R. Sandhya

Effects of temperature on the low cycle fatigue behaviour of nitrogen alloyed type 316L stainless steel

Abstract Strain-controlled low cycle fatigue tests have been conducted in air between 298–873 K to ascertain the influence of temperature on LCF behaviour of nitrogen-alloyed type 316L stainless steel. A strain amplitude of ± 0.60% and a symmetrical triangular waveform at a constant strain rate of 3 × 10−3 s−1 were employed for all tests. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The cyclic stress response at all temperatures was characterized by an initial hardening to the maximum stress, followed by gradual softening prior to attaining saturation. Temperature dependence of fatigue life showed a maximum in the intermediate temperature range. The drastic reduction in fatigue life at elevated temperatures has been ascribed primarily to the combined influence of dynamic strain ageing effects and oxidation-enhanced crack initiation, while the lower life at room temperature is attributed to detrimental effects associated with deformation-induced martensite.

High temperature time-dependent low cycle fatigue behaviour of a type 316L(N) stainless steel

Total strain controlled low cycle fatigue tests on 316L(N) stainless steel have been conducted in air at various strain rates in the temperature range of 773-873 K to identify the operative time-dependent mechanisms and to understand their influence on the cyclic deformation and fracture behaviour of the alloy. The cyclic stress response at all the testing conditions was marked by an initial hardening followed by stress saturation. A negative strain rate stress response is observed under specific testing conditions which is attributed to dynamic strain ageing (DSA). Transmission electron microscopy studies reveal that there is an increase in the dislocation density and enhanced slip planarity in the DSA regime. Fatigue life is found to decrease with a decrease in strain rate. 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.

Substructural recovery in a cold worked Ti-modified austenitic stainless steel during high temperature low cycle fatigue

Abstract Total axial strain controlled fatigue tests have been conducted in air at 823 and 923 K to ascertain the influence of cold work on LCF behaviour of a 15Cr–15Ni, Ti modified austenitic stainless steel, designated as Alloy D9. The LCF behaviour of the 20% cold worked alloy was compared with that of solution annealed alloy. A symmetrical triangular waveform at a constant strain rate of 3×10 −3 s −1 was employed for all the tests performed over the strain amplitude in the range of ±0.25 to ±1.00%. The cyclic stress response varied as a complex function of microstructure, temperature and strain amplitude. The cyclic stress response of solution-annealed alloy was generally characterized by initial hardening to the maximum stress followed by a regime of nearly stable peak stresses. The degree of initial hardening was higher at 823 K due to dynamic strain ageing effects. The cold worked alloy displayed a gradual softening prior to the attainment of saturation stress response stage. The plastic strain fatigue resistance of the cold worked alloy was inferior compared to that in solution-annealed condition at both 823 and 923 K. At low strain amplitudes, the cold worked alloy exhibited better total strain fatigue resistance at 923 K. The observed variations in fatigue life and cyclic stress response behaviour have been explained on the basis of crack initiation, development of substructure, precipitation behaviour and evolving changes in plastic strain during cycling.

Effect of mean stress and stress amplitude on the ratcheting behaviour of 316LN stainless steel under dynamic strain aging regime

Abstract The influence of dynamic strain aging (DSA) on the ratcheting behaviour of 316LN stainless steel was investigated at 823K as a function of mean stress (σm) and stress amplitude (σa). Test results obtained under different combinations of σm – σa were analysed in order to arrive at a map delineating different deformation regimes viz. ratcheting, strain burst and elastic shakedown. It was shown that the synergistic effect of σm and σa can be described by the ratio of mean stress and stress amplitude (σm/σa). A critical value of this ratio was identified to mark the transition between ratcheting and elastic shakedown and the same was used to predict deformation behaviour of the material in DSA under asymmetrical loading.

Effect of Application of Short and Long Holds on Fatigue Life of Modified 9Cr-1Mo Steel Weld Joint

Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone.

