Induction Hardening of GearsGear-performance characteristics (including load

condition and operating environment) dictate the re-quired surface hardness, core hardness, hardnessprofile, residual stress distribution, grade of steel,and the prior microstructure of the steel.

In recent years, gear manufacturers have gainedadditional knowledge about how technology can beused to produce higher quality induction hardenedgears and gear-like components with less distortion.The application of this knowledge has resulted in gearsthat are quieter, lighter, and lower cost with an in-creased load-carrying capacity to handle higher speedsand torques, while generating a minimum amount ofheat and requiring minimum or no post grinding.

Steels containing 0.40 to 0.60% carbon contentare commonly specified for induction hardening ofgears[1-3]; for example, AISI 1045, 1552, 4140,4150, 4340, and 5150. In some cases, steels withhigh carbon content are used (e.g., 52100). De-pending on the application, tooth hardness after tem-pering typically is in the 48 to 60 HRC range.

Gear Spin Hardening (Encircling Inductors)Spin hardening is the most popular approach for

hardening gears having fine- and medium-size teeth.Gears are rotated during heating to ensure an evendistribution of energy. Single-turn or multi-turn induc-tors that encircle the whole gear can be used[1].

When applying encircling coils, it is possible toobtain substantially different hardness patterns byvarying process parameters. Figure 1 illustrates a di-versity of induction hardening patterns that were ob-tained with variations in frequency, heat time, andcoil power[1].

As a rule, when it is necessary to harden only thetooth tips, a higher frequency and high power den-sity should be applied; to harden the tooth roots, usea lower frequency (Fig. 2). A high power density incombination with the relatively short heat time gen-erally results in a shallow pattern, while a low powerdensity and extended heat time produces a deep pat-tern with wide transition zones.

Quite often, to prevent problems such as pitting, spalling, tooth fatigue, and endurance, it is requiredto harden the contour of the gear, or to have gear-contour hardening (Fig.3). This often maximizes ben-eficial compressive stresses within the case depth andminimizes distortion of as-hardened gears.

In some cases, obtaining a true contour hardenedpattern can be a challenging task due to the differ-ence in current density (heat source) distribution andheat transfer conditions within a gear tooth. Two mainfactors must be optimized to obtain the contour hard-ness profile.

HEAT TREATING PROGRESS • OCTOBER 2009 9

PROFESSOR INDUCTIONby Dr. Valery I. Rudnev, FASM, Inductoheat Group

Professor Induction welcomes comments,questions, and suggestions for futurecolumns. Since 1993, Dr. Rudnev has been onthe staff of InductoheatGroup, where he currentlyserves as group director– science and technology.He has 28 years of experience in inductionheating. His expertise is in materials science, metallurgy, electromagnetics, heattreating, computer modeling, and process development. Credits include 21 patents and154 publications.Contact Dr. Rudnev atInductoheat Group32251 North Avis DriveMadison Heights,MI 48071;tel: 248/629-5055;fax: 248/589-1062;e-mail: rudnev@inductoheat.com;www.inductoheat.com.

Single-coil dual-frequency induction hardening of gears

Fig. 1 — Variety of induction hardening patterns obtainedusing variations in frequency, heat time, and coil power.

Fig. 2 — Frequency influence on eddy current flow within thegear when using an encircling coil[1].

Fig. 3 — Contour hardened gears.

High ffrequency LLow ffrequency

Gear range rrange

Coil

Eddy ccurrent

Gear

Coil

The first factor is that with encircling-typecoils, the root area does not have as goodelectromagnetic coupling with the inductorcompared with the coupling at the geartip. Therefore, it is more difficult to induceenergy into the gear root. Secondly, thereis a significant heat sink located under thegear root (below the base circle).

Traditional Gear Hardening Techniques

The power pulsing single-frequency con-cept was developed as the first step in over-coming difficulties in obtaining contour-likehardness pattern[1-3]. Power pulsing pro-vides the desirable heat flow toward theroot of the gear without appreciably over-heating the tooth tip, enabling the attain-ment of the desired metallurgical structuresand helping to obtain gear contour-like hard-ness pattern. Depending on the applica-tion, preheat can consists of several stages(preheat power pulses). Obviously, pre-heating reduces the amount of energy re-quired in the final heat.

After preheating, there might be a soaktime ranging from two to ten seconds toachieve a more uniform temperature distribution across the teeth of the gear.Final heat times can range from less than

one second to several seconds. The power pulsing dual-fre-

quency concept is a further im-provement in the attempt to ob-tain a true contour hardnesspattern and to minimize gear dis-tortion. Two different power sup-plies are required. The idea ofusing two different frequencies toproduce the desired contour-likepattern has been around since thelate 1950s. Since then, severalcompanies have pursued thisidea, and different names and ab-breviations have been used to de-

scribe it. Inductoheat built its first contour-hardening machine that applied a dualfrequency concept in 1986. Since thattime, the process has been refined. How-ever, regardless of the differences inprocess recipes, the basic idea remainsthe same.

The gear is induction preheated to atemperature determined by the processfeatures, typically being 350 to 100°Cbelow the critical temperature Ac1. The pre-heat temperature depends on the type andsize of the gear, tooth shape, prior mi-crostructure, required hardness pattern,acceptable distortion, and the availablepower source. Usually, preheating is ac-complished by using medium frequency(3 to 10 kHz). Lower frequency results ingreater eddy current penetration depths,which lead to more “in-depth” preheatingeffect. A high frequency (30 to 450 kHz)and high power density are applied duringthe final heating stage.

