Precipitation behavior of NbC in 9%Cr1%Mo0.2%VNb steel

Abstract

Dissolved Nb in both austetinic and ferritic phases of 9%Cr1%Mo0.2%VNb steel was measured using an inductively coupled plasma atomic emission spectroscope (ICP), and the microstructure has been characterized using a field emission scanning electron microscope of a ultra high resolution type and an analytical electron microscope. The dissolved Nb in austenitic phase measured by ICP is in agreement with that predicted by a phase equilibrium calculation system. However, the measured values of dissolved Nb in the specimens which are heat-treated for 2 h just below AC1 are much higher than the calculated values. It is newly confirmed by both chemical and physical analyses that the fine NbC particles which are preformed at 950°C and stable at this temperature are on one occasion re-dissolved into the matrix by the subsequent heat treatment at 800°C for 2 h. This fact suggests that high temperature tempering is recommended in order to improve creep resistance of high Cr heat resisting steels which contain strong carbo-nitride formers.