Showing papers by "Arnold R. Marder published in 1999"

Modeling solid-particle erosion of ductile alloys

Abstract: A model for solid-particle erosion behavior of ductile alloys is presented. The model incorporates the mechanical properties of the alloys at the deformation conditions associated with solid-particle erosion processes (i.e., temperature and strain-rate conditions), as well as the evolution of these properties during the erosion-induced deformations. An erosion parameter was formulated based on consideration of the energy loss during erosion and incorporates the material’s hardness, along with the high-strain-rate stress-strain response of the alloy. This erosion parameter shows a good correlation to experimentally measured erosion rates for a variety of industrially important materials. A methodology is presented to determine the proper mechanical properties to incorporate into the erosion parameter based on a physical model of the erosion mechanism in ductile materials.

Characterization of single and discretely-stepped electro-composite coatings of nickel-alumina

Abstract: Electrodeposition from a sulfamate bath has been used to produce single layer and discretely stepped electro-composites consisting of a metallic nickel matrix with second phase alumina (α-Al2O3) particles. Light optical microscopy (LOM), scanning electron microscopy (SEM), quantitative image analysis (QIA), and micro-indentation techniques were used to characterize the deposits. As previously seen, an increase in bath particle loading and decrease in plating current density increased the volume percent of alumina incorporated into the coating, with a maximum of 40 vol % being attained. For samples deposited above 1 A/dm2, a direct relationship between the alumina volume percent and coating hardness was seen due in part to the related decrease in interparticle spacing (IPS) at the higher vol %. However, the strengthening mechanism of the electro-composites may be more complex with both the metallic nickel grain structure and IPS being factors, as seen for samples deposited at 0.5 A/dm2. The incorporation of alumina into the electrodeposited nickel was also observed to affect the as-plated surface structure of the coating. Due to the particles inhibiting the formation of pyramidal features found on the surface of pure nickel electrodeposits, the electro-composite surfaces were observed to be relatively flat. Also, structure within the metallic nickel matrix appeared due to rapid growth of the nickel coating around the inert particles when plated at high current densities. In addition, discretely layered functionally graded materials were produced without alterations to the original deposition procedure of the single layer deposits. It was found that the various processing stages needed to produce the stepped coatings did not affect the structure or properties of the individual layers, when compared to that of the corresponding single-layered electro-composites.

Modelling mushy zones in welds of multicomponent alloys : implications for solidification cracking

Abstract: A method is proposed for calculating the variation in fraction liquid with distance in the mushy zone as an aid to determining the effect of alloy additions on solidification cracking susceptibility. The model combines the general liquidus equation of a multicomponent system, solute redistribution relations, and temperature gradient information. Calculations are presented for a range of niobium bearing superalloys and the results were found to reveal important relations between alloy composition, variation in fraction liquid with distance in the mushy zone, and cracking susceptibility as measured by the Varestraint test. In particular, the results directly show that the addition of carbon to these alloys is generally beneficial because it reduces the size of the crack susceptible mushy zone and limits the amount of terminal liquid available for the low temperature L → γ + Laves reaction. The modelling results and experimental data are also used to describe the influence of other alloy additions.

Corrosion behavior of weldable Fe–Al alloys in oxidizing–sulfidizing environments

Abstract: The objective of the present study was to investigate the corrosion behavior of weldable Fe–Al alloys in environments representative of low NOx gas compositions, i.e., high partial pressures of sulfur [p(S2)] and low partial pressures of oxygen [p(O2)]. Using thermogravimetric techniques, binary alloys with 0–12.5 wt% Al were exposed in oxidizing–sulfidizing environments [p(S2) = 10–4 atm and p(O2) = 10–25 atm] at 500–700°C for various times up to 100 h. Post-exposure characterization consisted of surface and cross-sectional microscopy in combination with energy dispersive spectroscopy and/or electron probe microanalysis. It was found that the Fe–Al alloys exhibited three different stages of corrosion behavior: inhibition, breakdown, and steady-state. Observance and/or duration of these stages was directly related to the aluminum content of the alloy. The inhibition stage was characterized by growth of a thin, gamma alumina scale that suppressed rapid degradation of the underlying substrate for al...

