Creep and creep rate curves of a 10cr-30mn austenitic steel during carbide precipitation
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INTRODUCTION
IT is well established that many microstructural changes can take place during high-temperature creep deformation of heat-resisting steels and other materials,t~] The microstructural changes include particle size and spacing of precipitates, dislocation networks, recovery of excess dislocations, recrystallization, and so on. The present authort2,3] reported for tempered martensitic 9Cr-W steels that the microstructural evolution (e.g., the recovery of excess dislocations, the agglomeration of M23C 6 carbides, and the coarsening of martensite lath subgrains) occurred with the aid of stress during creep at around 873 K and affected significantly the creep rates of the steels. Austenitic steels such as type 316 stainless steel are usually subjected to creep tests after solution annealing at high temperature. Therefore, the microstructural evolution during creep would be precipitation of carbides and intermetallic compounds from supersaturated solid solution, development of dislocation arrangements to form networks or sub-boundaries, and so on. Of the microstructural evolution, the precipitation of carbides and its effect on creep behavior are important, because carbides are one of the main strengthening factors for austenitic steels. The precipitation behavior of carbides in solution-annealed matrix of iron base alloys during aging and the effect of dislocations on the precipitation kinetics have been studied extensively.[4] However, at present, the understanding of the effect of carbide precipitation on creep rates is rather insufficient in commercial austenitic steels such as type 316 steel, because many kinds of precipitates form during creep. In particular, little is known about the effect of carbide precipitation on the time dependence of creep rate. The purpose of the present research is to investigate the
effect of carbide precipitation on creep and creep rate for 10Cr-30Mn austenitic steel. This steel is a proper material for the investigation of the effect of carbide precipitation, because the carbon addition up to 0.5 wt pct to this steel causes only one kind of carbide, M23C6 .[5'6] This simplifies the interpretation of the effect of carbide precipitation. Both Cr and Mn are known as M23C 6 stabilizers. Furthermore, high-Mn austenitic steels have attracted attention for fusion reactor structures, as will be described subsequently. In high-Mn austenitic steels, carbon is a very important alloying element. The carbon addition stabilizes the austenite phase but causes carbide precipitation which affects the mechanical properties at high temperature. In particular, the phase stability is one of the most serious problems for the development of high-Mn austenitic steels, because the austenite forming tendency of Mn is considerably less than that of Ni. In this research, the specimens containing different levels of carbon concentration (0.003 to 0.55 wt pct) were prepared and subjected to creep testing at 873 K for up to 30 Ms (8300 hours). Microstructural observations were made by transmission
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