CHARACTERIZATION OF THE XI-CARBIDE IN EX-SERVICE 1CR-0.5MO STEELS

The xi-carbide, also described as Fe2MoC and M(a)C(b), has been identi fied at ferrite grain boundaries in 1Cr-0.5Mo steels exposed to elevat ed temperatures (500-530 degrees C) for prolonged periods (65,000-170, 000 hr). The structure and composition of the phase have been characte rized using electron microdiffraction and energy-dispersive X-ray spec troscopy (EDXS) respectively. Analysis of microdiffraction patterns su pports the proposal that the xi-carbide has a monoclinic unit cell, ra ther than the orthorhombic unit cell first proposed. The ranges of met allic element concentrations in the xi-carbide (i.e. 48-59 at.% Fe, 22 -29 at.% Mo, 7-16 at.% Cr, 4-7 at.% Mn and 3-6 at.% Si) are unique com pared to those of other carbide precipitates identified in the 1Cr-0.5 Mo steels, which means that the xi-carbide may be identified rapidly o n extraction replicas using qualitative EDXS.