Experimental Research on Cyclic Response, Hold Effects and Fatigue of Stainless Steel

2017 ◽  
Author(s):  
Jussi Solin ◽  
Jouni Alhainen ◽  
Tommi Seppänen ◽  
H. Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Stainless Steel ◽  
Elevated Temperatures ◽  
Laboratory Data ◽  
Cyclic Hardening ◽  
Fatigue Performance ◽  
Life Extension ◽  
Normal Operation ◽  
Primary Circuit ◽  
Operational Conditions ◽  
Circuit Components

Our research on fatigue performance of stainless steel and transferability of laboratory data to nuclear power plant operational conditions continues. The focus is in quantification of time and temperature dependent damage relaxation during holds introduced within strain controlled LCF fatigue tests with niobium stabilized X6CrNiNb1810mod steel. These holds aim to simulate steady state normal operation between fatigue relevant cycles at start-up, shut-down or power changes in PWR primary circuit components, e.g. the pressurizer spray lines and surge line. Amplified cyclic hardening was observed at strain rates approaching zero at normal operation temperatures (≤325°C). Even more pronounced static hardening is consistently measured during holds in elevated temperatures (≥200°C). Beneficial effects of holds in material endurance were shown five years ago. The latest results suggest another beneficial change in component fatigue performance. In addition to improved material response, de-localization of strain is demonstrated in this paper. Our target is a thermodynamic prediction model for improved assessment of fatigue with normal operation periods. The model should quantify the life extension due to long periods in normal operation at operational temperatures.

Cyclic, Monotonic and Fatigue Performance of Stabilized Stainless Steel in PWR Water and Research Laboratory

2018 ◽  
Author(s):  
Jussi Solin ◽  
Jouni Alhainen ◽  
Tommi Seppänen ◽  
H. Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Stainless Steel ◽  
Endurance Limit ◽  
Elevated Temperatures ◽  
Laboratory Data ◽  
Test Method ◽  
Fatigue Performance ◽  
Austenitic Steels ◽  
Cyclic Behavior ◽  
Normal Operation ◽  
Environment Temperature

Strain controlled LCF testing extended to 10 million cycles revealed an abrupt endurance limit enforced by secondary hardening. In elevated temperatures the ε-N curve is rotated and endurance limit is lowered, but not vanished. When very low strain rates are applied at 325°C in simulated PWR environment, fatigue life is reduced, but far less than predicted according to NUREG/CR-6909. It is possible, but not probable that the difference is due to different stainless grades studied. We assume that the test method plays a more important role. We have repeatedly demonstrated in different tests campaigns that interruptions of straining with holds aiming to simulate steady state normal operation between fatigue relevant cycles can notably extend the fatigue endurance. Further proof is again presented in this paper. The suspected explanation is prevention of strain localization within the material microstructure and also in geometric strain concentrations. This actually suggests, that hold effects should be even more pronounced in real components. Cyclic behavior of austenitic steels is very complex. Transferability of laboratory data to NPP operational conditions depends on test environment, temperature, strain rate and holds in many ways not considered in current fatigue assessment procedures. In addition to penalty factors, also bonus factors are needed to improve transferability. Furthermore, it seems that the load carrying capacity of fatigued stainless steel is not compromised before the crack growth phase. Tensile tests performed after fatigue tests interrupted shortly before end-of-life condition in 325°C (N ≈ 0.85 × N25) showed strength and ductility almost identical to virgin material. This paper provides new experimental results and discusses previous observations aiming to sum up a state of the art in fatigue performance of German NPP primary loop materials.

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

2014 ◽  
Author(s):  
Jussi Solin ◽  
Sven Reese ◽  
H. Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Stainless Steel ◽  
Nuclear Power ◽  
Hold Time ◽  
Laboratory Data ◽  
Cyclic Strain ◽  
Hot Water ◽  
Fatigue Performance ◽  
Normal Operation ◽  
Operational Conditions ◽  
Thermal Transients

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.

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

2015 ◽  
Author(s):  
Jussi Solin ◽  
Sven Reese ◽  
H. Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Stainless Steel ◽  
Nuclear Power ◽  
Power Plants ◽  
Elevated Temperatures ◽  
Hold Time ◽  
Laboratory Data ◽  
Cyclic Stress ◽  
Lattice Defects ◽  
Normal Operation ◽  
Thermally Activated

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.

Numerical Approach for the Consideration of Hold Time Effects

2014 ◽  
Author(s):  
Sven H. Reese ◽  
Johannes Seichter ◽  
Dietmar Klucke ◽  
H. Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Hold Time ◽  
Austenitic Steels ◽  
Normal Operation ◽  
Primary Circuit ◽  
Published Data ◽  
Fatigue Lifetime ◽  
Time Effects ◽  
Positive Effects ◽  
Surge Line ◽  
Circuit Components

