Microstructural Evolution and Mechanical Properties of Short-Term Thermally Exposed 9/12Cr Heat-Resistant Steels
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INTRODUCTION
THE long-term creep rupture strength and microstructure stability of the 9/12Cr heat-resistant steels have been improved by a new alloying and optimized heat-treatment procedure.[1] The most recent improvements have been achieved by cobalt or tungsten alloying.[2–4] For example, it has been confirmed that cobalt is one of the important alloying elements to suppress the d-ferrite formation during the high-temperature normalizing process in 9/12Cr heat-resistant steels. In contrast, it is also believed that addition of 2 to 3 pct cobalt could drastically improve the short-term creep strength. Similarly, a lower creep rate and higher creep rupture strength in tungsten-containing heat-resistant steels can be attributed to the effects of both precipitation of Laves phase (Fe2W) or l phase (Fe7W6), as well as solid solution strengthening resulting from the addition of tungsten, which produces larger lattice misfit than molybdenum.[2] The Laves phase has a hexagonal crystal structure. However, during thermal exposure, the steels containing cobalt or tungsten will undergo a microstructural change, leading to the degradation of their mechanical properties. It was found that the M23C6 and MX carbides coarsened and Laves phase precipitated during long-term exposure of a 12Cr steel containing tungsten.[5] Most heat-resistant steels containing a high content of tungsten or cobalt show ductile-tobrittle transition with an increase of the rupture life, and WEI WANG, Research Associate, WEI YAN, Associate Professor, and YIYIN SHAN and KE YANG, Professors, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] WEI SHA, Professor, is with the School of Planning, Architecture & Civil Engineering, Queen’s University Belfast, Belfast BT9 5AG, U.K. Manuscript submitted December 16, 2011. Article published online June 13, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
it has been found that creep cavities are easily nucleated at coarse precipitates of Laves phase along grain boundaries. These findings suggest that, on the one hand, tungsten or cobalt can promote the precipitation during short-term thermal exposure, but on the other hand, it can accelerate the coarsening of the precipitates during the long-term exposure process. When the coarsened Laves-phase precipitates or carbides are greater than a critical size, the cavity formation is triggered and the consequent brittle intergranular fracture occurs.[6–9] During the exposure process, the precipitated particles may crack or become detached from the matrix, and cavities may form in the microstructure. These might eventually affect the fracture mode of the ferritic steels during subsequent deformation, or in mechanical testing, at ambient temperature. Previous works have often focused on the mechanical properties and microstructure evolution of 9/12Cr heatresistant steels after long-term exposure or under deformation creep. Few studies have discussed the change of microstructure under short-term
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