David Gandy

Weld repair of steam turbine casings and piping: An industry survey

As the U.S. fleet of fossil power plants age, utilities are forced to perform more and more repairs on such components as turbine casings, main and reheat piping, headers, and other components that have experienced high-temperature degradation. This paper presents information from two surveys on the weld repair technologies currently used by utilities and repair organizations to extend the life of high-temperature, high-pressure components. The initial survey included responses from 28 EPRI member utilities on various repair issues ranging from condition assessment to preheat/postweld heat treat to filler metals employed. The second survey was forwarded to repair vendors and OEMs to gain their perspective on utility industry repairs.

Systematic Evaluation of Creep-Fatigue Life Prediction Methods for Various Alloys

Failure under creep-fatigue interaction is receiving an increasing interest due to an increased number of start-up and shut-downs in fossil power generation plants as well as development of newer nuclear power plants employing low-pressure coolant. Such situations have prompted the studies on creep-fatigue interaction and the developments of various approaches for evaluating its significance in design as well as remaining life evaluation, but most of them are fragmental and rather limited in terms of materials and test conditions covered. Therefore, applicability of the proposed approaches to different materials or even different temperatures is uncertain in many cases. The present work was conducted in order to comparably evaluate the representative approaches used in the prediction of failure life under creep-fatigue conditions as well as their modifications, by systematically applying them to available test data on a wide range of materials which have been used or are planned to be used in various types of power generation plants. The following observations have been made from this exercise: (i) The time fraction model has a tendency to be nonconservative in general, especially at low temperature and small strain ranges. Because of the large scatter of the total damage, this shortcoming would be difficult to cover by the consideration of creep-fatigue interaction in a simple manner. (ii) The classical ductility exhaustion model showed a general tendency to be overly conservative in many situations, especially at small strain ranges. (iii) The modified ductility exhaustion model based on the redefinition of creep damage showed improved predictability with a slightly nonconservative tendency. (iv) Energy-based ductility exhaustion model developed in this study seems to show the best predictability among the four procedures in an overall sense although some dependency on strain range and materials was observed.

Performance of Repair Welds on Service-Aged 2-1/4Cr-1Mo Girth Weldments

This paper discusses the results of evaluations performed on service-aged piping using both conventional postweld heat treatments and temperbead repair techniques. The two repair weldments were accomplished on two 2-1/4Cr-1Mo pipe girth weldments which were removed from a utility hot reheat piping system in the fall of 1992 after 161,000 h of operation at 1,000 F (538 C). Each repair was performed around one-half of the diameter of a pipe girth weldment, while the remaining half of the girth weldment was left in the service-aged condition. Post-repair metallurgical and mechanical test results indicated that both weld repairs produced improved remaining lives when compared to the service-aged girth weldments. Since the two ex-service weldments that were utilized in weld repairs exhibited different stress rupture strengths to start with, the performance of temper bead and postweld heat-treated (PWHT) repair could not be compared directly. It was clear, however, that life extension periods exceeding 30 yr could be achieved by temperbead repairs, with improved toughness and with no loss of stress rupture ductility, tensile strength, or yield strength. The temperbead repair improved the toughness of the service-aged weldment, while the postweld heat-treated repair lowered the HAZ toughness.

Delamination failures of Stellite hardfacing in power plants: a microstructural characterisation study

Abstract In recent years, several incidents of cracking and failures have been observed in Stellite (Stellite is a registered trademark of the Deloro-Stellite Corporation) hardfacing used in valves of modern high temperature combined cycle gas fired power plants. These hardfacing layers are applied as an overlay onto a steel substrate, such as CrMo steel (i.e. Grade 22, WC9) or creep strength enhanced ferritic steel (i.e. Grade 91, C12A). Cracking has been observed in valve components at the Stellite/steel interface and in the weld dilution zone formed between the steel and clad. Ultimately, disbonding or delamination of the weld hardfacing from the valve body occurs and has resulted in collateral damage to components in the plant (such as to the turbine) or valve failure. In this study, the microstructure formed near the Stellite/steel interface is investigated. Based on thermodynamic modelling, microstructure formed at these regions is hypothesised and a simple methodology is proposed to predict the occurrence of these failures.

