John A. Siefert

Welding and weldability of candidate ferritic alloys for future advanced ultrasupercritical fossil power plants

Abstract Fossil fuels continue to be the primary source of energy in the world. The worldwide demand for clean and affordable energy will continue to grow, and a strong emphasis has been placed on increasing the efficiency and reducing the carbon footprint of new and existing fossil fired power plants. Throughout Asia, Europe and the USA, this demand is being met with programmes to develop advanced materials that have enhanced high temperature creep and corrosion properties. A new class of ferritic alloys, known as creep strength enhanced ferritic steels, has been developed to meet these requirements. This article focuses on the weldability of the advanced ferritic alloys used in boilers and boiler components of ultrasupercritical coal fired power plants. This review focuses on alloy selection; welding and weldability issues, including in service weld failure such as type IV cracking; welding of dissimilar metals; and weld repair. Future articles will address the welding and weldability issues of two other classes of materials, namely austenitic stainless steels and nickel base superalloys.

Weldability and weld performance of candidate nickel base superalloys for advanced ultrasupercritical fossil power plants part I: fundamentals

Abstract Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45% higher heating value (HHV) through an increase in steam temperature (700 to 760°C) and pressure (27.6 to 34.5 MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. In order to accomplish this, three classes of materials have been identified: creep strength enhanced ferritic steels, austenitic stainless steels and nickel base superalloys. Although new alloys have been designed and developed to meet this need, welding can have a significant and often detrimental effect on the required mechanical and corrosion resistant properties. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel base alloys will be discussed in a two part paper. In this paper, the primary focus will be on the fundamentals of welding and weldability of Ni base superalloys.

Weldability and weld performance of candidate austenitic alloys for advanced ultrasupercritical fossil power plants

Abstract Advanced ultrasupercritical steam conditions of up to 760°C and 34·5 MPa have been investigated in various programmes around the world over the last two decades. To date, much progress has been made, and three candidate materials, namely ferritic, austenitic and nickel base superalloys, have been investigated for high temperature strength, corrosion resistance and weldability. In an earlier published paper, welding and weldability of ferritic alloys were discussed. This paper considers the unique weldability characteristics for utilisation of austenitic stainless steels in future advanced ultrasupercritical fossil power plant designs and covers topics such as fundamentals of austenitic stainless steel welds, including weldability, filler metals and dissimilar metal welds, and discusses the prognosis for this class of materials for advanced ultrasupercritical fossil fired power plants.

Weldability and weld performance of candidate nickel based superalloys for advanced ultrasupercritical fossil power plants Part II: weldability and cross-weld creep performance

Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45%HHV (higher heating value) through an increase in steam temperature (700–760°C) and pressure (27.6–34.5 MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel based alloys will be discussed in a two-part paper. In this paper, the primary focus will be on the behaviour of candidate nickel based alloys that are being proposed in advanced ultrasupercritical power plants and with regard to weldability (Part I) and cross-weld creep performance (Part II).

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.

Microstructural Evolution and Creep-Rupture Behavior of A-USC Alloy Fusion Welds

Characterization of the microstructural evolution of fusion welds in alloys slated for use in advanced ultrasupercritical (A-USC) boilers during creep has been performed. Creep-rupture specimens involving INCONEL® 740, NIMONIC® 263 (INCONEL and NIMONIC are registered trademarks of Special Metals Corporation), and Haynes® 282® (Haynes and 282 are registered trademarks of Haynes International) have been analyzed via light optical microscopy, scanning electron microscopy, X-ray diffraction, and thermodynamic and kinetic modeling. Focus has been given to the microstructures that develop along the grain boundaries in these alloys during creep at temperatures relevant to the A-USC process cycle, and particular attention has been paid to any evidence of the formation of local γ′-denuded or γ′-free zones. This work has been performed in an effort to understand the microstructural changes that lead to a weld strength reduction factor (WSRF) in these alloys as compared to solution annealed and aged alloy 740 base metal. γ′ precipitate-free zones have been identified in alloy 740 base metal, solution annealed alloy 740 weld metal, and alloy 263 weld metal after creep. Their development during long-term thermal exposure is correlated with the stabilization of phases that are rich in γ′-forming elements (e.g., η and G) and is suppressed by precipitation of phases that do not contain the γ′ formers (e.g., M23C6 and μ). The location of failure and creep performance in terms of rupture life and WSRF for each welded joint is presented and discussed.

