Jussi Solin

Environmental Fatigue Factors (NUREG/CR-6909) and Strain Controlled Data for Stabilized Austenitic Stainless Steel

Experimental research on fatigue performance of niobium stabilized stainless steel (1.4550, X6CrNiNb1810 mod) relevant for German NPP primary piping has previously demonstrated good long life performance.Slow rate fatigue tests in 325 °C PWR water are first time presented and discussed in this paper. Good fatigue performance was measured also in hot water. Our experiments give consistently about doubled lives or 50% smaller Fen factors in compared to predictions by NUREG 6909.Transferability of the laboratory data, reference and design curves together with the proposed Fen evaluation procedure to component evaluation will be discussed.Copyright

Research on Hold Time Effects in Fatigue of Stainless Steel: Simulation of Normal Operation Between Fatigue Transients

In PVP2011-57942 we reported improved endurance in fatigue tests with intermediate annealing to roughly simulate steady state operation between fatigue transients in NPP components. Quantification of this effect is in focus of our continued research on fatigue performance of niobium stabilized stainless steel (1.4550, X6CrNiNb1810mod). Similar effect is expected in nuclear power plants during normal operation — e.g. in a PWR surge line or in pressurizer spray lines.Holds affect cyclic stress strain response. Stress amplitude, tensile mean stress and apparent elastic modulus are increased immediately after a hold, while decreased by cycles in between. Axial shortening is measured during hot holds at zero stress. This all suggest cyclic accumulation of lattice defects and recovery during holds. Recovery may occur through thermally activated dislocation migration together with diffusion, grouping and annihilation of lattice defects. More than one thermally activated processes control the rates of contraction during hold periods at elevated temperatures. Hold hardening delays crack formation by preventing plastic strain localization, in components also on macroscopic level.A mechanism informed model is sought for transferring laboratory data to real plant components in terms of improving accuracy of numerical fatigue usage assessment. Anticipated mechanisms behind gradual changes in material responses are discussed in relation to quantitative effects of holds.Copyright

Fatigue of Stainless Steel in Simulated Operational Conditions: Effects of PWR Water, Temperature and Holds

Our experimental research on fatigue performance of stainless steels and transferability of laboratory data to plant operational conditions focuses in niobium stabilized stainless steel (1.4550, X6CrNiNb1810mod) taken from a pipe manufactured as primary piping for a German NPP Good fatigue performance both in air and in PWR water was reported in previous PVP papers.The NUREG/CR-6909 report proposes Fen factors based on stroke controlled experiments in hot water for non-stabilized steels. Since PVP2013-97500 we have new data in 200°C PWR water to compare with predictions by NUREG/CR-6909. Our strain controlled tests in 325°C and 200°C PWR water give longer lives resp. smaller Fen factors. For the slowest tested strain rate 4·10−6 in 325°C water the prediction according to NUREG/CR-6909 goes just below the current ASME design curve, but our results remains well above. Including also the relevant design temperature effect, our result Fen = 4 is well below the predicted Fen = 14,5. The gap is smaller for higher strain rates and low Fen values.Simplified simulations of fatigue transients combined with normal operation indicated that relevant loading patterns as hold-time effects may result to notably longer lives than in standard laboratory tests. A concern was raised on transferability of data to thermal transients separated with months of normal operation. Cyclic strain (transients) followed by hot holds (normal operation) lead to time and temperature dependent hardening with reduction in cyclic plastic strain and fatigue usage, i.e. extension of life.This paper reports new data, challenges met and our progress towards developing realistic design factors for effects both reducing and extending fatigue endurance in nuclear power plant operational conditions.Copyright

Long Life Fatigue Performance of Stainless Steel

Fatigue of Niobium stabilized austenitic stainless steel (X6CrNiNb1810 mod) was studied using specimens extracted from a solution annealed and quenched primary piping material sample. This paper reports and discusses results of non-standard experiments to complement previously published test data. The NPP primary piping components spend long times in operation temperature between fatigue cycles originating from thermal transients. This was roughly simulated by fatigue tests periodically interrupted for intermediate annealing in elevated temperature. Fatigue endurance was notably increased when low strain amplitudes were used. The life extension is explained by the cyclic stress strain response. Hardening followed by slow cyclic softening was consistently observed after annealing. It is generally assumed that cumulative accumulation of fatigue damage occurs at a wide range of loading amplitudes. We performed two level and spectrum straining tests combining amplitudes above and below the (Nf > 107 ) endurance limit. The endurance limit seems to be effective also in variable amplitude loading. In terms of modified Miner rule, even “negative damage” was obtained in two level tests below and above the constant amplitude endurance limit. This behavior is linked to prominent secondary hardening of the steel.Copyright

Spectrum fatigue testing using dedicated software

A series of modular spectrum fatigue test control and data collection programs have been developed. Specially tailored modifications of the software are used for different types of tests. Simulated service spectra with predetermined amplitudes and frequencies for one or more control channels can be produced. Simultaneous monitoring of the test machine and specimen response permits run-time analysis of the actual spectrum, measuring of fatigue crack initiation or growth, and speed optimization. The full-scale testing of a locomotive bogie employing eight hydraulic actuators is described. The three-million kilometre service simulation verified the fatigue resistance of the structure. The importance of meaningful run-time analysis is emphasized.

