Non-destructive testing derived parameters for microstructure-based residual service life assessment of aging metallic materials in nuclear engineering

Abstract

Metallic components in nuclear engineering are exposed to extensive loads such as pressurization and temperature changes which can affect the properties of the material significantly depending on the load spectrum applied. In view of developing a procedure to evaluate the residual service life of metallic components in nuclear power plants aged during service, metastable austenitic steel AISI 347 (German designation: X6CrNiNb18−10) has been considered as an example. To this purpose, total strain-controlled fatigue tests were carried out under different environmental conditions and monitored by continuously measuring thermometric, resistometric, electromagnetic and electrochemical parameters. These parameters provide an information gain in terms of material characterization when compared to conventional strain measurements. Based on these parameters, the short time evaluation procedure StrainLife has been developed, which allows the determination of local S-N curves with a significantly reduced effort as compared with traditional procedures. This method has been implemented into the structural simulation program PROST for the integrity assessment of the components while considering local fatigue properties. This very effective method allows for the determination of local fatigue properties including the strain-specific local scatter of the metallic microstructure properties of the material which has not been possible by traditional means.

Microstructure-Based Lifetime Assessment of Austenitic Steel AISI 347 in View of Fatigue, Environmental Conditions and NDT

“…Metallographic investigations of the cross section including light microscopy show a squared stirring mark in the center of the bar, present, as explained before, due to its manufacturing process. Outside this area towards the sample edge, a strongly deformed and fine-grained austenitic structure is present, while inside, a strongly inhomogeneous multiphase structure can be recognized [10].…”

“…fine-grained austenitic structure is present, while inside, a strongly inhomogeneous mul-tiphase structure can be recognized [10].…”

“…This relationship is then corrected by the results obtained from two conventional load-controlled CAT usually performed at the stress level where the LIT fractured and at a stress level around the endurance limit respectively. This approach has been further extended and differentiated, being called the StressLife [9], StrainLife [10] and SteBLife [11] approach respectively.…”

“…Supposing material samples could be extracted from an aged structure to determine its fatigue properties, then the amount of material should be kept to a minimum. A way on how to get this done is by the application of so-called Short Term Evaluation Procedures (STEP) [8][9][10][11], where on the basis of a few (i.e., one to three) fatigue tests a complete S-N curve can be determined.…”

SteBLife, a New Approach for the Accelerated Generation of Metallic Materials’ Fatigue Data

“…Based on this idea and due to the symmetry of the specimen, it is possible to plot up to five cyclic deformation curves providing the database for cyclic stress material-response (σa-M) curves. In contrast to other STEPs such as PhyBaL [30], StressLife [26] or StrainLife [31], no pre-damage from load increase tests (LIT) and strain increase tests (SIT), respectively, due to the previous load steps as well as softening and/or hardening effects from the stepwise increased stress or strain amplitude has to be taken into consideration, whereby the as measured stress materialresponse relationship can be used, leading to a significant advantage. For the calculation, generalised Morrow [32] and Basquin [17] equations are combined in order to calculate the fatigue life and an S-N dataset for the material investigated.…”