Copper, Boron, and Cerium Additions in Type 347 Austenitic Steel to Improve Creep Rupture Strength
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INTRODUCTION
AUSTENITIC stainless steels are used extensively in several high-temperature utilities in which creep and oxidation resistance are prime concerns. There is an utmost need to increase the creep rupture strength of such steels to increase the efficiency of the utilities. Copper has been added to austenitic stainless steel to increase its creep deformation resistance. Copper has low solubility in the steel and precipitates out as nano-size particles. Precipitation hardening from the nano-size copper particles apart from the usual metal-carbonitride precipitates imparts high creep deformation resistance to the steel to increase its creep rupture strength.[1,2] Such steels usually possess low creep rupture ductility, and the enhanced creep cavitation deprives the steel of the advantage of high creep deformation resistance to increase its long-term creep rupture strength.[3] Creep cavitation proceeds with the nucleation, growth, and coalescence of intergranular creep cavities.[4,5] Stress concentrations developed at grain bound-
KINKAR LAHA, Head, is with the Creep Studies Section, Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Contact e-mail: [email protected] J. KYONO, Researcher, is with Materials Engineering Laboratory, National Institute of Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Ibaraki, Japan. NORIO SHINYA, Scientist, is with Nano-Materials Group, National Institute of Materials Science. Manuscript submitted November 23, 2010. Article published online October 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
ary irregularities because of grain boundary sliding, if not relaxed adequately, nucleate the creep cavity by decohesion of the particle from the matrix.[6,7] The cavity growth proceeds with the diffusive transport of matter from the cavity surface onto the grain boundary.[8–11] The control of diffusion along both the grain boundary and cavity surface is expected to be a useful way to enhance creep cavitation resistance and thereby to increase the creep rupture life and ductility of materials. Trace elements in a material can influence the intergranular creep cavity nucleation and growth processes critically because of their strong segregation tendencies to the grain boundary and nucleated cavity surface.[8,9,12] Holt and Wallace[13] classified the most common trace elements according to whether they have detrimental or beneficial effects on creep rupture strength. Among them, oxygen and sulfur can cause the most severe embrittlement during creep even at low levels. These trace elements in solid solution must be controlled within ppm levels in the material to improve its creep rupture properties. This can be achieved either by removing them during melting or by alloying the steel with suitable elements so as to precipitate them out. The minor addition of rare earth elements such as cerium is found to be highly effective in removing traces of soluble sulfur and oxygen in steel through the formation of ceriumoxysulfide (Ce2O2S).[14
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