Effect of Rare Earth on the Carburization of Steel

Materials Science and Engineering A267(1999)162–166

Letter

Effect of rare earths on the carburization of steel

Z.-X.Yuan a,Z.-S.Yu b,P.Tan a,S.-H.Song c,*

a Department of Materials,Wuhan Yejin Uni6ersity of Science and Technology,Wuhan,Hubei430081,People’s Republic of China

b Department of Materials Physics,Uni6ersity of Science and Technology Beijing,Beijing100083,People’s Rebubli

c of China

c Institute of Polymer Technology an

d Materials Engineering,Loughborough Uni6ersity,Loughborough,Leicestershir

e LE113TU,UK

Received9July1998;received in revised form25January1999

Abstract

The effects of rare earths(RE)on the carburization of steel are examined with four0.2%C steels doped and undoped with RE. Clearly,both RE in carburizer and RE in steel may accelerate the carburizing process.RE in carburizer is more effective at enhancing carburization than RE in steel.The mechanism for this enhancement of carburization is that the RE oxide enhances the medium-sample interface reaction during carburizing.?1999Elsevier Science S.A.All rights reserved.

Keywords:Rare earths;Carburization;Segregation;Grain boundaries

1.Introduction

As is well known,carburization is one of the effective approaches to enhancing the surface-layer strength and wear resistance of steel.It is to increase the carbon concentration in the surface-layer of steel components so that the components have adequate strength and strongly increased wear resistance in the surface-layer after quenching and tempering.

In the1980s,it was found[1,2]that rare earths(RE) added to gas carburizer could accelerate the carburizing process considerably by an activation reaction with the surface of components.Soon after,it was also found [3–5]that RE could accelerate the carbonitriding pro-cess by15–20%as compared to carbonitriding without the involvement of RE,simultaneously bringing about an increase in the fatigue strength of low-carbon steels. It is anticipated on the basis of the above information that RE doped in steel may be able to accelerate the carburizing process.The present work was aimed at investigating the effect of RE both in steel and in carburizer on the carburization of steel.2.Experimental procedure

Four0.2%C steels,with and without RE addition,in the form of mischmetal were prepared by vacuum induction melting.All ingots20kg each were forged into sheets13×13×1000mm in size and then cut into samples13×13×6mm.All the samples were nor-malised at900o C prior to carburization.Chemical com-position of the steels is shown in Table1.

The samples,well polished by mechanical polishing, were carburized in a pit-type gas carburizing furnace with a power of35kW at850and910o C for1,2,3, and4h,respectively.Details on the carburizing furnace may be seen in[6].The carburizer was kerosene,with or without Ce2O3oxides.It was made by the following method.A solution of15ml methanol and15ml acetone with8g Ce2O3is mixed with every3kg kerosene.In the course of carburization,the kerosene dropped into the furnace at a rate of50–70d/min (drops per minute)in the boost stage and30–40d/min in the diffusion stage.The carburizing atmosphere pres-sure was kept positive in the range of14–20mbar during carburizing.Approximate composition of the gas is listed in Table2.After carburizing,some samples were directly water-quenched and the others directly air-cooled.

*Corresponding author.Tel.:+44-1509-263171;fax:+44-1509-

223949.

E-mail address:s.song@https://www.360docs.net/doc/3f741706.html,(S.-H.Song)

0921-5093/99/$-see front matter?1999Elsevier Science S.A.All rights reserved. PII:S0921-5093(99)00061-1

Z.-X.Yuan et al./Materials Science and Engineering A267(1999)162–166163

Table1

Chemical composition of the experimental steels(wt.%)

S RE

P

Steel Mn

C Si

–

0.011

0.009

10.48

0.200.28

0.490.0130.0100.024 20.190.26

0.032

0.004

0.005

30.49

0.180.27

0.130

0.002

40.190.280.480.014There is no apparent difference in microstructure be-tween RE-free and RE-containing samples.

3.2.Carburizing kinetics

Fig.2represents the carburizing kinetics of the four experimental steels,with and without RE oxide in carburizer,at850and910o C,respectively.Clearly,the kinetic curves are parabolic.They may be divided into two stages.The?rst stage is from the beginning to1h and the second one is from1to4h.The curve slope in the?rst stage is larger than that in the second stage.RE in steel and RE in carburizer both have an enhancing effect on carburization.The case depth increases with increasing RE content in steel until0.032wt.%.When the RE content is greater than0.032wt.%,the case depth no longer increases.The enhancing effect of RE in carburizer is stronger than that of RE in steel.The combined enhancing effect of RE in steel and RE in carburizer is stronger than their individual effect.

