Effect of nitrogen content on microstructural aspects and creep behavior in extremely low carbon 9Cr heat-resistant steel

Effects of nitrogen content on microstructure and creep strength in extremely low carbon 9Cr ferritic steel were investigated, focusing on the dislocation structure and the distribution of precipitates. The creep strength of the high nitrogen steels was lower than that of the base steel, even though the initial lath width and prior austenite grain size in the high nitrogen steels are finer and larger, respectively. MX nitrides as well as large Cr2N particles were distributed along lath, block, packet and prior austenite grain boundaries in the high nitrogen steels after tempering. The large particles occupy the MX nitride precipitation sites. The coarsening of the MX nitride during creep exposure in the high nitrogen steels was faster than that of the base steel. The presence of large Cr2N particles after tempering and the higher coarsening rate of the MX nitride in the high nitrogen steels cause an increase in mean inter-particle spacing on boundaries, leading to a decrease in creep strength. The Z phase formation was observed after creep exposure in the gauge portion of all the steels. The Z phase formation in the high nitrogen steels occurs at relatively short term in contrast to the base steel. The short term precipitation of the Z phase in high nitrogen steels can contribute to a decrease in creep strength since the Z phase forms at the expense of the MX nitride which is a main strengthening obstacle.