Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625
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I. INTRODUCTION
ALLOY 625 is a wrought nickel-based superalloy strengthened by the solid-solution-hardening effects of chromium, molybdenum, niobium, and iron and precipitation-hardening effects of the intermetallic phases like bct , orthorhombic , and Pt2Mo-type Ni2(Cr, Mo).[1–9] The alloy has been developed for service at temperatures below 973 K, and it possesses high strength and excellent fabrication characteristics. The alloy is being used for a variety of components in aerospace, aeronautic, marine, chemical, petrochemical, and nuclear industries. It is also being used extensively in the form of tubes in ammonia cracker plants associated with heavy-water production. During normal service, the cracker tubes are exposed to a gas pressure of 14 MPa and temperatures in the range of 853 to 973 K. At these temperatures, the ammonia cracker tubes undergo creep deformation. Although the design life of the cracker tubes is 105 hours, some failures have taken place after about 60,000 hours of service, necessitating their premature replacement. Since replacement of all the tubes is very expensive, rejuvenation heat treatments have been suggested recently by Vani Shankar et al.[9,10] to regain partially the degraded properties. Complete rejuvenation of tensile properties through a solution-annealing treatment at 1423 K for 0.5 hours has been found to be beneficial with respect to tensile properties.[9,10,11] Dynamic strain aging (DSA) is known to occur in austenitic stainless steels and Ni-Fe–based superalloys over a range of temperatures and strain rates.[11–14] Serrated flow, one of the manifestations of DSA,[15] has been observed in the temperaVANI SHANKAR, Scientific Officer, Mechanical Metallurgy Division, M. VALSAN, Head, Fatigue Studies Section, Mechanical Metallurgy Division, K. BHANU SANKARA RAO, Head, Mechanical Metallurgy Division, and S.L. MANNAN, Associate Director, are with the Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India. Contact e-mail: [email protected] Manuscript submitted May 30, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
ture range of application of alloy 625 by the present authors. This may have a large impact on the mechanical properties and life of the cracker tubes. The various investigations of serrated flow suggest that the measurement of c, the critical strain for the onset/termination # of serrations,[16] and its dependence on strain rate () and temperature (T) is essential to understand the underlying mechanisms. This dependence is generally expressed as (mb) # K exp (Q/ kT ) [1] C where m and are the respective exponents in the relations for the variation of vacancy concentration (Cv) and mobile dislocation density (m) with plastic strain (Cv m and m , respectively), K is a constant, Q is the activation energy, k is the Boltzman constant, and T is the absolute temperature.[17,18] One can #obtain the exponent (m ) as the slope in the plot of ln vs ln c at a constant temperature. There are several methods for evaluating Q associated with
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