3.3.Potency of enhancement

The potencies of carburization enhancement caused by RE in steel and RE in carburizer individually are represented in Table3and Table4,respectively,and their combined potency in Table5.Here the potency of enhancement,P,is given by

P=x i?x o(1) where x o is the case depth of the RE-free sample without RE in carburizer for evaluation of the enhance-ment potency of RE in steel or the case depth of the RE-free and RE-doped samples without RE in carbur-izer for evaluation of the enhancement potency of RE in carburizer;and x i is the case depth of the RE-free and RE-doped samples,either without RE in carburizer for evaluation of the enhancement potency of RE in steel or with RE in carburizer for evaluation of the enhancement potency of RE in carburizer.

Table3,Table4and Table5may be summarised as follows.When the RE content is about0.032wt.% (Steel3)in steel,the enhancing effect is the strongest. In general,RE in carburizer is more effective than RE in steel in enhancing the carburization and the com-bined effect of RE in steel and RE in carburizer at 850o C is nearly the same as that at910o C except for the 1-h carburized sample.

3.4.Microhardness

Microhardness of the as-quenched4-h-carburized samples for Steels1and3are illustrated in Fig.3as a function of distance from the sample surface.Clearly, the microhardness of RE-doped Steel3is somewhat higher than that of RE-free steel at the same distance to

Table2

Approximate composition of the carburizing gas(vol.%)

N2

O2

H2

CO2

C n H2n+2CO

C n H2n

10–155

0.610–2050.460–7050.4

The distance from the sample surface to the50% pearlite position was taken as the case depth.The case depth was measured by optical microscopy for the as-air-cooled samples mechanically polished and etched by an ethanol solution of4%nitric acid.Fig.1gives an example for its determination.In the measurement of case depths,?ve samples were employed for each condi-tion and the arithmetic mean of data points acquired was taken as the measured result.The microhardness distribution from the sample surface to the centre was measured by an HX-500microhardness testing machine under constant load(100g)and loading time(15s) with?ve water-quenched samples for each condition.In the measurement of case depth,the arithmetic mean of data points obtained was taken as the measured result.

3.Results

3.1.Microstructure

For the sample air-cooled directly after carburiza-tion,microstructure in the carburized layer,as shown in Fig.1,is pearlite plus ferrite in the inner part and pearlite plus cementite in the outer part.This is be-cause,after carburization,the carbon concentration increases on going from the matrix to the surface.

Fig.1.Typical optical micrograph showing how to determine the case depth.

Z.-X.Yuan et al./Materials Science and Engineering A267(1999)162–166

164

Fig.2.The case depth as a function of carburizing time for(a)Steel1,(b)Steel2,(c)Steel3,and(d)Steel4air-cooled directly after carburizing at850and910o C(C1and C3:850and910o C without RE in carburizer,respectively;C2and C4:850and910o C with RE in carburizer, respectively).

the surface.The difference in microhardness between Steels1and3in the scenario without RE in carburizer is smaller for the850o C-carburized samples than for the 950o C-carburized ones(see Fig.3a).This is because the difference in the potency of carburization enhancement is smaller for the850o C-carburized samples than for the 950o C-carburized ones(see Table3and Table4).Also, the microhardness is more enhanced by RE in carbur-izer than by RE in steel because of the same reason as in enhancing the case depth,i.e.RE in carburizer is more effective than RE in steel in enhancing the carbur-ization,which will be discussed in Section4.It may be seen from Fig.2and Fig.3that,at the same distance from the surface,the microhardness increases generally with increasing case depth.This is reasonable because, at the same distance from the surface,the carbon concentration in as-quenched martensite within the car-burized layer would increase with increasing case depth.

4.Discussion

It is well known[7]that thermochemical treatment of the steel may be divided into four steps.They are reaction in medium,external diffusion,interface reac-tion,and internal diffusion.Of these four steps,the slowest step determines the rate of thermochemical treatment.In general,the reaction in medium and the external diffusion are quite fast.As a consequence,the rate of thermochemical treatment is controlled by either interface reaction or internal diffusion.It may be seen from Fig.2that the kinetic curves are parabolic,which follow the diffusion distance-time relationship,x= K(Dt)1/2,where x is the case depth,D is the diffusion coef?cient of carbon,t is the carburizing time,and K is a material constant.The microhardness distribution presented in Fig.3corresponds to the carbon distribu-tion from the surface to the matrix.It may be con-cluded that RE in steel and RE in carburizer both facilitate the carbon supply on the surface so as to have more carbon diffusing into the samples,i.e.RE in steel and RE in carburizer may both be able to speed up the interface reaction on the sample surface,but RE in carburizer is more effective than RE in steel due to the larger quantity adsorbed on the sample surface.

