Effect of roller burnishing parameters on roughness surface and hardness of steel S355J0 specimens by
using response surface methodology
Mohamed Tourab Mechanical Engineering Department
Badji Mokhtar University Annaba, Algeria [email protected]
Salim Belhadi
Mechanical Engineering Department 8 Mai 1945 University
Guelma, Algeria
Abstract—Burnishing is a cold working process with superficial plastic deformation, which is to exert an external pressure through a very hard and smooth roller or ball on a surface to occurs a uniform and work-hardened surface, makes it possible to reduce roughness, to increase the hardness and to produce residual stresses of compression The steel S 355 J0 specimens were machined on a conventional lathe to the proper dimensions, these machined specimens were then burnished by a simple locally designed and fabricated roller-burnishing tool. The main objective in this work is to determine a mathematical models statistically based on experimental design (response surface methodology) using central composite second-order rotatable design which allows to give the relationship between the two out parameters surface roughness and hardness representative of the superficial layer surface caused by the four internal roller- burnishing parameters namely: burnishing speed, force, feed and number of passes of the tool. The experimental results indicate that feed, burnishing force and speed are the most important and significant parameters to improve roughness surface and feed, speed, burnishing force and number of passes are the most important and significant parameters to improve superficial hardness of steel S 355 J0 specimens. The surface roughness and hardness were improved to 0.15µm from about 2.5µm and to 226 Hv from 176 Hv respectively.
Keywords—roller-burnishing; steel S355J0; surface roughness;
superficial hardness; response surface methodology.
I. INTRODUCTION
The roller burnishing is considered as a mechanical treatment process and also termed as a finishing machining process without chip removal is to say at constant volume, less expensive than the grinding [1], it fits on conventional and unconventional post-machining. It consists in applying an external pressure with a roller or a smooth hard ball on the machined surface of the workpiece (turned, milled or drilled), since all machined surfaces consist of a series of peaks and valleys of irregular height and spacing, the plastic deformation created by roller burnishing is a displacement of the material in the peaks which cold flows under pressure into the valleys, see Fig. 1. This produces significant changes on surface of the workpiece. The work in this field have confirmed the advantage of the application of burnishing process whatsoever
used alone as finishing operation[2- 6],or as a hybrid process, roller burnishing with milling [7] roller burnishing with electrochemical finishing [8-10] or as a process associated with another process such as laser process to treat hard materials, And with the shot peening process to ensure the surface layer of the reinforced surface treated by shot peening a good surface appearance [12]. Among the advantages of roller burnishing process: improvement of different criteria express the surface condition and surface hardness [2-6, 13], improved bearing capacity of the surface [14], the work of reference [15] showed that on the same material and with the same roughness depth, burnishing leads to high surface bearing ratio better than that given by grinding, it reduces friction between parts and increases wear resistance [16], generates compressive residual stresses [17-18] that tend to close the cracks thus delaying their propagation that improve resistance to fatigue and corrosion fatigue.
The burnishing was used as a solution for treating parts working in severs circumstances such as forging dies (tools) [20] and finishing of plastic injection molds steel to improve the appearance of the surface of molded products [21-22]. The roller burnishing can be applied to different types of materials provided they fulfill the following conditions: resistance to rupture R <130 kg/mm2, Rockwell hardness HRC <42 and an elongation > 6% [1, 23].
The main objective of this study is to determine a mathematical model giving the relation between the output parameters roughness and surface hardness representing the Hamid Hamadache
Mechanical Engineering Department Badji Mokhtar University
Annaba, Algeria
Fig. 1. Effect of roller burnishing on the surface : 1- machined surface, 2- burnished surface, 3- roller and P- burnishing force.
burnishing surface layer based on the four input burnishing process parameters namely: burnishing force, speed, feed and number of passes of the burnishing tool, based on a statistical study by applying experimental design (response surface methodology) using a central composite second-order rotatable design and also show the technical value of this method for some Algerian industrial firms specialized in the field of engineering production to avoid make finishing tools such as grinding wheels of the grinding that deform or become clogged over labor and require each time a diamond dressing to eliminate these defects, all ensuring a longer lifetime of the mechanical parts.