Temperature dependence of low cycle fatigue of 316(N) weld metals and 316L(N)/316(N) weld joints

Abstract The low cycle fatigue behaviour of 316(N) weld metals and 316L(N)/316(N) weld joints have been investigated in the temperature range of 300–873 K, at a strain amplitude of ±0·6% and a strain rate 3 6 10–3 s–1, to study the influence of dynamic strain aging (DSA). The 316(N) weld metal exhibited better fatigue life than the weld joint, though the weld metal has shown higher cyclic stress response and higher plastic strain accumulation than the weld joint. Significant features observed in the temperature regime of 300–873 K include the maximum in fatigue life at 573 K and DSA in the range of 673–873 K. Occurrence of DSA has been manifested through drastic reduction in fatigue life in the range of 673–873 K, associated with anomalous stress response. Dominant DSA effects have been observed at about 773 K in the weld joint and at 823 K in the weld metal. However, the effect of DSA is found to be nominal beyond 823 K where the reduction in fatigue life is attributed to the combined effects of oxidation and DSA. Secondary crack density measurements (in the range of 300–873 K) in the weld joint specimens revealed the severity of the heat affected zone (HAZ) in inducing fatigue damage. Parameters have been identified to determine the temperature corresponding to dominant DSA effects.

Influence of Secondary Cyclic Hardening on the Low Cycle Fatigue Behavior of Nitrogen Alloyed 316LN Stainless Steel

In this article, the occurrence of secondary cyclic hardening (SCH) and its effect on high-temperature cyclic deformation and fatigue life of 316LN Stainless steel are presented. SCH is found to result from planar slip mode of deformation and enhance the degree of hardening over and above that resulted from dynamic strain aging. The occurrence of SCH is strongly governed by the applied strain amplitude, test temperature, and the nitrogen content in the 316LN SS. Under certain test conditions, SCH is noticed to decrease the low cycle fatigue life with the increasing nitrogen content.

High temperature, low cycle fatigue behaviour of AISI type 316LN base metal, 316LN-316 weld joint and 316 all-weld metal

Abstract Studies on the strain-controlled low cycle fatigue (LCF) behaviour of type 316LN stainless steel (SS) base material, type 316 SS all-weld metal and 316LN-316 weldments prepared by the shielded-metal-arc-welding (SMAW) process at 873 K have been conducted at a strain rate of 3 × 10 −3 s −1 . The results indicated that the LCF resistance of all-weld metal was better than that of the base material while weldments displayed the least fatigue resistance. The better fatigue resistance of all-weld metal has been attributed to the beneficial effects associated with fine distribution of δ-ferrite in the austenite matrix while the poor fatigue resistance of the weldment has been ascribed to the detrimental effects associated with coarse grain size and surface intergranular cracking in the heat-affected zone. The cyclic stress amplitude varied with the material condition and strain amplitude in the LCF test.

On the Dynamic Strain Aging Effects during Elevated Temperature Ratcheting of Type 316LN Stainless Steel

Abstract Ratcheting experiments were carried out on type 316LN stainless steel (SS) at different temperatures in the range, 300–923 K using different mean stress (σm) – stress amplitude (σa) combinations. Occurrence of dynamic strain aging (DSA) in the intermediate temperature range led to an anomalous temperature dependence of ratcheting strain accumulation. While the peak DSA temperature was found to be 823 K for all mean stress-stress amplitude combinations, the temperature regime of occurrence of DSA was found to depend on the σm–σa combinations used.

On specimen geometry effects in strain-controlled low-cycle fatigue

Strain-controlled low-cycle fatigue tests have been conducted in air at room temperature on three different specimen geometries: (1) an hourglass specimen with a minimum diameter of 10 mm; (2) a cylindrical specimen with a gauge length of 50 mm and a gauge diameter of 10 mm; and (3) a cylindrical specimen with a gauge length of 25 mm and a gauge diameter of 10 mm. The effect of specimen geometry on the cyclic stress response and fatigue life is discussed. At all the strain ranges tested the hourglass specimens showed the highest life. The 50 mm gauge-length specimen showed the least life, while the 25 mm gauge-length specimens exhibited an intermediate life. The constants and coefficients in the Coffin-Manson, Basquin and cyclic stress-strain relationships are evaluated for the three geometries. The existing models for the prediction of the fatigue ductility exponent are also applied.