Z.-X.Yuan et al./Materials Science and Engineering A267(1999)162–166165

In general,the RE oxide is very stable and dif?cult to dissociate at carburizing temperatures and in carburiz-ing environments.As a result,the RE oxide itself cannot penetrate into the sample.RE involved in RE oxides can however activate the sample surface and enhance transport of carbon atoms from the carburizer into the sample[8],i.e.RE in carburizer can play a catalytic role in carburization.As is well known[9,10], the RE oxide has been employed as a catalyst in the petrochemical and automotive industries for quite a long time.It can accelerate transformation of CO to CO2.In our case,kerosene is a hydrocarbon and can decompose into CO and C n H2n+2during carburizing. CO and C n H2n+2have the following reactions on the medium-sample interface:

2CO [C]+CO2(2) C n H2n+2 n[C]+(n+1)H2(3) Since RE elements are very high electropositive and reactive,RE in steel may easily combine with oxygen adsorbed on the sample surface to form RE oxides so as to activate the surface.This,as described above,is bene?cial to carburization.In addition,RE and their oxides may be able to enhance transformation of CO2 (adsorbed on the surface)to CO and decomposition of CO or C n H2n+2(adsorbed on the surface)to active carbon atoms([C]).The above two processes are both bene?cial to carburization.

RE elements are very active to free surfaces and grain boundaries in steels and segregate strongly at these surfaces and boundaries[11–16].At carburizing tem-peratures,RE atoms segregated at grain boundaries diffuse to the surface through fast grain-boundary dif-fusion and then proceed to diffuse to all over the sample surface through even faster surface diffusion. RE on the surface can enhance the interface reaction as RE in carburizer.However,if the RE content is too high,RE and Fe may form brittle RE-Fe intermetallic compounds which do not contribute to RE segregation at grain boundaries.As a consequence,excessive RE in steel cannot contribute any more to the enhancement of carburization.Although there is no experimental evi-dence about RE distribution from the sample surface to the matrix,the preceding discussion should be reason-able from the standpoint of diffusion theories.

Table3

Potency of enhancement caused by rare earths(RE)for carburization at850o C,P(mm)

P for RE in carburizer

P for RE in steel

Time(h)

Steel3

Steel2

Steel1

Steel4

Steel3

Steel2

Steel1Steel4

0.03

10.05

0.060.080.210.290.28

00.050.080.090.130.250.370.34

2

0.110.090.200.270.380.34 300.08

0.38

0.140.100.270.270.41

400.09

Table4

Potency of enhancement caused by rare earths(RE)for carburization at910o C,P(mm)

P for RE in steel

Time(h)P for RE in carburizer

Steel3

Steel2Steel4

Steel3

Steel2

Steel1

Steel1Steel4

10.040.130.110.140.300.350.34

00.020.110.05

20.110.310.390.37

0.35

0.39

0.33

00.14

30.09

0.13

0.03

00.060.170.220.130.32

40.370.24

Table5

Combined potency of enhancement caused by rare earths(RE)both in steel and in carburizer for carburization at850and910o C,P(mm)

Time(h)P at910o C

P at850o C

Steel1Steel3Steel4Steel1Steel2Steel3Steel4

Steel2

0.330.140.34

0.240.48

0.350.45

0.08

1

0.450.42

0.50

0.33

0.11

20.130.300.43

0.200.43

30.350.44

0.52

0.36

0.14

0.49

0.350.550.480.130.380.54

0.270.46

4

Z.-X.Yuan et al./Materials Science and Engineering A267(1999)162–166 166

Fig.3.Microhardness distribution from the surface to the matrix in the as-water-quenched carburized samples for Steels1and3:(a) carburizing temperature=850o C and(b)carburizing temperature= 910o C(C1and C2:Steels1and3without RE in carburizer, respectively;C3and C4:Steels1and3with RE in carburizer, respectively;carburizing time=4h).5.Summary

Both RE in carburizer and RE in steel accelerate the gas carburizing process at850and910o C.The catalytic mechanism is that the RE oxide enhances the medium-sample interface reaction during carburizing.The most appropriate RE content in0.2%C steel may be about 0.032wt.%.The catalytic effect of RE in steel is weaker than that of RE in carburizer.

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In terms of the above discussion,that the difference in microhardness between Steels1and3in the case without RE in carburizer is smaller for the850o C-4-h-carburized samples than for the950o C-4-h-carburized ones(see Fig.3a)may be attributed to a lower diffusion rate of RE atoms at850o C leading the potency of carburization enhancement to be smaller for the850o C-carburized samples than for the950o C-carburized ones (see Table3and Table4).

.