Creep monitoring using permanently installed potential drop sensors

Creep is the primary life limiting mechanism of static high temperature, high pressure power station components. Creep state evaluation is currently achieved by surface inspection of microstructure during infrequent outages; a methodology which is laborious, time consuming and considered inadequate. The objective of this work is to develop a monitoring technique that is capable of on-load creep damage monitoring. A continuous update of component integrity will enable better informed, targeted inspections and outage maintenance providing increased power generation availability. A low-frequency, permanently installed potential drop system has been previously developed and will be the focus of this thesis. The use of a quasi-DC inspection frequency suppresses the influence of the electromagnetic skin effect that would otherwise undermine the stability of the measurement in the ferromagnetic materials of interest; the use of even low frequency measurements allows phase sensitive detection and greatly enhanced noise performance. By permanently installing the electrodes to the surface of the component the resistance measurement is sensitive to strain. A resistance - strain inversion is derived and validated experimentally; the use of the potential drop sensor as a robust, high temperature strain gauge is therefore demonstrated. The strain rate of a component is known to be an expression of the creep state of the component. This concept was adopted to develop an interpretive framework for inferring the creep state of a component. It is possible to monitor the accumulation of creep damage through the symptomatic relative increase in strain rate. By taking the ratio of two orthogonal strain measurements, instability and drift common to both measurements can be effectively eliminated; an important attribute considering the necessity to monitor very low strain rates over decades in time in a harsh environment. A preliminary study of using the potential drop technique for monitoring creep damage at a weld has been conducted. Welds provide a site for preferential creep damage accumulation and therefore will frequently be the life limiting feature of power station components. The potential drop technique will be sensitive to both the localised strain that is understood to act as precursor to creep damage at a weld and also the initiation and growth of a crack. Through the course of this project, two site trials have been conducted in power stations. A measurement system and high temperature hardware that is suitable for the power station environment has been developed. The focus of this thesis is the effective transfer of the technique to industry; the realisation of this is detailed in the final chapter.Open Acces

ECCC TEST PROGRAMME AND DATA ASSESSMENT ON GTD111 CREEP RUPTURE, STRAIN AND DUCTILITY

GTD111, a creep resistant Ni-based superalloy developed by GE, is widely used in land-based gas turbine first stage blades. However, there is little published information on its creep properties and microstructure. The European Creep Collaborative Committee (ECCC) Working Group 3C consequently selected GTD111 as a model material for testing and complementary data assessment. The aim of this paper is to present the results from the ECCC test program and data assessment, and to compare equiaxed (EA) and directionally solidified (DS) material performance. Testing and metallographic laboratories from six European nations collaborated to produce strain monitored creep rupture data on four EA and DS materials out to beyond 10,000 hours within a wide range of temperatures, 850-950°C, and stresses, 293-99 MPa. Available (generally short term) results from other sources were also included in the compiled, small but viable, 51-test data set. Assessment was carried out by three different assessors using different tools and adopting different prediction models. Conventional ECCC post-assessment techniques and novel “back-fitting” methods were used to identify a preferred model. It was shown that assessing all the EA and DS data together can lead to non-conservative predictions for EA materials, but separating the two classes creates small data subsets which cannot be modelled effectively. As a pragmatic compromise, the DS data and those EA data which also showed good ductility were included in a final "ductile GTD111" assessment. The resulting creep rupture material models and rupture strength predictions are presented up to 3 times the longest test duration. It was then shown that the performance of lower ductility EA materials can also be predicted effectively with the "ductile" model by truncating the rupture time at the measured fracture strain. For this exercise, a creep strain model based on rupture and time to strain data was fitted. In parallel, microstructural examination was performed to characterize the damage modes involved in the low ductility failures. It was thereby shown that the creep rupture strength shortfall of an EA material compared to its DS equivalent is not a constant factor, but is primarily governed by the reduced creep ductility. Hence, the shortfall varies between different EA casts, and tends to become greater in the longer term.JRC.F.4-Innovative Technologies for Nuclear Reactor Safet

Creep and life assessment of engineering components in power plants and process industries

