development of induction through-hardening and · pdf file-25-development of induction...

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NTN TECHNICAL REVIEW No.80（2012）

[ Technical Paper ]

Development of Induction Through-hardening and Induction Tempering Methodswith Temperature Control and Microstructual Control in Bearing Steel

1.Introduction

High-frequency induction heating (hereafter, IH) hasadvantages in safety, environmental impact, andheating efficiency over other heating methods. It isvery safe against fire because no combustion is used,environmentally friendly because there is no heatdissipation, and efficient because no fuel storagefacility is required. In addition, since this mechanismoffers quick heating taking advantage of electricity, itis suitable for any quick heating application. Recently,cooking devices that take advantage of thesecharacteristics, such as cooking heaters and electrickettles, are found in many homes. On the other hand, IH is attracting attention in

industrial production 1), 2), due to many advantages inproduction facilities, such as piece-by-pieceprocessing, facility space reduction, quick start/stop,and efficient processing of small-lot products. In thearea of wheel bearings, selective hardening by IH isapplied.IH has a lot of advantages in industrial production

activities. However, there were not many uses inthrough hardening for general bearings. There are tworeasons for this. One is the perception that theselective heating property of IH makes it difficult touniformly heat the entire product. The other is theconcern whether the high temperature and short

In this report, induction through-hardening and induction temperingfor bearing rings are introduced. Both technologies determine heatconditions in induction heating by utilizing temperature control andmonitoring of carbon solubility in bearing steel. This means the systemeliminates the need for qualified expertise in the heat condition designof induction through-hardening and induction tempering. The system

will promote replacements of conventional atmosphere furnaces with induction heating furnaces, andresult in a contribution to energy saving in the manufacturing process of rolling bearings andreduction of negative environmental impact.

Takumi FUJITA*

Nobuyuki SUZUKI*

* Advanced Technology R&D Center

heating could make penetration control of carbon intothe base metal with good reproducibility, which isnecessary for hardening of bearing steel. In order toclear these concerns, a design technique for theheating coil to provide uniform heating conditions anda method to control organization of the entire productare needed. In this paper, a method for high-frequency through

hardening and high-frequency induction tempering(hereafter, IH through hardening/tempering) that canminimize the heating time by controlling theorganization of the entire steel bearing raceway ring(JIS-SUJ2, hereafter SUJ2) is discussed.

2. IH through-hardening/temperingsystem

2.1 Overview of the systemPreformed material of SUJ2 with 1% carbon is

made by spherical disperse precipitation of carbidewith 15% of area ratio to improve machinability. Whenthe preformed product is heated to hardeningtemperature, carbon within the carbide becomes asolid solution in austenite. The hardening temperatureneeds to be adjusted so that the area ratio of carbidebecomes 6–8% for a good balance of fatigue strength,wear resistance, and dimensional stability 3). Forthrough hardening of bearing rings made of SUJ2 in

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Heating coilRadiation

thermometer (1)(temperaturecontrol side)

・High frequency power supply・Hardening water tank

Radiation thermometer (2)(heating stop timing side)Product

Temperaturemeasurement results

PID temperature control + hardening

¡IH unit

・Temperature control (PID)・Material prediction such as organization・Determination to stop heating

¡Personal computer

・PLC¡Controller

Fig. 1 Schematic of induction heating systemwith temperature control

an atmosphere furnace, sufficient soaking time shouldbe secured depending on the thickness so that theoverall carbide distribution is kept uniform. On theother hand, with IH, which is basically a selectiveheating technology, uniform heating is more difficultthan with an atmosphere furnace. Therefore,technology to control the entire bearing rings materialwithin the set criteria is necessary. In the following, amethod of controlling the entire material based on theresult of multiple-point measurement of bearing ringsis discussed.Fig. 1 shows a conceptual diagram of IH system

