The effect of thermal ageing on low cycle fatigue behaviour of 316 stainless steel welds Sunil Goyal, R. Sandhya, M. Valsan, K. Bhanu Sankara Rao * Materials Development and Characterisation Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India article info Article history: Received 18 December 2007 Received in revised form 14 July 2008 Accepted 16 July 2008 Available online 24 July 2008 Keywords: Low cycle fatigue Thermal ageing Austenitic stainless steel Weld metal abstract It is well known that welds are the weak links in any structure. Therefore, it is of out most importance to characterize the mechanical properties of welds. Moreover, the changes in the microstructure that occur in welds on exposure to high temperatures affect the mechanical properties and must be studied by age- ing the welds at high temperature. In this paper the low cycle fatigue behaviour of thermally aged 316 stainless steel weld metal is presented. Weld pads with single V configuration were prepared by the shielded metal arc welding process using 316 electrodes. Thermal ageing was done for 10,000 h at 823 and 873 K. Total strain controlled low cycle fatigue tests were conducted at a constant strain rate of 3 Â 10 À3 s À1 with strain amplitudes in the range ±0.25% to ±0.6% at 823 and 873 K. Weld metal exhibited initial hardening followed by cyclic softening prior to failure. The aged samples exhibited higher stress response as compared to the unaged samples. At both the temperatures and all strain amplitudes fatigue life was inferior to that of unaged samples. The metallography of the aged and tested material was stud- ied through optical, scanning and transmission electron microscopy. The effect of transformation of d-fer- rite to sigma phase and carbides in the weld metal on low cycle fatigue behaviour was evaluated. Ó 2008 Published by Elsevier Ltd. 1. Introduction Austenitic stainless steel, AISI type 316 and its modified grades like 316L(N) find applications as structural material in nuclear power plants for the construction of reactor vessel, piping and heat exchangers. The choice of this alloy is based on its excellent high temperature tensile, creep, fatigue, and creep-fatigue strengths in combination with good fracture toughness and fabricability. In the design of LMFBR components for high temperature service, resistance to low cycle fatigue (LCF) and creep-fatigue interaction are important considerations. Most of the service failures are ex- pected to occur either in the HAZ or in the weld metal. These fail- ures are more frequently associated with the presence of defects or microstructural inhomogeneities. Elevated temperature LCF behaviour of 316 and 316 (N) stainless steel (SS) welds and weld joints has received considerable attention in recent years [1–3]. Weld deposits of austenitic stainless steels [4] are heteroge- neous in nature due to the presence of segregated impurities at the d-ferrite/austenite interface boundaries, dendritic microstruc- ture and presence of secondary precipitates of M 23 C 6 carbides, chi and sigma phases, etc. Delta ferrite, rich in Cr and Mo, formed during the welding of austenitic stainless steels is required up to a limit of about 4–5 ferrite number (FN) in order to avoid hot crack- ing or microfissuring of the weld metal [4,5]. The ageing of such weld deposits, during exposure to high temperature service, leads to the transformation of d-ferrite. The transformation kinetics of the d-ferrite is complex and depends on the various factors such as material composition, size and morphology of the d-ferrite, etc. [6,7]. At elevated temperatures, d-ferrite transforms to austen- ite, carbides, r-phase, v-phase, Laves phase, R-phase etc. [8–10]. The nucleation and growth of the r-phase is easier in weld metal containing a low concentration of carbon [8,9]. Tensile studies of duplex weld metal indicated that at room temperature, d-ferrite strengthens the weld metal, in comparison with the base metal, as evidenced by increase in the yield strength and ultimate tensile strength accompanied by reduction in elonga- tion [11]. The competitive processes of annealing and transforma- tion of ferrite govern the changes in the mechanical properties during high temperature ageing of the weld metal [12]. The objective of the present study is to determine the effect of ageing on the low cycle fatigue (LCF) behaviour of 316 SS weld me- tal. A detailed examination of the microstructural changes and crack initiation and propagation modes has been conducted with a view to understanding the features which may influence the fatigue life. 2. Experimental details The chemical compositions in wt.%, of the 316L(N) base metal and basic coated 316 weld metal used in this investigation are 0142-1123/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.ijfatigue.2008.07.006 * Corresponding author. Tel.: +91 044 27480107; fax: +91 044 27480075. E-mail address: bhanu@igcar.gov.in (K. Bhanu Sankara Rao). International Journal of Fatigue 31 (2009) 447–454 Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue