Metallurgical Insights 7

8/9/2019 Metallurgical Insights 7

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Entries in the “Metallurgical insights for induction heat treaters” series alternate with those in the “Systematic analysis of induction coil failures” series.

Heat-treat practitioners sometimes observe un-usual effects in induction hardening, such as a

striping phenomenon, a barber-pole effect, fish-taileffect, soft spotting, and a snake-skin effect. The ap-pearance of different types of a striping phenomenonwas discussed in Part 6 of Metallurgical insights forinduction heat treaters[1]. The barber-pole, snake-skin,and fish-tail effects are discussed here.

Barber-pole effectThe appearance of the striping phenomenon that

occurs during induction heating has been discussedin Ref.1 and 2. According to this phenomenon, mul-

tiple hot and cold stripes can be observed by thenaked eye on the cylinder surface during its heating.Similar striping effects can also be observed uponquenching of uniformly heated workpieces using ei-ther a single-turn or multi-turn coil with or without partrotation.

Traditionally, the striping phenomenon that ap-peared after quenching is often called a quench-striping effect or a barber-pole effect[2,3]. The barber-pole effect that appears in heat treating might notrelate to the specifics of heating, but is pr imarilyassociated with the characteristics of quenching

including: Part rotation• Specifics of spray quench flow along the work-

piece surface after the spray quench impinged(stroked) its surface

• Scan speed• Presence of quench interruptions, or formation

of steam pockets, etc.Similar to the striping phenomenon type “B” that

appears during induction heating[1,2], the barber-poleeffect has never been obtained by mathematical mod-eling. It has only been observed in induction heating

applications. Barber pole stripes are usually of spiralshapes that could occur on the surface of an as-quenched steel or cast iron workpiece, and are typ-

ically related to a nonuniform surface oxidation andscale formation.

In some cases, the barber-pole effect might not beassociated with any appreciable microstructural vari-ations of as-hardened areas of the workpiece, andwould not affect part performance. Therefore, in cases

when the visual appearance of the part surface is notcritical, the barber-pole effect might only be aes-thetically unpleasing, but has no detrimental effecton heat-treated part performance.

However, in other instances[1,3,4], the barber-poleeffect can alter the microstructural homogeneity of the hardened pattern due to improper quenchingor interrupted (or partial) quench flow. Formationof upper transformation products (including lowerand upper bainite and pearlite) within the marten-sitic structure or the appearance of undesirable tem-pered martensite can be attributed to the barber-poleeffect as well, leading to a combination of hard and

soft (partially hardened) spirals. Experience showsthat improvement in quench flow, its severity and uni-formity, or applying a quench follower, as well asmaking improvements in the eccentricity of a rotatedpart and modifying the impingement of spray strokeshelps to avoid an appearance of the barber-poleeffect.

Induction heat treaters often refer to the barber-pole effect as a phenomenon that has no associationwith an appearance of any stripes, rings, or spi-rals. Sometimes, during induction heating of cylin-ders, a shifted, or squeezed temperature profile sud-

denly appears at the workpiece surface instead of astraight heating pattern (Fig. 1). This can take placewhen using single-turn or multi-turncoils, and it doesnot appear to berelated to the helixof a multi-turn coilwinding. This phe-nomenon is usu-ally quite unstable,and when the next

part is heated, thebarber-pole effectcould disappear

HEAT TREATING PROGRESS MAY/JUNE 2009 15

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

Professor Inductionwelcomes comments,questions, andsuggestions 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 expertiseis in materialsscience, metallurgy,electromagnetics, heattreating, computermodeling, and processdevelopment. Creditsinclude 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: [email protected];www.inductoheat.com.

Metallurgical insightsfor induction heat treatersPART 7: BARBER-POLE, SNAKE-SKIN, AND FISH-TAIL PHENOMENA

Fig. 1 — Appearance of a shif ted, orsqueezed, temperature profile

(right) instead of a straight heatingpattern (left) due to the occurrenceof a barber-pole effect.

Straightthermalpattern

Shif tedthermalpattern

Heatedcylinder



and may never be seen again. If this typeof barber-pole effect is steady, it can beeliminated by slightly reducing the scanspeed and changing power and/or partrotation.

