Effect of titanium on the creep deformation behaviour of 14Cr–15Ni–Ti stainless steel S. Latha a , M.D. Mathew a,⇑ , P. Parameswaran a , M. Nandagopal a , S.L. Mannan b a Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603 102, India b National Engineering College, Kovilpatti, Tamil Nadu 628 503, India article info Article history: Received 30 July 2010 Accepted 22 December 2010 Available online 30 December 2010 abstract 14Cr–15Ni–Ti modified stainless steel alloyed with additions of phosphorus and silicon is a potential can- didate material for the future cores of Prototype Fast Breeder Reactor. In order to optimise the titanium content in this steel, creep tests have been conducted on the heats with different titanium contents of 0.18, 0.23, 0.25 and 0.36 wt.% at 973 K at various stress levels. The stress exponents indicated that the rate controlling deformation mechanism was dislocation creep. A peak in the variation of rupture life with titanium content was observed around 0.23 wt.% titanium and the peak was more pronounced at lower stresses. The variation in creep strength with titanium content was correlated with transmission electron microscopic investigations. The peak in creep strength exhibited by the material with 0.23 wt.% titanium is attributed to the higher volume fraction of fine secondary titanium carbide (TiC) precipitates. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction 14Cr–15Ni–Ti modified stainless steel (14Cr–15Ni–Ti SS) has been selected as the clad and wrapper material for the initial cores of PFBR, which is being constructed at Kalpakkam. In order to achieve higher fuel burn-ups in subsequent cores, improved com- positions of this stainless steel (SS) are being developed by opti- mising the amounts of the elements phosphorus, silicon and titanium [1]. Fine carbide, phosphide and silicide precipitates asso- ciated with titanium, carbon, phosphorus and silicon additions in SS are known to suppress cavity growth (and thus void swelling) by trapping vacancies and dispersing helium gas atoms to precip- itate–matrix interfaces [2]. The current study aims to optimise the alloy composition with respect to titanium. Therefore, towards this alloy development programme, fifteen laboratory heats have been developed with 0.025 and 0.04 wt.% phosphorus, 0.75 and 0.95 wt.% silicon and 0.16–0.36 wt.% titanium. The compositions of the other elements were kept unchanged in all the heats. This paper presents creep properties of some of these heats of modified 14Cr–15Ni–Ti SS. 2. Experimental Fifteen laboratory heats were prepared by vacuum induction melting (VIM) of high purity charge of electrolytic grades of ferro- chrome, followed by vacuum arc remelting (VAR) process. The VAR ingots were hot forged and then hot rolled into plates of 12 mm thickness, subsequently solution annealed in the temperature range of 1323–1423 K, pickled and passivated. Creep properties were investigated for the heats with phosphorus 0.025, silicon 0.75 wt.% and with titanium contents of 0.18, 0.23, 0.25 and 0.36 wt.% in 20% cold worked condition. The chemical composition of the four heats of the material investigated is given in Table 1. The average grain size of the material was 40 ± 7 lm. Creep tests were carried out at 973 K. Optical, scanning and transmission elec- tron microscopic studies were conducted to characterise the microstructure, fracture behaviour and precipitation behaviour. 3. Results and discussion 3.1. Deformation behaviour In titanium-bearing austenitic stainless steels, titanium to carbon (Ti/C) ratio is considered to be a significant parameter to correlate the influence of titanium. In the present series of investi- gations, the carbon content was not varied and the differences in carbon content among the heats were minor. Creep properties of the various heats were studied at 973 K at three different stress levels, namely 175, 200 and 250 MPa. Figs. 1a–1c show the creep curves at 973 K at a stress level of 250, 200 and 175 MPa. The heat with 0.23 wt.% titanium exhibited a prolonged secondary stage than the other heats. The other heats with 0.18 and 0.36 wt.% titanium exhibited a shorter secondary stage and a well defined tertiary stage. The effect of titanium on the creep curves is more pronounced at lower stress levels. The variation of time fraction (the time spent in primary, secondary and tertiary divided by rupture life) with titanium content for 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.240 ⇑ Corresponding author. Tel.: +91 44 27480500x22271; fax: +91 44 274 80075. E-mail address: mathew@igcar.gov.in (M.D. Mathew). Journal of Nuclear Materials 409 (2011) 214–220 Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat