Denis Thibault
IREQ, Expertise mécanique, métallurgie et hydroéolien, Department Member
CA6NM and UNS S41500 martensitic stainless steels are widely used for manufacturing and repair of hydraulic turbine runners. They offer good mechanical properties and superior cavitation resistance when compared to mild steels. They are... more
CA6NM and UNS S41500 martensitic stainless steels are widely used for manufacturing and repair of hydraulic turbine runners. They offer good mechanical properties and superior cavitation resistance when compared to mild steels. They are also relatively easy to weld. However, when welded homogeneously, they require a post-weld heat treatment (PWHT) in order to temper the as-welded martensite. This PWHT is also beneficial for residual stresses reduction as it effectively lowers the stress peaks. To avoid this PWHT, austenitic filler metals are often used for repair. But omitting PWHT inevitably leaves weld-induced residual stresses in the assembly. In order to better understand the impact of the weld filler metal choice on the importance of residual stress, an experimental study has been conducted on three different filler alloys. The chosen alloys were: • 410NiMo, a martensitic grade having the same composition as the base metal (13%Cr-4%Ni-0.5%Mo) ; • 309L, an austenitic grade widely used for repair (24%Cr-13%Ni) ; • A proprietary low transformation temperature (LTT) martensitic grade (13%Cr-6%Ni). This paper compares residual stresses in the as-welded condition on welds of UNS S41500 (13%Cr-4%Ni) made using these filler metals. Residual stresses were measured using the contour method. Microstructural analysis was performed to identify the phases in the weld and the heat-affected zones (HAZ). Microhardness maps were done to see the hardness distribution of each weldment.
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L'objectif de la presente etude est de mieux cerner certains des principaux parametres regissant le comportement en fatigue des aciers inoxydables 13%Cr-4%Ni utilises pour la fabrication des roues de turbines hydrauliques. Ainsi deux... more
L'objectif de la presente etude est de mieux cerner certains des principaux parametres regissant le comportement en fatigue des aciers inoxydables 13%Cr-4%Ni utilises pour la fabrication des roues de turbines hydrauliques. Ainsi deux aspects sont traites dans cette these, soit les contraintes residuelles laissees par le soudage homogene de ces aciers et la transformation sous chargement cyclique de l'austenite de reversion contenue dans cet alliage. Les contraintes residuelles laissees apres soudage homogene ont ete caracterisees a l'aide de quatre methodes de mesure : la methode de percage, la diffraction des rayons-X, la methode du contour et la diffraction des neutrons. Ces contraintes ont ete mesurees dans deux types de joints soudes au metal d'apport 410NiMo avant et apres traitement thermique post-soudage. Une distribution de contraintes residuelles totalement differente de celle retrouvee dans les aciers de structures a ete mesuree. Ainsi, des contraintes residuelles de compression d'environ 400 MPa ont ete mesurees dans le dernier cordon de soudure. Des zones de tension residuelle ont ete retrouvees au pourtour de la zone affectee thermiquement et sous la derniere couche de soudure. Cette distribution inedite est expliquee par la transformation de phase austenite-martensite se produisant a basse temperature (~300oC) lors du refroidissement de la soudure. Les resultats de ces mesures montrent aussi que le traitement thermique post-soudage de revenu couramment utilise par l'industrie est tres efficace pour diminuer ces contraintes. Des contraintes maximales d'environ 150 MPa ont ete retrouvees apres ce traitement. L'austenite de reversion formee lors du traitement de revenu est par ailleurs instable mecaniquement. Les resultats presentes dans cette these montrent qu'apres essais de propagation de fissure, toute l'austenite situee pres de la surface de rupture a ete transforme en martensite sous l'effet du chargement cyclique. Ces mesures obtenues directement sur les surfaces de rupture par diffraction des rayons-X ont ete corroborees par les resultats obtenus en fatigue oligocyclique qui montrent que cette austenite se transforme graduellement et que cette transformation est completee apres 100 cycles. Le comportement en fatiguepropagation des aciers testes ne semble pas influence outre mesure par cette transformation qui a lieu a toute valeur de facteur d'intensite de contrainte. Les retombees pratiques de ces resultats pour la fabrication et la reparation des roues de turbines en inox 13%Cr-4%Ni sont discutees a la suite de la presentation des resultats.
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ABSTRACT Strains experimentally measured with strain gauges can differ from those evaluated using the Finite Element (FE) method. This difference is due mainly to the assumptions and uncertainties inherent to each method. To circumvent... more
ABSTRACT Strains experimentally measured with strain gauges can differ from those evaluated using the Finite Element (FE) method. This difference is due mainly to the assumptions and uncertainties inherent to each method. To circumvent this difficulty, we developed a numerical method based on Monte Carlo simulations to evaluate measurement uncertainties produced by the behaviour of a unidirectional welded gauge, its position uncertainty and its integration effect. This numerical method uses the displacement fields of the studied part evaluated by an FE analysis. The paper presents a study case using in situ data measured on a hydraulic turbine runner. The FE analysis of the turbine runner blade was computed, and our numerical method used to evaluate uncertainties on strains measured at five locations with welded strain gauges. Then, measured strains and their uncertainty ranges are compared to the estimated strains. The uncertainty ranges obtained extended from 74 μepsilon to 165 μepsilon. Furthermore, the biases observed between the median of the uncertainty ranges and the FE strains varied from -36 to 36 μepsilon. Note that strain gauge measurement uncertainties depend mainly on displacement fields and gauge geometry.
