The influence of microalloy elements niobium, titanium, yttrium and zirconium on the growth of crystal grains in austenitic stainless steel AISI 304 at annealing temperatures from 1050 ° C to 1200 °C and annealing times from 0 to 8 hours was investigated. The grain size was assessed under a light microscope using the ASTM E112 comparative method, the delta ferrite content was measured and Brinell hardness and microstructural analysis was performed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) for chemical analysis of inclusions and precipitates.
Titanium was the most effective microalloy element at inhibiting grain growth during annealing, but only up to the temperature of 1100 °C. The effect of zirconium on grain growth is slightly smaller than that of titanium, however, it was more effective at higher temperatures (up to 1150 °C). Yttrium was the only microalloy element that affected the size of the crystal grains when annealed at 1200 °C, i.e. when the crystal grains were already coarse. Niobium only had an effect at an annealing temperature of 1050 °C where it slightly slowed the growth of crystal grains and increased hardness due to the formation of Nb(C, N).
To describe grain growth in AISI 304 steel, we developed a model based on the Sellars Whiteman Arrhenius-type equation and used a model based on the modified Hall-Petch equation that describes hardness as a function of crystal grain size. In both models, we had a good match between the measured and calculated values. The annealing temperature also affected the delta ferrite content. With higher temperatures, the proportion of delta ferrite decreased. Its effect was greater at shorter annealing times.
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