The kinetics of austenite formation at high heating rates

Cinética da formação de austenita a altas velocidades de aquecimento

METALLURGY AND MATERIALS

The kinetics of austenite formation at high heating rates

Cinética da formação de austenita a altas velocidades de aquecimento

Marciano Quites Macedo^I; André Barros Cota^II; Fernando Gabriel da Silva Araújo^III

^IGeosol, M.Sc./REDEMAT [e-mail]

^IIPh.D., Physics Department /REDEMAT/UFOP [e-mail]

^IIIPh.D., Physics Department /REDEMAT/UFOP [e-mail]

ABSTRACT

The variation in critical temperatures for the formation of austenite when increasing the heating rate was studied by dilatometry. The analysis was performed at heating rates between 10 and 90ºC/s. Empirical equations are herein proposed for calculating Ac₃ with respect to the heating rate. The results showed that an increase in the heating rate had no influence on Ac₁, but Ac₃ increased 115ºC. The equations proved to be of a more general use, as they also predicted the results of works on other steels with high values of correlation coefficients.

Keywords: Austenitization, kinetics, fast heating, dilatometry.

RESUMO

Foi estudada por dilatometria a variação das temperaturas críticas para a formação de austenita, com o aumento da velocidade de aquecimento. A análise foi realizada a velocidades de aquecimento entre 10 e 90ºC/s. Foram propostas equações empíricas para o cálculo de Ac₃, em função da velocidade de aquecimento. O aumento da velocidade de aquecimento não afetou Ac₁, mas Ac₃ aumentou em 115ºC. As equações propostas se revelaram de uso mais geral, uma vez que também previram os resultados obtidos em trabalhos para outros aços com altos coeficientes de correlação.

Palavras-chave: Austenitização, cinética, aquecimento rápido, dilatometria.

1. Introduction

The formation of austenite in steels is employed in various industrial processes. Therefore austenitization is still a largely studied phenomenon, since the grain size and its distribution directly influence the final mechanicalt properties obtained (Morito et al., 2006). At continuous heating, the heating rates significantly alter the temperatures of steel processing and, consequently, its microstructures. Danon et al. (2003) studied the influence of the heating rate and austenitization temperature on the heterogeneous growth of austenitic grains in martensitic steels, and noted that the heating rate used to reach the austenitic phase had a strong influence on the austenitic grain size distribution.

Another important aspect to be considered is related to the type of steel because different original microstructures and different levels of carbon and alloy elements influence the austenitic microstructure produced, as well as the products of its tempering and annealing (Krauss, 1990; Oliveira et al., 2007).

Oliveira et al. (2007) observed that an increase in the heating rate increases the rate of austenite formation and affects its critical temperatures in low-carbon steel, for heating rates between 0.1 and 16ºC/s.

In some industries, due to the short treatment period and the ability to treat specific parts of the pieces, thermal treatment by electromagnetic induction is employed. In this process, because of losses from the Joule effect and hysteresis, an induced electrical current heats the piece to be treated. Usually the heating rates are extremely high, and may exceed 200ºC/s (Rudnev et al, 2003). Therefore, when electromagnetically induced thermal treatments are used, besides checking the previous microstructure and chemical composition, it is necessary to verify the critical temperatures for austenite formation to determine the heating rate that will be employed. Consequently, an examination of the influence of high heating rates at critical temperatures in the austenite formation is fundamental.

Dilatometric techniques have been used to study austenite formation under continuous heating, since during the steel's transformation phases , there is some volume variation due to changes in the crystalline structure (Garcia de Andrés et al., 1998).

This research investigated the influence of the variation of high heating rates at the starting and final temperatures of austenite formation, as well as its influence on the transformation period, in order to simulate thermal treatment conditions by electromagnetic induction.

2. Materials and experimental procedure

The material studied in this research was the SAE4130 steel, produced by V&M of Brazil S.A. The chemical composition of this steel, given in % by weight, was of 97.8Fe, 0.29C, 0.52Mn, 0.013P, 0.003S, 0.19Si, 0.01Ni, 0.97Cr, 0.16Mo and 0.008Cu. The steel microstructure at delivery condition was obtained by optical microscopy, on a sample etched with nital 2%.

The tests were conducted in an Adamel Ihomargy type LK02 dilatometer. The samples were cut from laminated and drawn tubes, with 0.6mm thickness, 2mm width and 12mm length. The critical starting and ending temperatures of austenite formation were determined by dilatometric curves. The critical temperatures were obtained for the heating rates of 10, 30, 50, 70 and 90ºC/s.

3. Results and analysis

The original steel microstructure is composed of ferrite and pearlite, with a volumetric ferrite fraction of V_α0=(48 ± 0.6)%, and a structure of bands, which is due to the laminating and drawing processes used for the manufacturing of the tubes from which the samples were taken.

Employing Andrews' equations, Ac₁ and Ac₃ values for the SAE4130 steel in this research, at slow heating, are: Ac₁=739ºC and Ac₃=814ºC. At continuous heating and at high heating rates, dilatometric tests were performed to determine the temperatures Ac₁ and Ac₃. Figure 1 displays the dilatometric curves, where ΔL=f(T) for the continuous heating and high heating rates of the steel under study, and the derivatives of the dilatometric curves, dΔL/dT=f(T), highlight the great differences among the final temperatures of austenite formation, Ac₃, under varying heating rates.

In Figure 1, with a low heating rate of 10ºC/s, it is possible to identify the starting temperature of the ferrite transformation into austenite (Af_i), which is of about 790ºC, after the pearlite dissolution; the temperatures Ac₁ and Ac₃ are also indicated.

