Effect of transformation temperature on the microstructure of an ASTM A335 grade P91 steel isothermally transformed to ferrite

Abstract

In this contribution the isothermal austenite-ferrite transformation in an ASTM A335 P91 (9Cr1MoVNb) steel was studied regarding the ferrite microstructure obtained after full transformation. First, using the information obtained from dilatometry experiments, the Temperature-Time-Transformation diagram was established for selected experimental conditions. The ferrite average grain size, the ferrite grain size distribution and the precipitated second phases nucleated and grown in the ferritic matrix during transformation were examined as a function of the transformation temperature in the range from 640°C to 760°C. For the size distribution of ferritic grains, a study was carried out using the kernel density estimation methodology. As transformation temperature increased, the average grain size showed an increasing tendency, and the ferrite grain size distribution broadened. M23C6, M2X and MX precipitated phases were identified in fully transformed samples, but differences in morphology, size and nucleation site of these precipitates were found according to the transformation temperature, i.e. below or above the “nose” of the TTT diagram. The presence of fiber morphology of the precipitates only at low and intermediate temperatures was associated to the grain-boundary nucleation of ferrite.