined component.

Another interesting publication is Matsumoto et al. [5],

who conclude that hard turning produces compressive resi-

dual stresses that contribute to a long fatigue life. They also

determine that edge geometry is the dominant factor decid-

ing the residual stress profile. Similarly, Abrao and Aspin-

wall [6] report that hard turned components improve fatigue

life compared to ground components. Furthermore Fujimoto

and Ohhata [7] reports fatigue life improvements of 3–7

times. A possibility to enhance product life would add to the

list of previously discussed advantages of hard turning. In

addition, for demanding applications where performance is

Journal of Materials Processing Technology 128 (2002) 318–323

* Corresponding author.

0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 4 7 2 - 7

critical, hard turning could provide increased protection

from rolling contact failure.

However, most of the prior studies on the effect of hard

turning on residual stress have been performed on 52 100, a

through-hardened bearing steel. There seems to be no work

done on the residual stress pattern from the hard turning of

bainite steel. Thus, an interesting research question is how

the bainitic heat-treated bainite will affect the results.

Changing the cutting speed is one of the most common

and easily done changes in the turning process. Therefore it

is of interest to know how this adjustment affects the surface

integrity. In this paper, the change of surface integrity when

adjusting the cutting speed while hard turning bainite steel

will be presented, in terms of residual stress, surface rough-

ness and microstructure.

2. Experimental procedure

Machining, residual stress measurements and the surface

roughness measurements were performed at Chalmers Uni-

versity of Technology, Gothenburg, Sweden. The micro-

structure was evaluated at SKF, Gothenburg, Sweden.

2.1. Machining conditions

Machining was done on a 2-axis lathe. The material of the

test rings was hardened steel B8 according to ISO 683-17.

Prior to the hard turning operation, the parts had been soft

turned and bainitic heat-treated to a hardness of HRC 58.

The chemical composition of bainite steel can be found in

Table 1. A 3-jaw chuck was used for the chucking. The outer

diameter of the rings was 310 mm, the inner diameter was

170 mm, and the width of the rings was 65 mm. The

operation performed was a facing operation with a depth

of cut of 0.1 mm from the inner diameter to the outer

diameter. The feed rate was kept to be 0.1 mm/rev for

all samples. The cutting speed was adjusted from 50 to

999 m/min for the test samples. All tests were performed

without coolant.

A CBN 100 insert constructed of cubic boron nitride

(CBN) and supplied by SECO was chosen for the test.

The full tool description is TNGN 110308-01015 CBN

100. The tool holder used for the tests had a �68 of rake

angle. Before each cut, the insert was changed to eliminate

the effect of tool wear.

2.2. Residual stress measurement

Residual stresses were measured with the X-ray diffrac-

tion method. Used equipment was a portable stress analyser

from Stresstech, Finland. A chromium tube calibrated on

ferrite was used to measure the {2 1 1} peak of steel. The

X-ray was generated using 35 kV and 7 mA. After several

test to evaluate the exposure time from 5 to 120 s it was

concluded that 15 s was enough to ensure good peak shapes.

The collimator size used was 2 mm, the calculation method

was cross correlation and the elastic constant applied in the

calculation was 204 GPa. In order to obtain the residual

stress beneath the surface, the material was chemically

etched away. Seven different measuring points were taken

along the depth profile. The stresses were evaluated both in

the feed and the speed direction.

2.3. Surface roughness measurement

The measurements were performed with a stylus probe

measurement equipment. The equipment is a Surfascan 3D,

and was delivered by Somicronic, France. The measured

surface size was 2 mm � 2 mm. The maximum height of the

profile (Rt) was measured.

2.4. Microstructure analysis

The samples were carefully sectioned in order to not

induce any transformation of the structure due to heat.

The samples were then mounted in a phenolic compound,

and ground and polished to a mirror-like surface with

different grit and diamond pastes. The next step was washing

and nital etching. Finally, the samples were inspected using a

100� to 1850� microscope.

3. Results

The results are presented in three different sections. First,

the evaluation of the residual stresses is presented. Second,

the surface roughness measurements are shown. Third, the

microstructure is presented. Combined, they provide a com-

plete picture of the surface integrity of the test samples.

3.1. Residual stress

Tables 2 and 3 show all measured values of residual stress.

When analysing the values, the following results were

obtained. On all samples, the compressive stresses increased

below the surface. Maximum compressive stress found

was 10–20 mm below the surface. A concentration of the

samples had a maximum around 10 mm. The maximum

value differed from �297 MPa on the sample with cutting

speed 50 m/min to �775 MPa on the sample with cutting

speed at 230 m/min. At a depth of 100 mm all samples

showed compressive stresses of less than �200 MPa.

