Issue 48

S.C.S.P. Kumar Krovvidi et alii, Frattura ed Integrità Strutturale, 48 (2019) 577-584; DOI: 10.3221/IGF-ESIS.48.56 578 in Type 316Ti with titanium additions by the formation of titanium carbide against chromium carbide precipitation [1]. SS 316Ti is widely used in bellows industry and is a candidate material for the bellows operating at high temperature in sodium cooled fast reactor (SFR) systems [2]. Bellows are generally used in piping system to absorb the differential thermal expansions and in bellow-sealed valves to prevent the entry of the process fluid to external environment. Typical applications of the bellows are given in Fig. 1. Bellows are subjected to pressure and movements (axial, lateral and angular). Bellows are specified mainly with their design pressure and fatigue life for the rated movements. Thickness of the bellows ply is decided based on the design pressure. However, the remaining design parameters such as pitch, depth of convolution, and number of convolutions are decided based on the fatigue life expectancy of the bellows. Design of the bellows in conventional system is carried out by using standards of expansion joints manufacturer’s association (EJMA) [3]. However, design methodology of the bellows SFR systems shall comply with standard design codes for nuclear systems such as RCC-MR or ASME boiler and pressure vessel code, section-III [4,5]. A typical operating temperature in SFR systems is around 550 o C. Design of the bellows against fatigue as per RCC-MR requires material properties such as cyclic stress-strain curve, design fatigue curve (strain range versus number of cycles). The material properties of SS316Ti are not available in RCC-MR. Creep-fatigue interaction (CFI) testing is generally carried out by imposing strain hold in continuous cycle fatigue either in tension or compression or both. The presence of hold time results in stress relaxation which in turn leads to creep damage in the material. The response of materials to application of hold depends upon the microstructure, duration of hold, location of hold, environment and temperature. More deleterious effects of tensile hold compared to equal compressive hold have been observed in many materials such as austenitic stainless steels (SS 304, SS 316 and their modified grades) and some superalloys [6-11] . Very limited data exist on the mechanical properties of 316Ti SS in open literature. The present investigation has been carried out to evaluate the mechanical properties such as tensile, low cycle fatigue, and creep-fatigue interaction of the material. Figure 1 : Typical applications of bellows in SFR systems (a) Bellows sealed sodium valve and (b) bellows in piping system to absorb thermal expansions. E XPERIMENTAL he 316Ti austenitic stainless steel (SS) has been used for the present investigation. The chemical composition of the material is shown in Tab. 1. The plate was received in solution annealed condition, with the chemical and the mechanical properties confirming to ASTM A240. The plate was heated to a temperature of 1311 K followed by water quenching. Tensile test was carried out at 823 K and at a strain rate of 3×10 -3 s -1 in air environment. The low cycle fatigue experiments were carried out on 316Ti SS at different strain amplitudes ranging from ± 0.25% to ± 0.8% in air environment at a strain rate of 3×10 -3 s -1 and 823 K. The gauge length and diameter of the specimen were 15 mm and 6 mm, respectively. The schematic of specimen is shown in Fig. 2. All the tests were carried out in air under fully reversed, total axial strain control mode employing a symmetrical triangular strain-time waveform using DARTEC servo hydraulic T