II. EXPERIMENTAL WORK
Specimens used in this experimental work are non-alloy steel S 355 J0. This material was selected because of its importance in the industrial field and it also fulfilled the conditions for it to be processed by burnishing [1, 23]. Table 1 shows the chemical composition and mechanical properties of steel S 355 J0.
This material was delivered in the form of a cylindrical rod of outer diameter equal to 30 mm and a length of 6000 mm.
After cutting, the specimens were prepared to the required dimensions on a classic conventional lathe type "n gallic 16"
with mixed against mounting chuck + revolving center (tailstock) to ensure a good rigidity of the specimens during the work, cutting conditions in turning were unified for all specimens with a frequency of rotation = 1000 rev/min, advance 0.18 mm/rev and a cutting depth = 1 mm. Each specimen was then divided into five parts, four parts turned and roller burnished with different regimes of burnishing process and the fifth part was only turned for comparison purposes Fig. 2. The initial surface roughness and surface hardness for more hand specimens after turning is about 2.5 µm and 176 HV respectively.
A simple burnishing tool with a new design and local manufacture Fig. 3, was used to perform the experimental tests, this tool consists of two parts: a) the tool body (shank)
“1” which serves firstly to fix the tool in the tool holder of the lathe and also allow calibration and display the values of force, it is controlled by setting the stiffness of the spring “2”
by means of a threaded rod “3”. b) Active part called roller
“4” in contact with the surface of the workpiece, made of steel 100 C 6 and has a hardness of 63 HRC, surface roughness of 0.015 microns and a radius curvature of 3 mm, its connection with the body “1” is ensured by means of a clevis “5” which is fixed on the main axis “6” with a hexagonal screw “7”. The calibration of different rolling forces is provided by the successive application of the weights of different masses while playing on the spring rate through the threaded rod to have a displacement of 5 mm of the spring corresponding to approximately 5 kgf.
The tool burnishing set-up used in this work is shown in Fig. 4. The turning and the burnishing of the test specimens were conducted without lubrication in this experimental work, but each time before the operation of burnishing, the surface of the workpiece turned and the surface of the roller must be cleaned to prevent small hard particles come into contact with the surface of the roller and that of the piece leaving deep scratches on the surface that result in damage to the surface burnished of the workpiece.
TABLE I. PROPERTIES OF STEEL S 355 J0 Chemical composition [weight %]
0.22C Si
0.6 Mn
0.55 P
0.045 S
0.045 Mechanical properties
Re [MPa]
363 Rm [MPa]
427 A %
34 KCV 0° [j/cm2]
35 HB
≈ 180
Fig. 3. Roller burnishing tool,a)view in perspectiveb)section view:1- shank, 2- spring,3- threaded rod,4- roller,5- clevis,6- main axis and7- hexagonal screw.
Fig. 4. Experimental set-up of burnishing process: 1- burnishing tool;
2-workpiece; 3-revolving center (tailstock); 4-chuck; 5- tool holder.
Fig. 2. Workpiece geometry and dimension
.
In this research the parameter of the surface roughness arithmetic Ra was selected as a criterion to measure the surface state of the unburnished and burnished surfaces as it has been used quite a few times in previous research [1-10]
and considered as the most commonly used criterion in workshops that classifies various surfaces, depending on the machining processes [24].
The average value of three measurements was taken for each interval of each specimen; a portable roughness Mitutoyo SJ-201 series with a special assembly on the lathe was used to measure the surface roughness. Relate the surface hardness, an universal hardness Innovatest NEXUS 7000 series with a digital display led a micrometric arm was used to measure the Vickers hardness (HV 10/15) along a cylindrical generator for each interval of each specimen. Note here according to the ISO 6507 each of the measured hardness value must be multiplied by a correction factor when the hardness measurement is performed on cylindrical surfaces.
