Demonstrating the Effect of Precipitation on the Mechanical Stability of Fine-Grained Austenite in Reversion-Treated 301LN Stainless Steel

Abstract

According to recent investigations, a huge difference exists in the mechanical stability of austenite between the grain-refined structure states obtained in reversion annealing at 800-700 • C or at 900 • C, in a 301LN type austenitic stainless steel. Precipitation of chromium nitride occurring at these lower temperatures has been argued to be the factor reducing the stability. To prove this argument, a fine-grained, very stable austenitic structure was created at 900 • C in 1 s, and subsequently annealed at lower temperatures between 850 and 750 • C, up to 1000 s. It was found that the subsequent annealing at 750 and 800 • C resulted in prominent gradual decrease of the mechanical stability under tensile straining, detectable after 10 s annealing duration and continued until 1000 s. Only minimal grain growth occurred, which decreased the stability very marginally. The degree of the stability drop followed the predicted kinetics of the Cr 2 N precipitation with regards as its dependence on annealing duration and temperature. Further, the tensile yield strength of the fine-grained structure increased slightly due to the annealing. The presence of nano-sized Cr 2 N particles was verified after 1000 s holding at 750 • C. These observations and predictions yield firm evidence for the imperative contribution of precipitation to the highly reduced mechanical stability of grain-refined austenite in this steel.

Reversed Microstructures and Tensile Properties after Various Cold Rolling Reductions in AISI 301LN Steel

“…much more stable than the 750-0.1 counterpart at a given CR reduction, in agreement with earlier studies [20,21]. The stability of various reversion-treated structures of 63% cold rolled 301LN steel has been discussed earlier, and the CrN precipitation was proved to be the controlling factor for the difference [30]. However, no previous data are available concerning the influence of the degree of the CR reduction on the stability.…”

“…Hence, the fraction and texture of the medium-sized grains cannot explain the stability difference between these structures created at these different annealing temperatures. The precipitation of CrN in the 750-0.1 structure seems to be the factor which also reduces the austenite stability in the 32CR structures, as confirmed earlier in the instance of the 63CR structures [30]. The mobility of all alloying elements in martensite is high, due to high defect concentration and pipe diffusion, shown to be 45 times higher than in the ferrite phase [35].…”

Deformation-induced martensite in austenitic stainless steels: A review

“…Moreover, a fast kinetics for the formation of α′-martensite during tension was reported for the UFG samples obtained by ECAP in the 301 grade, which was correlated to the effects of the preexisting microtwins and increasing the dislocation density during the formation of Lüders bands [128]. Furthermore, the chromium nitride precipitation during the formation of UFG structure in the 301LN grade was linked to the reduced stability of the austenite phase in the steel during subsequent deformation [2,166].…”

Processing and Properties of Reversion-Treated Austenitic Stainless Steels

“…Tomimura et al [39] found that the reversion was diffusional at 652 • C in an 18Cr-9Ni alloy (Ni/Cr = 0.5) in spite of a high heating rate of 300 • C/s. Somani et al [23,80], Kisko et al [27,28,[81][82][83] and Järvenpää et al [84][85][86][87] have used a high heating rate of 200 • C/s in their experiments. They found that in 301LN (Ni/Cr ≈ 0.4), the diffusion-controlled reversion started, depending on prior cold rolling reduction, around 650-700 • C and was completed within few seconds at 750-800 • C. Examples of the influence of cold rolling degree on the reversion kinetics in a 301LN ASS are illustrated in Figure 4, where also the data of Takaki et al [40] for an 18Cr-8.5Ni steel is included.…”

“…However, the A f temperature for a 301LN ASS has not been clearly reported, and due to the diffusion-controlled reversion, it is dependent on cold rolling reduction and annealing duration, as evident from Figure 4. In that data, about 5%DIM is left after 10 s holding at 700 • C after the 75% reduction, while after the 63% reduction, Järvenpää et al [84] reported 10% of DIM and Rajasekhara [26] 5% after 100 s and 20% after 10 s. Anyhow, in numerous studies, temperatures between 600 and 1000 • C have been used for reversion treatments of 301LN ASS [12,[22][23][24][25][26][84][85][86][87].…”