At continuous heating, the austenite formation rate reaches its maximum at about the final temperature of pearlite dissolution, and the increase in the heating rate raises the austenite formation rate (Oliveira, 2007). Therefore, at heating rates greater than 10ºC/s, the austenite formation rate will be so high in Afi that this temperature is not easily displayed graphically. Temperatures Ac₁ and Ac₃, for each heating rate at continuous heating, are listed in Table 1.

Figure 2 displays the variation of temperatures Ac₁ and Ac₃, as well as the time variation for austenite formation, with respect to the heating rates of 10, 30, 50, 70 e 90 ºC/s.

The starting temperature of austenite formation (Ac₁), defined as the temperature at which the thermal expansion has strayed from linearity (Garcia de Andrés et al., 1998; 2002), hardly varied with the heating rate, also displayed in Figure 1. The austenite formation derived from the ferrite and pearlite microstructure begins with the pearlite dissolution, as it becomes unstable at temperatures over Ac₁. The austenite nucleation occurs in the interfaces between the pearlite colonies and the ferrite-cementite interface, which implies in high formation rates, due to the narrow distances of carbon diffusion. Therefore, even with the increase in heating rates, the energy to the transformation, and consequently Ac₁, remain unaltered.

An increase is noticeable at the final temperature of austenitic transformation (Ac₃) with a raise in the heating rate. Figure 3 displays Ac₃ with respect to the transformation period. These results indicate that the increase in the heating rate reduces the austenite transformation period, hence, reducing the duration of the diffusion process, which demands greater energy for the transformation, and therefore a greater Ac₃ value.

An empiric equation is proposed, fitting the time variation at the end of the austenite formation with respect to the heating rates of Figure 2, which is expressed by:

where t₁₀₀ is the total time of austenite formation, Q and g are steel constants, which can depend on the starting micro-microstructure and chemical composition. In this case, for the SAE4130 steel, Q is about 109.1 and g is about 0.78, with a correlation coefficient of 0.999. Considering that the heating rate can be calculated by:

hence:

The results of Oliveira et al. (Oliveira, 2007) demonstrated that for heating rates amidst 0.1ºC/s and 16ºC/s, temperature Ac₃ raises with the rate, while Ac₁ suffers less influence of the increase in the heating rate. The adjustment of the figures of Oliveira et al. (Oliveira, 2007) to equation 1 proves that the equation is true also for low carbon steel microalloyed with niobium of that study, with a 0.999 correlation coefficient, and the Q and g constants at 135.6 and 0.95, respectively. Therefore, for both microalloyed steels, one should merely determine the temperature Ac₁ at continuous heating to find out Ac₃ at any heating rate.

For stainless steels, the results of Danon et al. (Danon, 2003) and Garcia et al. (Garcia, 2002) demonstrated that the temperature of Ac₁ suffers a greater influence from the heating rate. In fact, equations 1 and 3 are also able to correctly describe the time variation for the transformation and for the temperature of Ac₃ in respect to the increase of the heating rate for those steels.

4. Conclusions

For heating rates of over 30ºC/s, the starting temperature of the ferrite transformation into austenite (Af₁) is not easily identified by dilatometric curves. The increase in the heating rate amidst 10ºC/s and 90ºC/s had no influence in the starting temperature of the austenite formation (Ac₁), kept close to 740ºC, but changed its final critical temperature (Ac₃), which increased from 920ºC to 1035ºC. The set of empirical equations proposed from the dilatometric data successfully predicted Ac₃ as a function of the heating rate between 10 and 90ºC. The theoretical curves calculated from the proposed equations, for the alteration in transformation time and temperature Ac₃ with respect to the increase in heating rate and a correlation coefficient of 0.999, correspond to the experimental data obtained for the SAE4130 steel in this study, as well as to the data on steels analyzed in other studies, suggesting that it is an equation with general application.

5. Acknowledgments

To REDEMAT/UFOP, Geosol, Dr. Margareth S. Andrade and the Victor Dequech Foundation.

6. References

Artigo recebido em 17 de dezembro de 2009.

Aprovado em 14 de janeiro de 2011.

• DANON A., SERVANTE C., ALAMO A., BRACHET J. C. Heterogeneous austenite grain growth in 9Cr martensitic steels: influence of the heating rate and the austenitization temperature. Materials Science & Engineering A, p. 122-132, 2003.
• GARCIA DE ANDRÉS C., CABALLERO F.C., CAPDEVILA C., ÁLVAREZ L.F. Application of dilatometric analysis to the study of solid-solid phase transformations in steels. Materials Characterization, v. 48, p. 101-111, 2002.
• GARCIA DE ANDRÉS C., CABALLERO F.C., CAPDEVILA C., BHADESHIA H.K.D.H. Modelling of kinetics and dilatometric behavior of non-isothermal Pearlite-to-austenite transformation in an eutectoid steel. Scripta Materials; Scripta Materialia, v. 39, n.6, p. 791-796, 1998.
• KRAUSS G. Steels: heat treatment and precessing principles. ASM International, p. 145-256, 1990.
• MORITO S., YOSHIDA H., MAKI T., HUANG X. Effect of block size on the strength of lath martensite in low carbon steels. Materials Science & Engineering A, v.438-440, p. 237-240, 2006.
• OLIVEIRA F.L.G., ANDRADE M.S., COTA A.B. Materials Characterization, v.58, p.256-261, 2007.
• RUDNEV V. I., LOVELESS D. L., COOK R. L., BLACK M. R. Handbook of induction Heating Ed. Marcel Dekker, 2003. p. 11-136.

Publication Dates

• Publication in this collection
  27 July 2011
• Date of issue
  June 2011

History

• Accepted
  14 Jan 2011
• Received
  17 Dec 2009