Abstract
The effect of strain range and temperature on the low-cycle fatigue behaviour and microstructure change during cyclic deformation of Alloy 617 for use in very high temperature gas-cooled reactor components were studied at elevated temperature starting from ambient condition. Increasing the strain range and the temperature was noticed to reduce the fatigue resistance of nickel-based Alloy 617 due to facilitating the transformation behavior of the carbides in the grain interior, precipitates along the grain boundary, and oxidation behavior inducing surface connected precipitates cracking. Initial hardening behavior was observed at room temperature condition during cyclic due to the pile-up dislocation of micro-precipitates. The grain size was also taking a role due to the formation of an obstacle in the matrix. In the high temperature regime, the alloy 617 was found to soften for its entire life due to the fast recovery deformation, proved by its higher plasticity compared with lower temperature. The deformation behavior also showing high environmentally assisted damage. Oxidation behavior was found to become the primary crack initiation, resulting in early intergranular surface cracking.
Highlights
Low-cycle fatigue (LCF) is an important design consideration in structural components operating at high temperatures
Repetition of thermal stresses are generated as a result of temperature gradients which release on heating and cooling during start-up, shutdown and thermal transient conditions
LCF testing was completed in air at elevated temperatures to provide a baseline of understanding in cyclic deformation behavior
Summary
Low-cycle fatigue (LCF) is an important design consideration in structural components operating at high temperatures. Repetition (cyclic) of thermal stresses are generated as a result of temperature gradients which release on heating and cooling during start-up, shutdown and thermal transient conditions. LCF resulting from startups and shut-downs occurs under essentially strain-controlled conditions, since the surface region is constrained by the bulk of the component. A large steam turbine component may be undergoing power under peak-load conditions during its operation. Under these circumstances, the component must quickly respond from its stand-by state, and severe thermal stresses may result from the thermal transients induced by the start-up. In the other case of aircraft gas turbines, the normal operation of an engine on airlines is one implicating start and stop operation
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