III. EXPERIMENTAL DESIGN
The main objective in the present work is the study of the effects of fixed parameters of the burnishing process on the characteristics of the surface layer of the working material represented by the surface roughness (Ra) and the Vickers surface hardness (Hv). For this reason, among the three most important response surface design that were used in previous research work such as central composite design, Box-Behnken design and Doehlert design [25, 26], a central composite second-order rotatable design Box-Hunter in 1957 was selected because it has been found suitable for this study. The number of experiments to be performed in this work is determining according to the following (1):
N = Nf+ Nα+ N0 (1)
With:
Nf = 2k: Number of experimental runs for a full factorial design.
k: Number of input parameters studied.
Nα: Number of experimental runs for start design on the axes at a distance α of field center.
α = (Nf)1/4: Number of star points for each factor taken in this work.
N0: Number of experimental runs in the center of chosen field.
In this study, the selected experimental design contains four independent factors with five levels for each of them, which are illustrated in (table 2) in natural variables and coded, the coded units or coded variables resulting from the ratio of two quantities of the same physical unit, their interest is to present experiences of the plans in the same way regardless of the fields of study chosen and whatever factors.
The passing of the natural variables to Xi coded variables, and vice versa, is given by equations (2) to (5).
Burnishing force: X1= F – 15 / 5 (2)
Burnishing passes: X2= P – 3 / 1 (3)
Burnishing speed: X3= N – 600 / 200 (4) Burnishing feed: X4= f – 0.01 / 0.05 (5)
Output response “y” can be predicted according to the quadratic model described by (6):
Y = a0+ a1x1+ a2x2+a3x3+ a4x4+ a11x12+ a22x22+a33x32
+a44x42+a12x12+ a13x13+ a14x14+ a23x23+ a24x24
+ a34x34+ e (6)
So according to equation (1) the number of experiments to be performed for the central composite second-order rotatable design is 31 experiments that is to say the sum of 16 experiments of a factorial design “Nf” over 8 trials a star design “Nα” plus7 points in the field centre “N0”, with α = ± 2.
IV. RESULTS AND DISCUSSION A. Results
Table 3 shows the experimental matrix and results of 31 experiments in this work based on the central composite second-order rotatable design. These results are then used to deduce two mathematical models of the roughness and surface hardness for steel S 355 J0 depending to the four burnishing process parameters, by using Minitab 16 as calculation software.
TABLE II. NATURAL AND CODED VARIABLES OF BURNISHING PROCESS PARAMETERS
Parameters levels
-2 -1 0 1 2
Force [kgf] X1 5 10 15 20 25
Nb of passes X2 1 2 3 4 5
Rotation speed
[rev/min] X3 200 400 600 800 1000
Feed [mm/tr] X4 0,05 0,075 0,1 0,125 0,15
TABLE III. EXPERIMENTAL MATRIX AND RESULTS
Test
Parameters of burnishing process
C : coded value and N : natural value Response Force Nb. of
passes Speed Feed Ra HV
C N C N C N C N N N
X1 F X2 P X3 N X4 f Y1 Y2
1 -1 10 -1 2 -1 400 -1 0.075 0.51 203
2 +1 20 -1 2 -1 400 -1 0.075 0.21 208
3 -1 10 +1 4 -1 400 -1 0.075 0.39 193
4 +1 20 +1 4 -1 400 -1 0.075 0.20 206
5 -1 10 -1 2 +1 800 -1 0.075 0.16 226
6 +1 20 -1 2 +1 800 -1 0.075 0.33 198
