Machinability investigation in hard turning of AISI H11 hot work steel with CBN tool/Karsciui arsparaus plieno aisi H11 apdirbamumo rupiai tekinant CBN irankiu tyrimas.
Aouici, H. ; Yallese, M.A. ; Fnides, B. 等
1. Introduction
In hard turning, ferrous metal parts that are hardened usually
between (45-70 HRC) are machined with the single point cutting tools.
This has become possible with the availability of the new cutting tool
materials (cubic boron nitride (CBN) and ceramics). Since a large number
of operations is required to produce the finished product, if some of
the operations can be combined, or eliminated, or can be substituted by
the new process, product cycle time can be reduced and productivity can
be improved. The traditional method of machining hardened materials
includes rough turning, heat treatment, and then grinding process. Hard
turning eliminates the series of operations required to produce a
component and thereby reduces the cycle time resulting in productivity
improvement [1-3].
Various studies have been conducted to investigate the performance
of CBN tool in machining of various hard materials. Dilbag and
Venkateswara [1] have conducted the study on the influence of rake
angle, cutting speed, feed rate and nose radius are primary influencing
factors which affect the surface finish. The results indicated that feed
rate is the dominant factors affecting the surface roughness. Sahin and
Motorcu [2] show that feed rate was the main factor influencing the
surface roughness. It increased with increasing the feed rate but
decreased with increasing the cutting speed and the depth of cut,
respectively. In their experimental research work, Bouacha et al. [4]
investigated the effect of cutting speed, feed rate and depth of cut on
surface roughness and cutting forces using three level factorial design
(33) during machining of bearing steel (AISI 52100) with CBN tool. The
results show how much the surface roughness is influenced by feed rate
and cutting speed and that the depth of cut exhibits maximum influence
on the cutting forces as compared to feed rate and cutting speed. Horng
et al. [5] presented a model to evaluate the machinability of Hadfield
steel by applying response surface methodology (RSM) and analysis of
variance (ANOVA) techniques. The study indicated that the flank wear is
influenced principally by the cutting speed and the interaction effect
of feed rate with nose radius of the tool. Lima et al. [6] investigated
the machinability of hardened steels at different levels of hardness
using a range of cutting tool materials. More specific is the
machinability of hardened AISI 4340 high strength low alloy steel and
AISI D2 cold work tool steel. The results indicated that when turning
AISI 4340 steel surface roughness of the machined parts was improved as
cutting speed was elevated and deteriorated with feed rate. Depth of the
cut presented little effect on the surface roughness values. Flank wear
of mixed alumina tool increased with cutting speed and depth of the cut
increasing. Chou et al. [7] experimentally investigated the performance
and wear behaviour of different CBN tools in finish turning of hardened
AISI 52100 steel (DIN 100Cr6). In this study, it was established that
low CBN content materials provide the best performance in hard turning
in terms of tool life and surface finish. Zhou et al. [8] in their
investigation revealed that chamfer angle has a great influence on
cutting force and tool life. All the three force components increase
with the increase in chamfer angle. The optimized chamfer angle for
maximum tool life, as suggested by this study, is 15[degrees]. Luo et
al. [9] studied the wear behaviour in hard turning of the same alloy
steel by CBN and ceramic tools and they found that the flank wear was
reduced as work material hardness increased up a critical value of 50
HRC. In addition, wear mechanisms by diffusion, abrasion and adhesion
were discussed by Poulachon et al. [10] and usually it is concluded that
these mechanisms are prevalent during the wear process of CBN tools. The
major influencing factor on the tool wear is the presence of various
carbides in the steel microstructure. Hardness of these carbides varies
significantly, causing different wear rates when turning 100Cr6,
X155CrMoV5, X38CrMoV5 and 35NiCrMo16 steels. In these cases, the flank
wear on the tool has resulted in grooves caused by the major abrasive
action of carbides.
Fnides et al. [11] found that the temperature increases, which is
due to mechanical energy conversion into thermal energy because of
elastic strain friction of the chip on rake and relief surfaces of the
tool. The knowledge of the variation in temperature in the entire insert
and particularly to the interface tool chip will allow a better adequacy
between the cutting parameters, the characteristics of material to be
machined like those of the tool.
