Materials Science and Engineering A 372 (2004) 327–333 Creep properties of service-exposed Alloy 625 after re-solution annealing treatment M.D. Mathew ∗ , K. Bhanu Sankara Rao, S.L. Mannan Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 1 October 2003; received in revised form 15 January 2004 Abstract Creep rupture properties of alloy 625, that has been in service for 60,000 h at 993 K, have been evaluated after subjecting the service-exposed material to re-solution annealing treatment at 1433 K for 1 h. Creep tests were carried out at various temperatures between 923 and 1173 K, and the rupture times varied upto 32,000 h. Iso-stress and Larson–Miller parametric methods were employed to estimate the residual creep rupture life. Creep rupture strength and rupture ductility recovered substantially following re-solution annealing treatment. The variations in rupture life and rupture ductility with creep test variables have been rationalised on the basis of the complex microstructural changes that occurred in the material during creep. © 2004 Elsevier B.V. All rights reserved. Keywords: Alloy 625; Creep life extension; Thermal ageing; Microstructure; Rupture ductility 1. Introduction Alloy 625 is a nickel-base superalloy containing chromium, molybdenum and niobium. It is well suited for applications where high temperature strength and corro- sion resistance are required. It is resistant to oxidation, general corrosion, pitting and crevice corrosion, and is virtually immune to stress–corrosion cracking in chloride environments. These properties are derived from additions of molybdenum, iron and niobium to the alloy’s basic nickel–chromium composition. Alloy 625 has been used in heavy water plants where ammonia is cracked into nitro- gen and hydrogen. During normal operation, the cracker tubes are exposed to a temperature of about 993 K and gas pressure of 14 MPa. At these conditions, the cracker tubes undergo creep deformation. Although the original design life of the tubes was 100,000 h, some tubes failed after about 60,000 h of operation necessitating premature replacement. Studies have been carried out to determine the cause of premature failure and on ways to extend the life of the service-exposed tubes. It was established that a re-solution annealing treatment at 1433 K for 1 h was ben- ∗ Corresponding author. Tel.: +91-4114-280202; fax: +91-4114-280301. E-mail address: mathew@igcar.ernet.in (M.D. Mathew). eficial to restore the tensile properties. The yield strength of the material which increased from 325 MPa before ser- vice exposure to 925 MPa after service exposure, decreased to 470 MPa on re-solution annealing. Similarly, the tensile elongation increased from 6% after service exposure to 55% after re-solution treatment as compared to 53% before ser- vice. This paper presents the results of the study carried out to evaluate the creep properties of service-exposed material after re-solution heat treatment. Creep tests were carried out at various temperatures between 923 and 1173 K, and the rupture times were as long as 32,000 h. Microstructural changes and creep damage due to creep exposure have been systematically investigated to understand the degradation in creep properties of the material at elevated temperatures. Creep properties for such long durations, have not been so far reported for this material in the literature. 2. Experimental procedure The cracker tube [1] had a length of 13,500 mm with an outside diameter of 88.9 mm and wall thickness of 7.95 mm. Chemical composition requirements as per ASTM spec- ification [2] is shown in Table 1 along with the analy- sis of the cracker tube. Rectangular creep specimens with 35 mm gauge length, 7 mm gauge width and 5 mm gauge 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.01.042