Paul D. Jablonski

Homogenizing Advanced Alloys: Thermodynamic and Kinetic Simulations Followed by Experimental Results

Segregation of solute elements occurs in nearly all metal alloys during solidification. The resultant elemental partitioning can severely degrade as-cast material properties and lead to difficulties during post-processing (e.g., hot shorts and incipient melting). Many cast articles are subjected to a homogenization heat treatment in order to minimize segregation and improve their performance. Traditionally, homogenization heat treatments are based upon past practice or time-consuming trial and error experiments. Through the use of thermodynamic and kinetic modeling software, NETL has designed a systematic method to optimize homogenization heat treatments. Use of the method allows engineers and researchers to homogenize casting chemistries to levels appropriate for a given application. The method also allows for the adjustment of heat treatment schedules to fit limitations on in-house equipment (capability, reliability, etc.) while maintaining clear numeric targets for segregation reduction. In this approach, the Scheil module within Thermo-Calc is used to predict the as-cast segregation present within an alloy, and then diffusion controlled transformations is used to model homogenization kinetics as a function of time and temperature. Examples of computationally designed heat treatments and verification of their effects on segregation and properties of real castings are presented.

On the Relation Between Oxide Ridge Evolution and Alloy Surface Grain Boundary Disorientation in Fe-22 wt % Cr Alloys

cNational Energy Technology Laboratory, Albany, Oregon 97321-2198, USA Oxide ridges formed during the transient stage oxidation of the scale evolution in iron alloys containing 22 wt % Cr that were held at 800°C in dry air. The surface oxidation process was imaged in situ through a confocal scanning laser microscope, and the results were correlated with postexperiment characterization through scanning electron microscopy and the DualBeam system focus ion beam and electron beam analysis combined with three-dimensional reconstruction. The oxide ridges that formed on top of the Cr oxide scale overlapped the intersections of the underlying alloy grain boundaries with the Cr oxide scale. Ridges were generally very small on grain boundaries with disorientation angles of less than 15°, and it was suggested that the boundaries of the surface grains in the alloy may serve as bottlenecks for the transport of scale-forming elements. The effects of La 120 and 290 ppm and Ce 270 and 610 ppm additions during melt-stage processing were also investigated.

Fundamental Studies on the Transient Stages of Scale Growth in Fe-22 wt.% Cr Alloys

It is known that additions of reactive elements such as Ce, La or Y improve the properties of protective oxide-scales on Ni and Fe based alloys [ - ] by increasing oxide adhesion, decreasing the transient time until a continuous Cr2O3 layer is formed and decreasing the parabolic rate constant. Nevertheless, the precise roles played by these reactive elements to improve scales and the precise mechanisms by which they are incorporated into the scale during the surface treatment processes are unknown. Although they are believed to be associated with transport properties in the scale, it is not clear how this occurs or why it improves oxidation resistance. This project is aimed to gain understanding of the scale evolution in Fe-22 wt.% Cr alloys at 800 oC in dry air during the transient stage after 15 minutes of oxidation. The effect of La (120 and 290 ppm) and Ce (270 and 610 ppm) additions added during melt-stage processing are investigated. The surface oxidation process was imaged in-situ through a Confocal Scanning Laser Microscope (CSLM) and the results were correlated with post-experiment characterization through FEG-SEM and FIB-SEM combined with 3D reconstruction. The roles of rare-earth oxide particles on nucleation of Cr2O3 and blockage of short-circuit diffusion paths in the oxide scale and underlying metal are discussed.

Materials Performance of Modified 430 Stainless Steel in Simulated SOFC Stack Environments for Integrated Gasification Fuel Cell System Applications

The corrosion behaviors of a low silicon and aluminum 430 stainless steel with and without ceria surface treatment were investigated in a simulated coal syngas at 800 {degree sign}C and in air. Thermodynamic calculations were made to predict carbon activities for the coal syngas as a function of temperature. At 800 {degree sign}C, carbon activity is ~1.1, which indicates that carbon that forms could diffuse into the steel and induce carbon corrosion, e.g. carburization and metal dusting. The surface morphology was investigated with X-ray diffraction and scanning electron microscopy. In coal gas, the scale formed on bare steel consisted of Mn1.5Cr1.5O4 and Cr2O3 and on ceria treated steel (Fe, Mn)O, FeCr2O4, Cr2O3, and CeCrO3. Both materials underwent carburization, but not metal dusting. The results of oxidation in air using a thermogravimetric apparatus confirmed that the 430 sample was less resistant to oxidation than the 430 treated with ceria.

Mechanical Performance of Various INCONEL® 740/740H Alloy Compositions for Use in A-USC Castings

The use of Ni-based superalloys as structural components of advanced ultra-supercritical steam and CO2 turbines is becoming necessary due to the increasing performance requirements of future power plant designs. Although numerous Ni-based superalloys can respond to the demanding mechanical performance, very few present a combination of high-strength and creep resistance while maintaining good ductility and weldability. Furthermore, the improved performance is often associated with wrought alloys while the fabrication of large castings is the primary target for those applications. With the comparatively lower mechanical properties of cast alloys, new designs based on compositional changes are necessary to allow for the use of Ni-based superalloys in A-USC castings. This lack of performance is primarily associated with the large and inhomogeneous grain size as well as elemental segregation and other casting anomalies. This investigation presents various alloys with compositions within the range of INCONEL® 740 /740H designed to overcome the issues associated with the use of Ni-based superalloys in A-USC castings. Modifications to the chemistry were based on thermodynamic simulations and experimental results. Particular attention was given to reactive element additions, element partitioning, γʹ precipitate phase promoting elements and grain boundary carbides. The effects of the various compositional changes on the tensile and creep properties of INCONEL® 740 /740H will be discussed.

