The update of the ASME III design fatigue curve for stainless steel in conjunction with the Fen model described in the NUREG/CR-6909 report has been criticized since publication. Data used to develop curves and models raises more questions than it answers. Material testing in a simulated light water reactor environment is difficult due to the temperature and pressure involved. The experimental challenge makes it tempting to take shortcuts where they should least be taken. Facing and overcoming the challenges, direct strain-controlled fatigue testing has been performed at VTT using a unique tailored-for-purpose EAF facility. The applicable ASTM standards E 606 and E1012 are followed to provide results that are directly compatible with ASME Code Section III. Several earlier PVP papers (PVP2016-63291, PVP2017-65374) report lower than calculated experimental Fen factors for stabilized stainless steels. In this paper new results, in line with the previous years’ conclusions, are presented for nonstabilized AISI 304L tested with dual strain rate waveforms. To model environmental effects more accurately, an approach accounting for the damaging effect of plastic strain is proposed. A draft Fen model, similar in structure to the NUREG model but with additional parameters, is shown to significantly improve the accuracy of Fen prediction.

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The main objective of the performed Control and Exploitation of Thermal Distortions (DISCO) project was to seek an overall concept for the control of thermal distortions. The scope of the project was limited to structural steels and to the processes most important to the participating industry. The possibility to apply the inherent strain method for modelling thermal deformations was examined by establishing an inherent strain database for the target arc welding and thermal cutting situations. The project was executed in cooperation with Lappeenranta University of Technology and four Finnish enterprises. Osaka University, Japan, was consulted concerning the inherent strain method. The work focused on structural steels representing two strength levels, and GMAW, FCAW, SAW and MAG welding processes. The computational practices were revealed for treating thermal distortions. Further actions included testing and modelling of welded T-joint with various plate thicknesses. The relationship between plastic strains and angular distortion (out-of-plane deformation with the weld as the axis of rotation) of fillet welded plate T-joints were studied using numerical analyses. A three dimensional thermo-elastic-plastic analysis incorporating the effects of a moving heat source and non-linear material properties was performed to obtain the plastic strain distributions and deformations. Procedures to define a simplified strain distribution (inherent strain) and to compute deformations were presented. The comparison of detailed thermo-elastic-plastic computation and measurements yielded a good correspondence. The simplified inherent strain type analysis underestimated somewhat the deformations. The technical basis for the creation of an inherent strain database was presented.

Environmental effects of LWR coolant need to be factored in when defining cumulative fatigue usage of primary circuit components. The basis is a set of codified design rules and fatigue design curves, based on experimental data. To accurately quantify environmental effects, the reference curve in air to which fatigue life in water is compared shall be as reliable as possible. Literature studies and accumulated data at VTT reveal that the use of common reference curves for a wide range of austenitic stainless steel alloys and temperatures is unreliable. Some design codes already include measures to consider this but ASME III is not yet among them. The ASME III design curve is adopted from NUREG/CR-6909 and contains no consideration for dependence of temperature or stainless steel grade. Two different stainless steel grades, AISI 304L and 347, have previously been used in environmentally-assisted fatigue experiments at VTT. In this paper, reference curves for the AISI 304L heat are presented at room temperature and 325 °C to complement the curves already available for AISI 347. Demonstration of realistic environmental effect quantification is done using these reference curves as an alternative to the NUREG methodology.