Depending on the application, a single-coil design or two-coil arrangement (Fig.4top and bottom, respectively)can be used. A single-coil de-sign has many limitations.Some of them are related to

low reliability and maintainability of in-duction coil. With the second-coil arrange-ment, one coil provides preheating, andanother coil final heating. Both coils worksimultaneously if the scanning mode is ap-plied. In the case of a single-shot mode, atwo-step index-type approach is used (Fig.4, bottom).

In some cases, dual-frequency machinesproduce parts having lower distortion andhaving more favorable distribution ofresidual stresses compared with using single-frequency techniques. However, dependingon tooth geometry, the time delay betweenlow-frequency preheating and high-fre-quency final heating could have a detri-mental negative effect for obtaining trulycontour hardened patterns or in many cases,is not capable of minimizing distortion.

Single-coil Dual Frequency Hardening

Some induction practitioners have heardabout simultaneous dual frequency gearhardening, which uses two single-fre-quency inverters working on the same coilat the same time. Low frequency helps toaustenitize the roots of the teeth and highfrequency helps to austenitize the teethflanks and tips.

However, it is not advantageous alwayshave two different frequencies working si-multaneously all the time. Many times, de-pending on the gear geometry, it is prefer-able to apply lower frequency at thebeginning of heating cycle, and afterachieving a desirable root heating, thehigher frequency can complement the ini-tially applied lower frequency, completinga job by working together.

10 HEAT TREATING PROGRESS • OCCTOBER 2009

Fig. 4 — Conventional dual-frequency induction hardeningusing a single inductor (top) and two inductors (bottom).

Fig. 5 — Sketch of the circuitry of single-coil dual frequency inverter.Fig. 6 — Wave forms of coil current and coil voltage of a single-coil dual frequency hardening technology.

Inductor

Medium ffrequency HHigh ffrequencyLow ppower ddensity HHigh ppower ddensity

Quench rring

Medium frequency,low ppower

density

High frequency,high ppower

density

480 VV3 �

Disconnect CCs Cs

Ls

Cs Cs

L10C10

Induction coil

High-ffrequencymodule:600 WW,

150 –– 4400 kkHzCurrent ffed iinverter

Medium-ffrequencymodule:

600 WW, 110 kkHzVoltage ffed iinverter

Heating-ccoil vvoltage

1.530 mm 11.550 mm 11.570 mm 11.590 mm 11.610 mm 11.630 mmTime, mmsec

Heating-ccoil ccurrent

1.530 mm 11.550 mm 11.570 mm 11.590 mm 11.610 mm 11.630 mmTime, mmsec

For example, Inductoheot has recentlyshipped a single-<:oil dual frequency in­

duction gear hardening system to aleading supplier of automotive engine and

powertrain components. Figure 5 showsa sketch of the circuitry of the new Indue­

toheat gear-hardening system. Figure 6shows wave-forms aF coil current ond coil

voltage when two oppreciably different

frequencies ore applied ot the same time

to a single inductor. This machine was de­signed specifically far induction hardeningof internal wide-face, gear~ike component

having a minor gear diameter of 6.9 in.

(176 mm) and 0 major gear diameter of7.3 in. (186 mm) using a single·shot

heating mode.The totol power exceeds 1,200 kW

comprising medium-frequency (l0 kHz)

and high-frequency (120 to 400 kHz) mod­ules working not just simultaneously, but

in ony sequence desirable to optimize

properties of the gears heat·treated usingthis unique technology. Total heat time was

minimized to about 1.5 second. At the be­ginning of heating cycle, a medium fre-

quency is applied for about 0.8 s pro­viding the required root heating. For the

remainder of the heat cycle, two Frequen­cies were working together complementing

each other.Inductoheot's two-frequency induction

geor-hordening system (Fig.?) also has

some "auto-match" items to simplify luning.It is rugged and will be used far high­

volume single·shot hardening of severalpowertrain components, dramatically min­

imizing distortion of heat treated ports and

providing a superior hardness pattern withfavorable distribution af residual stresses.

lastly, recently patented Inductoheat IFP(independent frequency and pawer con­

trol) technology provided on opportunity

for using the variable frequency approachfor tailoring the hardening pottern per gear

geometry. HlP

References1. V. Rudnev, D. loveless, R. Cook, and M.Block, Handbook ofIndUe/ion Healing, MarcelDekker, 2003,2. V. Rudnev, Induction hardening aFgoors andcritical components, Pori 1., Gear Technology,

•-

Fig. 7 - IndUllaheal's single-loil dualfreqUeRly syslem comprises medium-frequency( 10 • Hz) and high·frequency (1 20 10 400 • Hz)madules wor.ing simultaneausly ar in anysequence desirable 10 oplimize properlies 01lhe heaHrealed gean; lolal power uueds1,200.W.

P 58-63, Sepl./Oct. 2008.3. V. Rudnev, Induction hardening oFgoors andcrilical components, Pori 2., Gear Technology,p 47-53, Nov./Dec. 2008.

HEAT TREATING PROGRESS. OCTOBER 2009 I I