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ABSTRACT A lack of observed data can lead to significant discrepancy between estimated and actual fatigue reliability. In this study, the load spectrum derived from the strain measurements and operation history of a hydroelectric turbine... more
ABSTRACT A lack of observed data can lead to significant discrepancy between estimated and actual fatigue reliability. In this study, the load spectrum derived from the strain measurements and operation history of a hydroelectric turbine are used to identify the information necessary to avoid major bias in the estimated fatigue reliability of such structure. Our results demonstrate that a limited number of parameters need to be considered. Any further simplifications may lead to major reliability estimate discrepancies. Furthermore, we observe that the parameters which influence initial reliability hide the influence of other parameters on the reliability failure rate associated with the structure’s life expectancy. We conclude that a typical sensitivity analysis made on parameter values needs to be complemented with a sensitivity study on the chosen assumptions in order to properly evaluate the risks associated with the operation of hydroelectric turbines.
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ABSTRACT The failure of hydraulic turbine runners is a very rare event. Hence, in order to assess the reliability of these components, one cannot rely on statistical models based on the number of failures in a given population. However,... more
ABSTRACT The failure of hydraulic turbine runners is a very rare event. Hence, in order to assess the reliability of these components, one cannot rely on statistical models based on the number of failures in a given population. However, as there is a limited number of degradation mechanisms involved, it is possible to use physically-based reliability models. Such models are more complicated but have the advantage of being able to account for physical parameters in the prediction of the evolution of runner degradation. They can therefore propose solutions to help improve reliability. With the use of such models, the effect of materials properties on runner reliability can easily be illustrated. This paper will present a brief review of the Kitagawa-Takahashi diagram that links the damage tolerance approach, based on fracture mechanics, to the stress or strain-life approaches. This diagram is at the centre of the reliability model used in this study. Using simplified response spectra obtained from on-site runner stress measurements, the paper will show how fatigue reliability is impacted by materials fatigue properties, namely fatigue crack propagation behaviour and fatigue limit obtained on S-N curves. It will also present a review of the most important microstructural features of 13%Cr- 4%Ni stainless steels used for runner manufacturing and will review how they influence fatigue properties in an effort to bridge the gap between metallurgy and turbine runners reliability.
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This study presents the results of residual stress characterization by neutron diffraction and the contour method on 13% Cr–4% Ni welds made using 410NiMo weld filler metal. The transverse, longitudinal and normal components of stress... more
This study presents the results of residual stress characterization by neutron diffraction and the contour method on 13% Cr–4% Ni welds made using 410NiMo weld filler metal. The transverse, longitudinal and normal components of stress were determined by neutron ...
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... Keywords: hammer-peening, residual stress, welding, steel, hole-drilling Abstract The strains induced by hammer-peening were measured by strain gauges on a mild steel plate during the hammer-peening operation. ... Fig. 1. Scompi robot... more
... Keywords: hammer-peening, residual stress, welding, steel, hole-drilling Abstract The strains induced by hammer-peening were measured by strain gauges on a mild steel plate during the hammer-peening operation. ... Fig. 1. Scompi robot hammer-peening the mild steel plate ...
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Abstract Cast low carbon CA-6NM (13%Cr-4%Ni) martensitic stainless steel has been used for hydraulic turbine runners manufacturing since the 60's owing to its high strength, toughness, corrosion, and cavitation-erosion resistance.... more
Abstract Cast low carbon CA-6NM (13%Cr-4%Ni) martensitic stainless steel has been used for hydraulic turbine runners manufacturing since the 60's owing to its high strength, toughness, corrosion, and cavitation-erosion resistance. These castings contain several types of defects which may have detrimental effects on the performance of the produced parts. Several aspects of the casting defects in these steels have not been specifically studied in detail. It is common in industry to evaluate the integrity of cast components by nondestructive testing such as X-ray radiography before putting them in operation. However, tortuous and complex geometry of macro-shrinkage porosities makes them difficult to be characterized precisely only by 2D radiographic results. In the present study, the relation between radiography mapping and actual macro-defect morphologies has been studied by the “salami” cross sectioning method in order to document the real shape and morphology of defects. Furthermore, metallographic characterization of cast microstructure in CA-6NM stainless steels has also been performed using several techniques to document both chemical composition and microstructure around macro-defects. Briefly stated, the main purpose of the investigation described herein is to study several severity levels of macro-defects in cast martensitic stainless steels and investigate some characteristics of real defects which can impact on the fatigue properties of cast parts at microstructural level.
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Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.