Compositional Design and Mechanical Properties of INCONEL ® Alloy 725 Variants

With a combination of high strength, toughness and resistance to corrosion, INCONEL® alloy 725 has been widely used in marine, aerospace, and land-based power industries. Typically, the alloy presents a conventional precipitate-strengthened γ-γ′/γ″ microstructure when appropriate aging treatments are employed. Although the corrosion resistance of INCONEL® alloy 725 is significant, its use is limited to relatively low temperatures when compared to other γ′ precipitate-strengthened Ni-based superalloys. This can limit the use of the alloy as turbine engine components or in other power-generation applications since future engine designs suggest increases in the operating temperature. This investigation aims at modifying the composition of the alloy to assess its high-temperature mechanical properties. Variations to the Ti/Al ratio were considered with respect to the precipitate phases stability as well as additions of Ta and Nb. Thermodynamic and kinetic predictions, such as phase fraction/stability and time–temperature–transformation diagrams, were used to help in the design process and were validated experimentally. The various compositions and relative aging treatments investigated produced microstructures differing in grain boundary phases and γ′ precipitate sizes and fractions. Tensile and creep testing were performed and the effect of the various compositions and microstructures on the mechanical performance of the modified INCONEL® 725 alloys was examined.

Melt Parameters and Resulting Characteristics in Laboratory-Scale Electroslag Remelting

Vacuum induction melting (VIM) and electroslag remelting (ESR) are techniques used to produce ingots of alloys with complex chemistries while lowering the amount of defects, inclusions or extent of elemental segregation. Those practices are widely employed in aerospace applications and more recently in fossil-fueled power plant components due to the increasingly demanding operating conditions. Consequently, research is ongoing to improve and control the melting of commercially available alloys for optimal performance in service. In this investigation, a laboratory-scale (200 mm–200 kg) ESR furnace was used to remelt various alloys with a focus on the ingot quality. Several approaches were considered to study and improve the melting characteristics. Targeted additions of minor elements in master alloys were found to improve the melt range which affected the melt pool volume and subsequently increased the remelting efficiency. Furthermore, the melt parameters during ESR of some select alloys were modified to improve the melting characteristics. Finally, the influence of the size of the ESR electrode was observed and provided a better understanding of the mixing mechanisms in the slag region and their effect on the voltage swing and melt rate. The results are presented using a combination of experimental and computational (thermodynamic and CFD-based) data.

Thermal Processing Design of Cast INCONEL® Alloy 740H for Improved Mechanical Performance

The increasing operating temperatures of turbine designs for power generation applications present challenges in selecting materials. With steam temperatures approaching 700 °C, and beyond, ferritic or martensitic steels are inadequate. Ni-based superalloys perform much better under these conditions and possess mechanical properties suitable for turbine and boiler applications. However, only limited Ni-based superalloys are available for large turbine castings where a combination of high-temperature strength (in particular, yield stress), long-term creep resistance, toughness, and weldability are essential properties. Research was undertaken at the National Energy Technology Laboratory to produce a cast INCONEL® 740H alloy with mechanical properties comparable to those of the wrought product. Since thermo-mechanical processing is not an option in castings, aging trials were performed to alter the grain boundary morphology after a computationally designed homogenization heat treatment. Thermodynamic simulations were used throughout to produce various grain boundary microstructures. Early investigations on cast INCONEL® 740/740H revealed less than desirable ductility. The modified microstructures altered the tensile and creep properties, and reinforced the possible use of cast INCONEL® 740H in power plants. The benefits associated with the alternate heat treatments were found to originate from the grain boundary phases present in the alloy controlled through the targeted aging treatments. Those results and the relations between thermal processing, microstructure, mechanical properties and failure mechanisms of cast INCONEL® 740H will be discussed.

Formation of β-NiAl Phase During Casting of a Ni-Based Superalloy

A high-refractory Ni-based superalloy prototype was melted on a research scale while simulating industry practices. Ingots were vacuum induction melted and subjected to a computationally optimized homogenization heat treatment prior to fabrication which consisted of forging and hot rolling. Failure of one of the ingots at the early stage of the forging process was attributed to the precipitation of the β-NiAl phase during melting which stabilized the eutectic constituent.

Effect of Molybdenum on the Corrosion Behavior of High-Entropy Alloys CoCrFeNi2 and CoCrFeNi2Mo0.25 under Sodium Chloride Aqueous Conditions

The corrosion behavior of high-entropy alloys (HEAs) CoCrFeNi2 and CoCrFeNi2Mo0.25 was investigated in 3.5 wt. percent sodium chloride (NaCl) at 25°C by electrochemical methods. Their corrosion parameters were compared to those of HASTELLOY® C-276 (UNS N10276) and stainless steel 316L (UNS 31600) to assess the suitability of HEAs for potential industrial applications in NaCl simulating seawater type environments. The corrosion rates were calculated using corrosion current determined from electrochemical experiments for each of the alloys. In addition, potentiodynamic polarization measurements can indicate active, passive, and transpassive behavior of the metal as well as potential susceptibility to pitting corrosion. Cyclic voltammetry (CV) can confirm the alloy susceptibility to pitting corrosion. Electrochemical impedance spectroscopy (EIS) elucidates the corrosion mechanism under studied conditions. The results of the electrochemical experiments and scanning electron microscopy (SEM) analyses of the corroded surfaces revealed general corrosion on alloy CoCrFeNi2Mo0.25 and HASTELLOY C-276 and pitting corrosion on alloy CoCrFeNi2 and stainless steel 316L.