The influence of submergedarc flux composition on the inclusion morphology and weld metal microstructures of low carbon steels is investigated. Systematic weld oxygen variations and changes in inclusion shape and size are obtained by changing the welding flux composition. The influence of inclusion content, morphology, and distribution on achieving specific types of weld metal ferrite is described.
The effect of ferrite content on the creep of a 12Cr-2Mo-0.08C, ferritemartensite, heatresisting steel was investigated. Creep tests were performed at 550, 600, and 650 °C for up to 10,000 hours on four steels containing, respectively, 51, 38, 23, and 12 pct ferrite. The variation in ferrite content was obtained through nickel additions of 0.6, 1.3, and 2.0 pct. Rupture time, minimum creep rate, elongation, and reduction in area are reported. The effect of ferrite content was found to be small but significant. At short rupture times, increasing ferrite content reduces creep strength. At long exposure times, however, increasing ferrite content increases creep strength. This effect is explained by the precipitation of Laves phase, Fe2Mo, and possibly Cr2N, in the ferrite during testing. This precipitation progressively strengthens the ferrite relative to martensite. In addition, martensite is continually weakening because of the coarsening of M23C6, the only precipitate found in the martensite. Nickel at the 2.0 pct level radically decreased creep resistance in these steels by accelerating aging of the precipitate structure in martensite.
Chromized and coextruded superheater tubings were welded and bent to construct the ultrasupercritical (USC) steam generator. Prior to the fabrication of the USC steam generator, welding and formability tests were performed to establish the fabrication procedures. In addition, mechanical and metallurgical investigations were also carried out. Welding of the chromized and coextruded tubings was satisfactorily accomplished and the joint showed sound structures and good strength properties, so that the established procedures were applied to the fabrication of the USC superheater. Bending formability for the chromized and coextruded thick wall tubings was evaluated, and the nocrackbending procedures for chromized tubing were developed.
A “Bread Pan” capsule has been designed which allows large numbers of tensile specimens to be simultaneously exposed to molten LiOH, its vapor, and vacuum. Capsules and specimens fabricated from the pure nickel alloy Ni-200 were annealed for 401 hours and 2500 hours at 775 K. Examination of the exposed materials revealed that little outward damage in terms of visible attack, weight change, or loss of room temperature tensile properties occurred. In particular the mechanical behavior of hydroxide contaminated alloy was essentially identical to that receiving a simple thermal exposure in vacuum. Examination of the microstructures revealed that LiOH did produce some nonuniform, shallow intergranular corrosion in Ni-200; however, the extent of the damage was insufficient to produce weakening or embrittlement.
High temperature oxidation behavior of a Nibase superalloy, PWA1480, developed for use as single crystal blades and vanes in advanced gas turbines has been assessed. Isothermal and cyclic oxidation tests up to 200 hours were conducted at 1323, 1410, and 1473 K. The alloy was evaluated as to sample weight change, type of scale formed, type and amount of spall, and microstructural changes. The oxidation attack was more severe in cyclic as compared with isothermal oxidation. Also, enormous growth of the γ’ precipitate was observed at 1423 K during cyclic oxidation.
Coefficients of hydrogen permeation and diffusion were determined electrochemically at constant current for commercial VP (Very Pure) iron, annealed AISI 1010, 1035, 1050, 1090, and heat treated 1050 steel between 273 and 303 K. Permeationcharge current measurements established that lattice diffusion was controlling transport and a pseudo Sieverts’ Law was applicable for correlation of experimental data. Coefficients of hydrogen permeation and diffusion in annealed steel decrease with increase in iron carbide volume fraction in the order VP iron, 1010, 1035, 1050, and 1090 steel. Coefficients of hydrogen permeation and diffusion decrease with heat treatments which refine the iron carbide distribution in the order spheroidized, annealed, normalized, austempered, and quench/tempered. Corresponding apparent solubility increases and lattice solubility decreases as the carbon content and iron carbide fineness increase.
Two exposures were carried out in the gasifier of the Westinghouse Process Development Unit to evaluate the performance of a wide variety of candidate construction materials for hostile high temperature, high pressure coal gasification environments. The test materials included highly alloyed iron-, cobalt-, and nickelbase alloys as well as packaluminized iron-base alloxs. The corrosion damage was assessed, and the corrosion processes taking place were investigated in order to interpret the data obtained from the surveillance tests. In addition, corrosion rates and mechanisms were compared between the two pilot plant exposures and a laboratory simulation test, to determine if laboratory data can be extrapolated to predict materials life in pilot plant environments. The results showed that although consistent kinetic data cannot be obtained by combining all three tests, materials performance can be evaluated qualitatively. Also, the corrosion mechanisms operating during the pilot plant exposures and the laboratory experiments were found to be similar.