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The fatigue reliability of a turbine runner is closely related to its dynamic behavior. Over the past few years, Hydro-Québec has performed measurement campaigns on many of its turbine runners. These measurements led to the evaluation of... more
The fatigue reliability of a turbine runner is closely related to its dynamic behavior. Over the past few years, Hydro-Québec has performed measurement campaigns on many of its turbine runners. These measurements led to the evaluation of the fatigue risk level of each runner in various operating conditions, which allows operating conditions with a higher risk of crack propagation to be avoided. This paper presents the results for turbine dynamic behavior assessed in steady-state conditions. Stress levels at strain gauge locations are used to evaluate the risk of fatigue cracking based on the Kitagawa-Takahashi diagram. Results show a good correlation between the calculated risk of cracking and real cases of cracks found in runners. Furthermore, the results comparison highlights a apparent tendency for recent designs to be more prone to cracking at speed-no-load operating condition than older designs. The paper gives an overview of the methodology used and discusses the conclusions d...
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The effect of shielding gas on the mechanical and microstructural characteristics of ER410NiMo martensitic stainless steel weldments was investigated. Three weldments with various inclusion contents were manufactured using different... more
The effect of shielding gas on the mechanical and microstructural characteristics of ER410NiMo martensitic stainless steel weldments was investigated. Three weldments with various inclusion contents were manufactured using different shielding gas compositions and welding processes: gas metal arc welding (GMAW) with 100% argon (Ar), GMAW 85% Ar/15% carbon dioxide (CO2), and flux cored arc welding (FCAW) 75% Ar/25% CO2. The inclusions in each weldment were characterized by means of scanning electron microscope observations and energy-dispersive spectroscopy analysis. The weldments underwent postweld heat treatment, after which the chemical composition and reformed austenite proportion were measured to account for microstructural effects. Hardness measurements, tensile tests, and impact toughness tests using the Charpy method were performed. The results showed that the Charpy V-notch (CVN) absorbed energy decreases with increasing inclusion content. The highest CVN absorbed energy, 195...
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Abstract Cast low carbon CA-6NM (13%Cr-4%Ni) martensitic stainless steel has been used for hydraulic turbine runners manufacturing since the 60's owing to its high strength, toughness, corrosion, and cavitation-erosion resistance.... more
Abstract Cast low carbon CA-6NM (13%Cr-4%Ni) martensitic stainless steel has been used for hydraulic turbine runners manufacturing since the 60's owing to its high strength, toughness, corrosion, and cavitation-erosion resistance. These castings contain several types of defects which may have detrimental effects on the performance of the produced parts. Several aspects of the casting defects in these steels have not been specifically studied in detail. It is common in industry to evaluate the integrity of cast components by nondestructive testing such as X-ray radiography before putting them in operation. However, tortuous and complex geometry of macro-shrinkage porosities makes them difficult to be characterized precisely only by 2D radiographic results. In the present study, the relation between radiography mapping and actual macro-defect morphologies has been studied by the “salami” cross sectioning method in order to document the real shape and morphology of defects. Furthermore, metallographic characterization of cast microstructure in CA-6NM stainless steels has also been performed using several techniques to document both chemical composition and microstructure around macro-defects. Briefly stated, the main purpose of the investigation described herein is to study several severity levels of macro-defects in cast martensitic stainless steels and investigate some characteristics of real defects which can impact on the fatigue properties of cast parts at microstructural level.
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This paper presents the results of a project aimed at evaluating the performance of ultrasonic techniques for detecting flaws in Francis turbine runners. This work is the first phase of a more ambitious program aimed at improving the... more
This paper presents the results of a project aimed at evaluating the performance of ultrasonic techniques for detecting flaws in Francis turbine runners. This work is the first phase of a more ambitious program aimed at improving the reliability of inspection of critical areas in turbine runners. Francis runners may be utilized to supply power during peak periods, which means that they experience additional load stress associated with start and stop sequences. Inspection during manufacturing is then of paramount importance to remove as much as feasible all flaw initiation sites before the heat treatment. This phase one objective is to collect initial data on a simplified mock-up and then to compare the experimental ultrasonic data with the results of simulations performed by CIVA, a computer simulation package. The area of interest is the region with the highest stress between the blade and the web. A welded T-joint coupon made of UNS S41500 was manufactured to represent this high-s...
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ABSTRACT
The objectives of the present study are to characterize some of the main parameters affecting fatigue behaviour of 13%Cr-4%Ni martensitic stainless steels used for hydraulic turbines manufacturing. Two aspects are studied: the residual... more
The objectives of the present study are to characterize some of the main parameters affecting fatigue behaviour of 13%Cr-4%Ni martensitic stainless steels used for hydraulic turbines manufacturing. Two aspects are studied: the residual stresses left after autogenous welding of these steels and the stress-assisted transformation of the reformed austenite contained in this alloy. The residual stresses induced by welding were
Research Interests: Neutron Diffraction, Fatigue crack growth, Stainless Steel, Stress Intensity Factor, X ray diffraction, and 9 moreResidual Stress, Stress distribution, Low Temperature, Stress Distribution, Martensitic Transformation, Low Cycle Fatigue, Fatigue Crack Propagation, Heat affected zone, and Post Weld Heat Treatment
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Thèse (M. Sc.)--Université Laval, 2000. Bibliogr.: f. 85-86.