Weld repair of 2-1/4Cr-1Mo service-aged header welds

The objective of this investigation was to evaluate the efficacy of different weld repair techniques as applied to service-aged 2-1/4Cr-1Mo steel weldments. A header which had been in service for 244,000 h at 1,050 F (565 C) was utilized for the study. Three girth welds were partially excavated and subjected to repairs using gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW) with postweld heat treatment (PWHT), and without postweld heat treatment using a temperbead technique. Results show that all the weld repairs improved the creep rupture lives of the ex-service weldments and that remaining lives of several decades could be achieved in the repaired condition. The SMAW-temperbead repairs resulted in increase of future life, tensile strength, and impact toughness compared to the SMAW-PWHT repairs. The GTAW-PWHT repairs also produced a superior combination of mechanical properties. Remaining creep rupture lives were a function of the extrapolation procedure and specimen size. These results are described here and discussed in comparison with results previously reported for a less severely degraded condition of the steel in order to delineate the effect of prior degradation on weld repair performance.

Evaluation of Nanocrystalline Coatings for Coal-Fired Ultrasupercritical Boiler Tubes

Cyclic oxidation behavior and microstructural degradation of nanocrystalline Ni-20Cr-xAl (where x=4 wt %, 7 wt %, and 10 wt %) coatings have been investigated. The coatings were deposited on Haynes 230 samples using a magnetron sputtering technique. Cyclic oxidation tests were conducted on the uncoated and coated samples at peak temperatures of 750°C and 1010°C for up to 2070 thermal cycles between the peak and room temperatures. The results showed that a dense Al 2 O 3 scale was formed on the external surface of all coatings after exposure at both temperatures. All three coatings showed no evidence of internal oxidation after exposure at 750°C. Among the three coatings, only the coating containing 4 wt % Al showed evidence of internal oxidation along the columnar grain boundaries after exposure at 1010°C. The Al 2 O 3 scale exhibited good spallation resistance during cyclic oxidation tests at both temperatures. As the Al content in the coating increased from 4 wt % to 7 wt % or 10 wt %, thermal exposure led to precipitation of coarse Al-rich particles at the coating/substrate interface. In addition, thermal exposure at both temperatures led to rapid depletion of Al in the coating and grain coarsening of the coatings. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are attributed to the ultrafine grain structure of the coating and oxide scale.

Corrosion-Fatigue Prediction Methodology for 12% Cr Steam Turbine Blades

The useful life of a steam turbine and the establishment of turbine outage schedules are often determined by corrosion fatigue to the low pressure (LP) blades in the phase transition zone (PTZ). Developing an effective corrosion damage prediction methodology is an important step to successfully reduce the number of unscheduled steam turbine outages.Tests with dual certified 403/410 12% Cr martensitic steel were performed to quantify the influence of corrosion pits on the fatigue life during testing in environments that are comparable to operational conditions. Threshold stress intensity factors ΔKth and fatigue limits Δσ0 were determined in air and two aqueous solutions. Additionally, stress-life tests were performed with pre-pitted specimens in air and aqueous solutions.The data for transition from a pit-to-a-crack have been correlated using the Kitagawa Diagram. This presentation of the data relates the steady stress, cyclic stress and pit width to the prediction of fatigue failure. Ultrasonic fatigue testing was an essential aspect of this program. This testing technique makes it possible to accumulate cycles at a rate of approximately 20 kHz. At this rate one billion (109) cycles are accumulated in less than 14 hours. One billion cycles has been used as the definition for non-progressive crack or specimen run-out life. All of the data for the survival and failure stress intensity factor was well represented by the El Haddad refinement to the Kitagawa Diagram.Based on these test results a comprehensive methodology has been developed to quantify the risk of corrosion-fatigue failure at a pit.Copyright

Oxidation Behavior of Sputter Deposited Nanocrystalline and Conventional Plasma Sprayed MCrAl(Y) Coatings

The MCrAl-type coatings with, or without, yttrium are widely used for oxidation and/or hot corrosion protection of hot section components of gas turbine engines. Recently, there has been great interest in developing nano or microcrystalline coatings since these coatings offer excellent oxidation and corrosion resistance compared to the conventional coatings. Cyclic oxidation and microstructral degradadation behavior of sputter deposited nanocrystalline Ni-20Cr-10Al coating has been investigated at 1010°C. The coating was deposited on Haynes 230 samples using a magnetron sputtering technique. This technique produced a coating with a grain size of ∼9 nm. The cyclic oxidation results showed that the sputter deposited Ni-20Cr-10Al coating exhibited better oxidation resistance in terms of weight loss kinetics compared to the conventional plasma-sprayed NiCoCrAlY and PWA 286 coatings. The Al content in the nanocrystalline coating was consumed in a relatively short time due to inward and outward diffusion of Al. The accelerated consumption of Al was presumably due to enhanced grain boundary diffusion resulting from the ultra-fine grain structure in the coating. Variation of oxide-scale spallation resistance during thermal cycling, internal oxidation of the coatings, and the rate of Al consumption due to inward and outward diffusion of Al between the nanocrystalline and plasma sprayed coatings is presented.Copyright

