The paper deals with influence of four temperatures of high temperature austenitization and two dwells on temperature on microstructure and substructure of heat-resistant 9CrNB steel. The microstructure consists of lathe martensite and bainite. The presence of delta ferrite was demonstrated at specific austenitizing temperatures. Transmission electron microscopy showed that the substructure at all tested heat treatment was formed by martensite and bainite laths. They were identified as rod-shaped respectively needle-shaped particles based on extraction of laths into carbon replicas. Carbide particles M3C-type were found by using diffraction analysis. Rod-shaped particles were excluded at the boundaries of the delta-ferrite grains, and these particles are MC type carbide particles according to the electron diffraction analysis. The presence of delta ferrite grains at the austenite grain boundaries effectively prevents their migration during austenitization and thus limits the growth of the mean austenite grain size in the austenitization temperature range of 1190 °C to 1230 °C. The presence of a larger amount of oval particles of the precipitate was identified at the austenitizing temperature of 1070 °C. It is very likely that they are NbC carbide particles according to the diffraction analysis. These particles should prevent a significant increase in austenite grain. This type of particles is observed only sporadically at a temperature of 1150 °C. The loss of the braking effect of NbC carbide particles on the migration of austenite grains caused the formation of significantly larger austenite grains at these austenitizing temperatures compared with the austenitizing state at 1070 °C.Keywords: Austenitizing temperature, low carbon 9Cr creep resistant steel, grain growth, heat treatment
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