INTRODUCTION
The industrialists need to optimize their production processes in order to
increase the productivity, to reduce the wear of the cutting tools (Ben
Salem et al., 2003). The improvement of the quality of the production
will remain always one of the paramount concerns for the industrial development.
Technological progress in the machining of metals is related to the improvement
of the behavior of materials to the cut. The increase in the performances on
the machine tools influences directly over the lifespan of the cutting tools
engaged in production. The selection of material of the tool is a delicate phase,
taking into account the conditioning of feasibility of the operation of machining.
Consequently the rational choice of a nuance of tool can be done only with experiments
quite specific to material given (Shultz and Moriwahim,
1992).
The materials of cut out of high speed steel, carburizes and cermet are largely
used in turning for the majority of machined materials (Schneider, 2002; Brandt
et al., 1990). However, their behaviour is limited and their use
is sometimes impossible when certain work pieces are characterized by a very
great hardness and an abrasion resistance very high (Fallbohmer
et al., 2000; Chou, 2003). It is the case
mainly special steels, pig iron and cast iron soaked, alloys refractory containing
nickel and of composite materials with metal matrix. We can also add the sintered
materials whose hardness’s exceed easily the 70 HRc. Technical progress
reveals new materials likely to be used to produce cutting tools answering these
requirements. The rational choice of these materials of cut can be done only
with experiments specific to each nuance. Thus their implementation requires
a sufficient control of the course of the process of cut, in particular the
evolution of the wear of the tools and required roughness. During the machining
of the materials whose hardness exceeds the 45 HRc, at cutting speeds raised,
the mechanical and thermal requests on the edge of cut of the tool, which constitutes
the active element carrying out the work of deformation, become significant.
Frictions and the high constraints which are exerted with the interfaces between
the part and the tool cause various forms of wear on the active part of tool
(Bauer et al., 2003; Luo
et al., 1999; Boulanouar and Andonov, 1996;
Maamar et al., 1994).
Morphologies of wear depend primarily on the nature of the tool, machined material,
the cutting conditions and the type of machining. According to standard NF E66-505,
the principal forms of wears met are characterized primarily by wears in skin
Vb and crater KT. Wear in skin or frontal wear is due to the friction
of the part on the face in skin of the tool (Remadna, 2001;
Gelin and Vincent, 1995). From the point of view practises,
frontal wear is most significant to consider since it determines the surface
quality of the part machined as well as dimensional accuracy (Kaufeld
and Torbaty, 1999). On the other hand, wear in crater is characterized by
a basin formed on the cutting face of the tool following frictions of the chip
(Remadna, 2001; Gelin and Vincent,
1995). This form of wear is allotted to the presence of strong pressures
and the release of a great quantity of heat in the zone of contact between the
chip and the tool. Consequently, the temperature rises and causes a significant
thermal diffusion process. The shape of the crater is defined by its depth KT.
Wear notches some occurs under certain cutting conditions following wrenching
of particles.
Several work (Amri, 1987; Gelin and
Vincent, 1995; Roumesy, 1975a-c)
proposed mathematical relations which express the lifespan of the tool T according
to the parameters of cut (Boulanouar et al., 2006).
Today hard machining is in full rise and the applications of hard turning include/understand
the gears (30%), the bearings (25%), the trees (20%), the casings (5%) and more
particularly those intended for the car industry which call upon the semi-completion
and the completion of the transmissions, the axles, the discs of brakes, etc.
(Poulachon et al., 2002; Defretin
and Levaillant, 1999). The turning of the chilled steels by hardening requires
the use of component materials having excellent properties of hardness at high
temperatures, wear resistance and chemical stability.
The latter had relatively high performances in turning for their substitution
for the operations of correction (Schultz, 1997; Schneider,
2002; Poulachon and Moison, 2003). The more of time,
the hard turning products of surfaces of qualities, minimizes the number of
operations and the rejected parts (Benchiheub and Boulanouar,
2006).
In end of this study is devoted to the comprehension of the damage mechanisms of the cutting tools in service.
