Leslie Bartosiewicz

Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron

Abstract An investigation was carried out to examine the influence of austempering heat treatments and the resultant microstructure of austempered ductile cast iron, on the fatigue crack growth rate, fatigue threshold, and high-cycle fatigue strength of the material. Two different approaches were used to study the fatigue behavior of this relatively new material, that is, a traditional S-N curve approach for determination of fatigue strength and a fracture mechanics-based approach for determination of the fatigue threshold. Compact tension and cylindrical specimens prepared from alloyed nodular ductile cast iron were given three different austempering heat treatments to produce three different microstructures. The fatigue threshold and high-cycle fatigue behavior of these specimens were studied in room temperature ambient atmosphere. The results of the present investigation demonstrate that the fatigue threshold of the material increases with increase in volume fraction of carbon-saturated austenite. The fatigue strength of the material, on the other hand, was found to increase with decrease in austenitic grain size. The crack growth process in the material was a combination of ductile striations and microvoid coalescence, and crack propagation by connecting the graphite nodules along its path.

Catalysis of carbon-steam gasification by ash components from two lignites

Abstract Ash transfer from a reactive to a less reactive coal is an interesting possibility for improving and equalizing gasification characteristics of coals. To assess the catalytic action of coal impurities in the steam gasification of carbon, three approaches were used. In the first series, the effects of different coal ashes on the gasification kinetics of graphite were compared. A parallel study was made by adding lignite ash to a coal of low reactivity. Finally, gasification rates of chars prepared from demineralized coals were measured. While it was found that ash from reactive coals can significantly enhance the gasification rates of chars derived from coals of lesser reactivity, it was not possible to distinguish clearly between a catalytic lowering of the activation energy and an increase in the number of gasification sites. The gasification enhancement by lignite ash may open practical possibilities for blending coals of different reactivity, and warrants further study to identify the constituents associated with this effect.

Catalytic steam gasification of graphite: Effects of calcium, strontium, and barium with and without sulfur

Abstract Alkaline earths strongly catalyze steam gasification of graphite. The catalytic effect increases in the order Ca

Effects of calcium, strontium, and barium as catalysts and sulphur scavengers in the steam gasification of coal chars

Effects of alkaline earths on the steam gasification of two coal chars, one of low and one of high intrinsic reactivity, were evaluated gravimetrically. The chars were derived from coal powders which had been impregnated with Ca, Sr, or Ba, pelletized, and pyrolysed in nitrogen. The additives increased the gasification rates in the order Ca < Sr < Ba. It follows from reaction kinetics that catalysis is caused by a large increase in the density of reaction sites, not by a lowering of the true activation energy. As shown by electron micrographs and elemental maps obtained by X-ray analysis, the strong catalytic effect is closely associated with the ability of the alkaline-earth species to spread over the char and to preserve contact with a freshly formed carbon surface as the gasification proceeds. The alkaline-earth catalysts are severely poisoned by hydrogen sulphide or sulphur dioxide from an external source.

Tensile behavior of a new single-crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures

Tensile behavior of a new single-crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures. The investigation also examined the influence of γ′ precipitates (size and distribution) on the tensile behavior of the material. Tensile specimens were prepared from single-crystal CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. These specimens were given three different heat treatments to produce three different γ′ precipitate sizes and distributions. Tensile testing was carried out at both room and elevated temperatures. The results of the present investigation indicate that yield strength and ultimate tensile strength of this material initially increases with temperature, reaches a peak at around 800 °C, and then starts rapidly decreasing with rise in temperature. Both yield and tensile strength increased with increase in average γ′ precipitate size. Yield strength and temperature correlated very well by an Arrhenius type of relationship. Rate-controlling process for yielding at very high temperature (T ≥ 800 °C) was found to be the dislocation climb for all three differently heat-treated materials. Thermally activated hardening occurs below 800 °C whereas above 800 °C thermally activated softening occurs in this material.

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined.The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.

