Analysis of Tensile Stress-Strain and Work-Hardening Behavior in 9Cr-1Mo Ferritic Steel B.K. CHOUDHARY, D.P. RAO PALAPARTI, and E. ISAAC SAMUEL Detailed analysis on tensile true stress (r)-true plastic strain (e) and work-hardening behavior of 9Cr-1Mo steel have been performed in the framework of the Voce relationship and Kocks- Mecking approach for wide range of temperatures, 300 K to 873 K (27 °C to 600 °C) and strain rates (6.33 9 10 5 to 6.33 9 10 3 s 1 ). At all test conditions, r-e data were adequately described by the Voce equation. 9Cr-1Mo steel exhibited two-stage work-hardening behavior character- ized by a rapid decrease in instantaneous work-hardening rate (h = dr/de) with stress at low stresses (transient stage) followed by a gradual decrease in h at high stresses (stage III). The variations of work-hardening parameters and h-r as a function of temperature and strain rate exhibited three distinct temperature regimes. Both work-hardening parameters and h-r dis- played signatures of dynamic strain aging at intermediate temperatures and dominance of dynamic recovery at high temperatures. Excellent correlations have been obtained between work-hardening parameters evaluated using the Voce relationship and the respective tensile properties. A comparison of work-hardening parameters obtained using the Voce equation and Kocks-Mecking approach suggested an analogy between the two for the steel. DOI: 10.1007/s11661-012-1385-0 Ó The Minerals, Metals & Materials Society and ASM International 2012 I. INTRODUCTION 9Cr-1Mo ferritic steel is an important high-tem- perature material for steam-generator (SG) applications in fossil-fired thermal and nuclear power plants. The choice of 9Cr-1Mo steel for steam-generator applica- tions is based on low thermal expansion coefficient and high resistance to stress-corrosion cracking in water- steam 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 sodium- cooled fast reactors. Experience with austenitic steels used for wrapper application indicates that the steel suffers from degradation in high temperature mechan- ical 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 econom- ical nuclear energy, 9Cr ferritic steel has been chosen as an alternate in view of its excellent resistance to irradiation creep and void swelling. [5,6] Tensile flow and work-hardening behavior are impor- tant 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-hard- ening behavior of metals and alloys, the Voce relation [11,12] has attracted more attention in view of the sound inter- pretation provided by Kocks-Mecking. [13–16] The strain- hardening law interrelating true stress (r) and true plastic strain (e) proposed by Voce [11,12] is expressed as r ¼ r S  r S  r I ð Þ exp  e  e I ð Þ e C   ½1 where r S is the saturation stress; r I and e I are true stress and true plastic strain at the onset of plastic deforma- tion, respectively; and e c is a constant. Equation [1] reduces to r ¼ r S  r S  r I ð Þ exp n V e ð Þ ½2 for initial plastic strain e I . = 0, with three constants r S , r I , and n V .= 1/e c . According to Voce, [12] the satura- tion stress r S is the asymptotic stress value attained after severe deformation. Therefore, r S is expected to be close to the value of ultimate tensile strength. The n V parame- ter 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 relation- ship for describing stress-strain behavior has been dem- onstrated for ferritic, pearlitic-ferritic, and austenitic stainless steels useful for engineering applications. [17–21] Further, the important gain from the applicability of the 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: bkc@igcar.gov.in Manuscript submitted June 30, 2011. Article published online September 8, 2012 212—VOLUME 44A, JANUARY 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A