Analysis of Tensile Stress-Strain and Work-Hardening Behavior in 9Cr-1Mo Ferritic Steel
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INTRODUCTION
9Cr-1Mo ferritic steel is an important high-temperature material for steam-generator (SG) applications in fossil-fired thermal and nuclear power plants. The choice of 9Cr-1Mo steel for steam-generator applications is based on low thermal expansion coefficient and high resistance to stress-corrosion cracking in watersteam systems compared to austenitic stainless steels, in addition to better elevated temperature mechanical properties compared to alternate 2.25Cr-1Mo steel.[1–3] 9Cr-1Mo steel offers good weldability, microstructural stability over long exposure at elevated temperatures, and a good combination of high creep strength and ductility.[4] Apart from steam-generator application, the steel has also emerged as favored core structural material for wrapper applications in future sodiumcooled fast reactors. Experience with austenitic steels used for wrapper application indicates that the steel suffers from degradation in high temperature mechanical properties and unacceptable dimensional changes due to irradiation creep and differential void swelling resulting in restricted fuel burn up. In order to overcome the problem and achieve high fuel burn up for economical nuclear energy, 9Cr ferritic steel has been chosen as
B.K. CHOUDHARY, Scientific Officer-H, D.P. RAO PALAPARTI, Scientific Officer-C, and E. ISAAC SAMUEL, Scientific Officer-E, are with the Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India. Contact e-mail: [email protected] Manuscript submitted June 30, 2011. Article published online September 8, 2012 212—VOLUME 44A, JANUARY 2013
an alternate in view of its excellent resistance to irradiation creep and void swelling.[5,6] Tensile flow and work-hardening behavior are important and attract continued scientific and technological interest in view of improving the appropriate conditions for material processing and for ensuring safe performance during service. Among the several flow relationships[7–12] proposed to describe tensile stress-strain and work-hardening behavior of metals and alloys, the Voce relation[11,12] has attracted more attention in view of the sound interpretation provided by Kocks-Mecking.[13–16] The strainhardening law interrelating true stress (r) and true plastic strain (e) proposed by Voce[11,12] is expressed as ðe eI Þ r ¼ rS ðrS rI Þ exp ½1 eC where rS is the saturation stress; rI and eI are true stress and true plastic strain at the onset of plastic deformation, respectively; and ec is a constant. Equation [1] reduces to r ¼ rS ðrS rI Þ exp ðnV eÞ
½2
for initial plastic strain eI. = 0, with three constants rS, rI, and nV. = 1/ec. According to Voce,[12] the saturation stress rS is the asymptotic stress value attained after severe deformation. Therefore, rS is expected to be close to the value of ultimate tensile strength. The nV parameter defines the rate at which the stress from its initial value tends to reach steady state value or saturation stress value.[12] The applicability of the Voce relationshi
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