The current article investigates the influence of cutting
parameters (cutting speed, feed rate and depth of cut) on cutting
forces, surface roughness and tool wear in turning of hot work steel
AISI H11 with CBN tools. Tool life model was obtained by the sowftwere
Design-Expert using RSM.
2. Experimental procedure
Turning experiments were performed in dry conditions using the
lathe type SN 40C with 6.6 KW spindle power. The workpiece material was
AISI H11, hot work steel which is popularly used in hot form pressing.
Its resistance to high temperature and its aptitude for polishing enable
it to answer the most severe requests in hot dieing and moulds under
pressure [12]. Its chemical composition is given in Table 1.
The workpiece is of 80 mm in diameter. It is hardened to 50 HRC.
Cutting insert is removable and offered eight squared working edges. The
chosen CBN tool in commercially known as CBN7020 and it is essentially
made of 57% CBN and 35% Ti(C, N). Its designation is SNGA12 04 08 S01020
and was manufactured by Sandvik. Physical properties of the CBN7020 tool
are summarized in Table 2. Tool holder is codified as PSBNR25x25M12 with
a common active part and tool geometry described by [X.sub.r] =
+75[degrees], [lambda] = -6[degrees], [gamma] = -6[degrees] and [alpha]
= +6[degrees]. Three component cutting force in X, Y and Z directions as
recorded using a standard quartz dynamometer (Kistler 9257B) allowing
measurements from -5 to 5KN. Instantaneous roughness criteria (Ra, Rt
and Rz) for each cutting condition were obtained by a Surftest 201
Mitutoyo roughness meter coupled with a radius and moved linearly on the
working surface. The length examined is 2.4 mm with a basic span of 3
mm. The measured values of Ra are within the range 0.05 to 40 um while
for Rt and Rz, they lay between 0.3 and 160 [micro]m.
Roughness measurements were directly performed on the same without
disassembling the turned part in order to reduce uncertainties due to
resumption operations. The measurements were repeated 3 times out of 3
generatrices equally positioned at 120[degrees] and the result is the
average of these values for a given machining pass. To measure maximum
temperatures in the cutting zone, we used a pyrometer with infra-red
model Raynger 3I, its interval is -30 to 1200[degrees]C.
3. Results and discussion
3.1. Evolution of the cutting forces
a) Effect of cutting speed
It can be seen in Fig. 1 that all components of the cutting force
decreased as the cutting speed was increased, with different slopes.
This is due to the rise in temperature in the cutting zone which makes
the metal machined more plastic and consequently the efforts necessary
for machining decrease [13].
[FIGURE 1 OMITTED]
It is noticed that the thrust force is dominating compared to both
others and that for all the cutting speed tested, probably due to the
work of tool exclusively with its nose radius is equal to 0.8 mm
([r.sub.e] > ap) and the negative rake angle ([gamma] = -6[degrees]).
The effects of cutting speed on the cutting forces are as follows: the
increase in cutting speed from 45 to 125 m/min increase the components
of the cutting force (Fr, Fv and Fa) successively of 39; 34.19 and
37.91%.
b) Effect of feed rate
The effect of feed rate on cutting forces is shown in Fig. 2. It
can be noted that the increase in feed rate resulted in the increase in
cutting forces.
[FIGURE 2 OMITTED]
If the feed rate increases, the section of sheared chip increases
because the metal resists rupture more and requires large efforts for
chip removal [14]. The effects of feed rate on the cutting forces are as
follows; the increase in feed rate from 0.08 to 0.24 mm/rev, increases
components of the cutting forces (Fr, Fv and Fa) successively of 156.24;
250.6 and 114.31%. It is noted that the tangential cutting force is very
affected by the feed rate.
c) Effect of depth of cut
Fig. 3 represents the influence of the depth of cut on the cutting
forces. With its increase, chip thickness becomes significant what
causes the growth of the volume of deformed metal and that requires
enormous cutting forces to cut the chip. For the depth of 0.05 to 0.75
mm, we successively recorded the increase in components of the cutting
forces (Fr, Fv and Fa) from 380.46; 559.20 and 915.15%. It is noted that
the axial force is very affected by the depth of cut.
