07-11-2012, 11:57 AM
Hot Machining of Hardened Steels with Coated Carbide Inserts
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Abstract:
Problem statement: The benefits of easier manufacture of hardened steel components can
be substantial in terms of reduced machining costs and lead times compared to the traditional route
involving machining of the annealed state followed by heat treatment, grinding/EDM and manual
finishing. But machinability of hard material through conventional machining is hindered due to
excessive wear of the cutting tools and differently in achieving desired quality of the machined surface.
In end milling the cutting tool is not in constant operation and so undergoes a heat cycle during the
intermittent cutting. This alternate heating and cooling of the inserts lead to the thermal cracks and
subsequently failure of the tool.
INTRODUCTION
Hardened steel is one of the difficult-to-cut
materials. During the last few years numerous studies
have been conducted to improve the machinability of
this kind of materials and to explore and develop new
techniques to minimize machining costs while
maintaining the quality requirements of the machined
parts. The benefits of direct manufacture of components
from hardened steel are expected to be substantial in
terms of reduced machining costs and lead times
compared to the traditional route of machining in the
annealed state followed by heat treatment, grinding or
Electrical Discharge Machining (EDM) and manual
finishing[1]. Recent advances in machine tool
technologies coupled with improved cutting tool inserts
have opened up new opportunities for investigation in
machining of hard materials especially for their bulk
removal. Hot machining process which includes
preheating of work-piece is gaining interest as it results
in reduced shear strength creating a condition
conducive to metal cutting[2]
MATERIALS AND METHOD
The machining operation was carried out on a
Vertical Machining Center (VMC) using a 40 mm
diameter tool holder fitted with Sandvik 1030 PVD
coated carbide inserts. End milling operation was
performed under dry cutting condition with a 5 mm
constant radial depth of cut. Experimental set-up for hot
machining of AISI D2 hardened steel is shown in Fig. 1.
One edge out of the four cutting edges of a tool insert
was used for each set of experimental conditions. Thus
machining was initiated with a new sharp edge of an
insert and continued for a 100 mm pass of cut followed
by checking of the flank wear. This procedure was
continued until the flank wear of the tool reached a
magnitude of 0.30 mm. Olympus tool maker
microscope was used to measure the flank wear with a
magnification of (20 x). The 0.30 mm flank wear
criterion was adopted in accordance with the ISO
standard (ISO standard 8688-2, 1989 for tool life
testing of end milling).
Selection of machining conditions:
The cutting
conditions were selected primarily by considering the
recommendations made by cutting tool manufacturer
(Sandvik tools) and the knowledge on practices
gathered through contemporary literatures on hard
machining. Selected three main parameters: Cutting
speed, feed and preheating temperature were changed
while the axial depth of cut, d was kept constant at 1
mm. The ranges of parameters used for experimentation
were: Feed, f: 0.02-0.044 mm tooth-1; cutting speed,
Work and cutting tool materials: The work material
as received from supplier was in the form of a block
hardened by oil quenching and tempered to a hardness
range of 56-62 HRC having 300´250´100 mm in
dimension. Hardness of work material was verified and
found to comply with the supplier’s specifications as
showed in Table 2.
As mentioned earlier the material used for
machining operation was AISI D2, the microstructure
(1000´ magnification) of which is shown in Fig. 3.
The end milling tool holder was a Sandvik
Coromill 390 Endmill: R390-020B20-11L employing
indexable inserts having code: Sandvik 1030 Coromill
290 R290-12T308E-PL. The TiAlN coated carbide
inserts having four sided cutting edges were used as
received from the supplier. Figure 4 shows a schematic
diagram indicating the geometry of tool insert
(Sandvik 1030) as coated through PVD method by
manufacturer with relevant dimensions in Table 3.
CONCLUSION
Through the end milling of preheated AISI D2
hardened steel by using TiAlN coated carbide cutting
tool it can be concluded that an overall enhanced
machinability is achievable by preventing catastrophic
damage of the cutting tool at higher levels of feed and
cutting speed.