Assessment of Creep Deformation, Damage, and Rupture Life of 304HCu Austenitic Stainless Steel Under Multiaxial State of
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TRODUCTION
FOR the reduction in carbon footprint in the generation of electricity from fossil fuel, curtailing the emission of CO2 gas causing greenhouse effect was required. The reduction in CO2 gas emission was linked with the increase in efficiency of the power plant to reduce fuel consumption.[1] Eventually, it led to the development of advanced ultra supercritical (AUSC) power plants, which aim to increase efficiency to more than 45 pct and, hence, to decrease CO2 emission. The increase in efficiency is associated with the increasing
K.C. SAHOO is with the Homi Bhabha National Institute, Mumbai 400094, India. Contact email: [email protected] SUNIL GOYAL, P. PARAMESWARAN, S. RAVI, and K. LAHA are with the Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, India. Manuscript submitted May 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steam temperature and pressure, which necessarily calls for materials having the elevated temperature creep strength. Conventional 18 pct Cr-8 pct Ni austenitic stainless steels possess better oxidation resistance than the ferritic steels. However, they do not have sufficient high-temperature creep strength for application in the AUSC power plants. The steels are generally alloyed with nitrogen, molybdenum, tungsten, niobium, titanium, copper, boron, etc. to increase creep strength through solid solution and precipitation strengthening and, thus, achieve creep cavitation resistance.[2] The 304HCu SS is one of the candidate materials to be used in AUSC power plants. It contains around 3 wt pct copper, certain amounts of niobium and nitrogen, and increased carbon content for enhancing creep strength.[3] Copper has relatively low solubility in the austenite matrix and is precipitated out as nanosized particles on thermal and creep exposures.[3,4] The presence of Cu particle and niobium carbonitrides causes precipitation hardening of the steel. The trace addition of boron in 304HCu SS suppresses the creep cavitation, which
increases its creep rupture strength to a much higher extent than the conventional austenitic stainless steels.[5] Evaluation of creep strength of materials is generally carried out under uniaxial state of stress. However, the boiler tubes made of 304HCu SS are subject to multiaxial state of stress due to internal pressure, bends, changes in section thickness, presence of weld joints having inhomogeneous structures, etc. The deformation and fracture behavior of material generally depend on state of stress.[6] Hence, it is important to study the response of the material under multiaxial state of stress. There are several ways of testing creep behavior of material under multiaxial state of stress.[7,8] At laboratory scale, circumferentially notched specimens of different notch root radius with the application of tensile load were used for multiaxial creep study.[9] In the research community, U-notch is preferred over V-notch, since U-notch has a certain volume of material under multiaxial state of stress, which facilita
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