Evaluation of Cutting Zone Machinability During
the Turning of a X2Cr16Ni8MoTiN Stainless Steel
Jozef Jurko
Abstract
In this study presents the some conclusions of the machinability tests on a stainless steel X2Cr16Ni8MoTiN, which
aplicated in food processing industry, and describes important concurrent parameters for the cutting zone during the process
of turning. The machinability of stainless steels is examined based on the cutting tests, cutting speed presented by vc -T
diagrams. The effect of cutting speed are analysed by chip formation and tool wear mechanisms. Based on the cutting tests,
cutting speeds of 40 to 200 m/min, feed rate of 0.04 to 0.2 mm and solid carbide indexable insert WNMG 080408-MF. Tool
wear criterion, VBK value of 0.3 mm. Chip were analysed by Semi Electron Microscopy (SEM).
Keywords: turning, cutting zone, stainless steels, machinability
1 Introduction
This article presents conclusions of machinability tests
on a new austenitic stainless steels X2Cr16Ni8MoTiN and
describes appropriate parameters for the cutting zone during
the process of turning. The article focuses on the analysis of
selected domains through four basic some parameters of
steel machinability. Austenitic steels are the most extensive
and thus the most important category of stainless steels.
Cutting zone is a summary term from the region during
cutting. To properly describe the cutting zone it is necessary
to describe the regions and test parameters: Primary plastic
deformation zone (primarily an examination of phenomena
associated with the creation and formation of chips, with the
effect of the components of cutting force-the state of strain
deformation, the location of the angle of the shear level,
chip compression, the temperature field, chip shape, chip
formation and separation, the effect of the components of
cutting force). Secondary plastic deformation zone (primarily an examination of phenomena associated with friction
and cutting wedge wear, and also with the generation of
heat and temperature-the location of the grain angle, the
contact length of the cutting wedge and the face plate,
friction stress and creation of Built Up Edge (BUE), friction, the generation of heat and temperature, the mechanism
of tool wear. Tertiary plastic deformation zone (primarily an
examination of the phenomena associated with the shaping
creation of the machined surface, its profile, morphology,
qualities and inherited traits-contact of the machined surface
and the worn side plate).
Researching the cutting zone (the interaction between
the tool, the workpiece, and the chips) is to capture its state
at the moment of the creation of the chip (the so-called root
of the chip), shown in Figure 1. The process of cutting is the
mutual interaction between the tool and the workpiece,
which is controlled by many phenomena. Figure 1 is a model of the cutting zone, which illustrates the underlying
phenomena in the separation of the cutting from the
material of the workpiece. It is important to define the shear
level in the cutting zone. Weber [8] stated, that the depth of
the shear level follows the formula 0.05h≤hSP≤0.1h, where h
is the thickness of the cut section and hSP is the depth of the
shear level. The size of this local region was determined
through the help of electron microscope analysis, and the
results are displayed in Table 2. We observe elements from
the cut layer in the shear layer that have been displayed
(they melt the cutting wedge). For austenitic steel it is our
opinion that chip formation most closely follows the method, which even accounts for the element of time in chip
formation.
2 Experimental Part and results
This article concerns itself with the evaluation of selected domains of machinability in compliance with EN ISO
3685 standards [11]. The experiments were performed in
laboratory conditions and verified in real conditions during
manufacture. The set-up used contained the following components: VMF -100 CNC machining centrum and WNMG
080408-MF cutting tool. The materials to be machined were
type of a new austenitic stainless steels with chemical composition listed in Table 1. The dimension of each piece was
of diameter (d) of 120 mm and length (l) of 200 mm. The
cut-ting process was employed axial dry machining (DM),
and the cutting speed was defined at intervals of vc =40 to
200 m per min, the feed was advanced from intervals of
f=0.04 to 0.2 mm per rev., and cutting depth ap=0.4 mm.
a)
b)
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c)
Figure 1 Cutting zone and model of chip formation during
turning of steel X2Cr16Ni8MoTiN
a-cutting zone, b-detail from (a) chip texture, c-chip
production model, 1-flow zone 2-stress hardening of the
surface at the point of cutting, 3-volume of driven material,
4-slippage region, 5-beginning of creation of solid segments
of material, (ΦI -angle of the shear level, Φ2 -chip angle)
field, d-slip lines, e-limit between the deformed and nondeformed material
Austenitic Cr-Ni steels are, as a result of their higher
ductility, more prone to surface strain hardening, which
compared to construction steel can be up to 1.5 times as
great by Jurko [3]. In a non-deformed state austenitic steels
are not as hard as C45 steel, but in cases of great deformation they are greatly harder than ferritic-perlitic steel by
Jurko [4]. Sintered carbide tools are not as sensitive to
temperature on touching plates as high-speed steel, and can
be used to attain higher performance, but in this case they
have greater pressure stress, which directly influences the
process of adhesive wear by Zum Gahr [10]. The criterion
VBK=0.3 mm was applied during evaluation. The cutting
process conditions were designed based on the needs of the
material and on the operation of the finished surface. The
results of the long-term test after exhaustive analysis of
selected data are show on Figure 3. The results of this
method is Taylor equation T =
30,74.103
, where the values
vc1, 4245
shown are appropriately acquired values from analysis of
measured values, Figure 3.
where ht is the width of cutting chip, hFZ is the depth of the
Flow zone, hPD is the depth of the plastically-deformed
material, hh is the depth of the hardened machined surface
and hSP is the depth of the shear layer.
