Enhanced Mechanical Properties through Reversion in Metastable Austenitic Stainless Steels

Abstract

A novel processing route of cold rolling and reversion annealing for enhanced mechanical properties has been investigated in metastable 17Cr-7Ni-type austenitic stainless steels, i.e., commercial grades AISI 301LN and AISI 301, and in some experimental heats. The investigation was essentially aimed at studying the possibility of processing nano/submicron-grained structure in these steels and to rationalize the possible effects of alloying elements on the reversion mechanisms. The steels were cold rolled to various reductions between 45 and 78 pct to induce the formation of martensite, and subsequently annealed between 600°C to 1000°C for short annealing times (mostly 1 to 100 seconds). Microstructure examinations of the reversion-annealed 301LN steel revealed that an ultrafine-grained austenitic structure was formed by the diffusional transformation mechanism within a short holding time above 700°C, even after the lowest cold-rolling reduction. In contrast, in 301 steel and experimental heats, the shear type of transformation occurred at temperatures above 650°C, but fine austenite grains were only formed by recrystallization at higher temperatures or longer holding times, e.g., at 900°C/100 s. An attempt has been made to determine the reversion mechanisms in various steels by modifying the criteria governing the Gibbs free energy change during the martensiteaustenite reversion in Cr-Ni alloys. The room temperature (RT)-tensile property evaluation showed that excellent combinations of yield or tensile strength and elongation are possible to achieve, depending mainly on annealing conditions both in the 301LN and 301 steels, but the experimental heats were too unstable for high ductility. Ultrafine grain size of austenite contributed to this in 301LN and shear-transformed high-dislocated austenite in 301. Upon reversion annealing, the reversion mechanism did not affect the texture. The texture of the reverted fine-grained austenite is very strong compared to the typical texture of commercially cold-rolled and annealed 301LN steel.

Dynamic shear response and evolution mechanisms of adiabatic shear band in an ultrafine-grained austenite–ferrite duplex steel

“…The grain size was found to have a strong effect on the transformation behavior of austenitic grains, and two contradictory conclusions could be found on this grain size effect in the previous research (Somani et al, 2009;Huang et al, 2011;Iwamoto and Tsuta, 2000;Shi et al, 2010b). Somani et al (2009) and Huang et al (2011) reported that martensite transformation could be enhanced significantly by ultra-fined austenite grains in 301LN stainless steel, which was contrary to the common observations that smaller austenite grains are more stable against transformation (Iwamoto and Tsuta, 2000;Shi et al, 2010b).…”

“…The grain size was found to have a strong effect on the transformation behavior of austenitic grains, and two contradictory conclusions could be found on this grain size effect in the previous research (Somani et al, 2009;Huang et al, 2011;Iwamoto and Tsuta, 2000;Shi et al, 2010b). Somani et al (2009) and Huang et al (2011) reported that martensite transformation could be enhanced significantly by ultra-fined austenite grains in 301LN stainless steel, which was contrary to the common observations that smaller austenite grains are more stable against transformation (Iwamoto and Tsuta, 2000;Shi et al, 2010b). Other factors, such as strain rate, temperature and stress triaxiality, were also found to have strong influences on TRIP effect: (i) at the low strain rate range (<1/s), TRIP effect happens at earlier strain for higher strain rate, while the maximum volume fraction of martensite decreases with increasing strain rate (Das and Tarafder, 2009;Lee et al, 2014;Prüger et al, 2014;Zaera et al, 2014); (ii) TRIP effect is suppressed with increasing temperature at the low temperature range (77-332 K) (Prüger et al, 2014;Zaera et al, 2014;Lebedev and Kosarchuk, 2000); (iii) increasing stress triaxiality intensifies TRIP effect (Lebedev and Kosarchuk, 2000;Jacques et al, 2007).…”

Effect of Nb Microalloying on Reversion and Grain Growth in a High-Mn 204Cu Austenitic Stainless Steel

“…Later Somani et al 7) extended the compositional dependence of the reversion type adopting the Cr-and Ni-equivalents including the elements Mo and C, N, Mn, Si, respectively. The influence of the reversion type on the microstructural evolution has been demonstrated in Cr-Ni steels.…”

“…The influence of the reversion type on the microstructural evolution has been demonstrated in Cr-Ni steels. 5,7,[10][11][12] However, the influence of microalloying has been investigated only by Sadeghpour et al 13) in a 201L-Ti steel and very recently by Shirdel et al 14) in 304L type steel microalloyed with Mo, Ti, V and Nb.…”

“…[4][5][6][7][8] An ultrafine grain size can be obtained by heavy cold deformation to create a high fraction of SIM that during subsequent proper annealing treatment reverts back to austenite. Martensitic reversion has already been investigated and reported during the 1970's, as referred by Tomimura et al 4) They also noted the formation of a fine austenite grain size obtained from reversed martensite in their experimental Cr-Ni steels.…”