prediction of austenite grain growth during austenitization of low alloy steels.pdf

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Prediction of austenite grain growth during austenitization of low alloy steels

Seok-Jae Lee, Young-Kook Lee *

Department of Metallurgical Engineering, Yonsei University, 134 Sinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

Article history:

Received 30 May 2007Accepted 13 March 2008

Available online 28 March 2008

Keywords:

Austenite grain growthSolute dragLow alloy steels

a b s t r a c t

Although the prior austenite grain size (AGS) of heat treatable low alloy steels is an influential factor inphase transformations during quenching and in mechanical properties, there are few equations to predictthe AGS considering alloying element effects. The purpose of this study was to investigate the effects ofalloying elements on austenite grain growth and to propose an empirical equation for predicting the AGSof global low alloy steels. The Arrhenius type equation was proposed based on the measured AGSs of the16 different low alloy steels and the predicted results were in a good agreement with the measured data.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

In the heat treatment process, the austenite grain size (AGS) be-fore quenching tremendously influences diffusive and diffusionlessphase transformations, precipitation, and mechanical propertiessuch as strength, hardness, toughness, and ductility. A plenty of re-

searches have been made to get better understanding and control-ling of the austenite grain size during austenitizing process of thesteels over the past half a century[18]. Even if the austenite grainsize is fine just after the reverse transformation during heat-up toaustenite region, the grain growth can easily occur especially inplain carbon steels to reduce the grain boundary free energy bythermally activated atomic processes. As alloying elements areadded in the carbon steels, the grain growth rate usually decreasesdue to the solute dragging effect of the alloying elements segre-gated into austenite grain boundaries [9,10]. If the alloying ele-ments precipitate as carbides or nitrides in austenite, theprecipitates also lower the grain growth rate by pinning the grainboundaries[11,12].

Up to date, several theoretical and semi-empirical models for

predicting austenite grain size during austenitization consideringthe alloying element effects have been suggested[18]. The theo-retical models contain thermodynamic parameters like grainboundary energy, activation energy for grain boundary diffusion,and so on, which are not easily obtainable. Some empirical equa-tions have been suggested simply as functions of temperatureand time of austenitization only for plain carbonmanganese steels[4,5,8], which are not suitable for low alloy steels. In this study, anempirical equation for predicting the AGSs of global low alloysteels is proposed from an industrial point of view. A newly devel-

oped equation for predicting the AGS of low alloy steels containingCr, Ni, and Mo was validated by comparing the measured and cal-culated AGSs of various low alloy steels.

2. Experimental procedure

The total 16 different low alloy steels were prepared using a vacuum inductionfurnace to investigate the effects of Ni, Cr, Mo, and C on austenite grain growth. Thechemical composition range of the low alloy steels used in this study was shown inTable 1. The ingots were homogenized at 1300C for 3 h and hot rolled at 1000 C to8 mm thick plates. The specimens were taken from the plate and austenitized innitrogen atmosphere at different temperatures from 850 to 1200 C for maximum3 h using a tube furnace, followed by water quenching. For the measurement ofAGS, the quenched specimens were etched by a saturated picric acid after mechan-ical polishing with 1 lm diamond suspension. Two methods by using the line inter-cept and the area of average grain were tried to measure the AGS of the etchedspecimens[13].

3. Results and discussion

Fig. 1shows optical microstructures of the quenched specimensof a CrMo steel held at different austenitizing temperatures for

10 min. The grains are almost equaxed even at 900 C and wereclearly coarsened with increasing austenitizing temperature. Thefine and coarse grains still coexist at such a temperature as highas 1050 C. On the basis of optical measurements, the averageAGS was quantitatively evaluated as functions of austenitizingtemperature and time. The linear proportion between the recipro-cal of temperature and the natural logarithmic values of AGS isshown inFig. 2a, while the exponential increment of AGS with aholding time is inFig. 2b, respectively.

The effects of Ni, Cr, and Mo on austenite grain growth arequantitatively compared based on the analysis of measured AGSsinFig. 3. The reduction in AGS with an addition of alloying ele-ments was greater at 1200 C. The Mo is more effective to prevent

0261-3069/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2008.03.009

* Corresponding author. Tel.: +82 2 2123 2831; fax: +82 2 312 5375.E-mail address:[email protected](Y.-K. Lee).

Materials and Design 29 (2008) 18401844

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Materials and Design

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austenite grain growth than Ni or Cr regardless of austenitizingtemperature. Similarly, the C addition has an effect on obstructingaustenite grain growth as shown inFig. 4. It has been reported thatthe alloying elements in a solute state play a role as obstacles to

the growth of austenite grains by the segregation of the alloyingatoms toward grain boundaries and that the difference in atomicsize between Fe and an alloying element affects the dragging effecton grain boundaries[14]. The difference in atomic radius betweenFe and Ni in austenite is 0.007 nm and 0.01 nm between Fe and Cr,and 0.034 nm between Fe and Mo, respectively[15]. Papworth andWilliams have investigated the segregation to austenite grainboundary in low alloy steels by X-ray mapping in the field emis-sion gun scanning transmission electron microscopy (FEGSTEM)and concluded that Ni, Cr, Mo, Mn and P segregate to austenitegrain boundary[16].

