Omer N. Dogan

Laboratory abrasive wear tests: investigation of test methods and alloy correlation

Abstract When screening materials, laboratory abrasive wear testing is a quick and inexpensive way of obtaining large quantities on information on wear rates and wear mechanisms. Typical laboratory abrasive wear tests approximate two- and three-body abrasion. The Albany Research Center, however, uses a suite of four laboratory abrasion, gouging–abrasion, and impact–gouging abrasion wear tests to rank materials for wear applications in the mining and minerals processing industries. These tests, and the wear mechanisms they approximate, are: (1) dry-sand, rubber-wheel (three-body, low-stress abrasion); (2) pin-on-drum (two-body, high-stress abrasion); (3) jaw crusher (high-stress gouging-abrasion); and (4) high-speed, impeller–tumbler (impact–abrasion). Subsequently, candidate materials can be ranked according to their performance for each of the wear tests. The abrasion, gouging–abrasion, and impact–abrasion test methods are described, highlighting the predominant wear mechanisms for each test. Data on a wide variety of irons and steels are presented with relative ranking of the materials according to the specific wear test.

Wear of Cast Chromium Steels With TiC Reinforcement

Wear resistance of a series of new titanium carbide reinforced cast chromium steels was investigated under various wear conditions. The steels which were melted in a vacuum induction furnace contained 12 Cr, 3-5 Ti, 1-2 C in weight percent. Microstructure of these materials was characterized using scanning electron microscopy, light optical microscopy, and X-ray diffraction. Microstructure of steels consisted of TiC phase dispersed in a martensitic matrix. High-stress and low-stress abrasion tests, and an erosion test, were utilized to understand the wear behavior of these materials under different environments. The steels were tested in as-cast and heat treated conditions. Wear rates of the cast Cr/TiC steels were compared to those of an AISI type 440C steel and P/M composites reinforced with TiC.

Erosive wear and impact damage of high-chromium white cast irons

Abstract High-chromium white cast irons are used extensively in environments where small particle impact causes considerable damage. In this study, three white cast irons with a variation in carbide volume fraction were eroded by spherical and angular particles. The white cast irons eroded at a higher rate when the erodent was WC–Co spheres than when the erodent was alumina particles. When eroded by spherical WC–Co particles, the eutectic white cast iron eroded at a higher rate than the hypoeutectic white cast iron. The results of the erosion experiments were interpreted in terms of a mechanism of platelet formation involving strain localization. The greater strain hardening rate of the hypoeutectic white cast iron can account for the better erosion resistance compared to the eutectic white cast iron, based on the strain localization mechanism. The larger size and greater density of the WC–Co spheres can account for the higher erosion rates of the white cast irons when eroded with WC–Co spheres compared to the erosion rate with alumina particles.

TiC-reinforced cast Cr steels

A new class of materials, namely TiC-reinforced cast chromium (Cr) steels, was developed for applications requiring high abrasion resistance and good fracture toughness. The research approach was to modify the carbide structure of commercial AISI 440C steel for better fracture resistance while maintaining the already high abrasion resistance. The new alloys contained 12Cr, 2.5–4.5Ti, and 1–1.5C (wt.%) and were melted in a vacuum induction furnace. Their microstructure was composed primarily of a martensitic matrix with a dispersion of TiC precipitates. Modification of TiC morphology was accomplished through changing the cooling rate during solidification. Wear rates of the TiC-reinforced Cr steels were comparable to that of AISI 440C steel, but the impact resistance was much improved.

Transient-Liquid-Phase Bonding of H230 Ni-Based Alloy Using Ni-P Interlayer: Microstructure and Mechanical Properties

Transient-liquid-phase bonding using Ni-P as an interlayer has been developed for H230 Ni-Cr-W solid-solution-strengthened Ni-based alloy. Two process parameters—composition of the interlayer and bonding time—have been varied to optimize the mechanical properties. H230 has been bonded into two sets of stacks (set I and II) for 8 and 4 hours using Ni-12P and Ni-6P interlayer, respectively, (wt pct) at 1423 K (1150 °C) and 12.7 MPa. The microstructure of both the stacks has three distinct regions—the joint centerline which showed the presence of pores, an isothermally solidified zone (ISZ) which did not have any carbide precipitates and base H230. Transmission electron microscopy and atom probe tomography showed a uniform microstructure, and an absence of any deleterious phases at the joint and in ISZ. Set I and set II had a yield strength of 76 and 86 pct of that of the H230 sheet, tested at 1023 K (750 °C). The measured elongation at fracture was negligible, but the fracture surfaces revealed a ductile cup-and-cone-type fracture occurring through the ISZ/joint region. Examination of broken tensile samples revealed that the plastic strain was constrained to within one joint region through which fracture occurred.

