South Asian Journal of Engineering and Technology Vol.2, No.22 (2016)

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1 Phone: Recrystallization Behaviour of Super 304H-Cu Austenitic Stainless Steel Saravanan. M, Prakash. P Department of Physics, Sasurie College of Engineering Tirupur , Tamilnadu, India Abstract Austenitic stainless steel having fcc crystal structure from room temperature to melting point and does not involve any solid state phase change up to melting. Hence their microstructure and mechanical properties cannot be modified by transformation induced by heat treatment unlike in the case of ferritic steels. Hence the only way to modify its property and microstructure is by hot or cold rolling, followed subsequently by suitable annealing. Hence a basic understanding of strain energy relaxation in the process of recovery, recrystallization and grain growth is essential in this particular material. Keywords:Recrystallization, ferritic, anealing INTRODUCTION In this chapter the results obtained through different characterization techniques have been presented. This is followed by a detailed discussion on the strain energy relaxation in the processes of recovery and recrystallization respectively. The detailed discussion on the present results are based on both quantification of stored energy variation using DSC and microhardness measurement and the subsequent microstructure evolution by the process of recovery and recrystallization in plastically deformed 304H SS uponthermal annealing both in isothermal and nonisothermal conditions. EXPERIMENTAL 218

2 XRD profile of as received sample The room temperature X-ray diffraction pattern of SS304H annealed at 1300ºC (1573 K) for 1 hour is shown in figure 4.1. In this the major diffraction lines correspond to (111), (200), (220) planes of face centered cubic structure which conforms to the typical austenitic structure of SS 304H.Some minor peaks in the XRD profile ensures thepresenceof M23C6 type of carbides even after the solution annealing treatment. The lattice parameter is found to be of the order of a= ± nm is fairly in agreement with data reported in literature for similar type of steel. Optical Microstructure of full annealed and Cold rolled SS 304H The optical microstructure of the SS304H sample annealed at 1200 o C for 2 hour/ Optical Micrograph super 304H annealed at1200oc for 2hours 219

3 Optical Micrograph for 80% cold rolled super 304H water quenched to room temperature is shown. In figure The microstructure shows uniform equiaxed grains with average grain size of 32 µ m. Appearance of Annealing twins reveals the characteristics of annealed austenitic steel. The micro hardness value is found to be around 175 VHN. It also shows that 1200 o C annealing could not dissolve all the primary NbCN phase, which are seen as black thick particles. However, most of M23C6 type carbideshave already seen full dissolution at the end of 1200 o C/2h treatment. This annealed sample of SS 304H with a initial dimension of 11cm x 4cm x 3cm was subsequently cold rolled at room temperature to 80% thickness reduction. In figure, the optical micrograph of 80% cold rolled microstructure is shown. The grains are considerably elongated in the rolling direction with sharp and irregular boundaries as indicated by the arrow mark. In general it is noticed that the deformation is highly inhomogeneous as it is expected in in case of severely deformed austenitic stainless steels. The average microhardness of the deformed microstructure is found to be 510VHN for 80% cold worked conditions, where the litreatuer value reported for 316 SS is about 502 VHN. Calorimetric investigation of strain energy relaxation A study of thermally activated strain energy relaxation has been extensively investigated because of its technical importance in various aspects. Both recovery and recrystallization belong to the class of composition invariant solid-state transformations, the dynamics of which is closely controlled by grain boundary structural characteristics, in particular their relation to mobility under thermal activation. The driving force for both recovery and recrystallization comes from the stored energy in the form of complex defect networks and its magnitude is relatively small. Hence accurate characterization of the driving force; i.e., stored energy, in terms of calorimetry is very difficult. Nevertheless, as can be inferred from results presented here, calorimetry also offers crucial information with regard to the kinetics of recovery and recrystallization process. 220

4 On-heating DSC thermogram showing distinct thermal arrests due to recovery and recrstallization events at different heating rates Energy release in the process of lattice softening at different heating rates The baseline corrected on-heating DSC thermogram for the 80% cold worked sample is shown at different heating rates.. The choice of 80% cold worked condition is based on the fact that higher the stored energy, the more prominent is the revelation of DSC signatures arising from defect relaxation phenomenon. The DSC outputs shown in the figure are normalized to the sample mass. This result brings out quite convincingly the sequence of major events that accompany the thermally activated lattice softening behavior of super304haustenetic stainless steel. The clear evidence of multi peaks for low heating rate experiments (10 K/min &20 K/min) shows that both the recovery and recrystallization occurs in different and sequential steps. The clear understanding of this behavior in case of lower heating rate experiments needs further investigation in terms of dislocation dynamics operating in this particular during annealing. In case of higher heating rate 221