The Paths of Small Fatigue Cracks in High-Strength Steels Initiated from Inclusions and Small Defects

In fatigue, the early growth mechanisms, paths, and rates of the microstructurally small cracks are not well known. Growth of subsurface—undetected—cracks cannot be monitored in real time, and postfracture fractography is complicated because of the statistical nature of local microstructure. Furthermore, hammering or sliding often damages the fracture surface before the test is stopped. We addressed this challenge with two approaches. This paper deals with fractographic details connected to local microstructure obtained by milling and imaging with a focused ion beam in a scanning electron microscope (SEM). The results provided input such as crack growth on adjacent planes and their coalescence, as well as formation of the “optically dark areas” that play a key role in the early growth and initiation of small cracks in fatigue. A subset of data consisting of eleven 100Cr6 bearing steel specimens loaded at the same stress amplitude, with fatigue lives (Nf) in the range of 10·106 < Nf < 650·106 loading cycles are studied and discussed. The second test series revealed that very early initiation and crack arrest are typical for small notches in the 34CrNiMo6 QT steel. The relation of crack path and microstructure, along with their connection to the optically dark area, is discussed.

Fatigue Performance of Stabilized Austenitic Stainless Steels: Experimental Investigations Respecting Operational Relevant Conditions Like Temperature and Hold Time Effects

Experimental research on fatigue performance of niobium stabilized stainless steel (1.4550, X6CrNiNb1810mod) relevant for German NPP primary piping demonstrated good long life performance.Fatigue tests periodically interrupted for holds indicated time and temperature dependent hardening during holds at 25°C to 325°C. Notable extension of fatigue life was measured when loading patterns consist of cyclic deformation in lower temperatures than hold annealing. Many NPP piping thermal transients separated by normal operation belong to this category and fatigue assessment based on standard fatigue data seems to underestimate fatigue endurance.Further results for stabilized stainless steel in air at various temperatures will be provided. A parallel paper will deliver unpublished data in PWR water. The influences of temperature and loading pattern will be discussed aiming to improve fatigue assessment of plant components and to reduce confusion concerning applicability of international design codes.Copyright

Fatigue Performance of Stainless Steel in NPP Service Conditions

Fatigue of Niobium stabilized austenitic stainless steel (X6CrNiNb1810 mod) is studied using specimens extracted from a solution annealed and quenched primary piping material sample. This paper reports and discusses new results complementing the data published in PVP2009 and 2011, where good long life performance was demonstrated and relevance of the new design curve in ASME III was questioned.Effects of temperature and operational loading sequences are in focus here. A typical case with steady state operation between fatigue cycles was roughly simulated in periodically interrupted tests. An extension of fatigue life due to hardening during the holds in elevated temperature was demonstrated and here we show that the hardening effect is generic. It occurs also in isothermal conditions, where straining and holds are both at 325 °C, irrespective of the phase within a cycle, where the hold is introduced, and also for other austenitic steel grades.Fatigue assessment based on standard fatigue data seems to underestimate fatigue performance of materials subjected to typical thermal transients in nuclear piping during operation.© 2012 ASME

Discussion on Fatigue Design Curves for Stainless Steels

The new stainless steel air curve endorsed in NRC RG 1.207 for new US designs only was recently adopted into ASME III without restrictions on applicability. We assume that the new (2009b) ASME curve may be applicable to some grades of stainless steel, but not to all. This paper reports contradictory data for stabilized austenitic stainless steels extending up to 10 million cycles in room temperature at air environment. Niobium and titanium stabilized stainless steel specimens were sampled from 100% relevant material batches fabricated for NPP primary piping. Additional research and more recent data for titanium stabilized steel suggest that our PVP 2009-78138 conclusions are not limited to one material grade. Therefore, the revised ASME design curve cannot be considered universally applicable.Copyright

Cyclic Behavior and Fatigue of Stainless Surge Line Material

Fatigue of Niobium stabilized austenitic stainless steel (X6CrNiNb1810mod) was studied using specimens extracted from a solution annealed and quenched φ360×32 mm pipe, which fulfils all the German KTA material requirements for primary components in BWR and PWR. An experimental strain life fatigue curve was determined as a base line for component specific evaluations and for comparison with the Langer and Chopra curves, which are the basis of the ASME III and NRC RG 1.207 design criteria. Stress strain responses in strain controlled tests were carefully measured to clarify the fatigue mechanisms. In the LCF regime our data lie within a common scatter band between the Langer and Chopra curves. When approaching HCF regime (> 100 000 cycles), the curves deviate and Chopra curve becomes highly conservative for this material. Strain controlled fatigue tests ranging to 10 million cycles in RT air indicated an endurance limit behavior for this alloy.Copyright