MATERIALS AND METHODS
Materials
Machined material: The cylindrical test-tubes of diameter Ø 80 mm and length L = 500 mm machined are out of steel XC38, of which a first series of test-tubes not soaked and a second series of test-tubes soaked. This steel is selected as material to be machined because it roughly presents 47% of thermically treated materials used in the manufacture of the parts (Table 1).
Materials of the tools: The cutting tool used is a turning tool with
removable plate brought back square, fixed on the tool by fastening. The square
brought back plates are out of metal carbide of nuance P30 having the following
geometry: Xr = 75°, α = 6°, γ = -6E, λ = -6E, whose the
radius of the nozzle used is Rε = 0.8 mm.
|
| Fig. 1: | Schematic
of wear to the specimen and the cutting tool |
The chemical composition of the plates is: WC = 82%, TiC = 8%, Co = 10%.
Methods
Models of lifespan and models of wear: The lifespan of a tool is the time
of cut total necessary to reach a criterion of specific lifespan. To evaluate
the lifespan, we refer to a limiting value of a direct criterion (Vb, KT).
The analysis of the damage is very often done by considering wear on the face
of skin because the latter influences directly the surface quality of the part
(Fig. 1). For a tool out of carbide, the following criterion
is recommended by the ISO standard: average height Vb = 0.3 mm From the criteria
of wear, we establish models of lifespan. Oldest and the most used is that of
Taylor (Eq. 1). Taylor was the first to be proposed a mathematical
model connecting the effective duration of cut T of a tool to the parameters
of modified cut or Taylor (Eq. 2) (Menegaux,
2004; Lelong-Ferrand and Arnaudies, 1977):
| C1 |
= |
Constant depends on the nuance of the tool and machined material |
| Vc |
= |
Cutting speed (m min-1) |
| n |
= |
Constant |
| C2 |
= |
Constant depends on the nuance of the tool and machined material |
| P |
= |
Depth of cut (mm) |
| a |
= |
Advance (mm tr-1) |
| x, y |
= |
Exhibitors depending on the nuance of the tool |
These equations describe the relation between the lifespan T and the parameters
of cut like the cutting speed VC the advance a and the depth of cut
P they utilize constants (C1, C2, n, x, y) which must
be identified in experiments for each couple tool/matter considered and each
process of machining. Contrary to the models of lifespan, the models of wear
describe the volume of matter lost in the zones of contact of the tool by connecting
them to physical parameters like the temperature or the pressures applied to
the tool (Roumesy, 1975a-c;
Roumesy and Bedrin, 1981, 1984).
PRESENTATION OF THE RESULTS
Case of the operations of slide-lathing of steel XC38 not hardened
Test 1: Test 1 consists to evaluate wear (Vb) and to determine the
behaviour of the tool out of metal carbide according to time for various cutting
speeds (Vc1 = 36 m min-1, Vc2 = 100 m min-1)
with an advance a = 0.08 mm tr-1 and a depth of cut p = 0.5 mm (Fig.
2, 3).
Figure 2 presents the variation of the wear of the tool on the surface of skin Vb according to time for different cutting speeds (Vc1= 36 m min-1, Vc2 = 100 m min-1), with a depth of constant cutting p = 0.5 mm and an advance of constant cut a = 0.08 mm tr-1. It is noted that the value cutting speed influences the value of the wear of the Vb tool for example with one 50 min duration of machining, with Vb = 0.28 mm and Vc2 = 100 m min-1. These two curves of wear have three parts. The first part is that of wear of grinding with a time of cut between 0 and 10 min. The second part presents a fair wear and tear of a time of cut between 10 and 90 min. The third part starts to 90 min is the part of catastrophic wear (Fig. 2). It is noticed that these three parts are not identical, because their zones are different.
The statistical processing of the results of the behaviours according to the cutting speed (Fig. 3) allowed the deduction of mathematical model, expressing the functional relation between the lifespan and the cutting speed according to the Eq. 3:
Figure 3 shows the behaviour T of the experimental tool for
the two different modes of cut (Vc1= 36 m min-1, a = 0.08
mm tr-1, p = 0.5 mm) and (Vc2 = 100 m min-1,
a = 0.08 mm tr-1, p = 0.5 mm).
|
| Fig. 2: | Variation
of cutting tool wear (Vb) according to time for different cutting speed
(a = 0.08 mm tr-1, p = 0.5 mm) |
|
| Fig. 3: | Experimental
behaviour for different cutting speed, Vb = 0.3 mm |
The first mode of cut gives a behaviour T = 140 min, on the other hand the
behaviour of the sec mode of cut is T = 56 min.