Nano-Technology Coatings for Erosion Protection of Turbine Components

Solid particle erosion (SPE) and liquid droplet erosion (LDE) cause severe damage to turbine components and lead to premature failures, business loss and repair costs to power plant owners and operators. Under a program funded by the Electric Power Research Institute (EPRI), TurboMet International (TMET) and Southwest Research Institute (SWRI) have developed hard erosion resistant nano-coatings and conducted evaluation tests. These coatings are targeted for application in steam and gas turbines to mitigate the adverse effects of SPE and LPE on rotating blades and stationary vanes. Based on a thorough study of the available information, most promising coatings such as nano-structured titanium silicon carbo-nitride (TiSiCN), titanium nitride (TiN) and multilayered nano coatings were selected. State-of-the-art nano-technology coating facilities at SwRI were used to develop the coatings. Plasma enhanced magnetron sputtering (PEMS) method was used to apply these coatings on various substrates. Ti-6Al-4V, 12Cr, 17-4PH, and Custom 450 stainless steel substrates were selected based on the current alloys used in gas turbine compressors and steam turbine blades and vanes. Coatings with up to 30 micron thickness have been deposited on small test coupons. Initial screening tests on coated coupons by solid particle erosion testing indicate that these coatings have excellent erosion resistance by a factor of 20 over the bare substrate. Properties of the coating such as modulus, hardness, microstructural conditions including the interface, and bond strength were determined. Tests are in progress to determine the effects of coatings on the tensile and high-cycle fatigue strengths of these alloys.Copyright

Erosion Resistant Nano Technology Coatings for Gas Turbine Components

Solid particle and liquid particle erosion in the compressor section of gas turbines and steam turbine vanes and blades lead to significant reduction in turbine efficiency over time. This results in increased downtime and operating cost of the power plants. Some of the conventional coatings and erosion protection shields used by the currently available commercial processes have limitations in their temperature and erosion protection capabilities. Under a project funded by the Electric Power Research Institute (EPRI), nano coatings with thickness within 40 microns (about 1.5 mils) have been produced on test samples using a state-of-the-art Plasma Enhanced Magnetron Sputtering (PEMS) technique. Five coatings were selected for the initial screening tests. Titanium silicon carbonitride nano-composite (TiSiCN), stellite and modified stellite, chromium carbide and Ti-TiN nano layered coatings are being studies in this project. The substrate selection is based on some of the alloys currently used in aeroderivative engine compressor blades, land based gas turbine compressor blades and steam turbine blades and vanes. They include titanium alloys and stainless steels. The PEMS coating technique differs significantly from the conventional techniques such as air plasma spray (APS), low-pressure plasma spray (LPPS), diffusion coatings, chemical or physical vapor deposition (CVD or PVD) used on blades and vanes. PEMS method involves a magnetron sputtering process using a vacuum chamber with an independently generated plasma source from which high current density can be obtained. This method used heavy ion bombardment prior to and during deposition to increase the coating adhesion and limit columnar growth in the coatings. Single-layered thick nitrides coatings up to about 80μm and thick carbonitride coatings of TiSiCN about 30μm have been obtained by this process. A novel method using trimethylsilane gas instead of solid targets was successful in producing this nanocomposite. Initial tests conducted on some of the coated titanium alloy samples produced thus far show significant improvement in the erosion resistance in laboratory sand erosion tests. It was observed that TiSiCN exhibited the best low-angle erosion resistance — nearly 25 times higher than the uncoated Ti-6Al-4V and about 5–10 times higher than all other nitrides. This paper covers a brief description of the deposition technology and the properties of the coatings. Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS), and X-Ray diffraction (XRD) analysis were used to study the microstructure and morphology of these coatings. Nanoindentation was conducted to determine the hardness and Young’s modulus, while sand erosion tests were conducted to rank the erosion resistance of the coatings produced using several processing variables.Copyright