with temperature control. Two radiation thermometers are set at the heater to

measure the temperature at the surface close to thecoil and far from the coil. The point closer to the coil isheated faster since more flux penetrates it. On theother hand, the point far from the coil is heated slowlysince less flux penetrates it. Now, the position of theradiation thermometer closer to the coil (1) is calledthe temperature-controlling side and the position ofthe radiation thermometer far from the coil (2) is calledthe heating-stop-timing side. The characteristic of thistechnology is the material prediction method based onthe temperature measurement results of the materialat these positions. Together with PID control of thetemperature at the temperature-controlling side, thematerial at that position is predicted in real time, with aPC based on the temperature measurement results. Inaddition, the temperature of the heating stop timingside is also measured to predict the material at thatposition, as well. If the bearing rings are cooledimmediately after the material prediction results ofboth the positions reach the control criteria of material,hardening can be accomplished in the shortest time.In this system, the material parameter to be predictedat hardening is set to carbide area ratio, and the

Fig. 2 SEM image of carbide distribution of SUJ2quenched by atmosphere furnaces

material parameter to be predicted at tempering is setto hardness after tempering. Furthermore, the configuration of PC and controller

in Fig. 1 can also be used in the configuration withprogrammable PLC, fast-response AD and DAconversion unit, controlling unit with PID control, and atouch panel.

2.2 Prediction of carbide area ratio at IH hardeningFig. 2 shows the carbide distribution of SUJ2

hardened product made with an atmosphere furnace.The carbide area ratio in the figure is 6.3%. With thistechnology, the carbide distribution is predicted fromthe temperature measurement results using twothermometers. The prediction can be made by solvingthe diffusion equation 4) or using empirical formula.The method using empirical formula was adopted forthis system. We considered that the predictionaccuracy is higher with the method from experimentalresults than with the diffusion equation model becausethe latter needs several assumptions. The equations(1) and (2) show the empirical formula.

M = M0 exp (−(kt)n)………………………… (1)k = A exp (−E/RT)………………………… (2)

M : Carbide area ratio (%)t : Heating timeM0 : Carbide area ratio (%) of raw materialT : Temperature (K), R: Gas constantA, k, n, E: Constants obtained from experiment

A, k, n, and E in the formula are obtained byregressing the carbide area ratio of the test sampleshardened under various conditions with equations (1)and (2).

10μm


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Development of Induction Through-hardening and Induction Tempering Methods with Temperature Control and Microstructual Control in Bearing Steel

Fig. 6 SEM image of carbide distribution of SUJ2 afterquenching. The austenitization condition and thepredicted carbide area ratio are shown in Fig. 5

The carbide area ratio is obtained by analyzing theelectron microscopic image of the organization of testsamples. Fig. 3 shows the results from the calculationof the relation between heating conditions and carbidearea ratio, using the regression formula obtained fromcarbide area ratios of 50 samples of different types.The carbide area ratio significantly reduces in a shorttime when the heating temperature is higher.

Equations (1) and (2) predict the change of carbidearea ratio at constant temperature over time; however,since the temperature of bearing rings while beingheated is not constant, some ingenuity is required forcalculating the carbide area ratio from the temperaturemeasurement results. Fig. 4 shows the schematic oftemperature measurement results. When the graph isenlarged, you can see that the temperature changesin steps. The carbide area ratio Mi can be calculatedusing equations (3) and (4) based on temperature Ti

at each step.

ti indicates the time necessary for heating to reachthe temperature of the next step, converted from thechange of the carbide area ratio of the current step. Δtis the sampling period for temperature measurement.

002468

1012141618

500 1000 1500 2000

800˚C

850˚C1000˚C

900˚C950˚C

2500 3000

Time sec

Carb

ide a

rea

ratio

%

Fig. 3 Carbide area ratio as a function of holding timebefore quenching of SUJ2

0500

600

700

800

900

1000

1100

1200

20 40 60 80 100 1200

5

10

15

20

25Temperature control side (temperature)

Heating stop timing side (carbide)

Heating stop timing side (temperature)

Upper limit10%

Temperature control side (carbide)

