gear heating patterns- control distortion during heat treatment
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32 GEAR SOLUTIONS JULY 2007 gearsolutionsonline.com
Combining process monitoring
and a diagnostic tool to develop
a process for contour (profile)
hardening gear teeth or tooth-
like objects on parts having gear-like geom-
etry can eliminate costly and most often
destructive quality checks. The process toolis an extension of the dual pulse induction
hardening (DPIH) process.
A high-value gear requires a hard wear-
resistant surface with a soft core. A gear
transmits torque, so its teeth are subjected
to a combination of cyclic bending, contact
stresses, and different degrees of sliding or
contact behavior. This makes it critical for
a gear to have a proper case/core structure,
a condition that can be achieved using a
surface hardening process, such as induc-
tion hardening, which is discussed in thisarticle.
Contour gear hardening processes are
currently performed using one or two power
supplies. The process goal is to achieve a
uniform case depth both at the root, as well
as along the pitch diameter and the tip. This
pattern uniformity is achieved by maintain-
ing a definite range of temperatures between
the tip and the root area, which depends on
the geometry, pitch, and other gear charac-
teristics.
Contour Hardening ProcessDevelopment
Figure 1 illustrates the dual-pulse induc-tion hardening process for gear profilehardening. Other known methods ofprofile hardening gears are dual frequencyheating, variable frequency, and simultane-
ous frequency. A common process variablein all of these processes is that the partmust be rotated at a high speed for uniformsurface heating during the short heatingcycle.
The profile nature of the case depths at
the root area and the tip area can be adjusted
for a given gear geometry by changing pro-
cess variables; these are preheat (time and
power), soak (time), and final heat (power
and time). There virtually is no scientific
method to alter these variables to predict
the K ratio, defined as the ratio of the casedepth at the tip of the gear to the case depth
at the root (Fig. 2). Typically, a process is
derived by means of a trial and error method
to produce a profile hardness pattern with a
K ratio close to 1. Table 1 lists the values for
preheat, soak, and final heat (in seconds)
and the temperatures achieved for the steps
in the development of a typical profile or
contour hardening process.
A number of tests were conducted using
the various process steps mentioned above.
Samples were sectioned and the K rat
were measured. Table 2 shows case dep
at two critical areas and their correspondi
ratio. Sample number 15 is an ideal pro
hardened sample, while number 18 has
case at the root, and sample number 20 h
a through hardened gear pattern. Figurshows two examples of gears having diff
ent K ratios. The gear at the top has a hea
case at the tip, similar to sample numb
18, while the gear at the bottom has a mo
pronounced case at the tip (also known
through hardened) similar to sample nu
ber 20.
Gears are sectioned in all contour ha
ening processes to examine the harden
profile until achieving a profile pattern
in Fig. 2. This destructive process is n
only part of process development (Fig. but also it is used during a production ru
where parts are cut at regular intervals
monitor quality.
Proposed ProcessDevelopment Method
The tool used for this process is a hig
speed infrared camera with software to c
brate, freeze, and record temperatures at
end of preheat, the end of soak, and at t
end of final heat cycles (Fig. 5). By contr
DIAGNOSTIC TOOLHELPS DEVELOP PROFILE FOR
INDUCTIONHARDENING
PROCESS
A high-speed infrared camerawith software to calibrate, freezeand record temperatures aftereach heat cycle is used as adiagnostic tool to develop aprocess for profile hardeninggears or gear-like objects. The method caneliminate costly and most often destructive
examination.
**Member ASM International and member, ASM Heat Treating So
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Figure 4 Typical setup procedures to
achieve a contour pattern
Table 2 Test results from sectioned contour hardened
gears using different process variables
Figure 7 Monitoringtemperatures during cotour hardening; uniformheating of root and tip(a) and heat pattern duing the heating cycle (b
A
B
Figure 3 Different hardening patterns on gears resulting from using different proces
variables
Figure 5 Diagnostic tool used for pro-cess development consists of a high-speeinfrared camera with software to cali-brate, freeze, and record temperatures atthe end of preheat, the end of soak, and athe end of final heat cycles
Figure 6 Visible heatpattern on gear duringthe final heating stage the hardening process
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gearsolutionsonline.com JULY 2007 GEAR SOLUTIONS
ling the settings (power and time),
an exact heat cycle can be defined
without cutting gear samples to ver-
ifying case depths. It is possible to
achieve the desired profile harden-
ing pattern in gears by monitoring
the surface temperatures at speci-
fied regions. Figures 6, 7 and 8 illus-
trate the different stages of heating
and soaking cycle during processing
of a gear. Regions of nonuniformheating at the surface of the gear are
shown in Fig. 8, which indicate that
the process is far from optimum.
The DPIH process or any other
gear profile hardening processes
can be optimized using the pro-
posed type of diagnostic tool,
which will result in a uniform tem-
perature between the root area and
tip region. Figure 9 shows such a
process developing step.
The DPIH process was used toheat treat gears at various levels of
preheat time, soak time and power
levels. The high-speed camera mea-
sured temperatures at four critical
areas of the gear (Fig. 10). By com-
paring the temperatures of the four
areas, an optimum process setting
was possible for that particular gear.
The process was verified by section-
ing samples at different heat treat
process variables. Figure 11 shows
cross sections of gears having dif-
ferent hardened profiles resultingfrom using different power settings
for each gear.
The other characteristics of this process is a closed-loop system that
sends a signal to the power supply to alter the process frequency (variable
frequency process), or to adjust the ratios of the amount of high to lower fre-
quency (simultaneous frequency process) at different power settings. This
allows the process to be altered during the heating cycle. Figure 12 shows a
series of photos at different stages during profile hardening of a gear using
simultaneous dual frequency. Both the frequencies and amount of power
were altered to achieve uniform surface temperatures at critical regions to
achieve a uniform contoured hardening pattern.
Figure 11 Gear cross sections show various hardening
profiles resulting from different power settings.
Figure 8 Nonuniform heating
at the surface of a gear
Figure 9 Temperature record-
ed by infrared camera shows
more uniform heating at the
gear tip and root
Figure 10 Temperature is
measured at the gear root, tip,
flank, and corefour critical
locationsto establish opti-mum process settings.
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SummaryThe proposed method depends on measur-
ing temperatures of critical areas and com-
paring them with established temperatures
required to obtain a contoured gear pattern.
Temperature profiles from these regions send
signals through a closed-loop system to the
power supply to alter power level, heat time,
and also the frequency of the power supplies.
By adjusting the frequencies of the powersupplies, the power levels, and the heat times,
a contoured or a profile gear pattern can be
achieved (Fig. 13). The process is a nonde-
structive method.
FOR MORE INFORMATION:
Figure 12 Different stages during profile hardening of a gear
using simultaneous dual frequency; root heating (a), tip heating
(b), tip heating and more intense root heating (c), and uniform
heating along the gear contour by adjusting process variables (d)
Figure 13 Uniform
hardened pattern
1. U.S. Patent 4,639,279 (example); Chatterjee, M.S.
2. U.S. Patent 5, 428,208; Chatterjee, M.S., et. al.
3. GPC 2002; Dinwiddie, R. and Chatterjee, M.S.
REFERENCES:
Madhu S. Chatterjee is Director of Special Projects, Inductoheat Inc., 32251 N. Avis Dr., Madison Heights., MI 48071; tel: 248-585-9393; fax: 248-58
1062; e-mail: [email protected]; Internet: www.inductoheat.com
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