Effect of Cyclic Thermal Process on Ultrafine Grain Formation in AISI 304L Austenitic Stainless Steel

Abstract

As-received hot-rolled commercial grade AISI 304L austenitic stainless steel plates were solution treated at 1060°C to achieve chemical homogeneity. Microstructural characterization of the solution-treated material revealed polygonal grains of about 85-lm size along with annealing twins. The solution-treated plates were heavily cold rolled to about 90 pct of reduction in thickness. Cold-rolled specimens were then subjected to thermal cycles at various temperatures between 750°C and 925°C. X-ray diffraction showed about 24.2 pct of strain-induced martensite formation due to cold rolling of austenitic stainless steel. Strain-induced martensite formed during cold rolling reverted to austenite by the cyclic thermal process. The microstructural study by transmission electron microscope of the material after the cyclic thermal process showed formation of nanostructure or ultrafine grain austenite. The tensile testing of the ultrafine-grained austenitic stainless steel showed a yield strength 4 to 6 times higher in comparison to its coarse-grained counterpart. However, it demonstrated very poor ductility due to inadequate strain hardenability. The poor strain hardenability was correlated with the formation of strain-induced martensite in this steel grade.

Processing and Properties of Reversion-Treated Austenitic Stainless Steels

“…In a more recent study, Sun et al [140] reported that in two-stage annealing of a 304 ASS containing 47% DIM and 53% DA after two-stage cold rolling (67% + 47%, respectively) at 800 and 850 • C, non-uniform grain structure was created, where the fraction of ultrafine grains (<1 µm) decreased and larger austenite grains (up to 3 µm) increased with prolonged annealing. Ravi Kumar et al [18,146,147] and Ravi Kumar and Raabe [148] have obtained bulk ultrafine-grained ASS with mono-(maximum at GS ≈ 0.6 µm) and bimodal-type (minimum at GS ≈ 0.5 µm and maximum at GS ≈ 1.40-1.65 µm) GS distributions by 4-stage cyclic thermal processing in a 90% cold-rolled AISI 304L grade. This enhanced significantly the elongation.…”

Large Delta T Thermal Cycling Induced Stress Accelerates Equilibrium and Transformation in Super DSS

“…A schematic illustration of the thermal cycling is presented in the plot of Figure 1 [24]. A short holding period of 1 min at the peak temperature was selected mainly to inhibit the grain growth [20]. Short soaking periods at the annealing temperature of about 1 min are generally recommended for reducing costs from an industrial viewpoint [18,25].…”

“…Particularly, the grain boundary angles of most γ grains were concentrated between 53 • and 63 • ; they were not affected by the thermal cyclic number (Figure 6b). It has been reported that newly reverted γ assembles the lath martensite at high-angle boundaries after repeated thermal cycles [20,24]. The complexity increases because of the simultaneous presence of δ and γ phases in DSS.…”

“…Before the thermal cycling tests, 304L steel was solid solution treated for one hour at 1060 • C. Nano-sized grains of 260 nm to 500 nm were obtained by thermal processing of four cycles between 25 • C and 775 • C with 30 s of holding. Cycling between 25 • C and 925 • C for two cycles with 30 s of holding yielded the finest average grain size of approximately 240 nm [20]. Overall, an ultrafine grain austenite microstructure can be obtained with the appropriate cyclic temperature range ∆T (the difference between the peak and the bottom temperature) combined with the appropriate number of cycles.…”

“…Overall, an ultrafine grain austenite microstructure can be obtained with the appropriate cyclic temperature range ∆T (the difference between the peak and the bottom temperature) combined with the appropriate number of cycles. The good strength combined favorable ductility of the nanostructured stainless steel is attributed to its ability to be work hardened through the transformation of strain induced martensite (SIM) during deformation [20]. Two kinds of reversion processes in low-carbon steels: (1) cyclic transformation between martensite and austenite, the so-called thermal cycling, and (2) reversion from tempered and cold-rolled lath martensite, can refine the austenite grain sizes smaller than 5 µm.…”