In recent years the Environmentally Assisted Fatigue (EAF) became an item, which has to be considered additionally in terms of ensuring a conservative determination of the actual component’s health status resp. the CUF. For practical application, the consideration of the so called Fen-factor leads to the reduction of the admissible cycles in fatigue calculations. Beyond that the influence of elevated temperatures has been identified as one parameter having a negative influence on the admissible cycles as well. For example the German KTA 3201.2 defines for austenitic steels separate fatigue curves for temperatures above 80°C and for temperatures below 80°C. In summary on the one hand parameters influencing component’s lifetime negatively have to be considered in terms of conservative calculations. On the other hand, there are other parameters which influence the component’s fatigue lifetime in a positive manner. As such positive effects are neglected so far, CUF allowing for EAF tend to become over conservative leading to oversized components. Therefore, positive effects should be considered as well in the framework of a comprehensive and detailed analysis making sure not to overdesign components. When taking a closer look on the operational behavior of primary circuit components, fatigue loading is mainly defined by long steady-state periods with no significant changes in the loadings and by normally short outage periods with no thermal loading. For example fatigue of a PWR surge-line is mostly caused by short in-surge and out-surge events during start-up or shut-down of the plant. Normal operation transients mostly not cause fatigue relevant events in the surge-line. Fatigue of PWR spray-lines is primarily generated by very few spray-events during a one-year period of operation. Spray events are mainly caused by significant load ramps. Subsequently the fatigue status of primary circuit components is controlled by long periods with no fatigue relevant loading at operating temperature and few additional loading patterns in between. Experimental investigations have shown that hold time effects have a positive influence on fatigue lifetime of austenitic stainless steel materials. Anyhow, no quantification of these effects has been published in recent years. Within this publication an engineering based approach will be developed to quantify the hold time effect based on literature and published data. On the basis of a practical example the influence of hold time effects will be quantified and a direct comparison to lifetime reducing effect of EAF and temperature will be drawn.

Grade and Temperature Dependent Reference Curves for Realistic Fen Quantification of Austenitic Stainless Steels

2020 ◽  
Author(s):  
Tommi Seppänen ◽  
Jouni Alhainen ◽  
Esko Arilahti ◽  
Jussi Solin
Keyword(s):  
Stainless Steel ◽  
Environmental Effects ◽  
Austenitic Stainless Steels ◽  
Primary Circuit ◽  
Reference Curve ◽  
Aisi 304L ◽  
Common Reference ◽  
Wide Range ◽  
Reference Curves ◽  
Circuit Components

Abstract 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.

scholarly journals Chemical Forms of Important Fission Products in Primary Circuit of HTR-PM under Conditions of Normal Operation and Overpressure and Water Ingress Accidents: A Study with a Chemical Thermodynamics Approach

2019 ◽  
Vol 2019 ◽  
pp. 1-12
Author(s):  
Chuan Li ◽  
Wenqian Li ◽  
Lifeng Sun ◽  
Haoyu Xing ◽  
Chao Fang
Keyword(s):  
High Temperature ◽  
Fission Products ◽  
Point Of View ◽  
Normal Operation ◽  
Primary Circuit ◽  
Chemical Thermodynamics ◽  
Chemical Forms ◽  
Operational Conditions ◽  
Water Ingress ◽  
High Temperature Gas

The chemical forms of important fission products (FPs) in the primary circuit are essential to the source term analysis of high-temperature gas-cooled reactors because the volatility, transfer, and diffusion of these radionuclides are significantly influenced by their chemical forms. Through chemical reactions with gaseous impurities in the primary circuit, these FPs exist in diverse chemical forms, which vary under different operational conditions. In this paper, the chemical forms of cesium (Cs), strontium (Sr), silver (Ag), iodine (I), and tritium in the primary circuit of the Chinese pebble-bed modular high-temperature gas-cooled reactor (HTR-PM) under normal conditions and accident conditions (overpressure and water ingress accident) are studied with chemical thermodynamics. The results under normal conditions show that Cs exists mainly in the form of Cs2CO3 at 250°C and gaseous form at 750°C, and for I and Ag, Ag3I3 and Ag convert to gaseous CsI and AgO, respectively, with increasing temperature, while SrCO3 is the only main kind of compound for Sr. It is also observed that new compounds are generated under accidents: I exists in HI form when a water ingress accident occurs. Regarding tritium, the chemical forms of FPs change little, but compounds need higher temperature to convert. Furthermore, hazard of some FPs in different chemical forms is also discussed comprehensively in this paper. This study is significant for understanding the chemical reaction mechanisms of FPs in an HTR-PM, and furthermore it may provide a new point of view to analyze the interaction between FPs and structural materials in reactor as well as their hazards.

