Gear Heating Patterns- Control distortion during heat treatment

8/13/2019 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.

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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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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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Gear Heating Patterns- Control distortion during heat treatment