Identification of M 5 C 2 carbides in ex- service 1Cr-0.5Mo steels
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I.
INTRODUCTION
L O W - a l l o y ferritic steels are used extensively in the power-generating industry for boiler components, such as headers, attemperators, and superheater tubing. Lowalloy chromium-molybdenum steels commonly enter service in the normalized condition, with a mixed ferritepearlite microstructure. Initial strength at elevated temperature is achieved primarily through alloying elements (chromium, molybdenum, and manganese) in solid solution in the ferrite. However, with increasing time at elevated temperature, these alloying elements precipitate from solid solution as fine-scale carbides or nitrides, and the loss in initial solid-solution strengthening is balanced and supplemented by precipitation hardening, ttj A sequence of transformations to the most thermodynamically stable precipitates and precipitate coarsening eventually leads to the degradation of the creep properties of chromium-molybdenum steels during high-temperature exposure. A clear understanding of these microstructural changes is important both for preserving plant integrity within the existing design life and for assessing the potential remanent life of a plant that is approaching the end of its nominal design life. The dominant carbide in normalized 1Cr-0.5Mo steels* is initially pearlitic M3C. **[2,3,41 During service, *Low-alloy creep-resistant steels are commonly described by the nominal concentrations (in weight percent) of the principal alloying elements. **Carbides in alloy steels are commonly described by general formulas, such as M~C or M2.~C6, where M represents a mixture of metal atoms (e.g., Fe, Cr, Mo, and Mn).
the M3C in the pearlitic regions spheroidizes slowly and the Cr and Mn concentrations in the M 3 C increase at the S.D. MANN, Ph.D. Student, D.G. McCULLOCH, Postdoctoral Fellow, and B.C. MUDDLE, Reader, are with the Department of Materials Engineering, Monash University, Clayton, Victoria 3168, Australia. S.D. Mann, Research Engineer, is with HRL Technology Pry. Ltd., Mulgrave, Victoria 3170, Australia. Manuscript submitted December 6, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
expense of the Fe content.ts-81 Finely dispersed needles of M2C form within the ferrite grains during elevatedtemperature exposure, t3'4'8 ~q and it is commonly considered that these M2C carbides are the most important precipitate for conferring creep strength in low-alloy Cr-Mo steels3 ~.~~ Other carbides that have been reported in 1Cr-0.5Mo steel after extended exposure at elevated temperatures include M7C3, [2'4'8'9] M23C6,13'4'91 and M6C.14.8, 9]
During the present study, analytical transmission electron microscopy has been used to characterize the microstructures of ex-service 1Cr-0.5Mo steels and samples of 1Cr-0.5Mo steel that have been subjected to creep testing and unstressed isothermal aging in the laboratory. In many cases, coarse particles with a rod morphology were observed within the ferrite grains after extended elevated-temperature exposure. Although some of these rods were identified as M3C carbides, the majorit
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