Dynamic Recrystallization of Austenite in 18-8 Stainless Steel and 18 Ni Maraging Steel

Abstract

SynopsisThe dynamic recrystallization behavior of austenite in an 18-8 stainless steel and an 18 Ni maraging steel was studied mainly by microstructural observations of the specimens which were water-quenched immediately after the tensile deformation to various strains at temperatures ranging from 800 to 1200 °C and at strain rates of 10'3 10-~ s-1, The changes in austenite grain size and hardness at room temperature due to the dynamic recrystallization were also studied.The deformation conditions for the occurrence of dynamic recrystallization in both steels were made clear as a function of deformation temperature (T), strain rate (~) and strain (e). Dynamic recrystallization takes place under the deformation conditions with Z (Zener-Hollomon parameter) less than the critical value of Z~. Z~ increases as the amount of strain given by the hot deformation is increased or the initial austenite grain size is decreased. The addition o f Cr seems to have the retarding e ffect on the occurrence of dynamic recrystallization of austenite. The dynamically recrystallized grain size (D) is determined only by the deformation condition z, and does not depend on the strain and the initial grain size. D is decreased with increase in Z. The room temperature hardness of dynamically recrystallized austenite in 18-8 stainless steel and the martensite formed from the dynamically recrystallized austenite in 18 Ni maraging steel is higher than that of conventionally heat-treated specimens. However, such an increment of hardness due to dynamic recrystallization is not so large.

Ultrafine Grain Structure through Dynamic Recrystallization for Type 304 Stainless Steel.

“…The initial grain size, D o , does not have much significance. Hence, we compare the present data with the data provided by Maki et al 10) Their D o was between 76 and 250 mm and ours was about 30 mm. Figure 8 rearranges the D s data as a function of the Z-H parameter.…”

Regularities of Deformation Microstructures in Ferritic Stainless Steels during Large Strain Cold Working

“…This suggests that the size of ultrafine grains that develop by continuous dynamic recrystallization during severe deformation can be reduced down to some limit, which depends on physical deformation conditions such as temperature and strain rate. This is similar to the phenomenon of discontinuous dynamic recrystallization during hot working, when the dynamic grain size can be expressed by a power law function of temperature compensated strain rate, [10][11][12][13][14] although responsible structural mechanisms seem to be different. In the case of continuous dynamic recrystallization, the final grain size approaches the size of dislocation subgrains or cells, which reaches a limit in the early stages of deformation and changes only slightly thereafter.…”

“…The main characteristic of dynamic recrystallization taking place at elevated temperatures have been fairly clarified in a number of papers. [10][11][12][13][14] The dynamic grain size that can be obtained after completion of dynamic recrystallization decreases with a decrease in deformation temperature. Therefore, a considerable refinement of the microstructure can be achieved by plastic working at relatively low temperatures.…”

Dynamic Restoration Process of Ni-30Fe Alloy during Hot Deformation.

“…Namely, a higher strain is necessary for the DRX with a decrease in the deformation temperature or an increase in the strain rate. Figure 9 replots the grain sizes of the equiaxed microstructures obtained in the present study as a function of the Z parameter together with the similar data for the SUS304L steel by Salvatori et al 14) and Maki et al 16) Their data were for the DRX grains of the SUS304L steel. It is interesting that the empirical equation suggested by Salvatori et al 14) shows a good fit with the present results as far as the DRX is concerned.…”