Low Cycle Fatigue Behavior of 429EM Ferritic Stainless Steel at Elevated Temperatures

2004 ◽  
Vol 261-263 ◽  
pp. 1135-1140 ◽  
Author(s):  
Keum Oh Lee ◽  
Sam Son Yoon ◽  
Soon Bok Lee ◽  
Bum Shin Kim
Keyword(s):  
Stainless Steel ◽  
Elastic Modulus ◽  
Fatigue Life ◽  
Elevated Temperatures ◽  
Low Cycle Fatigue ◽  
Fatigue Behavior ◽  
Cyclic Hardening ◽  
Dynamic Strain ◽  
Temperature Structure ◽  
Cycle Fatigue

In recent, ferritic stainless steels are widely used in high temperature structure because of their high resistance in thermal fatigue and low prices. Tensile and low cycle fatigue(LCF) tests on 429EM stainless steel were performed at several temperatures from room temperature to 600°C. Elastic modulus, yield stress and ultimate tensile strength(UTS) decreased with increasing temperature. Considerable cyclic hardening occurred at 200°C and 400°C. 475°C embrittlement observed could not explain this phenomenon but dynamic strain aging(DSA) observed from 200°C to 500°C could explain the hardening mechanism at 200°C and 400°C. And it was observed that plastic strain energy density(PSED) was useful to predict fatigue life when large cyclic hardening occurred. Fatigue life using PSED over elastic modulus could be well predicted within 2X scatter band at various temperatures.

Effects of Hot Water and Holds on Fatigue of Stainless Steel

2016 ◽  
Author(s):  
Jussi Solin ◽  
Jouni Alhainen ◽  
Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Elevated Temperatures ◽  
Water Environment ◽  
Fatigue Tests ◽  
Hot Water ◽  
Primary Circuit ◽  
Primary Coolant ◽  
Fatigue Assessment ◽  
Atom Diffusion ◽  
Environment Temperature ◽  
Circuit Components

Direct strain controlled LCF data for solid specimens is still very rare. In PVP2013-97500 and PVP2014-28465 we reported results for niobium stabilized X6CrNiNb1810mod steel (type 347) fatigued in 325°C and 200°C PWR water according to VGB water chemistry specification. New data in this paper further confirms the conclusions: we are unable to repeat as high Fen factors or short lives as predicted according to NUREG/CR-6909. The slowest strain rate used 4·10−6 in 325°C water would predict Fen > 12, i.e. laboratory specimen data below the current ASME design curve, but our results are superior for this steel generally used in German NPP’s. However, the difference is not necessarily grade specific. Use of 100% relevant fabricated material batch and standard LCF methodology are regarded to play an important role. Notable hardening can be measured, when long duration holds in elevated temperatures are introduced between blocks of cyclic strains at lower temperatures. This is the case for thermal gradient loaded primary circuit components, e.g. the PWR pressurizer spray lines or surge line, which connects the pressurizer to primary coolant line. In PVP2011-57942 we reported improved endurances in fatigue tests aiming to roughly simulate steady state operation between fatigue transients in such NPP components. New test types have been introduced to generalize the results. Mechanisms of time and temperature dependent relaxation of fatigue damage and/or improvement of material fatigue performance during holds are not yet fully revealed, but the rate controlling thermal activation energy is below shown to be near that for vacancy and interstitial atom diffusion. This allows us to draft a thermodynamic prediction model. Improved accuracy of fatigue assessment helps in focusing optimally scheduled nondestructive testing to the most relevant locations and maintaining high level of reliability without excessive cost and radiation doses for inspection personnel. This paper provides previously unpublished experimental results and proposes methods to improve transferability of laboratory test data to fatigue assessment of NPP components. The effects of material, water environment, temperature and service loading patterns are discussed.

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

2013 ◽  
Author(s):  
Jussi Solin ◽  
Sven Reese ◽  
H. Ertugrul Karabaki ◽  
Wolfgang Mayinger
Keyword(s):  
Stainless Steel ◽  
Hold Time ◽  
Austenitic Stainless Steels ◽  
Fatigue Tests ◽  
Fatigue Performance ◽  
Normal Operation ◽  
Experimental Investigations ◽  
Fatigue Assessment ◽  
Thermal Transients ◽  
Loading Patterns

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.

Hidden Roles of Time and Temperature in Cyclic Behavior of Stainless Nuclear Piping

2018 ◽  
Author(s):  
Jussi Solin ◽  
Tommi Seppänen ◽  
Wolfgang Mayinger ◽  
H. Ertugrul Karabaki
Keyword(s):  
Stainless Steel ◽  
Stress Responses ◽  
Laboratory Data ◽  
Water Environment ◽  
Fatigue Performance ◽  
Cyclic Behavior ◽  
Design Curve ◽  
Environment Temperature ◽  
Unexpected Findings ◽  
Cyclic Straining

Unexpected findings on time and temperature dependent behavior have been recorded during our research on fatigue performance of niobium stabilized stainless steel. Cyclic straining at 325°C and low strain rates resulted in higher stress responses than in higher rate tests. This effect is particular strong in PWR water environment. Subsurface bulk effect in environment is in contrast to the assumption on similar responses in air and environment, which is the foundation of the ‘companion specimen’ method where the strain in environment is measured from a parallel specimen similarly tested in air. Our data shows that environmental effects caused by PWR water cannot be isolated as a separate issue. Environment, temperature and strain rate are factors, which interactively affect the cyclic response and fatigue performance of stainless steel in relevant temperatures and loading conditions. The current ASME Code Section III design curve is based on different translation of the laboratory data than that made by Langer et al. The resulting effect is not as radical as caused by replacement of the original test data to a new highly scattered data base. But also the procedural changes have detectable effects and open a door for a provocative question: “is the design curve still compatible with the code?”

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