[FIGURE 3 OMITTED]
3.2. Evolution of the surface roughness
Characterization of the machined surface quality was limited to the
criteria of total roughness (Rt), arithmetic mean roughness (Ra) and
mean depth of roughness (Rz).
a) Effect of cutting speed
Fig. 4 shows the evolution of surface roughness according to the
cutting speed.
[FIGURE 4 OMITTED]
The three criteria of roughness present a decrease when the cutting
speed increases. This can be related to the growth of temperature in
cutting zone and consequently, friction becomes less important. This
graph indicated that at cutting speeds lower than 180 m/min (zone I),
the criteria of roughness (Rt, Rz and Ra) fall successively from 37.73;
50.25 and 48.50%.
In the second zone, surface roughness is stabilized slightly
because of reduction in the cutting forces stabilizing the machining
system. Chen [15] explains this stability which returns to the weak
deformation of the workpiece for higher speed (this is with the rise in
temperature in the zone of cut which makes metal machined more plastic
and consequently the efforts necessary to the cut decrease).
b) Effect of feed rate
Fig. 5 illustrates the evolution of surface roughness according to
the feed rate. The analysis of the graphs shows that this parameter has
a very significant influence, because its increase generates helicoids
furrows the result of tool shape and helicoids movement tool-workpiece.
These furrows are deeper and broader as the feed rate increases. For
this reason, we must employ weak feed rate during turning. In practice
it is noted that roughness's (Rt, Rz and Ra) are minimal for the
weakest feed rate. But they increase with the rise in this one. We note
the increase of approximately 159% of Rt, and 173.71% for Rz and 197%
for Ra, when the values of the feed rate pass for 0.08 to 0.24 mm/rev.
[FIGURE 5 OMITTED]
In the literature, Habak [16] and Remadna [17] show that the
equations [R.sub.a] = [f.sup.2]/32 [r.sub.[epsilon]] and [R.sub.t] =
[f.sup.2]/8[r.sub.[epsilon]] are not appropriate to hard turning. This
is shown though the results presented in Table 3. The experimental
results are either lower, or higher than the computed values. One can
conclude that in hard turning, the value of roughness depends on several
parameters: geometry of tool (major cutting edge angle, rake angle,
...), the process of machining, and hardness of the workpiece [3].
c) Effect of depth of cut
Fig. 6 shows the evolution of surface roughness according to the
depth of cut. The three parameters of roughness show that this parameter
has a very weak effect compared to that of feed rate.
[FIGURE 6 OMITTED]
For cut depth 0.05 to 0.75 mm, we recorded the increase in (Rt and
Rz) respectively in 64.67 and 35.30%. On the other hand roughness (Ra)
remains stable 13.79%.
3.2. Evolution of the cutting temperature
a) Effect of cutting speed
Fig. 7 presents the change of temperature in cutting zone according
to the cutting speed for the machining time of 25 seconds. With the
increase of the cutting speed, frictions increase, this induces
temperature increase in the cutting zone. It is noted that for the speed
of 45 m/min, the maximum temperature is 114[degrees]C. For the variation
of cutting speed of 45 to 335 m/min, we record the increase in
temperature in the cutting zone of 240.5%.
[FIGURE 7 OMITTED]
[FIGURE 8 OMITTED]
b) Effect of feed rate
Fig. 8 presents the change of temperature in the zone of cut
according to the feed rate. The results of the influence of the feed
rate on temperature, show an increase in this last. With the increase in
the feed rate section of the chip increases and consequently friction
increases, which involves an increase in the temperature. For the feed
rate from 0.08 to 0.24 mm/rev, we record temperatures which vary from
200.5[degrees]C to 293.5[degrees]C. It represents the increase of
46.38%.
c) Effect of depth of cut
Fig. 9 shows the maximum temperature in the cutting zone according
to the depth of cut. We record an increase which is worth ~176% when the
depth of cut varies from 0.05 with 0.70 mm.
* For the depth of cut of 0.05 mm the maximum temperature recorded
in the zone of cut is 122[degrees]C. If the depth of cut increases up to
0.15 mm, (either 3 times), the value of the maximum temperature becomes
220[degrees]C, which represents the increase in temperature of 81.96%.
* For the depth of cut of 0.45 mm, (that is to say 9 times), the
value of the maximum temperature in the zone of cut reached
271[degrees]C, which represents the increase in temperature of 122.13%.