Table 1

Chemical composition of the experimental steels (wt.%)

C Mn Si Ni Cr Mo

Min. 0.15 0.73 0.20 0.00 0.00 0.00Max. 0.41 0.85 0.25 1.80 1.45 0.45

Fig. 1. Variation of austenite grain size in a water quenched CrMo steel held at different austenitizing temperatures for 10 min: (a) 900C, (b) 950 C, (c) 1000C,(d) 1050C, (e) 1100 C, and (f) 1150 C.

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The relation between austenite grain growth and austenitizingcondition has been explained by a thermally activated atomic jumpprocess, which is typically expressed by the following Arrheniustype equation:

d Aexp QRT

tn 1

whered is grain size in micrometer, A a constant, Qactivation en-

ergy (J/mol) for grain growth, R gas constant (8.314 J/mol/K), Taustenitizing temperature in Kelvin, taustenitizing time in second,

and n time exponent. The activation energy can be obtained byso-called Arrhenius plot using experimental data. The Arrheniustype equations for predicting the AGS in plain carbonmanganesesteels have been previously proposed [4,5,8] and described inEqs.(2)(5).

d 1:324 105exp 94000RT

t0:194 2

d 9:1 106exp 126000RT

t0:18 3

d 7:9 104exp 69000RT

t0:19 4

d 4:1 107

exp 141000

RT

t0:12

5The equations cannot be applied to low alloy steels but to plain

carbonmanganese steels because of the solute dragging effect ofalloying elements (Ni, Cr, and Mo) segregated into the austenitegrain boundaries. Thus, the 89 measured data were used to devel-op an Arrhenius type predictive equation of the AGS in low alloysteels. Considering that the activation energy (Q) for grain growthis affected by the amount and kind of the alloying elements, it canbe as follows:

Q Q0 X4i1

@Qi@Xi

Xi

6

whereQ0is the activation energy (J/mol) for austenite grain growth

of an alloy containing almost constant amounts of Mn and Si, asshown inTable 1,Qiis the increment in activation energy by addinganother alloying element i such as Ni, Cr, Mo, and C, and X iis theconcentration ofi element in weight percent. The empirical equa-tion for predicting the AGS of low alloy steels during austenitizationhas been made based on the Arrhenius type equation by fitting themeasured AGS data as functions of alloying elements, temperature,and time (Eq.(7))

d76671exp 890983581C1211Ni1443Cr4031MoRT

t0:211

7The predicted AGSs using Eqs.(2)(5)and(7)were compared to

the measured ones of low alloy steels in Fig. 5. Two comparing

parameters (Dand E) for the accuracy of the equation are used inthis figure. The parameter D is the average absolute distance

Fig. 2. Effects of (a) austenitizing temperature holding for 1 h and (b) austenitizingtime at 900C on austenite grain size of a Cr steel.

Fig. 3. Effects of the alloying elements on preventing austenite grain growth atdifferent austenitizing temperatures for 1 h.

Fig. 4. Effect of carbon on preventing austenite grain growth at different austen-itizing temperatures for 10 min in CrMo steels.

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between the line in the middle of the figure and markers, while theparameterEis the average, signed distance between the line in themiddle of the figure and markers.

D XNi1

1

ffiffiffi2p dcal dexp

!,N 8

Fig. 5. Comparison of the austenite grain size calculated by the previous and proposed equations with the measured one: (a) Eq. (2), (b) Eq.(3), (c) Eq.(4),(d) Eq.(5),and (e)Eq.(7).

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EXNi1

1ffiffiffi2

p dcal dexp !,

N 9

Here, Nis the number of experimental data and both D and Ehave units in ASTM grain size number. The smaller values ofD indi-cate a better match between predicted and experimental resultswhileEindicates whether a equation is, on average, predicting val-ues that are too high (positive values) or too low (negative values).As expected, the proposed equation in this study shows the small-est D and Evalues, because it was made by fitting the measureddata in Fig. 5. The same predicted AGSs paralleled to x axis inFig. 5ad indicate the absence of alloying element effect in the pre-viously proposed equations. Eq.(7)is expected to be used for pre-dicting the AGSs of most of commercial low alloy steels.

4. Conclusion

To predict the change in AGS of global low alloy steels duringaustenitization, the 16 different low alloy steels were austenitizedat different temperatures of 8501200 C for maximum 3 h and theAGSs were measured. The addition of Mo was found more effectiveto prevent austenite grain growth than Cr or Ni. The empiricalequation for predicting the AGS of low alloy steels was suggestedconsidering the alloying element effects on the activation energyfor grain growth using the measured AGSs and the steel chemistry.In a technical point of view, this AGS equation would be contrib-uted to the better prediction of the AGS of global low alloy steels.

Acknowledgements

This research was supported by the National Core ResearchCenter (NCRC) program from MOST and KOSEF (No. R15-2006-022-01002-0).

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