Lower limit 6%

Tem

pera

ture

˚C

Time sec

Carb

ide a

rea

ratio

%

Fig. 5 Specific example of carbide area ratio calculatedby equations 3 and 4

Mi

Ti

M2

T2M1

T1

TimeΔt

Tem

pera

ture

Fig. 4 Schematic of time-temperature chart

In addition, the calculation of the carbide area ratio isstarted from the austenite temperature when thecarbide starts penetrating.Fig. 5 shows an example of calculation of carbide

area ratio from the temperature measurement resultsat the temperature-controlling side and the heating-stop-timing side. PID is controlled so that thetemperature-controlling side remains constant at880˚C. The carbide area ratio decreases after thetransformation point is passed along with the rise oftemperature. If we set the allowable range of carbidearea ratio to 6–10%, the shortest heating time, withwhich each temperature measurement positionreaches the allowable range, is 120 seconds. Fig. 6shows the carbide distribution of the heating-stop-timing side of the bearing rings hardened under theconditions of Fig. 5. The carbide area ratio of Fig. 6 is9.5% and matched, in general, with the calculationresult of Fig. 5.

10μm

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■■■■■■■■■■■■■■■■■■■■■■■■

■■■■■■■■■■■■■■■■■■■■■■■■

■■■■■■■■■■■■■■■■■■■■■■■■

■■■■■■■■■■■■■■■■■■■■■■■■

■■■■■■■■■■■■■■■■■■■■■■■■

■■■■■■■■■■■■■■■■■■■■■■■■

■■■■■■■■■■■■■■■■■■■■■■■■

……………… (3)

……… (4)

=Mi

=ti

M0 exp(－(ki (ti＋Δt))n)

ln (M0 /Mi-1) / Anexp(nE /RTi)n

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(a) Outer ring

(b) Inner ring

Heating coil

Heating coil

Fig. 8 Measuring position of Vickers hardnessFig. 7 Relationship between hardeness and tempering time

056

58

60

62

64

66

68

30

Hard

ness

afte

r tem

perin

g H

RC

60 90

180˚C

230˚C

330˚C

280˚C

120 150Heating time min

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2.3 Prediction of hardness with IH temperingIn tempering, the hardness after tempering is

estimated in real time from the temperaturemeasurement results 5). Equations (5) and (6) showthe empirical formula for obtaining hardness aftertempering.

X=X0−(X0−Xf )(1−exp(−(k' t)n' ) ………… (5)k' = A' exp(−E' / RT ) ………………………… (6)

X : Hardness after tempering (HRC)t : Heating timeX0 : Hardness after hardening (HRC)Xf : Hardness of raw material (HRC)T : Temperature (K), R: Gas constantA', k', n', E': Constants obtained from experiment

Fig. 7 shows the relation of heating time andhardness after tempering for each temperingtemperature. This relation is obtained, in the sameway as Fig. 3, by regression analysis of the results ofhardness measurement of test samples obtainedunder systematically changed tempering conditions. Inaddition, the real time calculation for hardness aftertempering for IH is the same as that of casehardening.

No

1

2

3

4

5

678

Outer ring

HardeningLocation Tempering

↑ ↑

900˚C IH heatingCarbide area ratio 6–8%Room temperaturesoluble water cooling

900˚C IH heatingCarbide area ratio 6–8%Room temperaturesoluble water cooling

180℃×2hThermostatic chamberheating

Outer ring

Inner ring

Inner ring ↑ ↑

830～ 860℃Atmosphere furnace heating90˚C semi-hot oil cooling

230˚C IH uniform heatingHardness HRC62

Outer ring

Inner ring ↑ ↑

230˚C IH uniform heatingHardness HRC62

Outer ringInner ring

Standard product (hardening/tempering with atmosphere furnace)

Table 1 Heat condition for inner and outer rings of 6206ball bearing

No

12345678

Outer ringInner ring

Hardening Tempering

IH Furnaceheating

Furnaceheating IH

IH IH

Furnaceheating

Furnaceheating

Hardness HVAverage Standard deviation

763 7.5734 24.8710 6.5721 6.1702 5.3710 11.9743 8.3738 5.5

Location

Outer ringInner ringOuter ringInner ringOuter ringInner ring

Table 2 Vickers hardness for inner and outer rings of6206 ball bearings2.4 Material examination results

