Tensile Work Hardening Behavior of Thin-Section Plate and Thick-Section Tubeplate Forging of 9Cr-1Mo Steel in the Framew
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9Cr-1Mo ferritic steel and its modified versions are favored structural material for high-temperature steam generator (SG) applications in thermal and nuclear power generating industries. The choice of 9Cr-1Mo steel for all the components, i.e., tubes, shell, and thick-section tubeplates of SGs of sodium-cooled fast reactors (SFRs) is based on low thermal expansion coefficient and high resistance-to-stress corrosion cracking in water–steam systems compared with austenitic stainless steels and superior elevated temperature mechanical properties than the alternate 2.25Cr-1Mo steel.[1,2] 9Cr-1Mo steel also offers a good combination of high creep strength and ductility, good weldability and microstructural stability over long exposures at elevated temperatures.[1–3] Owing to its high hardenability, 9Cr steel displays the ability and tolerance to offer nearly uniform microstructure over large section size, and this in turn is expected to provide only small variations in mechanical properties with increasing thickness, and between the surface and the center of thick-section products.[4] Further, the use of 9Cr-1Mo steel as a single structural material in the SGs of SFRs enhances the reliability of the components including critical tube-to-tubeplate welds. In addition to the above, the steel with close control on trace elements
B.K. CHOUDHARY, Scientific Officer-H, and J. CHRISTOPHER, Scientific Officer-D, are with the Mechanical Metallurgy Division Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Contact e-mail: [email protected], [email protected] Manuscript submitted February 8, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
such as sulfur and phosphorous has also emerged as an important candidate material for in-core application as wrapper tubes in future SFRs because of its higher resistance to irradiation creep and void swelling compared with austenitic stainless steels.[5,6] Recent investigation on the comparative evaluation of elevated temperature tensile properties of plate and thicksection tubeplate forging of 9Cr-1Mo ferritic steel indicated a systematic reduction in the yield and ultimate tensile strength values accompanied with higher ductility of the forging.[7] The inferior strength values of the forging were ascribed to the effects associated with coarseness of microstructure.[7] Further, the steel in both the product forms exhibited a gradual decrease in strength values with increase in temperature from 300 K (27 C) to intermediate temperatures up to 748 K (475 C) followed by rapid decrease in the strength values at high temperatures.[7,8] 9Cr-1Mo steel in both the product forms exhibits serrated flow, an important manifestation of dynamic strain aging (DSA) at intermediate temperatures.[7–10] Based on the measurements of activation energy for serrated flow, diffusion of interstitial carbon has been suggested to be responsible for the DSA in the steel.[8–10] The other notable manifestations of the DSA affecting tensile properties of plate and tubeplate forging of the steel include
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