The results of microstructural and tribological studies performed on sputter-deposited coatings of ZrO2·2OY2O3, TiB2 and B-18wt.%Si are reported. In each case, deposition parameters were identified that offered good tribological properties while maintaining low deposition temperatures (below 400 °C) to avoid distortion and softening of previously heat-treated substrates. Zirconia deposits were prepared with a pronounced columnar microstructure that provided excellent thermal fatigue and wear resistance, making them candidates for heat engine applications. The TiB2 coatings were the hardest coatings prepared. These provided good wear resistance on steel substrates. The B-Si coating displayed excellent wear resistance and good adherence to copper and aluminum alloys.
Encapsulated isotopic heat sources for use in generating electrical power for space applications require flight-quality hardware material. Iridium is the chosen material for such applications, and Oak Ridge National Laboratory has been the prime supplier of iridium alloy forming blanks 52 mm in diameter by 0.66 mm thick (2.0 by 0.026 in.). Prior to the work reported here, these blanks were ultrasonically examined by using 0.9-mm-diameter (0.0305-in.) simulated flaw standards. However, as a result of this effort, the sensitivity of our ultrasonic pulse-echo test system has been increased. The improved ultrasonic test system permits blank inspection at the 0.5-mm-diameter (0.020-in.) simulated flaw detection level. This test system was successfully demonstrated on the initial blanks provided via an improved processing route (consumable arc-melting, extruding, and rolling). The equipment modification and/or selection and the specific-focused search unit immersion technique developed to provide this capability are described. The improved flaw detection capability also provides data maps of a common type of defect in iridium (delaminations).
Heattoheat variations in GTA weld penetration of six austenitic stainless steels are shown to be controlled by the CaO content in oxide inclusions. The three test sheets with shallow weld pool penetration had relatively high CaO contents (> 10 pct) in numerous inclusions, but two readily weldable sheets included practically no CaO, and the third one contained CaO and a great amount of oxides improving penetration. The maximum welding speeds of two test sheets were improved by shot blasting with glass beads, by a thin layer of waterglass, and by utilizing Ar + 0.3 pct CO-2 as a shielding gas, the latter two methods being the most effective.
Current stateoftheart thermal barrier coating (TBC) systems consist of partially stabilized zirconia coatings plasma sprayed over a MCrAlY bond coat. Although these systems have excellent thermal shock properties, they have shown themselves to be deficient for a number of severe diesel and aircraft applications. New ternary ceramic plasma coatings are discussed with respect to their possible use in TBC systems. Ceriayttria stabilized zirconia (CYZ) coatings have been developed with low thermal conductivities, excellent thermal shock resistance, and erosion resistance comparable to baseline yttria stabilized zirconia (YSZ) coatings. In addition, dense zirconiatitaniayttria (ZTY) and chromiatitaniasilica (CTS) coatings have shown particle erosion resistance properties exceeding conventional stabilized zirconia coatings. Coatings have been evaluated in conjunction with a NiCrAlCoY2O3 bond coat. Also, multilayerhybrid coatings consisting of a bond coat with subsequent coatings of ceriayttria stabilized zirconia (CYZ) or yttria stabilized zirconia (YSZ) and top coats of zirconiatitaniayttria (ZTY) or chromia titania silica (CTS) have been evaluated. These coating systems combine the enhanced thermal shock performance characteristics of CYZ or YSZ with the improved erosion resistance of ZTY or CTS coatings.
The feasibility of producing high purity homogeneous welding fluxes by the solgel process and the effects that these fluxes have on the welding process were determined. Reagent grade solgel welding fluxes were produced by making systematic variations of the SiO2-CaO-TiO2-l pct Na2O flux system. The resulting fluxes were made into flux cored wires and used to make bead on plate welds on a niobium microalloyed HSLA steel. Solgel fluxes were shown to have excellent homogeneity, low residual hydrogen content, and no apparent water adsorption. The welding behavior of the solgel fluxes was shown to have superior arc stability compared to a commercial flux cored wire and very low weld metal hydrogen content. The chemical behavior of this flux system was characterized with respect to elemental transfer. The weldments exhibited a primarily acicular ferrite microstructure. Analysis of nonmetallic inclusion size distributions was compared to previous investigations and found to be consistent with the formation of high toughness weld metal microstructure.
An important item in the development of the MHD generator is the development of a durable electrode. This paper investigates the compatibility of metal electrodes to the oil fired MHD environment by examining the degradation mechanisms of type 304 steel and SHOMAC® steel anodes and W-Cu and WC-Ag cathodes. For purpose of clarification, we wish to state that the cathode is the electrode that is bombarded by positive ions from the MHD plasma; the anode is bombarded by negative ions. It is shown that type 304 steel anode loss is due to both corrosion by current and abrasion by combustion gas flow, and can be predicted by using empirical equations, if only the surface temperature is given. Abrasion appears if the surface temperature is over about 700 K. General corrosion also occurs on both type 304 steel and SHOMAC steel anodes and is accompanied by the formation of solid oxide scales. On the other hand, W-Cu cathode loss is mostly by abrasion in the current region below about 0.3 A/cm2. However, above that current value, corrosion by current increases rapidly due to the generation of intensive arc spots.
