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

A neural network approach to elevated temperature creep–fatigue life prediction

Strain-controlled fatigue tests have been conducted in impure helium, simulating the primary-circuit coolant of a high temperature gas-cooled reactor to ascertain the influence of strain rate ( e = 4 × 10 −3 to 2 × 10 −5 s −1 ), hold condition at peak strains (tension-only, compression-only and tension-plus-compression holds) and hold time (up to 120 min) on the low cycle fatigue behaviour of Inconel 617. A strain range of 0.6% and a temperature of 950°C were employed for all the tests. Microstructural changes which occurred during fatigue deformation were evaluated and damage mechanisms which influence fatigue life identified. A small reduction in fatigue life was found with decreasing e. Irrespective of the position of hold at peak strain in a cycle, the hold time always reduced the fatigue life in comparison with continuously cycled tests at lower strain rates but of equal cycle duration. Tensile holds were found to be most damaging, followed by compression holds. Symmetrical tension-plus-compression holds led to fatigue lives which were very close to those of the continuously cycled tests. In the continuously cycled tests with strain rates down to e = 6.7 × 10 −5 s −1 , failure was always transgranular with no indication of creep damage. However, for tests with tensile holds, creep damage was evidenced by grain boundary cavitation and oxidation, which were responsible for a reduction in the fatigue life. The formation of thick oxide scales at the surface observed at longer hold times led to chromium-depleted surface zones in which the carbide precipitates were dissolved. The loss of carbides increased grain boundary sliding which enhanced the formation of grain boundary cracks. These cracks shortened the critical length of surface fatigue cracks which, during fracture of the specimen, linked with the intergranular creep cracks in tensile hold tests. The damaging effect of compression hold was attributed to increased inelastic strain and deformation ratcheting found in the cycle; failure occurred by local accumulation of tensile plastic strain which finally caused tensile necking.

Creep-fatigue interaction of inconel 617 at 950°C in simulated nuclear reactor helium

The development of microstructural damage during high temperature creep–fatigue of a nickel alloy

Total strain controlled low cycle fatigue (LCF) tests with and without hold times were performed at temperatures ranging from 816 to 982 °C in laboratory air on a nickel-based superalloy, HAYNES 230®. The influence of hold time on the cyclic stress response and fatigue life was studied. Fracture surfaces and metallographic sections were evaluated in terms of the crack initiation and propagation modes, i.e., transgranular or intergranular. In general, the fatigue life decreased as the temperature increased. However, at total strain ranges higher than 1.0% and without a hold time, the LCF life was longer at 927 °C than at 816 °C. This “abnormal” behavior was found to result from the smaller plastic strain amplitude at half-life at 927 °C than that at 816 °C. The introduction of a hold time led to a decrease in fatigue life. At both 816 and 927 °C, the material exhibited a cyclic hardening/softening behavior at higher total strain ranges and a cyclic hardening/saturation behavior at lower total strain ranges. An increase in temperature and/or the introduction of a hold time decreased the hardening rate and increased the softening rate. The introduction of a hold time and/or the increase of the test temperature progressively changed the fracture mode from transgranular to mixed trans/inter-granular, then to intergranular. Within the two phases of the fatigue process, crack initiation was more severely influenced by the change of hold time and/or temperature.

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Hold time effects on low cycle fatigue behavior of HAYNES 230® superalloy at high temperatures

Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(e = 4 × 10-6'to 4 × 10-3 s-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing e at 850 °C and with increasing temperature at e = 4 × 10-5 s-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (e = 4 × 10-5 s-1) and in the tests at higher e at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M23C6 carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates e≥4× 10-4 s-1. At 850 and 950 °C even at strain rates of 4 × 10-5 s-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries.

H. Nickel

Papers

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617