IDENTIFICATION OF M(5)C(2) CARBIDES IN EX-SERVICE 1CR-0.5MO STEELS

Rod-shaped precipitates up to 6 mu m long and 0.25 mu m wide, observed as a common feature within proeutectoid ferrite grains of ex-service 1Cr-0.5Mo steels, have been characterized using electron microdiffract ion, energy-dispersive X-ray spectroscopy, and electron energy loss sp ectroscopy. The majority of the rods have been identified as M(5)C(2) carbides, although some were M(3)C. The M(5)C(2) carbide, also known a s the Hagg or chi-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M (5)C(2) rods appeared to nucleate heterogeneously on M(2)C carbides an d persist in ferrite regions from which the needlelike M(2)C carbides had disappeared. This suggests that the M(5)C(2) carbide is more stabl e thermodynamically than M(2)C in 1Cr-0.5Mo steels under typical servi ce conditions. The metallic element compositions of the rodlike carbid es varied, but the average compositions were in the range 48 to 56 at. pet Fe, 32 to 42 at. pet Cr, 8 to 12 at. pet Mn, and about 1 at. pet Mo. The Mn content of the rods varied systematically with exposure tem perature and thus might be applied to the estimation of the effective service temperature of 1Cr-0.5Mo steel components.