Using this technology, IH through hardening andtempering was applied to 6206 ball bearing inner/outerrings made of SUJ2. Table 1 shows the list of testsamples. Vickers hardness of 8 types of test samplesin Table 1 was measured at 9 points shown in Fig. 8.Table 2 shows the average hardness and standarddeviation at all the measurement points. Standardhardness of bearings made of SUJ2 was, in mostcases, HRC 60-63 (HV696-772). The variance ofhardness in products was within 2 points in HRC

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(approx. 50 points in HV). The quality of 6206 ballbearing inner/outer rings processed with thistechnology was good, well within these ranges.

3. Characteristics of IH through-hardening/tempering product

3.1 Rolling element fatigue testRolling fatigue tests were conducted using the 6206

ball bearing life tester shown in Fig. 9 under theconditions in Table 3. Fig. 10 and 11 show the testresults under clean oil lubrication and lubrication withcontamination. Under rolling fatigue tests with clean oillubrication, all three subjects completed testingwithout any damage for 5984 h. Assuming the Weibullslope is the same as the standard products, 10% lifeof IH through hardening/tempering is calculated to2115 h 6), 7), with 90% of reliability, and is equivalent tothe life level of 1624 h for standard bearings. Also, the life of IH through-hardening/tempering

products under contaminated lubrication showed nodifference from the standard products, after thesignificance test 6), 7).

3.2 Static crush test and ring crack fatigue testTable 4 shows the static crush test result of 6812

ball bearing outer rings. IH through-hardening/tempering products showed the samestrength against static crush as the standard products.Fig. 12 shows the results of a crack fatigue test

using rings of 37.8 mm outer diameter × 29.8 mminner diameter × 15 mm width. The continuous dropsingle logarithm curve model, standard from theSociety of Materials Science, Japan, 8) was applied tothe data obtained from the fatigue test for analysis.The 10% fatigue strength of IH through-hardening/tempering product after 107 times wasabout 50 MPa lower than the standard product.However, since there was no significant differencewith a 5% risk factor, both can be considered thesame in strength.

Loadingspring

Test bearings

Driving pulley

Fig. 9 Rolling contact fatigue (RCF) test rig for 6206 ball bearing

1000.1

0.51.0

5.010.0

50.070.090.099.0

1000 10000Time h

Standard

IH through-hardening/tempering

Cumu

lative

dama

ge pr

obab

ility

%

Fig. 10 Comparison of RCF lives for 6206 ball bearingtreated by induction heating and conventional heating

(clean lubrication)

10.1

0.51.0

5.010.0

50.070.090.099.0

10010 1000Time h

Standard


Cumu

lative

dama

ge pr

obab

ility

%

Fig. 11 Comparison of RCF lives for 6206 ball bearingtreated by induction heating and conventional heating

(contaminated lubrication)

BearingMaximum contact surface pressure

Clean oil lubrication6206C33.2GPa

Z45˚=0.16mm3000min-1

126h (no foreign objects)

Lubrication with foreign objectsItem

Maximum shear stress depthRotational speed

Lubrication

Foreign objects

Size of foreign objectsHardness of foreign objectsEstimated life

Turbine oil with noadditivesVG56 circulationlubrication

NTNstandard foreign objects0.4g/LTurbine oil with noadditivesVG56 circulationlubrication

HV700～800100～181μｍ

KHA30Gas-atomized powder

Table 3 Conditions of RCF life testing for 6206 ball bearing


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3.3 Peeling test and smearing testUnder thin lubrication conditions, the rolling

elements and raceway rings are in direct contactcausing surface damage with small peeling andcracking. Also, in applications with slide bearings,surface damage caused by smearing can beobserved. These surface damages are evaluatedusing a two-cylinder tester shown in Fig. 13. Table 5 shows the peeling test conditions. The

peeling test is conducted by driving the driver-sideshaft with a motor, allowing free rotation of thefollower side. The resistance against peeling wasevaluated with the average of peeling area ratios atthree positions on the rolling contact surface of thedriving side. Fig. 14 shows the peeling test results.The peeling area ratio of the IH through-hardening/tempering products was equivalent with thestandard products.Table 6 shows the smearing test conditions. For the

smearing test, the driver side and follower side aredriven with different motors, in order to give sliding atthe contact surface. The test samples are fixed onboth shafts under a predetermined load. Both testsamples are driven for 3 min at 200 min-1 for runningin, then maintaining the rotation speed of the followerside at 200 min-1. The rotation speed of the driver side

