14-01-2014, 03:28 PM
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.
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).
RESULTS
Tool wear and tool life: Maintaining tool wear to a
minimum level is a great challenge in machining of
hardened steel. Experiments in this study have
demonstrated that tool wear in end milling of AISI D2
hardened steel without preheating could be quite severe
and catastrophic. As shown in Fig. 5, the flank wear
exceeded the limiting value of 0.3 mm in less than 7.33
min of machining time after cutting a length of 330 mm
with a cutting speed of 56.57 m min−1, feed of 0.1
mm/tooth and depth of cut of 1.0 mm. In this case the
tip of cutting tool insert ended up with breakage.
However, under the same cutting speed but with a
lower feed (f =0.044 mm tooth−1) the situation
improved and the tool wear reached the limiting value
of 0.30 mm after about 35 min of machining (Fig. 6).
Preheating of work-piece has been found to further
increase the tool life by reducing the tool wear rate
quite significantly.