Engineering components in thermal power plants such as boiler tubes, headers, main steam pipes, HP, IP and LP cylinders, rotors etc. made of Cr-Mo, Cr-Mo-V steels and in process industries - steam cracker furnace, reformer tubes, process heater tubes etc. made of HP40, IN 519, HK40 alloys operate in a complex environment involving high temperature, pressure and corrosive atmosphere to perform specific functionsfor a minimum specd period of time. During service depending upon the operating conditions, several mechanisms such as creep, fatigue, corrosion, oxidation etc. become operative. Accumulation of microstructural damages in the components due to prolong operation decreases their load bearing capacity thereby limiting the lives of the components.When the load bearing capacity falls below a critical level determined by component geometry and loading, failure occurs.This paper describes some aspects of creep and life assessment technology and a few case studies related to changes in micros tructure due to ageing, creep and remaining life of components in power plants and process industries

Oxidation behavior during prolonged service of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V heat resistance steels

During service of thermal power plant (TPP) units, different components are exposed to high temperature due to technological cycle of TPP unit. Service lifetimes of these components, especially boiler heating surfaces, may be limited due to creep, fatigue or oxidation, but materials designed for use at high temperatures have been developed primarily for their creep properties and microstructural stability during long term exposures at elevated temperatures. Having in mind that oxidation in steam environment on the inner surface of boiler tubes and in flue gass on the outer surface of boler tubes could lead to a different consequences regarding service life of tubes, either directly through metal wastage or indirectly through raising local temperatures due to the lower thermal conductivity of the oxide scale, the oxidation behaviour of a different heat resistant steels become very important characteristics. In this paper are presented some data about the oxidation behavior of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V steel after service of approximately 130.000 and 200.000h in two 620MVV TPP units. Characterization of oxide scales on the inner side on tubes made of two steels with different chromium content, after two different prolonged periods of service, were compared and also their influence on the service life of tubes and kinetics of oxide scale growth were analyzed

Oxidation behavior during prolonged service of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V heat resistance steels

During service of thermal power plant (TPP) units, different components are exposed to high temperature due to technological cycle of TPP unit. Service lifetimes of these components, especially boiler heating surfaces, may be limited due to creep, fatigue or oxidation, but materials designed for use at high temperatures have been developed primarily for their creep properties and microstructural stability during long term exposures at elevated temperatures. Having in mind that oxidation in steam environment on the inner surface of boiler tubes and in flue gass on the outer surface of boler tubes could lead to a different consequences regarding service life of tubes, either directly through metal wastage or indirectly through raising local temperatures due to the lower thermal conductivity of the oxide scale, the oxidation behaviour of a different heat resistant steels become very important characteristics. In this paper are presented some data about the oxidation behavior of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V steel after service of approximately 130.000 and 200.000h in two 620MVV TPP units. Characterization of oxide scales on the inner side on tubes made of two steels with different chromium content, after two different prolonged periods of service, were compared and also their influence on the service life of tubes and kinetics of oxide scale growth were analyzed

Oxidation behavior during prolonged service of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V heat resistance steels

During service of thermal power plant (TPP) units, different components are exposed to high temperature due to technological cycle of TPP unit. Service lifetimes of these components, especially boiler heating surfaces, may be limited due to creep, fatigue or oxidation, but materials designed for use at high temperatures have been developed primarily for their creep properties and microstructural stability during long term exposures at elevated temperatures. Having in mind that oxidation in steam environment on the inner surface of boiler tubes and in flue gass on the outer surface of boler tubes could lead to a different consequences regarding service life of tubes, either directly through metal wastage or indirectly through raising local temperatures due to the lower thermal conductivity of the oxide scale, the oxidation behaviour of a different heat resistant steels become very important characteristics. In this paper are presented some data about the oxidation behavior of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V steel after service of approximately 130.000 and 200.000h in two 620MVV TPP units. Characterization of oxide scales on the inner side on tubes made of two steels with different chromium content, after two different prolonged periods of service, were compared and also their influence on the service life of tubes and kinetics of oxide scale growth were analyzed

Thermomechanical fatigue in 9-12Cr steels: Life prediction models and the effect of tensile dwell periods

This paper is concerned with the assessment of life prediction models for thermomechanical fatigue (TMF), with specific application to P91 steel. A program of TMF tests, including dwell periods, are performed to determine the role of thermomechanical loading on fatigue life. As expected, fatigue life under conventional TMF testing (no dwells) is governed by maximum applied stress and inelastic strain-range. However, with the introduction of dwell periods, at maximum tensile stress during TMF loading, in-phase loading becomes the life-limiting case. This is attributed here to increased microstructural degradation and oxidation, associated with the dwell at peak temperature. Analysis of commonly used TMF life prediction models shows that the effect of dwell periods currently cannot be predicted for in-phase loading. Thus, it is concluded that physically-motivated approaches are required to successfully predict fatigue life under more complex (service) thermomechanical loading histories.Keyword