The form and the shape of the curves of Fig. 2 and 3
resemble the curves obtained by other research tasks to work out by other researchers
(Schultz, 1997; Schneider, 2002;
Poulachon and Moison, 2003) but they are not identical
because the working conditions are not the same ones, in particular the mode
of cut (Vc, a, P), the matter with machined, the nature of the reported plates
of the cutting tools, the machine tools used are not identical. We can say that
the cutting speed Vc is a significant parameter influencing the damage and the
behaviour of the tool.
Test 2: Evolution of wear (Vb) and determination of the behaviour of
the tool out of metal carbide according to time for various depths of cut (Fig.
4, 5):
|
| Fig. 4: | Variation
of cutting tool wear (Vb) according to time for different depth of feed
(Vc = 100 m min-1, a = 0.08 mm tr-1) |
|
| Fig. 5: | Experimental
behaviour for different depth of feed, Vb = 0.3 mm |
For a cutting speed and advance constants, one varies the depth of cut. The advance a = 0.08 mm tr-1, the cutting speed Vc = 100 m min-1 and the depths of cuts (p1 = 0.5 mm, p2 = 1.5 mm).
Figure 4 presents the wear of the tool according to the depth of cutting (p1 = 0.5 mm and p2 = 1.5 mm), on the other hand the cutting speed (Vc) and advance (a) are constant. If the depth of cutting increases the wear of the tool example increases if we takes t = 50 min of machining, we obtains the wear of the tool equal to Vb = 0.4 mm for p = 1.5 mm but if we takes p = 0.5 mm, the wear of the tool is of Vb = 0.28 mm.
It is noted that the increase the depth of cutting increases the wear of the tool.
The statistical processing of the results of the behaviours according to the depth of cut (Fig. 5) allowed the deduction of mathematical model, expressing the functional relation between the lifespan and the depth of cut given by the Eq. 4:
|
| Fig. 6: | Variation
of cutting tool wear (Vb) according to time for different cutting advancement
(a) |
|
| Fig. 7: | Experimental
contains for different advancement, Vb = 0.3 mm |
Test 3: Evolution of wear Vb and determination of the behaviour of the
tool out of metal carbide according to time for various advances of cut (Fig.
6, 7):
For a cutting speed and a depth constants, we varies the advance of cut (a = 0.08 mm tr-1, a = 0.32 mm tr-1) with a cutting speed Vc = 100 m min-1 and a depth p = 1.5 mm.
Figure 6 allows even influence of the variation of the parameter of the advance of cut on wear Vb of the tool. It is noted that if the value of the advance of cut increases the value of wear also increases, for example, for one duration of machining of 20 min, Vb wear is 0.2 mm with an advance of 0.08 mm tr-1, on the other hand for a value in advance of 0.32 mm tr-1, wear increases with 0.3 mm.
The statistical processing of the results of the behaviours according to the advance (Fig. 7) allowed the deduction of the mathematical model expressing the functional relation between the lifespan and the advance in the form Eq. 5:
The behavior T of the tool according to the advance according to the deduction of the mathematical model connecting the behaviour of the tool compared to the advance Eq. 5. It is noted that the behaviour of the tool decreases when we increases the advance (Fig. 7).
Multifactorielle method: The tests carried out allowed the follow-up
of the evolution of wear according to time for the various combinations (Fig.
2, 4, 6).
The curves of the Fig. 8 have the results of different experimental combinations from the modes of cut (cutting speed, advances, depth of cutting) according to time. Following these results, we notices the same experimental results of the tests previously presented. For two identical experiments, if we increases the cutting speed (Vc1 = 36 m min-1, Vc2 = 100 m min-1), the wear of the tool increases. Same manner if we increases the advance of cut (a1 = 0.08 mm tr-1, a2 = 0.32 mm tr-1), the wear of the tool increases also even for the increase the depth of cutting (P1= 0.5 mm, P2 = 1.5 mm).