Verification of Thin-Wall Ductile Iron Test Methodology

Modifications need to be made to standard test procedures to determine the actual properties of thin-walled ductile iron castings. A series of experiments was performed on test bars to examine the effects of surface finish, elongation measurement techniques, and the amount of material removed during grinding. A paired t-test was used to show that strength and ductility increased as a result of removing the as-received (as-cast) surface. This is attributed to inaccuracies in measuring the cross section area of the rough surface as well as an actual increase in strain through removal of surface effects. A nontraditional method of measuring elongation (to retain the fracture surface) was compared to the standard ASTM method. The nontraditional method was found to be a conservative measure of elongation that is highly correlated with plastic strain as measured by the stress-strain curve. The amount of surface removed during grinding was found not to affect the mechanical properties with the exception of hardness, a surface measurement. It is suggested that thin-walled castings be tested in the condition the casting will be used, i.e., if the casting will be used with the as-cast surface, testing with the as cast surface will accurately reproduce the decrease in strain caused by the surface effects. If the casting will be ground before use, any amount of surface can be removed to make the testing convenient.

The role of metal vacancies during high-temperature oxidation of alloys

An improved understanding of high-temperature alloy oxidation is key to the design of structural materials for next-generation energy conversion technologies. An often overlooked, yet fundamental aspect of this oxidation process concerns the fate of the metal vacancies created when metal atoms are ionized and enter the growing oxide layer. In this work, we provide direct experimental evidence showing that these metal vacancies can be inseparably linked to the oxidation process beginning at the very early stages. The coalescence of metal vacancies at the oxide/alloy interface results initially in the formation of low-density metal and eventually in nm-sized voids. The simultaneous and subsequent oxidation of these regions fills the vacated space and promotes adhesion between the growing oxide and the alloy substrate. These structural transformations represent an important deviation from conventional metal oxidation theory, and this improved understanding will aid in the development of new structural alloys with enhanced oxidation resistance.Oxidation: active vacanciesInstead of annihilating, vacancies created early during high-temperature oxidation of a nickel superalloy coalesce into low-density regions. A team led by Richard Oleksak from the National Energy Technology Laboratory in Oregon, USA, used scanning transmission electron microscopy and atom probe tomography to characterize the surface of a nickel superalloy early in the oxidation process. Under the oxide layer, the metal substrate had many voids, and was rich in aluminum oxide. Contrary to conventional metal oxidation theory, vacancies coalesced to form clusters and nanovoids near the oxide/alloy interface while aluminum was selectively oxidized, creating a region of low-density metal rich in aluminum oxide under the surface, which was then incorporated into the growing surface oxide. Research into early and long-term oxidation mechanisms will help better design stable alloys.

Technical Note: Characterization of Corrosion Films Formed on Austenitic Stainless Steel in Supercritical CO2 Containing H2O and O2

Future technologies require structural alloys resistant to corrosion in supercritical CO2 (sCO2) fluids containing impurities such as H2O and O2. Traditional pipeline steels are potentially unsuita...

Foreword: Materials for High-Temperature Applications: Next Generation Superalloys and Beyond

Nickel-based superalloys possess an excellent combination of mechanical properties and environmental resistance at elevated temperatures, and have beenwidely used in challenging environments prevalent in aircraft engines and land-based power generation gas turbines, as well as in nuclear power systems and chemical plants. The ever-increasing demand for higher operating temperatures to achieve better fuel efficiency has been driving the development of the next generation of superalloys, in which the higher temperature capability has been achieved by increasing additions of refractory elements, optimizingprocessing conditions, and through the applicationof coatings.However, there is a strongneed formaterials that can make a disruptive change in temperature capability beyond that provided by current materials.

Notched Bar Izod Impact Properties of Zinc Die Castings

Notched bar Izod impact testing of zinc die cast Alloy 3, Alloy 5, ZA-8, and AcuZinc 5 was performed at five temperatures between -40\mDC and room temperature in accordance with ASTM E23 for impact testing of metallic materials. A direct comparison between ASTM D256 for impact testing of plastics and ASTM E23 was performed using continuously cast zinc specimens of Alloy 5 and ZA-8 at -40\mDC and room temperature. There are differences in sample sizes, impact velocity, and striker geometry between the two tests. Bulk zinc tested according to ASTM E23 resulted in higher impact energies at -40\mDC and lower impact energies at room temperature then did the same alloys when tested according to ASTM D256.