Effect of Precipitation Control on Strength and Impact Toughness in Advanced Pressure Vessel Steel
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TRODUCTION
GENERATION IV nuclear reactors such as sodiumcooled fast reactor (SFR) and very-high-temperature reactor (VHTR) should be operated during 60 years under higher temperature condition than the reactors of the previous generation.[1–5] Therefore, low-alloyed MnMo-Ni steels applied to conventional light-water reactors are inadequate to SFR and VHTR. Instead, ASME SA387 grade 22 (2.25Cr1Mo) steel has been used for reactor pressure vessel material owing to considerable application experience and its good performance in the power-generation and petro-chemical industries under high temperature.[1,6] Meanwhile, 2.25Cr-1.5W-VNbTi steel was developed by alloying W instead of Mo and alloying MX carbo-nitrides formers such as V, Nb, Ti. This steel has been usually applied to boiler tube/pipe components in ultra-supercritical coal-fired fossil power plant. Also, it has received ASME code approval (ASME SA1017 grade 23) for application to pressure vessel and piping. SA1017 grade 23 has elevatedtemperature strength comparable to that of SA387 grade 91 (9Cr-1Mo-VNbTi) steel. Reactor pressure vessels also should withstand neutron damage, so that
HYUN JE SUNG is with the Technical Research Laboratories, POSCO, Pohang 37859, South Korea. SUNG-JOON KIM is with the Graduate Institute of Ferrous Technology, POSTECH, Pohang 37673, South Korea. Contact e-mail: [email protected] Manuscript submitted January 27, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
SA1017 grade 23 can be one of the reduced activation ferritic/martensitic steels because of solid solution element W that substitutes for highly radioactive Mo.[7,8] Furthermore, newly introduced SA1017 grade 23 steel with bulk V content ([V]) > 0.4 wt pct has higher creep strength than SA1017 grade 92 (9Cr-1.5W-0.5MoVNbTi) steel.[9,10] The Gibbs free energy difference for M23C6 carbide formation becomes positive when [V] > 0.4 wt pct at the tempering temperature of 923 K (750 C); this would inhibit the precipitation of easily coarsened M23C6 carbide and encourage formation of only nano-sized MX carbo-nitrides in the steel with [V] > 0.4 wt pct during tempering. Low-angle subboundary walls can be stabilized by MX carbo-nitrides on sub-boundaries, so coalescence of neighboring subboundaries is suppressed.[11] Prohibited sub-grain coarsening can help sub-boundary hardening continue during creep. Meanwhile, in addition to creep strength of pressure vessel steels, temper embrittlement in the steels during long-term aging at high temperature determines the service life of the vessel. Ferritic/martensitic steels can be embrittled and failed with intergranular fracture mode by P segregation, leading to subsequent decrease in grain boundary cohesive strength.[9,12–15] Therefore, this research utilizes step-cooling heat treatment[16] which can promote temper embrittlement within 250 hours, instead of time-consuming thermal aging, and quantifies how impact toughness of SA1017 grade 23 steel is affected by elimination of easily coarsened M23C6 carbide, MX carbo-nitride precipitation,
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