* For the depth of cut of 0.70 mm, (that is to say 14 times), the
value of the maximum temperature in the zone of cut reached
337[degrees]C, which represents the increase in temperature of 176.22%.
It is noted, if the depth of cut increases, the section of the chip
increases and friction chip tool increases, which leads to an increase
in temperature.
[FIGURE 9 OMITTED]
3.3. Evaluation of the tool life
Tool life is a crucial factor for evaluating machinability of
materials. In order to determine this important factor, we have realized
wear tests according to machining time at three cutting speeds and three
feed rates.
Table 4 presents experimental results of tool life (T) for various
combinations of cutting regime elements (cutting speed and feed rate)
according to [3.sup.2] full factorial design. Minimal values of tool
life (T) were obtained at [V.sub.c] = 240 m/min and f = 0.16 mm/rev.
Maximal values of tool life were registered at [V.sub.c] = 120 m/min and
f = = 0.08 mm/rev. In order to achieve better tool life, the lowest
level cutting speed and lowest level feed rate are recommended.
3.4. Modeling of the machining parameters for tool life
The response surface methodology (RSM) is the procedure for
determining the relationship between the independent process parametres
with the desired response and exploring the effect of these parametres
on responses.
In the current study, the relatioship between the cutting
conditions and the machinability aspect is given as
Y = [phi]([V.sub.c], f) (1)
where Y is the desired machinability aspect and [phi] is the
response function.
The approximation of Y is proposed by using a non-linear
mathematical model, which is suitable for studying the interaction
effects of process parameters on machinability characteristics.
In the present work, the RSM based second order mathematical model
is given by
Y = [a.sub.0] + [a.sub.1][V.sup.c] + [a.sub.2]f +
[a.sub.12][V.sub.c] x f + [a.sub.11][V.sup.2.sub.c] +
[a.sub.22][f.sup.2] (2)
where Y is the desired response of the tool life [a.sub.0],
[a.sub.1], [a.sub.2], [a.sub.12], [a.sub.11] and [a.sub.22] regression
coefficients to be determined for each response.
The resultas of analysis of variance (ANOVA) for tool life (T) are
shown in Table 5. This Table also shows the degrees of freedom (df), sum
of squares (SC sq.), mean square (MS), F-values and probabilty
(P-value), in addition to the contribution (Cont. %) of each factor.
ANOVA results for T are indicated in Table 5. It can be noted that
the cutting speed affects T in a considerable way. Its contribution is
91.57%. The second factor influenceing T is feed rate. Its cobtribution
is 3.83%. The interactions feed rate/cutting speed and cuting
speed/cutting speed are significant but the interaction feed rate/feed
rate is not significant. Respectively, their contributions are 2.04;
2.398 and 0.036%. (Values of Prob > F less than 0.0500 indicate model
terms are significant.
Using the data presented in Table 4, a quadratic model regression
can be obtained by equation (3), which gives the time required to reach
VB = 0.30 mm as a function of cutting speed (Vc) and feed rate (f) with
[R.sup.2] = = 0.9862
T = 213.03 - 1.269[V.sub.c] - 321.375f + 1.796[V.sub.c] f +
+1.833x[10.sup.-3][V.sup.2.sub.c] -509.375[f.sup.2] (3)
The obove mathematical model can be used to predict the values of
tool life (T) within in the limites of the factors studied.
The differences between measured and predicted responses are
illustrated in Fig. 10.
The results of comparison were proved to predict the values of tool
life close to those readings recoreded exprementally with a 95%
confidence interval. Good agreement is observed between these values as
seen in Fig. 11.
[FIGURE 10 OMITTED]
Fig. 12 gives the main factor plots. The tool life appears to be an
almost-linear decreasing function of cutting speed. This result
contradicts with common expectation that tool life usually decreases
with increasing cutting speed. But the feed rate has little effect on
tool life.