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Load

Lubrication feltTest piece on the follower side

Test piece on the driver sideFollower side

Driver side

Fig. 13 Ring to ring type test rig

Fig. 14 Results of peeling test for SUJ2 rings treated byinduction heating and conventional heating

1.E+04700

800

900

1000

1100

1200

1300

1400

1.E+05 1.E+06 1.E+07Loading cycle

10% fatigue strength after 107 times 853.7MPa10% fatigue strength after 107 times 808.9MPa

IH through-hardening/temperingStandard

Stre

ss

MPa

Fig. 12 SN Curves of SUJ2 rings treated by inductionheating and conventional heating. The stress ratio is 0.1.

TesterTest piece on the driver side φ40×t12，Axial curvature R60

Two-roller testing machine

φ40×t12，Axial straight2000 min-1

1245N2.3GPa1.48×0.71 mm4.8×105

Room temperatureTurbine oil with no additives VG46Felt oiling


Test piece on the follower sideRotational speedRadial loadMaximum contact surface pressureContact ellipseLoading cycleAtmosphere temperatureLubricating oilOiling method

Standard02468

10121416

Peeli

ng ar

ea ra

tio %

Table 5 Condition of peering test

Fig. 15 Results of smearing test for SUJ2 rings treatedby induction heating and conventional heating

TesterDriver side/follower side test piece φ40×t12，Axial curvature R60

Two-roller testing machine

980N2.1GPa1.37×0.66 mmTurbine oil with no additives VG46Felt oilingStep up from 200 min-1 in increments of 100 min-1

Constant at 200 min-1

Radial loadMaximum contact surface pressureContact ellipseLubricating oilOiling methodRotationalspeed

Driver sideFollower side

0

100

200

300

400

500


Standard

Time t

o sme

aring

se

c

Table 6 Condition of smearing test

Heat treatmentAverage

1.50 0.13

1.48 0.02Standard deviation

Strength (kN)Type

6812 Ball bearingouter ring

Φ78×Φ60×10


Standard deviation

Table 4 Compressive strengths of SUJ2 rings treated byinduction heating and conventional heating

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size of facility are required for application to smallproducts.For larger products, the difference in productivity

between IH and atmosphere heating will be reduced;however, it should be noted that different challengeswill surface, such as reduced advantages due tolarger facilities, decarburization and generation ofscale due to increased uniform heating time in theatmosphere, and more difficult uniform heating.

4.2 Effect of IH on reduction of environmental loadTable 8 shows the estimates from the energy-

saving and CO2-reduction viewpoints on the effect ofIH on reduction of environmental load. Its powerconsumption can be reduced to 74% of powerconsumption of electric furnace per kg of productweight (compared to our existing product). Also, CO2emission can be reduced to 56% of the emission ofheaters that use city gas. In the calculation, CO2emission per consumed electric power unit wasassumed to be 0.341 kg/kWh, which is electricity-CO2emission equivalent in the Chubu region in FY2010.

is increased from 200 min-1 in 100-min-1 steps every30 seconds. The smearing is detected by vibration,and resistance against smearing was evaluated by theduration of smearing. Fig. 15 shows the smearing testresults. The resistance against smearing of IHthrough-hardening/tempering products was equivalentwith the standard product.

3.4 Dimensional stabilityHigh-dimensional stability is required for the

raceway rings for rolling bearings. Fig. 16 shows thedimensional change due to aging when they were keptat 230˚C for two hours. IH through-hardening/tempering products had smallerdimensional change than standard products.

