Derivation of Design Fatigue Curves for Austenitic Stainless Steel Grades 1.4541 and 1.4550 Within the German Nuclear Safety Standard KTA 3201.2

Abstract

Titanium and niobium stabilized austenitic stainless steels X6CrNiTi18-10S (material number 1.4541, correspondent to Alloy 321) respectively X6CrNiNb18-10S (material number 1.4550, correspondent to Alloy 347) are widely applied materials in German nuclear power plant components. Related requirements are defined in Nuclear Safety Standard KTA 3201.1. Fatigue design analysis is based on Nuclear Safety Standard KTA 3201.2. The fatigue design curve for austenitic stainless steels in the current valid edition of KTA 3201.2 is essentially identical with the design curve included in ASME-BPVC III, App I (ed. 2007, add. July 2008 respectively back editions). In the current code revision activities of KTA 3201.2 the compatibility of latest in air fatigue data for austenitic stainless steels with the above mentioned grades were examined in detail. The examinations were based on statistical evaluations of 149 strain controlled test data at room temperature and 129 data at elevated temperatures to derive best-fit mean data curves. Results of two additional load controlled test series (at room temperature and 288°C) in the high cycle regime were used to determine a technical endurance limit at 107 cycles. The related strain amplitudes were determined by consideration of the cyclic stress strain curve. The available fatigue data for the two austenitic materials at room temperature and elevated temperatures showed a clear temperature dependence in the high cycle regime demanding for two different best-fit curves. The correlation of the technical endurance limit(s) at room temperature and elevated temperatures with the ultimate strength of the materials is discussed. Design fatigue curves were derived by application of the well known factors to the best-fit curves. A factor of SN = 12 was applied to load cycles correspondent to the NUREG/CR-6909 approach covering influences of data scatter, surface roughness, size and sequence. In terms of strain respectively stress amplitudes in the high cycle regime, for elevated temperatures (>80°C) a factor of Sσ = 1.79 was applied considering and combining in detail the partial influences of data scatter surface roughness, size and mean stress. For room temperature a factor of Sσ = 1.88 shall be applied. As a result, new design fatigue curves for austenitic stainless steel grades 1.4541 and 1.4550 will be available within the German Nuclear Safety Standard KTA 3201.2. The fatigue design rules for all other austenitic stainless steel grades will be based on the new ASME-BPVC III, App I (ed. 2010) design curve.

Experimental characterization and numerical assessment of fatigue crack growth under thermo‐mechanical conditions

“…These new parameters (IfW parameters) can be compared with the ANL parameters, which are valid in air at temperatures up to 371°C [2], and with KTA parameters for the austenitic stainless steels X6CrNiTi18-10 (1.4541, AISI 321) and X6CrNiNb18-10 at room temperature (T ≤ 80°C) and at elevated temperature (T > 80°C), Table 2. More details on the KTA curves can be found in [4]. Figures 2-4 provide an overview of the results of the above-mentioned isothermal experiments (the fatigue life N is defined by 5% load drop).…”

Modeling short crack propagation under variable structural and thermal loadings

“…In addition to calculations with PJ‐Orig and PJ‐RifoT2, calculations with models based on the damage parameter Z D 38 were performed 19 . Some codes, for example, KTA 3201.2 1,57 or the ASME Code Section VIII, 2 use only Wöhler curves for strain ranges without considering load sequence and mean stress effects. Therefore, calculation results are presented for comparison based on the strain life curve of Langer 58 or according to the equation of Manson–Coffin 59,60 and Basquin 61 proposed by Morrow 62 : …”

Environmental Fatigue Analysis of Nuclear Structural Components: Assessment Procedures, Loads, and a Case Study

“…The KTA approach [32] includes a fatigue curve, as well as a method for incorporating environmental calculations. The KTA air data fit is based on a Langer fatigue equation with a total least-square method [33]. It is worth noting that two fits for high or low temperature are carried out, as a clear temperature effect was shown on Titanium-stabilized austenitic stainless steels [7,33].…”

“…Based on these mean air models, factors on life and strain amplitude were applied. Concerning the factor on life, the same approach as in NUREG/CR-6909 was applied [33] (factor of 12). Concerning the factor on strain amplitude, the factor of 1.79 was obtained as the result of the multiplication of EN-13445 factors for surface roughness (f S ), thickness (f e ), and mean stress (f m ) and a coefficient on data scatter of 1.27.…”

“…The actions encompass online monitoring, experimental testing, and analytical calculations. In the case of analytical calculations, the NUREG/CR-6909 method can be used in conjunction with realistic boundary conditions [33]: these include approaches such as the one presented in RCC-M or the introduction of a transferability factor determined by experimental work [8], which includes beneficial and aggravating effects (hold times, transients, etc.). The overall approach is summarized in Table A1.…”