The role of martensitic transformation on bimodal grain structure in ultrafine grained AISI 304L stainless steel

Abstract

In the present study, metastable AISI 304L austenitic stainless steel samples were subjected to different cold rolling reductions from 70% to 93%, followed by annealing at 700 º C for 300 min to form ultrafine grained (UFG) austenite with different grain structures. Transmission electron microscopy (TEM) and nanoindentation were used to characterize the martensitic transformation, in order to relate it to the bimodal distribution of the austenite grain size after subsequent annealing. The results showed that the martensite morphology changed from lath type in the 60% rolled sample to a mixture of lath and dislocation-cell types in the higher rolling reductions. Calculation of the Gibbs free energy change during the reversion treatment showed that the reversion mechanism is shear controlled at the annealing temperature and so the morphology of the reverted austenite is completely dependent on the morphology of the deformation induced martensite. It was found the austenite had a bimodal grain size distribution in the 80% rolled and annealed state and this is related to the existence of different types of martensite. Increasing the rolling reduction to 93% followed by annealing caused changing of the grain structure to a monomodal like structure, which was mostly covered with small grains of around 300 nm. The existence of bimodal austenite grain size in the 80% rolled and annealed 304L stainless steel led to the improvement of ductility while maintaining a high tensile strength in comparison with the 93% rolled and annealed sample.

Gas tungsten arc welding and friction stir welding of ultrafine grained AISI 304L stainless steel: Microstructural and mechanical behavior characterization

“…8. The calculated parameters using equations (4)(5)(6)(7)(8) are summarized in Table 3. Also, the austenite grain size in the stired zone after FSW is finally related to the Zenner-Hollomon parameter in Fig.…”

“…Meanwhile, their relatively low hardness and yield strength is not sufficient to meet the requirements for some applications [3]. Moving towards ultrafine and nanocrystalline grain structure is one of the most effective ways to improve their mechanical properties, and as a result, ultra-fine grained (UFG) steels are currently being studied worldwide [4][5].…”

“…Although these methods have been somewhat successful, deterioration of mechanical properties in the weld zone is still A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 4 common. Recently, solid state welding techniques are being developed to weld advanced materials [10][11][12].…”

Microstructural Evolutions During Annealing of Plastically Deformed AISI 304 Austenitic Stainless Steel: Martensite Reversion, Grain Refinement, Recrystallization, and Grain Growth

“…[1,9,11] The martensite phase is not stable at elevated temperatures and its reversion to fine-grained austenite can occur by annealing at elevated temperatures. [1,16] While the reversion process has received a considerable attention in recent years for enhancement of mechanical properties, [1,[17][18][19][20][21][22] much more works are required on this subject to identify the different stages of microstructural evolution to enable its control during thermomechanical treatment. Moreover, it has been reported that for obtaining a marked grain refinement, [1,11] the availability of great amounts of martensite before reversion might be required.…”

Influence of testing mode on the fatigue behavior of <111> austenitic grain at the nanometric length scale for TRIP steels

“…Talonen and Hänninen [32] found a close correlation between the shear bands and α'-martensite formation. Also, the relation between shear bands and α'-martensite, may be related to blocked-shape martensite structures form by nucleation and coalescence of α'-martensite embryos within a single shear band [33][34][35][36].…”