The role of aluminum on the weldability and sulfidation behavior of iron-aluminium cladding

Abstract: Single-pass welds and multiple-pass cladding of Fe-Al alloys were deposited on carbon steel substrates using the gas tungsten arc and gas metal arc welding processes. The effect of alloy composition on cold cracking susceptibility was assessed using a dye penetrant technique. The high-temperature (600 C) sulfidation behavior of low-Al alloys (5--10 wt.% Al), which exhibited good weldability, was examined using a thermogravimetric balance in a moderately reducing 0.1% H{sub 2}S--3.0% H{sub 2}-bal. Ar gas. Microstructural characterization was conducted by light optical microscopy, scanning electron microscopy, electron probe microanalysis and Knoop hardness testing. By varying the welding parameters, a range of dilution levels was achieved that resulted in fusion zone compositions with 3--30 wt.% Al. Under these processing conditions, cracking of the Fe-Al cladding is directly related to the aluminum concentration within the deposit. Cracking of the cladding was seen to have occurred by both inter- and transgranular modes for deposits containing greater than 10 wt.% Al. Below this composition limit, cracking did not occur. According to the Fe-Al phase diagram, this composition indicates the microstructural transition from the single-phase region of disordered solid solution ({alpha}) to the two-phase region of {alpha} + ordered Fe{sub 3}Al. The ordered structures of Fe{submore » 3}Al and FeAl have been found to be more susceptible to environmental embrittlement, which has been seen to be the source of the cladding cracking problem, in terms of sulfidation behavior, alloys containing 5--10 wt.% Al exhibited corrosion rates significantly lower than carbon steel and 304 stainless steel. The results of this work indicate that Fe-Al cladding containing 5--10 wt.% Al shows potential promise for applications requiring a combination of weldability and sulfidation resistance in moderately reducing environments.« less

Investigation of Iron Aluminide Weld Overlays

Abstract: Conventional fossil fired boilers have been retrofitted with low NO(sub)x burners in order for the power plants to comply with new clean air regulations. Due to the operating characteristics of these burners, boiler tube sulfidation corrosion typically has been enhanced resulting in premature tube failure. To protect the existing panels from accelerated attack, weld overlay coatings are typically being applied. By depositing an alloy that offers better corrosion resistance than the underlying tube material, the wastage rates can be reduced. While Ni-based and stainless steel compositions are presently providing protection, they are expensive and susceptible to failure via corrosion-fatigue due to microsegregation upon solidification. Another material system presently under consideration for use as a coating in the oxidation/sulfidation environments is iron-aluminum. These alloys are relatively inexpensive, exhibit little microsegregation, and show excellent corrosion resistance. However, their use is limited due to weldability issues and their lack of corrosion characterization in simulated low NO(sub)x gas compositions. Therefore a program was initiated in 1996 to evaluate the use of iron-aluminum weld overlay coatings for erosion/corrosion protection of boiler tubes in fossil fired boilers with low NO(sub)x burners. Investigated properties included weldability, corrosion behavior, erosion resistance, and erosion-corrosion performance.

Corrosion resistance of weldable Fe-Al alloys in reducing environments

Abstract: Low aluminum Fe-Al alloys are being investigated for use as protective coatings in reducing environments. Intermetallic compositions have already demonstrated excellent high temperature corrosion resistance in both oxidation and sulfidation environments, however, they are limited in use due to low room temperature ductility and poor weldability arising from hydrogen embrittlement. Using Gas Tungsten Arc and Gas Metal Arc Welding process without the use of pre-heat and post weld heat treatments, a weldability regime below approximately 10 wt% Al in the overlay has been found. The high temperature sulfidation behavior of alloys located near this crack/no crack boundary (5 to 12.5 wt% Al) was examined using a thermogravimetric balance in a reducing gas mixture of O.l% H{sub 2}-1.0% H{sub 2}S-balance Ar gas at 700 C. Light optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy were used to analyze the surface and cross-sectional morphologies. It was seen that the Fe-Al alloys exhibited lower weight gains than bare water wall tubes (carbon steel) and 309 stainless steel. Aluminum additions to the iron were found to extrinsically affect the iron sulfide reaction product morphology. Low carbon steel, with no aluminum, formed a continuous, and somewhat dense, iron sulfide scale that provided some protection.more » Small additions of aluminum (5--7.5 wt% Al) resulted in the growth of iron sulfide as nodules or a porous surface scale. Larger additions (above 10 wt%) were found to limit the growth of iron sulfide and significantly reduce corrosion rates. These results indicate that Fe-Al claddings containing 5 to 10 wt% Al show potential promise for applications requiring a combination of weldability and sulfidation resistance in moderately reducing environments.« less