J. N. DuPont

Fabrication of functionally graded TiC/Ti composites by Laser Engineered Net Shaping

Abstract Crack-free functionally graded TiC/Ti composite materials were fabricated by laser engineered net shaping (LENS), with compositions changing from pure Ti to approximately 95 vol% TiC. By delivering the constituent materials from different powder feeders and through process control, the LENS process can be used for the fabrication of functionally graded materials.

Solidification of an alloy 625 weld overlay

The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy 625 weld overlay deposited on 2.25Cr - 1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy, electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification at ≈ 1216 °C by aγ/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification temperature range (≈170 °C) and formation of interdendritic (γ+Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent (γ+Laves) constituent with the Scheil equation by treating the overlay system as a simpleγ-Nb “binary” and using an experimentally determinedkNb value from electron microprobe data.

Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Abstract Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information was used to determine the partition coefficients k of the elements in each dissimilar weld. The dilution level was found to decrease as the ratio of volumetric filler metal feedrate to net arc power increased. Reasons for this behaviour are discussed in terms of the distribution of power required to melt the filler metal and base metal. In addition, the segregation potential of Mo and Nb was observed to increase (i.e. their k values decreased) as the Fe content of the weld increased. This effect is attributed to the decreased solubility of Mo and Nb in austenite with increasing Fe additions. Since the Fe content of the weld is controlled by dilution, which in turn is controlled by the welding parameters, the welding parameters have an indirect influence on the segregation potential of Mo and Nb. The results of the present work provide practical insight for corrosion control of welds in super austenitic stainless steels.

Modeling solute redistribution and microstructural development in fusion welds of Nb-bearing superalloys

Abstract Solute redistribution and microstructural evolution have been modeled for gas tungsten arc fusion welds in experimental Ni base and Fe base Nb-bearing superalloys. The multi-component alloys were modeled as a ternary system by grouping the matrix (Fe, Ni, Cr) elements together as the “solvent” to form the γ component of the γ-Nb–C “ternary system”. The variation in fraction liquid and liquid composition during the primary L→γ and eutectic type L→(γ+NbC) stages of solidification were calculated for conditions of negligible Nb diffusion and infinitely fast C diffusion in the solid phase. The proposed model is based on modifications of solute redistribution equations originally developed by Mehrabian and Flemings. Results of the calculations were superimposed on the pseudo-ternary γ-Nb–C solidification surfaces to predict the solidification reaction sequences along with the total and individual amounts of γ/NbC and γ/Laves eutectic-type constituents which form during solidification. Comparison was made to experimentally measured values, and reasonable agreement was found. The model results permit quantitative interpretations of composition–microstructure relations in these Nb-bearing experimental alloys and should provide useful insight into comparable commercial alloy systems as well.

The effect of second phase volume fraction on the erosion resistance of metal-matrix composites

Abstract Metal-matrix composites that consist of a ductile metal-matrix and hard ceramic particles are often used as materials of choice for protection against solid particle erosion. In this study, a model Ni–Al 2 O 3 system was chosen to analyze the effect of hard second phase particles on erosion resistance. This system consists of hard Al 2 O 3 particles dispersed within a ductile Ni matrix. Two processing techniques were used to fabricate the Ni–Al 2 O 3 composites. First, a hot isostatic pressing (HIP) technique was used to produce bulk Ni–Al 2 O 3 alloys. These composite samples contained 0–45 vol.% of Al 2 O 3 with an average particle size of 12 μm. Second, an electrodeposition technique was developed and Ni–Al 2 O 3 coatings with various volume fractions of Al 2 O 3 (0–39 vol.%) were produced on a pure Ni substrate. In contrast to the bulk powder composites, the electrodeposited composites contained much smaller Al 2 O 3 particles (≈1 μm). Erosion testing was conducted at impact angle of 90° using angular alumina. It was found that for both type of composites, an increase in Al 2 O 3 content led to an increase in erosion rate of the composites and pure Ni showed the best erosion resistance. However, the electrodeposited Ni–Al 2 O 3 alloys exhibited better erosion resistance than the powder processed Ni–Al 2 O 3 alloys. For erosion test conditions used, the smaller Al 2 O 3 particles in the Ni matrix were more beneficial in terms of erosion resistance than the large Al 2 O 3 particles.