5 experiments like30 K/min and 40 K/min., clearly distinguished peaks are observed for recovery and recrystallization respectively. The tail of this recovery peak is followed by the onset of principal recrystallization thermal arrest at nearly 1046 K and 1062 K respectively for 30 K/min and 40 K/min heating rates. One of the most important and fundamental advantages of calorimetric techniques for the study of lattice relaxation is that a quantification of average stored energy release is possible separately for recovery and recrystallization by calculating the area under the DSC thermogram. This particular method has been adopted here to find out the relative variation of stored energy in both recovery and recrystallization processes under different heating rates. From the above figure it is pretty clear that the energy released both in recovery and recrystallization increases with heating rate. It is important to note that at a higher heating rate 40 K/min the total energy released was found to be less than that for other lower heating rates. The exact reason for this unusual behavior is unknown at the present time. It is believed that this may be due to insufficient time available for relaxation in case of a higher heating rate experiment. A detailed study with higher heating rate experiments may be possible to find out this peculiarity. Kissinger Model for activation energy of Recovery and recrystallization Traditionally the kinetics of recovery and recrystallization is being modeled by empirical models like Kolmogoov-Johnson-Mehl-Avrami (KJMA) which gives quick estimates for the overall activation energy for the process. But the original KJMA model was formulated based on a homogeneous nucleation of product phase from the parent phase. But in the process of recovery and recrystallization, the enormous amount of the stored energy present in the form of complex defect networks is found to be inhomogeneous in nature and hence the nucleation of strain free grains were believed to be site saturated. In this case a model free kinetic analysis called Isoconversional method (Kissinger, Ozawa like models) is generally employed for estimation of activation energy. 222

6 Kissinger plot for recovery and recrystallization in super 304H k(t)=a exp (-Q eff /RT) where α is rection Progress, k(t) is the temperature factor, A is the pre exponentiel factor, Qeff is the activation energy, T is absolue temperature and R is the gas constant. Isoconversional methods are based on the so called isoconversional principle saying that the reaction rate at constant reaction progress α is only a function of temperature. These methods allow determination of the activation energy (or dependence Q on ) without assuming the explicite form of f(α). Kissinger like methods can be obtained from the above equation as follows. Ln { /TP 2 } = ln (AR/E)} - E/RTP A plot between ln( /TP 2 ) verses (1/RTP) gives a straight line whose slope gives the activation energy. for the process. This particular method has been employed here to calculate the activation energy of recovery and recrystallization separately. Figure shows the Kissinger plot with corresponding activation energy of 86 and 50 KJ/mol respectively for recrystallization and recovery respectively. Summary of the study In this study, Recrystallization behaviour of super 304H-Cu austenitic stainless steel has been studied using dynamic calorimetry technique with metallography and hardness measurement as supplementing contributions. The summary of the present work is as follows: typical XRD of annealed ss304h has clearly revealed that it is of fcc structure which conforms to the austenitic structure and the lattice parameter is found to be nm. From the microstructural characterization of isothermally and isochronally annealed samples, it is obvious that the effect of temperature and time both has a significant impact on the recrystallization behavior of SS304H Cu. In this case a model free kinetic analysis called Isoconversional method (Kissinger, Ozawa like models) is 223

7 generally employed for estimation of activation energy. References 1. William D.Callister, Jr, Material Science and Engineering - an Introduction, Wiley Student edition, sixth edition. 2. R.E. Smallman and A.H.W. Ngan, Physical Metallurgy and Advanced Materials, Elsevier Ltd, Seventh edition V. Raghavan, Materials Science and Engineering, Prentice Hall of India, New Delhi, Second edition (). 4. Sidney H. Avner, Introduction to Physical Metallurgy, Tata McGraw Hill, New Delhi, Second edition 5. R. Visawnathan, W. Bakker, Materials for ultra supercritical coal power plants boiler materials: Part 1, JMEPEG 10 (2001) T. Kan, Y. Sawaragi, Y. Yamadera, H. Okada, Proceedings of 6th Liege Conference on Materials for Advanced Power Engineering, Part-1, 1998, pp Akira Tohyama, Hitoshi Hayakawa, and Yusuke Minami- NKK technical review No. 84(2001) 8. Minami et al. Iron and Steel. Vol. 71, No. 13, pp. S1447 (1985). 9. F.J. Humphreys and M. Hatherly, Recrystallization and Related Annealing Phenomena, Pergamon Press, Oxford,