With the tests of Fig. 8, we could deduce the life spans
from the cutting tools represented by the Fig. 9 according
to the model of Taylor. The graphic treatment of these results led to the determination
of the life spans corresponding to the various conditions of tests. The statistical
processing of the behaviours allowed the deduction of mathematical model of
the modified equation of Taylor 2. The latter express the functional relation
between the lifespan and the elements of mode of cut in the form of Eq.
6 (Lelong-Ferrand and Arnaudies, 1977).
Figure 9 is a histogram which recapitulates the various experiments realized previously according to the various combinations of the parameters of cut (cutting speed, advances, depth of cutting). We notes that the greatest behaviour (lifespan of the tool) is according to the parameter cutting speed smallest (Vc = 36 m min-1), then that of the smallest advance (a = 0.08 mm tr-1) and in end the depth of the smallest cutting (p = 0.5 mm).
Micrography of wear Vb of the plates P30 after machining on a steel XC38
not hardened according to time: Figure 10a-d
show the various wears Vb recorded on plates out of metal carbide P30 according
to time on steel XC38 test-tubes not hardened.
The photographs on the Fig. 10 they is images made with
the MEB of stop work of the plates of the tools for each test carried out in
order to even the degree of wear.
|
| Fig. 8: | Variation
of cutting tool wear (Vb) according to time for different combinations |
|
| Fig. 9: | Experimental
behavior for different combinations, Vb = 0.3 mm |
The micrographic one of the wear of the plates to pay out of metal carbide
according to variation of the time of machining of steel XC38 not soaked shows
that the wear of the plate is not significant and that the tools resisted the
criteria of wear well.
Case of the operations of slide-lathing of hardened steel XC38
Heat treatment (Hardening, Returned): According to the percentage
out of carbon and their volume, the steel XC38 test-tubes have were put in a
furnace heated until a temperature of 850EC, after 2 h, the parts are soaked
in water. Then, the test-tubes have undergoes an income to know: we gives the
test-tubes in the furnace, at the temperature of 250EC left we them during 2
h with the furnace then cooled with the free air with 20EC. The conditions of
the heat treatment are presented in the Fig. 11.
|
| Fig. 10: | Micrographics
of wear Vb for metallic carbide layer to P30 nuance at Vc = 100 m min-1,
a = 0.32 mm tr-1 and p = 1.5 mm. (a) : tc = 10 min, (b) : tc
= 20 min, (c) : tc = 40 min, (d) : tc = 60 min |
The average hardness of the test-tubes obtained at the end of the operations
is of 112 HRB.
We take again the same tests carried out for the case of the operations of slide-lathing of the steel soft centre, respectively.
The first test consists in varying the cutting speed (Vc1 = 36 m min-1, Vc2 = 100 m min-1) and to keep the advance a = 0.08 mm tr-1 and the depth of cut p = 0.5 mm constant.
One on the other hand notices on the Fig. 12 that for a time of 2 min machining, wear strips Vb of them is 0.18 mm for a cutting speed of 36 m min-1, Vb = 0.34 mm for a cutting speed of 100 m min-1. Also, the increase cutting speed influences the value of wear. The value of wear is larger with that of the test of the Fig. 2 because of the hardness of the machined part (the soaked part).
From Fig. 12, we deduces the histogram from the behaviours (lifespan) (Fig. 13) the functional relation between the lifespan and the cutting speed is given by Eq. 7:
We notes on the Fig. 14, a much reduced time of cut compared to the case of the parts not soaked with almost identical values of wear.
|
| Fig. 12: | Variation
of cutting tool wear (Vb) according to time, (a = 0.08 mm/tr, p
= 0.5 mm) |
|
| Fig. 13: | Experimental
behaviour for different cutting speed, Vb = 0.3 mm |
The functional relation between the lifespan and the depth of cut is in the form Eq. 8:
The experimental behaviours with various depths of cut for the soaked parts do not exceed 4 min (Fig. 15) compared to those of the parts not soaked which exceeds the 50 min (Fig. 5).
|
| Fig. 14: | Variation
of cutting tool wear (Vb) according to time and depth of feed, a = 0.08
mm tr-1, Vc = 100 m min-1 |
|
| Fig. 15: | Experimental
behaviour for different depth of feed, Vb = 0.3 mm |
Figure 16 shows the influence of the variation of the parameter of the advance of cut on wear Vb of the tool, we notes that if the value of the advance of cut increases the value of wear also with a time of cut does not exceed the 3 min increases a much reduced time of cut.
