CHARACTERIZATION OF PEARLITIC CARBIDE IN 1CR-0.5MO STEELS

The pearlitic carbide formed during isothermal decomposition of austen ite in a commercial 1Cr-0.5Mo steel has been studied using electron di ffraction and energy dispersive X-ray spectroscopy. The pearlitic carb ide was found to be composed of both M3C and M23C6, with the M23C6 car bide becoming more common as the transformation temperature increased. The M23C6 carbide was richer in molybdenum (2.5-6.3 at.-%) than the M 3C carbide (<2.5 at.%). The average manganese and chromium concentrati ons in the pearlitic M3C and M23C6 formed at a given transformation te mperature were similar. The average manganese concentrations (similar to 2.5 at.-%) did not vary significantly with transformation temperatu re, but the chromium concentration increased from similar to 9 at.-% a t a transformation temperature of 690 degrees C to similar to 18 at.-% at 730 degrees C. This indicates that the initial manganese concentra tion in the pearlitic carbide of commercial products is unlikely to be influenced strongly by the cooling rate following austenitisation, bu t the chromium concentration may be more sensitive. The pearlitic carb ide in exservice 1Cr-0.5Mo steels from two superheater outlet headers and a virgin 1Cr-0.5Mo steel has also been characterised to confirm th at the pearlitic M23C6 carbide does occur, although not commonly, in s teels subjected to commercial austenitising treatments.