Role of microstructural degradation in the heat-affected zone of 2.25Cr-1Mo steel weldments on subscale features during steam oxidation and their role in weld failures (original) (raw)

1998, Metallurgical and Materials Transactions A

Microstructural degradations in the base metal adjacent to the weld pool, i.e., the heat-affected zone (HAZ), caused during welding of 2.25Cr-1Mo steel, were characterized by electron and optical microscopy of different regions of the weldments. In order to study the influence of the microstructural degradations on scaling kinetics in steam and the resulting subscale features, samples of the base metal, the HAZ, and weld metal specimens were extracted from the weldment and oxidized in an environment of 35 pct steam ϩ nitrogen at 873 K for 10 hours. Oxide scales formed in the three regions and the underlying subscales were characterized using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Influence of the ''free'' chromium content in the three weldment regions on protective scale formation and on the subscale features has been investigated. As the principal achievement, this study has clearly shown the occurrence of oxidation-induced void formation in the subscale zone and grain boundary cavitation in the neighboring area during steam oxidation of the HAZ. This article also discusses the possible role of oxidation-induced void formation and grain boundary cavitation in the inferior service life of welds in 2.25Cr-1Mo steel components. I. INTRODUCTION LOW-ALLOY ''Cr-Mo'' ferritic steels, viz., 2.25Cr-1Mo and 1Cr-0.5Mo steels, are used extensively in the steam generating and handling systems of power plants (in the temperature range of 623 to 873 K) because they satisfy the required mechanical properties, weldability, formability, and corrosion resistance. [1,2] Common applications of 2.25Cr-1Mo steel include reactors for refining and processing of petroleum and high-temperature/high-pressure vessels for thermal reforming, polymerization, alkylation, and hydrocracking. [3-6] This material is also a strong candidate for the fabrication of pressure vessels used for the gasification and liquefaction of coal. [7] The microstructures of Cr-Mo ferritic steels are very susceptible to thermomechanical treatments. This microstructural susceptibility is often exploited in order to develop carbide precipitates of a required chemistry, morphology, and distribution to effect precipitation hardening. However, due to the metastable nature of the chemical composition and the morphology of the strengthening precipitates, the secondary precipitates undergo undesirable transformations during elevated temperature service and/or thermomechanical treatments experienced during fabrication, viz., welding, forging, hot rolling, etc. The strength of the weldments of these steels is generally reported to be inferior, [7,8] to the extent that the creep rupture of the welds is often the life-limiting factor. In fact, about 80 pct of the in-service failures are reported to take place in the weld region of low-Cr ferritic steel compo