Evaluation of options for weld repair of Grade 91 piping and components: metallographic characterisation

Abstract Creep strength enhanced ferritic (CSEF) steels such as Grade 91 are the preferred materials for many of the high temperature boiler tubing and piping components used in modern power generating plants. Validation of temperbead welding techniques is particularly important for Grade 91 steel as accurate post-weld heat treatment (PWHT) has proven to be difficult in field applications. Even a well-controlled PWHT can degrade the base material strength, and a poorly performed heat treatment can lead to subsequent creep properties being below the minimum expected by codes. Successful demonstration of temperbead techniques for Grade 91 steel would alleviate these concerns and create additional options in designing a ‘well-engineered’ repair. This article presents the results from a project evaluating options for field repair of Grade 91 components. Metallographic characterisation of the welds produced is described with future publications providing results from cross-weld creep testing and post-test examination.

Optimization of Vickers Hardness Parameters for Micro- And Macro-Indentation of Grade 91 Steel

Hardness is being assessed as a potential life-assessment tool for tracking microstructural degradation and remaining life in creep strength enhanced ferritic (CSEF) steels. Such methodology is already being utilized for the CSEF steel Grade 91, which has been widely implemented in both replacement parts and new construction over the last two decades. Additionally, research into the complex microstructural features in welded joints often utilizes hardness surveys to characterize changes caused by thermal processing. New automated hardness testing equipment now affords the ability to develop statistically relevant datasets for these uses. However, proper application and understanding of the data requires knowledge of limitations and variations with the load and spacing of the hardness indents and the potential role of different material conditions in this complex steel. In this work, we have examined the effect of load in three Grade 91 material conditions, as well as the effect of spacing in as-received material to determine optimum parameters for both macrohardness (i.e., >1 kg) and microhardness mapping (i.e.,

Microstructural Characterization of the Heat-Affected Zones in Grade 92 Steel Welds: Double-Pass and Multipass Welds

The microstructure in the heat-affected zone (HAZ) of multipass welds typical of those used in power plants and made from 9 wt pct chromium martensitic Grade 92 steel is complex. Therefore, there is a need for systematic microstructural investigations to define the different regions of the microstructure across the HAZ of Grade 92 steel welds manufactured using the traditional arc welding processes in order to understand possible failure mechanisms after long-term service. In this study, the microstructure in the HAZ of an as-fabricated two-pass bead-on-plate weld on a parent metal of Grade 92 steel has been systematically investigated and compared to a complex, multipass thick section weldment using an extensive range of electron and ion-microscopy-based techniques. A dilatometer has been used to apply controlled thermal cycles to simulate the microstructures in distinctly different regions in a multipass HAZ using sequential thermal cycles. A wide range of microstructural properties in the simulated materials were characterized and compared with the experimental observations from the weld HAZ. It has been found that the microstructure in the HAZ can be categorized by a combination of sequential thermal cycles experienced by the different zones within the complex weld metal, using the terminology developed for these regions based on a simpler, single-pass bead-on-plate weld, categorized as complete transformation, partial transformation, and overtempered.

Weldability and weld performance of candidate nickel based superalloys for advanced ultrasupercritical fossil power plants

Fossil fuel will continue to be the major source of energy for the foreseeable future. To meet the demand for clean and affordable energy, an increase in the operating efficiency of fossil fired power plants is necessary. There are several initiatives worldwide to achieve efficiencies >45%HHV through an increase in steam temperature (700–760°C) and pressure (27.6–34.5 MPa). Realising this goal requires materials with excellent creep rupture properties and corrosion resistance at elevated temperatures. Two previous papers addressed the welding and weldability of ferritic and austenitic stainless steels. Welding and weldability of nickel based alloys will be discussed in a two-part paper. In this paper, the primary focus will be on the behaviour of candidate nickel based alloys that are being proposed in advanced ultrasupercritical power plants and with regard to weldability (Part I) and cross-weld creep performance (Part II).