Identification of M5C2 carbides in ex- service 1Cr-0.5Mo steels

Abstract

Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M5C2 carbides, although some were M3C. The M5C2 carbide, also known as the Hagg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M5C2 rods appeared to nucleate heterogeneously on M2C carbides and persist in ferrite regions from which the needlelike M2C carbides had disappeared. This suggests that the M5C2 carbide is more stable thermodynamically than M2C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components.