Strain Controlled Low Cycle Fatigue and Creep-fatigue interaction behaviour of A type 316(L) Stainless Steel

Abstract

Type 316L(N) stainless steel (SS) is the currently favored structural material for several high temperature components in the primary side of the liquid metal cooled fast breeder reactor. The choice of this material is primarily based on its resistance to sensitization and adequate high temperature mechanical properties. In liquid metal cooled fast breeder reactors, the components are often subjected to temperature gradient induced thermal stresses, which are cyclic in nature as a result of start-ups, shutdowns and transients. Furthermore, steady state loading at elevated temperatures in combination with cyclic loading leads to creep-fatigue interaction. Therefore, resistance to low cycle fatigue and creep-fatigue interaction is important factors in the design of liquid metal cooled fast breeder reactor components. In view of this, it is necessary to understand the cyclic deformation and fracture behaviour of this alloy under various loading conditions. Such understanding must not only include low cycle fatigue behaviour at the maximum operating temperature but also at various strain amplitudes and temperatures encountered during transients. In addition, prior cold work might be introduced in the alloy unintentionally during erection or fabrication of plates and sheets into vessels, tanks, pipings etc. Therefore, to ensure reliable performance of stainless steels in service, studies related to effect of prior cold work on strain controlled low cycle fatigue behaviour assume significance. In this thesis, the following aspects of strain controlled low cycle fatigue and creep-fatigue interaction behaviour of type 316L(N) SS have been critically examined : (i) The effects of temperature and strain amplitude on low cycle fatigue behaviour of solution annealed alloy, (ii) Time dependent low cycle fatigue behaviour under various strain rate and temperature conditions, (iii) The influence of prior cold work on elevated temperature cyclic behaviour and (iv) Creep-fatigue interaction behaviour of the alloy employing hold times at peak cyclic strains. The effects of total strain amplitude (+ 0.25% to + 1.0%), and temperature (298K to 923K) on fatigue behaviour of the solution annealed alloy were studied in detail. The cyclic stress response of the alloy was characterized by cyclic hardening to peak stress followed by softening, which ended in a stable stress response until failure. The regime of stress saturation was longer at lower strain amplitudes of testing. The peak stress amplitude decreased initially from 298 K to 573K. The peak stress increased rapidly with increasing temperature between 573 and 873K. Beyond 873 K, the peak stress decreased again. The alloy exhibited a peak in life at intermediate temperatures around 573 K. Life decreased drastically with increase in temperature beyond 573 K. The temperature dependence of stress response and fatigue life have been explained on the basis of several interacting phenomena which include strain induced transformation of austenite to martensite, substructural recovery, dynamic strain ageing (DSA) and oxidation. The fatigue life at elevated temperatures is significantly influenced by testing parameters such as strain rate and waveform due to combined effects of various time dependent deformation and damaging processes. The effects of strain rate (3x1 O'5 s'1 to 3x1 O'2 s’1) on low cycle fatigue behaviour of 316L(N) SS have been examined at 773, 823 and 873 K. Cyclic stress amplitude at half-life has been found to increase with decreasing strain rate at 773 and 823 K indicating the negative strain rate sensitivity of cyclic stresses. The negative strain rate sensitivity of half-life cyclic stress amplitude at 873 K occurred only over a limited strain rate range. Peak tensile stress amplitude developed as a function of strain rate has been partitioned to friction stress and back stress components to establish the mechanism of cyclic hardening. The rapid hardening and negative strain rate sensitivity of cyclic stress was attributed to DSA. Transmission electron microscopy studies revealed that there is an increase in the dislocation density and enhanced slip planarity in the DSA regime. At all the temperatures, fatigue life decreased with decrease in strain rate. The factors responsible for the reduction in life have been identified and the life reduction was explained on the basis of operating deformation and damage mechanisms. The degradation in fatigue resistance is attributed to the detrimental effects associated with DSA and oxidation. Quantitative measurement of secondary cracks indicate that both transgranular and intergranular cracking are accelerated predominantly under conditions conducive to DSA. Creep-Fatigue interaction tests have been carried out at 873 and 923 K to evaluate the influence of duration of hold time (1 to 90 min) and position of hold (at peak tensile strain, peak compression strain and peak tension plus compression strain). The alloy under hold time conditions exhibited lower cyclic stresses compared to those obtained in solution annealed state. The decrease in cyclic stress response with hold time is attributed to enhanced recovery of the substructure and increase in grain boundary damage accumulated during stress relaxation period. It has been observed that tensile hold was more damaging than the compression hold and fatigue life decreased with increase in duration of hold time in tension. Creepfatigue interaction was noticed to be more prominent only at lower strain amplitudes of testing. The effects of oxidation and creep in causing reduction in life under holdtime conditions are discussed. Quantitative assessment of cracking behaviour has been made under hold time conditions on longitudinal sections of the low cycle fatigue tested samples. The role of 20% prior cold work at 873 K on low cycle fatigue properties of solutionised 316L(N) SS, at a strain rate of 3x1 O'3 s'1 , has been studied in detail. Cyclic stress response of the alloy was characterized by a short period of hardening followed by gradual softening, which ended in a stable stress response at lower strain amplitudes of testing. Interrupted tests were carried out at different stages of cyclic stress response and substructural changes were characterized using transmission electron microscopy to determine the underlying mechanisms causing such response. The fatigue life of material in prior cold worked condition was lower at higher strain amplitudes of testing, whereas at lower strain amplitudes (< ± 0.4%), prior cold worked material exhibited higher life compared to solution annealed alloy. The lower life of prior cold worked alloy at higher strain amplitude has been attributed to lower ductility and higher cyclic response stresses. Higher fatigue endurance of PCW material at lower strain amplitudes of testing is attributed to