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

A grain boundary phenomenon observed, after metallographic polishing and etching, in a 1Cr-0.5Mo steel is shown to be due to the selective etching of the boundaries. The phenomenon is a feature only of material that has been subjected to strain-controlled fatigue cycles incorporating a dwell time at peak tensile stress (creep fatigue) and is largely confined to boundaries whose orientation is approximately normal to the direction of the principal tensile stress. The structural feature responsible has not been identified, although it has been established that it is not a physical discontinuity, such as a void cavity or a decohered carbide particle. The phenomenon develops concurrently with the precipitation of M3C and M7C3 carbides at the boundaries, which also is a feature of creep fatigue, and develops at the interface between these particles and the ferrite matrix as well as at the ferrite grain boundaries themselves. It seems to have been initiated in a very thin layer at these interfaces, perhaps in a layer only of the order of atom layers in thickness.

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 Hägg 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.

The ξ-carbide, also described as Fe2MoC and MaCb, has been identified at ferrite grain boundaries in 1Cr-0.5Mo steels exposed to elevated temperatures (500–530°C) for prolonged periods (65,000–170,000 hr). The structure and composition of the phase have been characterized using electron microdiffraction and energy-dispersive X-ray spectroscopy (EDXS) respectively. Analysis of microdiffraction patterns supports the proposal that the ξ-carbide has a monoclinic unit cell, rather than the orthorhombic unit cell first proposed. The ranges of metallic element concentrations in the ξ-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 compared to those of other carbide precipitates identified in the 1Cr-0.5Mo steels, which means that the ξ-carbide may be identified rapidly on extraction replicas using qualitative EDXS.

The precracking structures developed in a creep-ruptured Cr-Mo ferritic steel have been investigated metallographically using surface preparation methods that can be expected to produce truly representative surfaces. It is concluded that the structures are grain boundary carbides that have decohered from the matrix ferrite during creep straining. Traditional void cavities were not present. It is established, however, that several commonly used metallographic preparation procedures develop artifacts at the sites of decohered carbides, which could be mistaken for void cavities. The development of these artifacts greatly enhances the detectability of decohered carbides but gives a false impression of their size and destroys the evidence on which the nature of the precracking structures could be elucidated. Methods of reliably investigating the structure of decohered carbides, which is somewhat difficult, or of merely detecting their presence, which is comparatively simple, are discussed in the light of this new information, as is also the consequence of their presence instead of voids on the methods currently used in estimating the residual life of affected components. The implications to theoretical considerations of creep failure are also discussed, a number of questions being raised that require further investigation. Foremost among them is the frequency with which, and the reason why, carbide decohesion occurs without subsequent growth of significant voids. The morphology of the boundary carbides that grow during the earlier stages of creep appears to be central to these matters, and this, in turn, is in need of investigation. It apparently has to be accepted until these questions are settled that two mecanisms of creep failure are possible in these steels.