The functional relation between the lifespan and the advance is represented by Eq. 9:
The behaviour T of the tool according to the advance according to the deduction of the mathematical model connecting the behaviour of the tool compared to the advance Eq. 9. It is noted that the behaviour of the tool decreases when the advance at cutting speed and constant depths of cut are increased, do not exceed 1 min (Fig. 17).
Multifactorielle method: The tests carried out allowed the follow-up
of the evolution of wear according to time for the various combinations of the
experimental design (Fig. 12, 14, 16).
|
| Fig. 16: | Variation
of cutting tool wear (Vb) according to time and advancement. Vc = 100
m min-1, p = 1.5 mm |
|
| Fig. 17: | Experimental
behaviour for different advancement, Vb = 0.3 mm |
The functional relation between the lifespan and the elements of mode of cut
is in the form:
The curves of Fig. 18 have the results of different combinations
experimental from the mode of cut (cutting speed, advances, depth of cutting)
according to time. We note the checking of the various experimental results
(curved) to find previously with knowing: for two experiments identical, if
we increase the cutting speed (example Vc1 = 36 m min-1
and Vc2 = 100 m min-1), the wear of the tool increases.
Same manner, if we has two experiences identical but with different advances
of cut (example a1 = 0.08 mm tr-1 and a2 =
0.32 mm tr-1), the wear of the tool Vb1 is an inferior
with Vb2. It is the same case to note for various depth of cutting
(example p1 = 0.5 mm and p2 = 1.5 mm). In end, for three
tests identical relatively, we notes that the value of wear is according to
the cutting speed initially, then of the value of the advance in second and
end of the value the depth of cutting.
|
| Fig. 18: | Variation
of cutting tool wear (Vb) according to time and different combinations |
|
| Fig. 19: | Experimental
behaviour for different combinations of experiences plan, Vb = 0.3 mm |
By comparison of Fig. 8 and 18, the value
of wear is different following the increase from the hardness of the soaked
test-tube. It is noticed that the values of the wear of the tool are definitely
higher than those with the experiments ridges with a test-tube not hardening.
The behaviour of the tool for the various experiments realized according to the various combinations of the parameters of cut (cutting speed, advances, depth of cutting), shows that the greatest lifespan of the tool is according to the parameter cutting speed smallest, then that of the smallest advance and in end the depth of the smallest cutting.
In end, if we compare the results of the behaviour tool presenting it on Fig. 19 with those obtained and presenting on Fig. 9, we notice that the behaviour of the tools for these experiments is not the same bus the hardness of the test-tubes is not the same one, although the mode of cut is the same one.
Micrography of wear V B of the plates P30 after machining on a steel XC
38 hardened of hardness 112 HRB: Figure 20a-d
show, the micrographic variation of the wear of the plate out of metal carbide
is according to the variation of the time of machining initially, the mode of
cut and the hardness of the work piece. We notice that wear is significant as
the time of machining increases is that the hardness of the test-tube increases.
DISCUSSION
The analysis of these results shows that the cutting speed has a significant
influence on wear, indeed with the increase in the latter; wear increases in
a remarkable way (Fig. 2, 12).
It comes out from this analysis that the effect the cutting speed over the
lifespan is more pronounced for speeds high (Fig. 3, 13)
it is explained by the fact why when one works with higher cutting speeds:
| • |
The machining system becomes unstable because of the great vibrations
recorded during machining |
| • |
The temperature in the zone of cut increases (in particular with the interfaces
chip surfaces of attack and part surfaces of principal skin, which results
in a loss of hardness as well as the manifestation of the various mechanisms
of wear of the tool |
As the Fig. 4, 6, 14
and 16 show it that it is in the case of the machining of
steel XC38 hardened or not, it is noted that wear increases with the increase
in advance and the depth of cut. This rise in wear induced to a degradation
of the surface quality of the part machined according to time. The increase
in the advance enormously decreases the behaviour of the tool which does not
exceed 1 min (Fig. 17). Amongst other things, the wear of
the tool presented by the Fig. 10 and 20
show the variation of the damage of the tool according to time with selected
modes of cuts.