7 -1 10 +1 4 +1 800 -1 0.075 0.50 216
8 +1 20 +1 4 +1 800 -1 0.075 0.28 209
9 -1 10 -1 2 -1 400 +1 1.25 0.15 177
10 +1 20 -1 2 -1 400 +1 1.25 0.43 196
11 -1 10 +1 4 -1 400 +1 1.25 0.15 184
12 +1 20 +1 4 -1 400 +1 1.25 0.50 210
13 -1 10 -1 2 +1 800 +1 1.25 0.53 191
14 +1 20 -1 2 +1 800 +1 1.25 1.31 190
15 -1 10 +1 4 +1 800 +1 1.25 0.38 196
16 +1 20 +1 4 +1 800 +1 1.25 1.27 202
17 -2 5 0 3 0 600 0 0.1 0.36 190
18 +2 25 0 3 0 600 0 0.1 0.95 201
19 0 15 -2 1 0 600 0 0.1 0.36 196
20 0 15 +2 5 0 600 0 0.1 0.31 201
21 0 15 0 3 -2 200 0 0.1 0.15 190
22 0 15 0 3 +2 1000 0 0.1 0.45 199
23 0 15 0 3 0 600 -2 0.05 0.15 222
24 0 15 0 3 0 600 +2 0.15 0.78 197
25 0 15 0 3 0 600 0 0.1 0.27 198
26 0 15 0 3 0 600 0 0.1 0.17 191
27 0 15 0 3 0 600 0 0.1 0.20 191
28 0 15 0 3 0 600 0 0.1 0.20 194
29 0 15 0 3 0 600 0 0.1 0.19 202
30 0 15 0 3 0 600 0 0.1 0.25 203
31 0 15 0 3 0 600 0 0.1 0.30 203
B. Mathematical models
This part of the work was devoted, for the analysis of variance “ANOVA” in order to determine a very large grandeur “standard deviation” was used as a yardstick to assess the significance of the coefficients in the two models obtained by the central composite rotatable design, that is to say the most significant coefficients on both responses (roughness and surface hardness) having a t-test Student higher than standard critical t-test of Student and which are illustrated in (table 4) by a asterisk.
Therefore the two final models are calculated again with the retained coefficient deducted after analysis of variance and the Student t-test are as follows:
Ra = 0.2735 + 0.1225x1+ 0.1175x3+ 0.1417x4
+ 0.1053x12+ 0.0578x42+ 0.0925x13
+ 0.1775x14+ 0.1438x34 (7)
HV= 196.974 + 2.2917x1+ 1.54175x2+ 2.87x3
– 6.7917x4+ 3.201x42+ 2.6875x12– 5.8125x13
+ 4.1875x14+ 3.0625x24 (8)
The final models were also tested by analysis of variance
“ANOVA” that is to say by Fisher-ratio (table 5). It was found that the two final models of the roughness and surface hardness depending the four parameters of the burnishing process are adequate and possess respectively a coefficient of
TABLE IV. STUDENT’S T-TEST OF MODELS COEFFICIENT
Coefficient Surface roughness Ra Value of
coefficient Standard
deviation t- Student p-value a0 0.2257* 0.03596 6.276 < 0.0001 a1 0.1225* 0.01942 6.307 < 0.0001
a2 -0.0025 0.01942 -0.129 0.899
a3 0.1175* 0.01942 6.05 < 0.0001 a4 0.1417* 0.01942 7.294 < 0.0001 a11 0.1102* 0.01779 6.195 < 0.0001
a22 0.0302 0.01779 1.699 0.109
a33 0.0214 0.01779 1.208 0.245
a44 0.0627* 0.01779 3.526 0.003
a12 -0.0063 0.02379 -0.263 0.796
a13 0.0925* 0.02379 3.889 0.001
a14 0.1775* 0.02379 7.462 < 0.0001
a23 0.01 0.02379 0.42 0.68
a24 -0.0175 0.02379 -0.736 0.473
a34 0.1438* 0.02379 6.043 < 0.0001
Coefficient Hardness surface HV Value of
coefficient Standard
deviation t- Student p-value a0 197.4286* 1.5142 130.384 < 0.0001
a1 2.2917* 0.8178 2.802 0.013
a2 1.5417* 0.8178 1.885 0.078
a3 2.875* 0.8178 3.516 0.003
a4 -6.7917* 0.8178 -8.305 < 0.0001
a11 -0.3467 0.7492 -0.463 0.65
a22 0.4033 0.7492 0.538 0.98
a33 -0.5967 0.7492 -0.797 0.437
a44 3.1533* 0.7492 4.209 0.001
a12 2.6875* 1.0016 2.683 0.016
a13 -5.8125* 1.0016 -5.803 < 0.0001
a14 4.1875* 1.0016 4.181 0.001
a23 0.5625 1.0016 0.562 0.582
a24 3.0625* 1.0016 3.058 0.008
a34 -1.6875 1.0016 -1.685 0.111
The standard critical value of Student test: t0.05 - 16= 1.746.