Carbon oxidation catalyzed by lead

Abstract The pronounced catalytic effect of lead on the reaction of carbon and oxygen was studied by gravimetric measurements of reaction rates. In addition, the lead distribution was investigated with a scanning electron microscope. Impregnation of graphite by 0.001–1 atom% of lead resulted in a reaction rate increase by up to five orders of magnitude. Reaction parameters were derived from Arrhenius plots. Data analysis shows that the rate increase by lead is primarily caused by a massive increase in the density of reaction sites. No lowering of the activation barrier associated with the carbon oxidation is indicated. The apparent activation energy remains constant at 196 ± 20 kJ/mol for graphite with and without lead. Based on absolute rate theory the site density is consistent with the geometry of the graphite crystals and indicates that every carbon atom in the carbon structure can be transformed into a reaction site. The conclusion is supported by a comparison of graphite topography and the elemental maps of lead on the carbon crystals. The study includes carbons of lesser structural perfection.

Application of a new model for fatigue threshold in a structural steel weldment

Abstract A new model for fatigue threshold has been proposed. The model takes into consideration the influence of material strength, grain size, and load ratio on fatigue threshold. Fatigue crack growth behavior of a structural steel weldment (ASTM Grade A 514) was investigated to examine the applicability of this model. The investigation also examined the influence of welding procedure [submerged arc welding (SMAW) versus metal inert gas welding (MIG)] on the resultant microstructure and the near threshold fatigue crack growth behavior of the material in room temperature ambient atmosphere. Compact tension specimens were prepared from ASTM Grade A 514 structural steel weldment in such a way that cracks propagated through the heat affected zone (HAZ), base metal and weldments. This arrangement permitted the influence of HAZ on fatigue crack growth rate to be determined in both linear and threshold regions of the Paris curve in room temperature ambient atmosphere. Welding was accomplished by two different techniques, i.e. SMAW and MIG processes. The results of the present investigation demonstrated that the near threshold fatigue crack growth rate was lowest and fatigue threshold was highest when the crack propagated through the weldment. The fatigue threshold values predicted by the model were in good agreement with the experimentally determined values. The MIG welding process was found to be beneficial compared to the SMAW process as far as the fatigue threshold is concerned for weldment.

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)* was investigated. The investigation also examined the influence of heat treatment on resultant microstructures and the near-threshold fatigue crack growth behavior. In addition, the influence of load ratios(R), material strength, and grain size on fatigue threshold was studied. Compact tension specimens prepared from Incoloy 825 with transverse-longitudinal (TL) orientation in the as-received, as well as two different heat treated conditions, were used. The heat treatment studies revealed a peak hardness condition after solution treatment at 1200 °C for 1/2 hr, followed by aging at 600 °C for 434 hr. Among all the heat treated conditions, the fatigue threshold was the highest and the near-threshold crack growth rate was lowest in this peak aged condition. Fatigue threshold values were observed to decrease with an increase in load ratio, whereas an increased grain diameter resulted in a higher fatigue threshold. An earlier mathematical model was found applicable to characterize the relationship between load ratio and fatigue threshold. Preferential etching of grain boundary suggests formation of a thin film of carbide precipitation along the grain boundary region in the aged specimens. This carbide precipitation facilitated intergranular crack growth in these samples, resulting in higher roughness-induced crack closure. The highest fatigue threshold in the peak aged condition can be attributed to this large roughness-induced crack closure process.

Effects of catalyst addition to coal on CO2 adsorption kinetics of coal and char

Abstract The surface area of Illinois No. 6 coal impregnated with a series of carbon gasification catalysts was measured by carbon dioxide adsorption before and after pyrolysis. Chars were prepared by pyrolysing the coal in flowing nitrogen at a low controlled rate. The adsorbate uptake was described by a logarithmic time dependence. The amounts of CO 2 adsorbed at a fixed pressure within 1 minute and at maximum coverage were used to describe the surface area associated with micropores larger than 0.5 nm in diameter and with the total surface area, respectively. Catalyst addition decreased the surface area of coal accessible to CO 2 in all cases. After pyrolysis, some of the chars showed a considerable increase, others a massive decrease, in their adsorption capacity.