Snake-skin effectand soft-spotting phenomena

The snake-skin phenomenon representsone of variations of soft spotting. It appearsas alternating soft and hard spots withinthe as-quenched structure (looking some-what similar to the skin of the snake) andcan often be seen by the naked eye on theworkpiece surface (Fig. 2). Soft spots rep-resent regions where upper transforma-tion products were formed. This phenom-enon usually takes place when small

coil-to-workpiece gaps are applied in com-bination with a substantial quenchant pres-sure and sparsely located spray nozzles.

To improve coil electrical efficiency, in-experienced inductor builders who lackadequate theoretical knowledge might de-sign MIQ (machined integral quench) in-ductors having a substantially small coil-

to-workpiece air gap. It is important to re-member that induction hardening is a two-part process: heating and quenching.Lower than expected hardness readings(soft spots) can occur due to insufficientquenching, or trapped hot quench pockets.

Properly designed MIQ inductors or useof quenching blocks with slightly enlargedcoil-to-workpiece gaps and quench holeseliminate this undesirable phenomenonwithout having any noticeable reductionin coil electrical efficiency. Under identicalheating and quenching conditions, castirons are usually more prone to this phe-nomenon than carbon steels, because ironshave lower thermal conductivity than steels.Different variations of the soft-spotting phe-nomenon are observed when heating com-

plex-shaped parts with an interruptedquench, or if quench flow is deflected orunintentionally blocked due to geometricalcomplexity. Presence of steam pockets canalso result in appearance of the soft-spot-ting effect.

All components of an induction heattreating system, including tooling and fix-

tures, should be reviewed when soft spot-ting occurs. For example, worn bearingscan result in part wobbling during its ro-tation. That a part is rotating is sometimesdeceiving, because it creates an illusionthat workpiece rotation automatically pro-vides required uniformity of both heating

and quenching stages. However, it shouldbe recognized that worn bearings canlead to a situation where regardless of thepart rotation, certain regions can alwaysbe positioned closer to the induction coil,and during heating, those areas will ex-perience more intense heating. In addi-tion, the same areas of the part will also

16 HEAT TREATING PROGRESS • MAY/JUNE 2009

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Fig. 2 — Snake-skin pattern[2] .



be located closer to quenching deviceduring quenching, often resulting in moreintense quenching.

In contrast, if the part wobbling is meas-urable, opposite regions of the workpiececould experience appreciably less inten-

sive heating and a noticeably milderquench severity. Both factors potentiallycan result in the appearance of surfacesoft-spotting and reduced hardness casedepth.

When hardening gears and splines, itis also important to appreciate a “quench-bouncing” phenomenon where teeth of gears and splines can function as paddlesbouncing off the quenching strikes, re-sulting in nonuniform quenching and softspots.

To eliminate soft-spotting requires re-designing the spray-quench system bytaking into consideration the geometricalfeatures of the particular heat treated part.Part rotation should be smooth without anysignificant wobbling.

Fish-tail effect (field-fringing effect)Conventionally designed single-turn in-

ductors and some multi-turn coils haveareas where there is an inevitable distor-tion of the magnetic field, which could leadto an appearance of regions with a heat

deficit and where undesirable microstruc-tures can be produced. One region is re-lated to an area where copper busses thattransmit electrical current from a powersource are connected to an induction coil.These connectors sometimes have theshape of a fish tail, and the region is oftencalled a fish-tail region of the inductor.Figure 3 shows fish tail-shaped connection

copper buses used in the fabrication of ashaft hardening single-turn inductor (top)and multi-turn strip heating coil (bottom).

Figure 4a shows the location of the fish-tail effect and an instantaneous orienta-tion of the electrical current (arrows shown

in magenta color). A magnetic field isformed by incoming and outgoing elec-trical currents oriented in opposite direc-tions at the areas where connection busses(coil terminals) join an inductor copper.This results in a magnetic field distortionthat leads to a heat deficit within the cor-responding workpiece region, and is oftenreferred as the fish-tail effect (or flux-fringingeffect). One of the typical solutions to com-pensate for a fish-tail effect is to rotate thepart during heating, ensuring that all re-

gions of the workpiece absorb same en-ergy during the entire process cycle.In recent years, some induction heat-

ing manufacturers developed and paten-

HEAT TREATING PROGRESS • MAY/JUNE 2009 17

Fig. 3 — Fish tail-shaped buses of inductor terminals: (inset) single-turn inductor for hardening shafts;(bottom) multi-turn inductor for heating strip.



ted [5,6] a variety of advanced means to ef-fectively control magnetic flux, thus pro-viding compensation for the fish-tail ef-fect and magnetic field fringing atinductor terminals. To illustrate, Fig. 4band 4c show two simplified approaches

that allow compensating for the fish-taileffect and eliminate the need to rotate theheated workpiece. In both cases, im-proved electromagnetic coupling at thefish-tail region of inductor allows com-pensating for magnetic field fringing atthat location.

