Effects of deformation microbands and twins on microstructure evolution of as-cast Mn18Cr18N austenitic stainless steel
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Hot deformation behavior and microstructure evolution of as-cast Mn18Cr18N austenitic stainless steel were investigated by isothermal compression experiments. The results indicate that the microstructure evolution of the as-cast Mn18Cr18N steel is sensitive to strain rates. Discontinuous dynamic recrystallization, characterized by nucleation and growth controlled by grain boundary migration, occurs at lower strain rates. However, higher strain rates result in higher adiabatic temperature rise, which could be contributed to dynamic recrystallization (DRX) nucleation and growth by acceleration boundary migration. In addition, at higher strain rates, a large number of deformation microbands in the interior of coarse columnar grains were observed, which would provide potential nucleation sites for DRX. Meanwhile, a great number of R3 twins were observed, which reveals that twinning accelerates the separation of subgrains from bulging grain boundaries, and the iterative processing among R3 twins and its variants promotes the transformation from specific CSL grain boundaries to random high-angle boundaries.
I. INTRODUCTION
Austenitic stainless steels with excellent properties are widely used in nuclear power industries, power plants, and medicine equipments.1,2 Mn18Cr18N austenitic stainless steel with high nitrogen content, as a special kind of developed structural material used in the production of the retaining ring of power generators,3 has been paid increasing attention. With the development of metallurgical technology, electro-slag re-melting (ESR) has become an attractive method used for producing consolidated, homogenized, and pure ingots of austenitic stainless steel. However, coarse grains, especially coarse columnar grains, are more prominent.4 Meanwhile, the hot deformation activation energy of as-cast Mn18Cr18N steel is about 672 kJ/mol, which is higher than that of conventional austenitic stainless steel, such as 304 (426 kJ/mol) and 316LN (459 kJ/mol).5 It implies that the deformation resistance for as-cast Mn18Cr18N steel is high, and the recrystallization is more difficult during hot deformation process. The cogging and forging of the ingot become key steps during the primary process of large structural forgings and play a decisive role on the performance of the large structural part.6 Therefore, hot deformation parameters for the ingot cogging and forging need to be optimized, and the microstructure evolution mechanism needs to be clarified for as-cast Mn18Cr18N austenitic stainless steel. Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.389
Stress–strain curves can be used to describe the hot deformation behavior and predict the microstructure evolution to a certain extent. The hot deformation behavior of austenitic stainless steels reflects the dynamic equilibrium between work hardening and flow softening at elevated temperatures.7 In addition, hot deformation results in increasing of the stored energy within the deforme
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