The Figure 1 shows the project of the model of chip
production in the cutting zone and it is the result of the
analysis of the examined cutting zone. Another phenomenon in the cutting zone is the marked destruction of the
X2Cr16Ni8MoTiN steel in front of the cutting edge, as
shown in Figure 2a. The result of the cutting zone
evaluation for cutting conditions (vc=180 m per min, ap=0.4
mm a f=0.1 mm per rev.) the defining of the shear plane
angle Φ1 and the texture angle Φ2. For the X2Cr16Ni8
MoTiN steel, the angle is Φ1= 29o the angle Φ2= 36o. The
important values of size of these areas are: the area of the
plastically deformed material (chip), the flow zone area and
the area of the hardening of the machined surface.
Figure 2 Plastic deformation in the cutting zone
for the X2Cr16Ni8MoTiN steel
a-deformation material before cutting wedge, b-primary
zone of plastic deformation, c- plastic deformation - slip
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Tool life log T [min]
1000
100
10
R2 = 0,9645
1
10
100
1000
Cutting speed log v c [m/min]
Figure 3 Relation between the tool life of log T and cutting
speed log vc
Damage to the cutting edge of the tool in conditions
greater than (vc=150 m per min, ap=0.4 mm and f=0.1 mm
per rev.) is characteristic of Figure 4. As has already been
shown, machining of austenitic stainless steels involves very low thermal conductivity, which dissipates heat very
slowly from the cutting zone. This often contributes to the
channel formation on the face plate.
Figure 4 Face plate damage to cutting edge
Constituent analysis was obtained with the aid of a
tricomponent piezoelectric dynamometer KISTLER 9257A.
The results are interesting from two standpoints: at smaller
values of chip cross-section (feed values up to 0.1 mm) the
specific cutting force was much higher, achieving values up
to 18500 MPa. For greater values of chip cross-section (for
feed values above 0.1 mm per rev) the value of the specific
cutting force peaks at 3200 to 6600 MPa. The region of slip
contact has an important role in turning. One of the phenolmena that results from the interaction of the workpiece and
the cutting portion of the tool is the creation of a hanging
layer on the faceplate and side plates of the tool that we call
the BUE. The BUE is one of the important factors which
influences a change in the specific cutting force, which is
formed with steel X2Cr16Ni8MoTiN mainly at cutting
speeds between 70 and 80 m per min.
Heat in the cutting zone influences deformation and
friction in the removal of the cut layer. The magnitude of
the heat depends on the cutting time, but after a certain
interval it no longer increases in the tool (this is characteristic of turning). This effect of heat has an important significance in the study of damage to the tool cutting edge and
cutting wedge wear. The heat produced is associated with
the heat field distribution in the chip (58-68 %), on the
machined surface (16-17 %), on the tool cutting edge (68%) and into air (20-7%). The author of Dautzenberg [2],
shows a dependency of average cutting wedge temperature
(as measured by thermal camera) during axial turning of
Cr20Ni10Ti steel in the form Θ m = 14,25.vc0,68 oC. Table 3
shows the measured and calculated values for X2Cr16Ni8
MoTiN steel.
The generation of heat during the cutting process causes
a change in temperature. The temperature field is defined in
spite of the lack of temperature gradient, as described in
Wei Yen, Wright [9].
For evaluating the outer surface after turning and defining the cutting process conditions, the following parameters were used in the investigation: the outer surface roughness parameter Ra [µm] was measured on measuring tools
a HOMEL TESTER 1000 C. The measured results are documented in Figure 5.
[ μm]
6,00
f=0.04 mm per rev.
5,00
f=0.06 mm per rev.
Surface roughness Ra
f=0.08 mm per rev.
4,00
f=0.1 mm per rev
f=0.2 mm per rev.
3,00
2,00
1,00
0,00
0
50
100
150
Cutting speed v c [m/min]
200
250
Figure 5 Dependence surface roughness Ra on the cutting
speed, X2Cr16Ni8MoTiN steel
The shaping and forming of chips in the cutting zone belong among the basic information regarding the investigation of cutting theory and have been dealt with by many
authors, for example Bailey and Boothroyd [1], Loladze
[5]and others. The authors’ theories are varied, although no
model for the shaping of chips can be applied to all machined materials by Usui and Hirota [7]. The process of forming chips by turning austenitic stainless steel is very problematic and is documented in intensive plastic
deformation. Small section chips (feed ƒ=0.08 to 0.1 mm
per rev, cutting depth ap=0.4 mm) are characterized by chip
sections with a section similar to Figure 6.
Figure 6 Plastic deformation in a chip section,
X2Cr16Ni8MoTiN steel, slip lines to slip fields on the chip
Sections of chips document very intensive plastic deformation in the chips themselves. The physical and mechanical characteristics of these steels predetermine their processing. In spite of their excellent qualities, they also have
their drawbacks. As a result, all information gained through
real interpretation and undertaken research on local hardening can push science in the given field further. [6]
4 Conclusions
It is important for both theory and practical applications
that essential conclusions come from measurement and analysis. Results were acquired under laboratory conditions and
performed in praxis. The conclusions are as follows: defined tool life equation following Taylor, defined the equation for the cutting strength components, designed a model
to generate chips, thermal analysis for the cutting process in
the cutting zone, confirmation of surface roughness after
cutting. Defined coefficients for kinetic machining of austenitic stainless steels, whereby X2Cr16Ni8MoTiN steel Kv =
0.52-0.55, for C45 steel, the coefficient of kinetic machining was Kv =1.0. On the basis of experience, the authors
recommend, for machining these types of steels, selecting
criteria for automated production process based on the
following order: For finished work, the criteria are set in the
following manner: outer surface quality after cutting, forming of chips, kinematic processes and dynamic processes.
This article is the result or much research work on the part
of the authors in this field and the article presents actual
conclusions that are currently being successfully implemented in machine shops.
Acknowledgement
The authors would like to thank in words the VEGA
grant agency at the Ministry of Education SR for supporting
research work and co-financing the projects: Grant work
#03/7166/2009.
To be continued on page 38
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