4. Consideration

4.1 Advantage of IH in productionAs shown in Table 7, IH has many advantages over

an atmosphere furnace. However, of special note isthe fact that the stop/start-up time of the facility isalmost zero and that no flammable gas is required.These points open the possibility of elimination of 24-hour operation and elimination of use of flammablegas, which are today instituted in the conventionalheat treatment plants of rolling bearings. The advantages listed in Table 7 assume the

production volume, facility cost, and facility space areequivalent to atmosphere furnace. IH shows highproductivity of quick heating in applications such assurface hardening, providing equal or moreadvantages over atmosphere furnace heating.However, for through hardening, since relatively longheating is required, more units are required forproduction of the same volume. Therefore, reductionof cycle time, reduction of facility cost and compact

0

51015

2025

30Condition：230˚C×2h


StandardRatio

of d

imen

siona

l cha

nge

due

to a

ging

×

10-5

Fig. 16 Dimensional change of SUJ2 rings treated byinduction heating and conventional heating

Features

Safety/environment

・Inline integration with other facilities・Heat history tracing of each product・Efficient treatment of small-lot products・Short shutdown/star-up time・High freedom of layout

・Low soot・Flammable gas facilities are not required

Table 7 Merits of induction heating on manufacturing

Heating methodAtmosphereElectricity for heating kWh/kgCity gas Nm3/kgRX gas Nm3/kgCO2 emission g/kg

IH Electrical City gasNitrogen gas Nitrogen gas RX gas

0.934 1.261 00 0 0.1850 0 0.464

318 430 566

Table 8 Reduction effect of using induction heating onnegative environmental impact


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5. Conclusion

In this paper, a method for achieving IH throughhardening/tempering in the minimum processing time,while controlling the quality of the entire bearing ringswithin the control criteria, was discussed. Thistechnology is a proprietary system of NTN, which evenallows engineers inexperienced with IH to optimallydesign the processing conditions. We believe thissystem will accelerate the use of IH for heat treatmentof rolling bearings, resulting in improved environmentof production sites and energy saving.

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Photo of authors (titles are at the time of development)

Takumi FUJITAAdvanced Technology

R&D Center

Nobuyuki SUZUKIAdvanced Technology

R&D Center

Reference1) Takao Yamazaki: Review of High FrequencyInduction Heat Treating Technology during Past 50Years, Netsu Shori 50，6 (2010)580-588.

2) Kazuhiro Kawasaki, Yoshitaka Misaka, FumiakiIkuta: Towards the High Performance InductionHeat-Treatment Accompanied with W-Eco(Ecological & Economical) Effect, Netsu Shori, 50，4 (2010) 368-376.

3) Saburo Shiko, Kazuo Okamoto, Shozo Watanabe:Effect of Metallographical Factors on the RollingFatigue Life of Ball Bearing Steel, Tetsu- to –Hagane, 54，13 (1968)1355-1366.

4) Takumi Fujita, Nobuyuki Suzuki: High FrequencyOverall Heating Hardening Method for BearingRace under Temperature Control, 61st Meeting ofthe Japan Society of Heat Treatment, presentationmaterial, (2005) 41-42.

5) Takumi Fujita, Nobuyuki Suzuki: High FrequencyOverall Heating Hardening Method for BearingRace made of SUJ2 under Temperature Control,62nd Meeting of the Japan Society of HeatTreatment, presentation material, (2006) 25-26.

6) Takumi Fujita: Rolling Contact Fatigue Life TestDesign and Result Interpretation MethodsMaintaining Compatibility of Efficiency andReliability, NTN Technical Review, 76 (2008) 31-38.

7) T. Fujita: Rolling contact fatigue life test design andresult interpretation methods maintaining compatibilityof efficiency and reliability, J. ASTM international, 7, 6(2010)Paper ID JAI102492.

8) The Society of Materials Science, Japan,Committee on Fatigue of Materials, Committee onReliability Engineering, Metal Materials FatigueReliability Evaluation Standard JSMSSD-6-02 –SNCurve Regression Method, The Society of MaterialsScience, Japan (2002).

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development of induction through-hardening and · pdf file-25-development of induction...

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