Dilution in single pass arc welds

A study was conducted on dilution of single pass arc welds of type 308 stainless steel filler metal deposited onto A36 carbon steel by the plasma arc welding (PAW), gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and submerged are welding (SAW) processes. Knowledge of the arc and melting efficiency was used in a simple energy balance to develop an expression for dilution as a function of welding variables and thermophysical properties of the filler metal and substrate. Comparison of calculated and experimentally determined dilution values shows the approach provides reasonable predictions of dilution when the melting efficiency can be accurately predicted. The conditions under which such accuracy is obtained are discussed. A diagram is developed from the dilution equation which readily reveals the effect of processing parameters on dilution to aid in parameter optimization.

Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

Abstract Dissimilar metal welds between ferritic and austenitic alloys are used extensively in power plants. Premature failure of such welds can occur below the expected creep life of either base metal. This article reviews microstructural evolution in dissimilar welds and describes factors that contribute to premature failure. The microstructure in the as welded condition consists of a sharp chemical concentration gradient across the fusion line. Martensite forms within this gradient due to high hardenability and rapid cooling rates from welding. Upon aging, carbon diffuses down the chemical potential gradient from the ferritic steel toward the austenitic alloy. This can lead to formation of a soft carbon denuded zone in the ferritic steel, and nucleation and growth of carbides in the austenitic steel that produce high hardness. These differences in microstructure and hardness occur over distances of about 50–100 μm. Failure is attributed to the steep microstructural and mechanical property gradients, the large difference in coefficient of thermal expansion, formation of interfacial carbides that promote creep cavity formation, and preferential oxidation of the ferritic steel. Information is also provided on available creep rupture properties, remaining life estimation techniques, current best practices and research in progress.

Weld overlay coatings for erosion control

Abstract Research was conducted to develop a criterion for selecting weld overlay coatings for erosion mitigation in circulated fluidized beds (CFBs). Initially, 11 weld overlay alloys were deposited on 1018 steel substrates using the plasma arc welding process and erosion tested at 400°C. Erosion resistance was evaluated by determining the steady state erosion rate, and the microstructure of each coating was characterized before erosion testing. The steady state erosion rates for several weld overlay coatings (Ultimet, Inconel-625 and 316L SS) were considerably lower than the other coatings evaluated (Armacor-M, B-60, TS-2, Stellite-6, Hastelloy-22, high Cr iron and 420 SS). No correlations were found between the room temperature hardness of the weld overlay coatings and their erosion resistance at elevated temperature. Microhardness tests were performed on the eroded samples below the erosion surface in order to determine the size of the plastically deformed zone, and it was found that some coatings deformed plastically as a result of erosion while others did not. Possible erosion mechanisms for these groups of coatings were analyzed.

The effect of water vapor on passive-layer stability and corrosion behavior of Fe-Al-Cr base alloys

Although it has been reported that additions of water vapor to high-temperature corrosion environments accelerate corrosion, the role of water vapor on increasing the corrosion rate of Fe–Al–Cr alloys and coatings is not very well understood. In order to better understand these effects, multiple Fe–Al–Cr base alloys were tested at 500°C for 100 hr in three different multi-component corrosive gases with and without water vapor. The three gases were a sulfidizing gas, a mixed oxidizing/sulfidizing gas, and an oxidizing gas. Thermogravimetric testing showed that the corrosion kinetics increased when water vapor up to 6% was added to the atmospheres. The surfaces of the exposed samples were considered carefully to determine if the addition of water vapor changed the morphology of the corrosion products and, more importantly, affected the passive layer. It has previously been shown that the formation of nodules can be caused by the inability of the passive layer to re-heal itself after the presence of a defect allowing fast growing non-protective corrosion products to externally grow from the surface. In this study, the amount of external nodules that formed on the surface of the alloys was shown to increase with the addition of water vapor. An increase in the amount of external nodules present due to additions of water vapor gives an indication that water vapor accelerates the passive-layer breakdown. It was observed that the scale morphology correlated well with the corrosion kinetics for the exposed alloys, which showed that water vapor increased the corrosion rates of the alloys.

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.