*The significant coefficient.
determination R2= 93.1% (for surface roughness) and R2 = 89.8% (for surface hardness).
C. discussion
Analysis of the final equations of the roughness and hardness of models has identified the most significant parameters of the roller burnishing and influence their interactions on the two responses. It is notable that the predicted roughness (7) is highly influenced by the feed, the burnishing force and the rotation speed of the workpiece. The roughness increases proportionally with these three factors represent the most significant parameters of roller burnishing.
The hardness is more affected by the advance of the tool whose increase reduces the predicted value (8). To a lesser degree of influence, the speed, the force and number of passes of the burnishing tool are the other most influential parameters respectively. Their increases are favorable for hardness.
Both models predictions show that the interaction of the burnishing force with the speed (F-N) and feed (F-f) and the speed with feed (N-f) can influence the evolution of the roughness.
When the burnishing force interacts with the number of passes (F-P), advance (F-f) and speed (F-N) or when the number of passes is combined in advance (P-f) is hardness that is affected in one way or another.
In order to study the effect of each burnishing process parameter on the surface roughness and hardness, curves were plotted represent the effect of one parameters of burnishing process while the other parameters were kept constant at their mean values (table 2) on surface roughness and hardness of the workpiece.
D. Effect of burnishing force on surface roughness and hardness
Fig. 4 illustrates the effect of burnishing force on surface roughness, the curve shows that for low values of burnishing force the surface roughness is high, but when the burnishing force increases more the surface roughness decreases, that is to say each increase in of the burnishing force will allow the tool to penetrate into the surface, producing a good crushing surface asperities and the peaks of the surface were filling better the surface valleys, leading to a good flattening of the surface similar to polishing. But the flaking phenomenon occurs due to the capacity for cold working of the material when the force exceeds certain limit above of 15kgf.
Fig. 6 shows the effect of burnishing force on the surface hardness, with the increase in the burnishing force, the surface hardness increases, this significant evolution is mainly due to the high burnishing forces applied. So the plastic deformation and hardening that followed were substantial. Furthermore, TABLE V. FISHER-RATIO TEST OF FINAL MODELS
Surface roughness Ra Source Sum of
squares Degree of
freedom Mean
square Fisher-ratio First-order
term 1.17317 3 0.391056 45.78
second-order
term 1.36439 5 0.272878 31.94273
Residual Error 0.18794 22 0.008543 Total 2.7255 30 (N-1)
The standard valued of Fisher-ratio for significance α = 0.05 and degree of freedom 3 et 22 is F0.05 (3, 22)= 3.10 and at degree of freedom 5 et 22 is F0.05 (5, 22)= 2.71.
Hardness surface HV Source Sum of
squares Degree of
freedom Mean
square Fisher-ratio First-order
term 1488.5 4 372.13 23.9
second-order
term 1388.13 5 277.63 17.83
Residual Error 327.05 21 15.57 Total 3203.68 30 (N-1)
The standard valued of Fisher-ratio for significance α = 0.05 and degree of freedom 3 et 22 is F0.05 (4, 21)= 2.87 and at degree of freedom 5 et 22 is F0.05 (5, 22)= 2.71.
Fig. 5. Effect of burnishing force on surface roughness.
