Effects of Austenitizing Temperature and Cooling Rate on Precipitation Behavior and Tensile Properties of Pressure Vessel Steels

Title & Authors
Effects of Austenitizing Temperature and Cooling Rate on Precipitation Behavior and Tensile Properties of Pressure Vessel Steels
Shin, Jae Woong; Lee, Sang Min; Kim, Yong Jin; Lee, Sang Woo;

Abstract
The effects of austenitizing temperature and cooling rate on precipitation behavior and tensile properties were investigated in an Mn-Mo-Nb-V pressure vessel steel. During austenitizing, it was shown that the austenite coarsening was somewhat suppressed by undissolved NbC. After cooling from austenitizing, the microstructure of all the steels mainly consisted of upper bainite. However, the steel comprised a little lower bainite and martensite in the case of aqua oil quenching from $\small{1000^{\circ}C}$, which would be due to increased hardenability by partly dissolved Nb and comparatively large austenite grains. The average size of NbC in austenite at higher temperature was analyzed to be smaller than that at lower temperature because of the more dissolution. It was found that the NbC did not grow much during fast cooling from austenitizing. Meanwhile, the NbC grew much during slow cooling, probably due to wide temperature range of cooling and sufficiently long time for NbC to grow. It was conjectured the V precipitates newly formed and/or grew during cooling from austenitizing and during tempering. On the other hand, the formation of NbC was almost completed before tempering and little more precipitated during tempering. Among the tempered steels, the steel which was fast cooled from $\small{1000^{\circ}C}$ showed the highest tensile strength, which seemed to come from the microstructure of fine upper bainite and some low temperature phases as well as the comparatively fine NbC precipitates.
Keywords
Pressure vessel steel;Austenitizing temperature;Cooling rate;Extraction residue analysis;Precipitation;Tensile strength;
Language
Korean
Cited by
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