Effect of Grain Refinement on Thermal Stability of Metastable Austenitic Steel

Abstract

In martensitic steels, it is well known that a certain chemical driving force (about 180 MJ/m 3 ) is required to start martensitic transformation (Ms), and additional driving force has to be charged further to complete the transformation (Mf). In the case of metastable austenitic steels with Ms temperature at around room temperature, however, only the chemical driving force needed to start martensitic transformation has been stored at room temperature. Hence, the state of austenite is very unstable thermally. It has already been known that such a metastable austenite undergoes a partial martensitic transformation during isothermal holding at room temperature or cooling to a low temperature. It is very convenient to investigate the behavior of martensitic transformation of austenite. In this study, the effect of austenite grain size on martensitic transformation is introduced from the viewpoint of microstructural analysis and thermo-dynamics. The steel used in this investigation is an Fe-16 mass%Cr-10 mass%Ni ternary alloy, which has Ms temperature at around room temperature. The grain size of this steel can be controlled from 0.8 mm to 80 mm using the technique of reversion of deformation induced martensite. In the material with coarse grain size (80 mm), about 18% of martensite was detected at room temperature and the amount of martensite was increased to 50% by the following subzero treatment to 77 K. However, martensite was hardly detected in the material with ultra fine grains (0.8 mm) even after the subzero treatment. It was found that such a stabilization occurs in the materials with the grain size below 10 mm and the stabilization was reasonably explained by considering the relation between austenite grain size and elastic strain energy which is required on the single variant martensitic transformation.

Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

“…Takaki et al pointed out that as PAGS is reduced, the number of variants forming during martensitic transformation is reduced. [32] This agrees with current observations in martensitic steel. The number of packets per parent austenite grain is reduced with decreasing PAGS as well as the ratio d PAGS /d PS , which approaches unity.…”

“…Takaki [32] explained austenite stabilization with refinement of grain size by the increasing difficulty of multivariant transformation. Unlike coarse grains in which multivariant blocks form to minimize the local shear distortion accompanied with the martensitic transformation, in fine PAGs, the relaxation of volume expansion is not enough and the elastic strain energy becomes large.…”

Microstructures of Pearlite and Martensite Transformed from Ultrafine-Grained Austenite Fabricated through Cyclic Heat Treatment in Medium Carbon Steels

“…Additionally, the blocks transformed from the ultrafine-grained austenite tend to exhibit rather equiaxed morphology compared with those transformed from the coarse-grained austenite. Takaki et al 6) reported that the ultrafine-grained austenite with grain sizes smaller than 10 µm transformed to single variant martensite or single packet (variants of martensite with the identical habit plane) in an austenitic stainless steel. As shown in the EBSD orientation maps of Figs.…”

“…The high thermal stability of bulky polycrystalline austenite with ultrafine grains against martensitic transformation was also confirmed in an FeNi alloy 5) and an austenitic stainless steel. 6) In addition, it was reported that the crystallographic features, such as variant selection rule and orientation relationship, of martensite transformed from ultrafine-grained austenite were different from those transformed from coarse-grained austenite.…”

Ridging-free Ferritic Stainless Steel Produced through Recrystallization of Lath Martensite

“…Since the martensitic transformation proceeds so as to accommodate anisotropic lattice displacement by forming several fine grains with different crystallographic orientations (variants), 12) the banded structure can be subdivided effectively and the anisotropy is reduced through the multi-variant transformation on cooling, even if the coarse austenite colonies have been formed during hot rolling after solidification. As a result, the recrystallized ferritic structure formed from such a martensitic structure is also expected to have crystallographic isotropy.…”