The effect of feed rate (f) and cutting speed (Vc) on the tool life
(T) is shown in Fig. 13. This figure displays that the value of tool
life (T) decrease with the increase of cutting speed and feed rate. The
decrease is approximately 75.86% of T,
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
[FIGURE 13 OMITTED]
4. Conclusions
The tests of straight turning carried out on grade AISI H11 steel
treated at 50 HRC, machined by a Cubic Boron Nitride tool (CBN7020),
enabled us to study the influence of the following parameters: feed
rate, cutting speed and depth of cut on cutting force, surface
roughness, temperature in the cutting zone and tool life. The
conclusions of research are as follows.
* Tangential cutting force is very sensitive to the
variation of cutting depth what affects the feed (axial) forces in
a considerable way.
* Thrust force is dominating compared to both others and that for
the entire cutting regime.
* Surface roughness is very sensitive to the variation of the feed
rate. We record the increase of approximately 159% of Rt, 173.71% for Rz
and 197% for Ra when the values of the feed rate passed from 0.08 to
0.24 mm/rev.
* Temperature is strongly influenced by the cutting speed
(240.50%).
* RSM technique has the advantage of investigating the influence of
each machining variable on the values of technological parameters.
* Cutting speed is the most significant factor with 91.57%
contribution in the total variability of model (T), whereas feed rate
has a secondary contribution of 3.83% in the model.
Acknowledgements
This work was completed in the laboratory LMS (University of
Guelma, Algeria) in collaboration with LaMCos (CNRS, INSA-Lyon, France).
The authors would like to thank the Algerian Ministry of Higher
Education and Scientific Research (MESRS) and the Delegated Ministry for
Scientific Research (MDRS) for granting financial support for CNEPRU
Research Project--LMS: N[degrees]: 0301520090008 (University of Guelma).
Received May 28, 2010
Accepted December 07, 2010
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H. Aouici *, M.A. Yallese *, B. Fnides *, T. Mabrouki **
* Mechanics and Structures Laboratory (LMS), Department of
Mechanical Engineering, May 08th 1945 University, Guelma 24000, Algeria,
E-mail: aouici_hamdi@yahoo.fr
** Universite de Lyon, CNRS, INSA--Lyon, LaMCoS, UMR5259, F69621,
France
Table 1
Chemical composition of AISI H11
Composition (Wt %)
C 0.35
Cr 5.26
Mo 1.19
V 0.50
Si 1.01
Mn 0.32
S 0.002
P 0.016
Other components 1.042
Fe 90.31
Table 2
Physical properties of CBN
Material Hardness V, Tenacity, Young's
daN/[mm.sup.2] MPa [m.sup.1/2] modulus, GPa
CBN 7020 2800 4.2 570
Material Density, Grain size,
g/[cm.sup.3] [micro]m
CBN 7020 4.3 2.5
Table 3
Comparison of measured and predicted values of the
surface roughness
Roughness measured Roughness predicted
f, Rt, Ra, Rt, Ra,
Vc = 180 m/min mm/rev [micro]m [micro]m [micro]m [micro]m
ap = 0.15 mm
[r.sub.[epsilon]] 0.08 2.17 0.34 1 0.25
= 0.8 mm 0.16 4.30 0.93 4 1
0.24 5.62 1.01 9 2.25
Table 4
Tool life for VB = 0.30 mm and ap = 0.15 mm (AISI H11)
Vc, m/min F, mm/rev Tool life T, min
180 0.08 40.00
120 0.08 76.00
180 0.12 37.00
180 0.16 32.67
120 0.16 56.60
240 0.12 20.00
240 0.16 18.34
120 0.12 67.50
240 0.08 20.50
Table 5
Analysis of variance for tool life
Source df SC sq. MS F-value Prob>F Cont. %
Model 5 3627.59 725.52 528.53 0.0001 --
f, mm/rev 1 139.11 139.11 101.34 0.0021 3.83
Vc, m/min 1 3325.73 3325.73 2422.75 <0.0001 91.57
f x Vc 1 74.30 74.30 54.13 0.0052 2.04
[f.sup.2] 1 1.33 1.33 0.97 0.3978 0.036
[Vc.sup.2] 1 87.12 87.12 63.47 0.0041 2.398
Residual 3 4.12 1.37 0.113
Total 8 3631.77 100
Source Remark
Model Signifiant
f, mm/rev Signifiant
Vc, m/min Signifiant
f x Vc Signifiant
[f.sup.2] Not signifiant
[Vc.sup.2] Signifiant
Residual
Total