These results are similar to those published by Schultz
(1997), Schneider, (2002) and Poulachon
and Moison (2003), whose authors prove that the parameters of cut play a
very significant role in the surface quality of the parts more the surface quality
of the tool after machining
|
| Fig. 20: | Variations
Micrographiques de l=usure Vb du carbure métallique de nuance P30,
à Vc = 100 m min, a = 0.32 mm/tr, et p = 1.5 mm pendant l = usinage
de l = éprouvette en acier XC38 trempée : (a) : tc= 10s,
(b) : tc = 20s, (c) : tc = 30 sec, (d) : tc = 1 min |
After having evaluated wear Vb for procedures of tests which take into account
the variation of the various parameters of cut (Vc, a and P), we noted that:
| • |
The tool out of metal carbide of nuance P30 resists wear for
the machining of half-hard steel XC38, to reach acceptable wear Vb = 0.3
mm. It him took a time of cut T = 60 min for a cutting speed Vc = 100 m
min-1 and T = 140 min for Vc = 36 m min-1 |
| • |
Hardened steel XC38 of hardness 112 HRB is very difficult to machine,
the tool of nuance P30 to reach acceptable wear very quickly (T = 1 min
for Vc = 100 m min-1 and T = 4 min for Vc = 36 m min-1) |
| • |
The surface quality obtained in the case of the machining of hardened
steel XC38 is better compared to that obtained during the machining of steel
XC38 not hardened |
The modes of wear observed are those of abrasion and diffusion which appears on surface in skin and the surface of attack.
The analysis of the curves of follow-up of wear according to time shows that the pace of the latter obeys the universal law of the wear of any machine element.
The influence of the advance and that the depths of cut on the behaviour are not very significant compared to the cutting speed.
The mathematical model suggested expresses the qualitative and quantitative relation between the wear and the elements of the mode of cut. It is necessary for optimization and the industrial exploitation.
Following the experiments carried out on the soaked test-tubes, we noted and
checks the same observations quoted in the references (Schultz,
1997; Schneider, 2002; Poulachon
and Moison, 2003) that the surface quality is better and does not ask for
an operation of correction.
This comparative study made it possible to establish the variations of performance of the tool out of metal carbide of nuance P30 during the machining of two series of hardened steel XC38 test-tubes and not soaked with the same modes of cut.
NOMENCLATURE
| L |
= |
Length (mm) |
| Ø |
= |
Diameter (mm) |
| % C |
= |
Percentage of carbon |
| % Mn |
= |
Percentage of manganese |
| % Si |
= |
Percentage in silicon |
| Rm |
= |
Maximum resistance to traction (MPa) |
| Re |
= |
Normal load with the elastic limit (MPa) |
| Rp,0.02 |
= |
Conventional load with the elastic limit (MPa) |
| Amin |
= |
Elongation at the fracture of the sample (%) |
| α |
= |
Clearance angle (°) |
| γ |
= |
Angle of attack (°) |
| λ |
= |
Angle of inclination of edge (°) |
| Rε |
= |
Radius of nozzle (mm) |
| WC |
= |
Carbon of tungsten |
| TiC |
= |
Carbon of titanium |
| Co |
= |
Cobalt |
| Vb |
= |
Cutting tool wear(mm) |
| KT |
= |
Wear in crater (mm) |
| T |
= |
Behaviour of the tool (lifespan) (min) |
| C1 |
= |
Constant depends on the nuance of the tool and machined material |
| n |
= |
Constant |
| Vc |
= |
Cutting speed (m min-1) |
| C2 |
= |
Constant depends on the nuance of the tool and machined material |
| P |
= |
Depth of cut (mm) |
| a |
= |
Advance (mm tr-1) |
| tc |
= |
Time of cut (min) |
| x, y |
= |
Exhibitors depending on the nuance of the tool |
| P30 |
= |
Nuance of carbide of the tool |
| XC38 |
= |
Steel XC38 |
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