The fish-tail effect was one of the chal-lenges solved during development of aninnovative non-rotational crankshaft hard-ening technology. Figure 5 shows the in-ductor used in Inductoheat’s CrankPro

machine implementing patented non-ro-tation crankshaft and camshaft surfacehardening technology[2, 5-7].

According to the patented non-rotationalhardening process, an inductor consistsof two coils (Fig. 5): a top (passive) coiland a bottom (active) coil. The bottom coil,being active, is connected to a medium-or high-frequency power supply, while thetop coil represents a short circuit (a loop).The bottom coil is a stationary coil, while

a top coil can be openedand closed. Each coil has

two semi-circular areaswhere the appropriate

crankshaft journals are lo-cated.

After loading a crank-shaft into the heating posi-

tion, the top coil moves into aclosed position and power is ap-plied to the bottom coil. The cur-

rent starts to flow in the bottom coil.Being electromagnetically coupled to

the top coil, current flowing in the bottom

coil induces eddy currents that start to flowin the top coil. The induced eddy currentsare oriented in the opposite direction com-pared with a source current similar to atransformer effect. Any heated feature of

the crankshaft “sees” the inductoras a classical fully encir-

cling, highly electri-cally efficient coil withtwo fishtail regions.The fish-tail effect wascompensated for and

effectively controlledusing patented coil design in-

novations[5, 6]. Figure 6 shows

a transverse cross section of the surfacehardened crankshaft main journal. Due tothe advanced inductor design, there is

no indication of soft spots or case depthloss, which typically occurs using a con-ventional design. Non-rotational induc-tion hardening and tempering technologyprovides several benefits such as simpleoperation, superior reliability, quality,maintainability, cost reduction, andquality of induction hardened crankshaftsand camshafts[2, 5-7].

The patented approaches to control thefish-tail effect can be successfully used inother induction heat treating applications,particular where single-turn inductors are

used. When long, multi-turn inductors areused (for example, forging coils), the fish-tail effect is not pronounced and usually itis not necessary to compensate for it. HTP

References1. V. Rudnev, Metallurgical insights for induc-tion heat treaters. Part 6: Striping phenomena,Heat Treating Progress, ASM Intl., p 21-22,Nov./Dec., 2008.2. V.Rudnev, D.Loveless, R.Cook, and M.Black,Handbook of Induction Heating, Marcel Dekker,

2003.3. R.Haimbaugh, Practical Induction Heat Treating, ASM Intl., 2001.4. G.E.Totten, et.al. Bath maintenance and trou-bleshooting of polymer quench-related prob-lems for induction heat treating, Proc. ASM HeatTreat Show, p 951-956, 1997.5. D.Loveless, V.Rudnev, et. al., Induction heattreatment of complex-shaped workpieces, USPatent No. 6,274,857, Aug. 14, 2001.6. V.Rudnev, D.Loveless, Induction heat treat-ment of complex-shaped workpieces, US PatentNo. 6,859,125, Feb. 22, 2005.

7. G.Doyon, et. al., Taking the crack out of crankshaft hardening, Ind. Htg., p 41-44, Dec.,2008.

Electric current

Inductor Fish-tail(fringing)

ef fect

InductorShaft terminals

(a)

Inductor Compensatedfish-tail

ef fect

InductorShaf t terminals

(b)

Inductor Compensatedfish-tail

ef fect

InductorShaf t terminals

(c)

Fig. 4 — Appearance of fish-tail effect and itscorrections.

Fig. 5 — Patented non-rotationalcrankshaft hardening inductor.Courtesy of Inductoheat Inc.

Fig. 6 — Transverse cross section of a sur face-hardened crankshaft main journal does notreveal any traces of a fish-tail effect as a resultof using patented design corrections.

18 HEAT TREATING PROGRESS • MAY/JUNE 2009

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Metallurgical Insights 7