Table 1

Chemical composition of steel B8 according to ISO 683-17

C 1.00

Mn 0.80

Si 0.50

Cr 2.0

Mo 0.60

Ni 0.10

Cu 0.15

M. Jacobson et al. / Journal of Materials Processing Technology 128 (2002) 318–323 319

It can also be concluded that the cutting speed effect

the amount of residual stress generated in hard turning. In

Figs. 1 and 2, the cutting speeds from 50 to 230 m/min are

compared. In Figs. 3 and 4 the cutting speeds from 230 to

999 m/min are compared. All other cutting conditions and

the cutting speed is kept constant. Figs. 1–4 clearly show

that the compressive residual stress reach a maximum at

around 230 m/min. Both measuring directions, speed and

feed direction show the same pattern.

3.2. Surface roughness

Due to the particular geometry of lathe machining, there

is a direct relationship between turning parameters and

the roughness of the machined surfaces. For the theoretical

Table 2

Residual stress (MPa) measured in speed direction at cutting speeds from 50 to 999 m/min

Depth (mm) Cutting speed (m/min)

50 80 110 170 230 290 500 999

0 �179 166 87 247 255 46 350 299

10 �245 �507 �605 �627 �775 �659 �542 �393

20 �319 �393 �420 �690 �570 �326 �252 �160

30 �178 �253 �274 �526 �326 �176 �126 �105

50 �84 �157 �146 �238 �140 �167 �121 �123

70 �74 �143 �135 �150 �51 �191 �101 �122

100 �79 �127 �161 �106 �54 �154 �139 �134

Table 3

Residual stress (MPa) measured in feed direction at cutting speeds from 50 to 999 m/min

Depth (mm) Cutting speed (m/min)

50 80 110 170 230 290 500 999

0 �203 �71 �165 �159 �131 �231 �22 �25

10 �297 �430 �508 �636 �680 �515 �387 �297

20 �297 �318 �360 �606 �415 �275 �185 �111

30 �213 �218 �215 �404 �252 �192 �133 �91

50 �177 �164 �159 �246 �110 �163 �149 �72

70 �102 �157 �128 �133 �123 �209 �170 �110

100 �111 �144 �133 �167 �72 �145 �171 �100

Fig. 1. Residual stress at cutting speeds from 50 to 230 m/min.



320 M. Jacobson et al. / Journal of Materials Processing Technology 128 (2002) 318–323

calculation of Rt the following formula from Capello et al.

[8] was used:

Rt �f 2

8re(1)

where Rt is the surface roughness from top to bottom, f the

feed rate, and re the tool nose radius.

The theoretical surface roughness from Eq. (1) was then

compared with the measured values (Table 4) and displayed

in Fig. 5. The Rt parameter was found to differ with the

cutting speed. Fig. 5 clearly shows how the surface rough-

ness reaches a minimum at the cutting speed of 170 m/min.

Fig. 6 shows how the surface looks when using the

optimum cutting speed of 170 m/min. The 3D illustra-

tions provide a good picture of how the surfaces look.

The feedmarks can be clearly seen. This 3D pictures did

not show any extremes in the sense that material is missing

or that there are high-ploughed hills.

3.3. Microstructure

The structure of the test samples was evaluated. In Figs. 7

and 8 the photos of the microstructure can be seen. Fig. 7


Table 4

Surface roughness (Rt) measured in feed direction at cutting speeds from

50 to 999 m/min

Cutting speed (m/min) Rt

50 2.8

80 2.7

110 2

170 1.7

230 1.9

290 2.6

500 3.9

999 4.5

Fig. 5. Surface roughness at cutting speeds from 50 to 999 m/min.

Fig. 6. 3D picture of surface, cutting speed 170 m/min.

Fig. 7. Microstructure at surface, cutting speed 50 m/min (1850�).

Fig. 8. Microstructure at surface, cutting speed 50 m/min (1850�).


shows no alteration to the surface. However on the same test

samples thin layer of changed structure could be found, see

Fig. 8. This white layer was less then 1 mm deep when using

cutting speed at 50 m/min. This type of transformed surface

was approximately found on less then 5% of the surface. The

reason for evaluating the test sample from 50 m/min was that

it showed the highest compressive stress values. It is there-

fore believed that the lower speed gives less heat and less

transformation of the surface layer. The white layer normally

gives tensile stress.

4. Discussion

The surface generated by hard turning bainite steel

depends on the machining conditions. This study has shown

how the cutting speed affect residual stresses. This study is a

small part of a bigger investigation on how different machin-

ing parameters affect the result generated in hard turning.

More then 100 residual stress profiles have been measured

with different machining parameters. This larger study

support that the cutting speed has this effect on residual

stress generated. The amount of error on an individual

sample deviates, depending on how well aligned the sample

is with the X-ray diffraction equipment. An average of

�20 MPa on the measured value is the normal error. The

stylus probe measurement equipment used for roughness

measurement showed stable results for roughness measure-

ments. The tests were conducted with machines and tools

used for normal production. The purpose was to test with

conditions as close as possible to normal production.