Corrosion of high temperature metal surfaces in waste incineration systems results primarily from compounds of chlorine, sulfur, and metals such as lead, zinc, and tin. The presence of such compounds in municipal refuse and chemical wastes can result in severe metal wastage in energy recovery systems. The corrosion mechanism involves interaction of sulfur oxides with chlorides in deposits to generate HCl and chlorine at the metal surface. Metal chlorides also can contribute by forming low melting eutectics. Reducing atmospheres, particularly carbon monoxide, in the combustion gases also appear to be a factor in corrosion. Corrosion rates of carbon and low alloy steels increase significantly with both metal temperature and gas temperature. The rates for stainless steels initially decrease as the metal temperature increases, and are less sensitive to gas temperature. Corrosion by chlorine can be inhibited by maintaining a sufficiently high concentration of sulfur or silica in the fuel. The results of corrosion probe exposures in waste-fueled boilers are presented to illustrate these mechanisms.
Methods of analyzing liquid metal penetration data are discussed and compared. Data on the corrosion of 2.25Cr-1Mo steel by liquid lithium at 480 ‡C are reanalyzed as an example, and a previously reported activation energy for a secondary penetration mechanism is corrected.
The materials issues in fluidized bed combustion systems are in many cases essentially similar to those in conventional pulverized coal systems. Those that are related to the technology itself include the erosion and corrosion of in-bed components, particularly the heat exchanger; the erosion of above-bed components; the wear (perhaps accelerated by corrosion) of the solids handling systems, particularly the ash recirculation system; and, in the case of pressurized combustors, the erosion and hot corrosion of the expander turbine. For steam-generating systems, the issue of in-bed corrosion is probably not as serious as had once appeared, provided care is taken in materials selection, in design, and in operation. In-bed erosion now appears to be the most important problem to be solved. The expander turbine problems define the degree of particulate removal from the hot gas that has to be achieved, and limit the maximum permissible bed temperature. However, both the erosion and corrosion may be easier to deal with in the low-temperature expander in the “pressurized boiler” concept.
Several metallurgical coatings have been developed that provide good tribological performances in high-temperature liquid sodium and that are relatively unaffected by neutron fluences to 6 × 1022 n/cm2 (E >0.1 MeV). The coatings that have consistently provided the best tribological performance have been the nickel aluminide diffusion coatings created by the pack cementation process, chromium carbide or Tribaloy 700 (a nickelbase hardfacing alloy) applied by the detonation-gun process, and chromium carbide and other hardfacing materials applied by the electro-spark deposition process. The latter process is a relatively recent development for nuclear applications and is expected to find wide usage. Other coating processes, such as plasma-spray coating, sputtering, and chemical vapor deposition, were candidates for use on various components, but the coatings did not pass the required qualification tests or were not economically competitive. The advantages and limitations of the three selected processes are discussed, the tribological performance of the coatings is reviewed, and representative applications and their performance requirements are described.
This paper describes a mechanical process of applying rapidly solidified (RS) metal coatings. The process comprises spraying a mixture of RS metal powder and peening particles at high velocity against the surface of substrates. By the impact of peening particles, the metal powder will be bonded to the surface and form a protective coating. Parts thus treated are expected to have not only improved corrosion resistance due to RS metal coating but increased fatigue strength by peening also. The apparatus and coating procedures are presented, and specimens of brass substrate coated with RS tin powder are shown as examples.
The surface mechanical and chemical properties of ion implanted ceramics reflect changes in the microstructure produced by the bombarding ions. Metastable solid solutions which contain large concentrations of lattice defects have been produced in Al2O3, SiC, and TiB2. Such structures exhibit a higher micro-indentation hardness, fracture toughness, and wear resistance than unimplanted materials. Amorphous surfaces have been formed on Al2O3 and SiC crystals by low temperature, high fluence, or certain ion species implantation. These surfaces have a lower hardness but higher toughness than unimplanted crystals.
Microstructural heterogeneities are observed to have a dual influence on the creep rupture behavior of a polycrystalline alumina. The duality is related to a transition in rupture mechanism. The heterogeneous regions in the microstructure provide the cracks which cause creep crack propagation controlled failures at small strains. Additionally, shear bands nucleate at the heterogeneities, and thereby contribute to damage controlled failure at large strains. The transition in rupture mechanism is associated with a high incidence of creep blunting of cracks at a threshold stress intensity factor.