Fig. 6. Effect of burnishing force on surface hardness.
according to some research [15], the increase in the burnishing force causes the appearance of significant residual stresses involved, in turn, to increase the surface hardness.
E. Effect of number of passes on the surface roughness and hardness
Fig. 3 shows the Effect of number of passes on the surface roughness, at the low numbers of passes, the surface roughness is high, when, the number of passes of the tool increases, the surface roughness decreases gradually. The tool, by returning several times to the surface, has more chance to crush the asperities and reliefs of the previous process (turning). But to a value greater than 3 passes, the surface roughness increases gradually this can be explained by the fact that each material has a definite capacity for cold working beyond any further cold working will produce flaking on the surface of the workpiece.
Fig. 7 illustrates the influence of the number of passes on the surface hardness, we can say that the surface hardness increases gradually when increased the number passes of burnishing tool, because the burnishing with this condition causes a good plastic flow of the surface material which results in a high hardening, and it gave a substantial surface hardening of the workpiece.
F. Effect of burnishing rotation speed on the surface roughness and hardness
Fig. 1 shows the effect of burnishing rotation speed on surface roughness, the surface roughness increases in proportion as the burnishing speed is increased. This can be explained by the poor contact between the surface of the workpiece and that of the tool due to the high burnishing speeds, thus causing the chattering phenomenon. Good surface roughness are obtained when burnishing speeds are low, a good contact between the surface of the workpiece and the tool thus promoting an important crushing of surface asperities which thus forcing peaks of surface to fill better the valleys.
This action leads to an improvement in surface finish.
Fig. 8 shows the effect of burnishing rotation speed on the surface hardness, on the contrary of the surface roughness, progressively as the burnishing speed is increased, the surface hardness decreases gradually. High surface hardness is achieved when the speed is at its lowest value; this is due to a good contact between the surface of the workpiece and the burnishing tool and the absence of chattering phenomenon.
That is not the case for higher burnishing speeds.
Fig. 7. Effect of number of passes force on surface roughness.
Fig. 8. Effect of number of passes force on surface hardness.
Fig. 9. Effect of number of burnishing rotation speed on surface roughness.
Fig. 10. Effect of number of burnishing rotation speed on the surface hardness.
G. Effect of burnishing feed on the surface roughness and hardness
Fig. 2 shows that the effect of the burnishing feed on the surface roughness causes an increase in the roughness of the surface as it increases. The best roughness is registered with low feed because the distance between two consecutive traces of the roller burnishing tool is small which allows the tool to crush more surface asperities and to erase the traces of the previous process (turning). When the burnishing feed is high, the distances between the successive burnishing traces will be more important, and much asperities and reliefs misses by the burnishing tool, which gives less improvement in the surface finish of the workpiece.
Fig. 5 illustrates the effect of burnishing feed on the surface hardness, it can be seen from this curve that increasing in burnishing feed causes a decrease to a minimum value of surface hardness, because the good values of hardness are related with low values of burnishing feed that is attributed to the decrease of the distances between the successive burnishing traces. This will cause an increase in the plastic deformation action on the surface of the workpiece, which leads to increase in surface hardness.
V. CONCLUSION
In this present work the roller burnishing tool with a new design and local manufacture, used to perform the experimental tests in this work, was improvement with successfully the surface roughness about 88.5% and hardness about 22% of the S355 J0 steel workpiece . It was found that after analysis of variance “ANOVA” the two final models of the roughness and surface hardness depending of the four burnishing process parameters are adequate and possess respectively a coefficient of determination R2= 93.1% (for surface roughness) and R2= 89.8% (for surface hardness). The surface roughness is highly influenced by the feed, the burnishing force and the rotation speed of the workpiece. The roughness increases proportionally with these three factors represent the most significant parameters of roller burnishing.
The hardness is more affected by the advance of the tool whose increase reduces the predicted value. To a lesser degree of influence, the speed, the force and number of passes of the burnishing tool are the other most influential parameters respectively. Their increases are favorable for hardness.
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