Results presented in this study are valuable since they

show that compressive residual stress can be generated when

bainite steel is hard turned. It is also clear that the amount of

compressive stresses generated can be altered depending on

the cutting speed. When residual stress is generated there is

an effect from both the heat generated and mechanical work

going in to the surface and subsurface. When increasing

cutting speed one also increase the strain rate in the process

which gives more mechanical work in to the process, leading

to compressive stress. When increasing the cutting speed

more heat is introduced in the process, which has a tendency

to produce tensile stress at the surface. This could explain

why the compressive stress first increase then later decrease

while increasing the cutting speed. The two opposing phe-

nomena counteract to create the final result.

The amount of residual stress that is beneficial for

improving bearing fatigue life differs with each application.

For a theoretical case Broszeit et al. [9] concluded that the

minimum equivalent stress maximum occurs when the

residual stress is �0.21 of the maximum Hertzian contact

pressure. Schreiber et al. [10] have a more practical

approach. In their study, they introduced different degrees

of residual stress by using different time periods when cold-

working bearing balls. The cold-worked balls were then

assembled into a 6206 deep groove ball bearing. After

assembly, a rolling contact fatigue test was performed.

The rig used a Hertzian contact pressure of 3500 MPa. This

more practical approach resulted in the conclusion that an

optimum of residual stress is achieved at around �600 to

�1000 MPa. Liu and Mittal [4] also concluded that in order

for a residual stress profile to have a favourable effect, at

every point on the service stress profile the residual stress

must be of the same sign as the service stress. Also, the

magnitude of the residual stress should be less than or equal

to twice the magnitude of the service stress. Service stress in

rolling contact is primarily compressive in nature. There-

fore, for the initial residual stresses to be beneficial, they also

must be compressive.

Bearing this in mind, it is clear that hard turning of bainite

steel can be favourable in terms of improving fatigue life.

The measured values fall in the range mentioned above, and

can be altered to suit different applications.

The equation for Rt (Eq. (1)) only accounts for the

geometric portion of the roughness. The geometric rough-

ness and secondary roughness have been well described by

Capello et al. [8]. The real surface finish is a combination of

the geometric roughness and the secondary roughness

derived from the plastic flow of the material, the removal

process, the tool wear, the tool, and workpiece chatter

among others. The equation is useful for medium and high

levels of surface roughness, since the geometric portion

dominates the roughness value. For lower levels of rough-

ness, the model systematically underestimates the real sur-

face roughness, because in this case the secondary roughness

becomes more significant than geometric roughness. When

analysing the surface it becomes clear that when the cutting

speed is low, too low temperature is reached in the cutting

zone and the material is hard to cut. On the other hand if the

cutting speed is to high the amount of heat generated makes

the material soft enough to escape the cutting edge. When

cutting with 170 m/min the right temperature is reached and

roughness depends only on the geometric roughness. The

surface roughness value Rt then fits Eq. (1) well.

A white layer is a result of microstructure alteration. It is

called a white layer because it resists standard etchants and is

white under an optical microscope. In addition, white layers

often produce tensile stresses at the surface. An interesting

question is however this very thin layer has a detrimental

effect on fatigue life. White layer produced in grinding and

in hard turning is not the same thing, this is well described by

Konig et al. [11].

With the information available concerning the surface

integrity, there is indication that hard turning of bainite steel

is a possible process. An interesting next step would be to

perform a fatigue test with hard turned components. Mittal

and Liu [3] have already shown that an extremely good

surface finish can be obtained with hard turning. The

robustness of the process also must be documented. Konig

et al. [12] showed how the surface of a 52 100 component at

first is compressive, but soon turns to tensile when tool wear

appears.

322 M. Jacobson et al. / Journal of Materials Processing Technology 128 (2002) 318–323

5. Conclusions

Tests were conducted to find the effect of hard turning on

the surface integrity of bainite steel. The outcomes were

reported as residual stress, surface roughness and micro-

structure analysis. From the analysis of experimental data

and discussions, the following conclusions can be drawn:

� Hard turned bainite steel consistently shows the same

residual stress profile along the depth. The amount of

compressive stress increases to a maximum of �775 MPa

around 10 mm below the surface. At a depth of 100 mm,

the effect from hard turning on residual stress is small.

� The cutting speed clearly affects the amount of residual

stress generated in hard turning. Maximum compressive

stress is generated at a cutting speed of 230 m/min.

� The level of compressive residual stress produced is,

considering the literature, at the right level to have a

positive effect on rolling fatigue life.

� The cutting speed affects the surface roughness. At a

speed of 170 m/min a minimum Rt value is found. This

correlates well with the surface roughness that can be

predicted from Eq. (1).

� The microstructure analysis showed little or no alteration

to the surface or subsurface.

Acknowledgements

The authors would like to thank SECO and SKF. The

authors is also grateful for the funding received from Vinnova.

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Cutting speed influence on surface integrity of hard turned bainite steel

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