James C. Rawers

Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.

Nitrogen addition to iron powder by mechanical alloying

Abstract Nitrogen was alloyed into iron (a) by mechanical processing in a nitrogen gas environment, and (b) by mechanically alloying with iron-nitride powders to characterize resulting nano-structure and nitrogen distribution. Although the infused nitrogen concentration was significantly greater than the thermodynamic equilibrium solubility of iron, no nitrides formed, even for nitrogen concentrations as high as 4.1 wt.% However, a bct Fe phase did form. Lattice expansion calculations indicate that the sum of the interstitial bcc-Fe and bct Fe nitrogen concentrations was significantly less than the total measured nitrogen concentration. A considerable portion of the mechanically infused nitrogen was determined to be associated with nanograin boundaries.

Microstructure and tensile properties of compacted, mechanically alloyed, nanocrystalline Fe-AI

Data on mechanical properties of nanocrystalline materials have been limited, due in part to the difficulty in producing consolidated nanocrystalline materials of sufficient quantity for characterization and evaluation. A second problem is consolidation and retention of the nanostructure. A vacuum hot-pressing consolidation program has been developed to produce full-dense compacts from attrition milled, mechanically alloyed, nanograin micron-size particles of Fe-2 wt pet Al powder. The resulting compacts were of sufficient size to allow evaluation of microstructure, density, hardness, and tensile properties. The compacted microstructure was a composite of pure iron submicrograins and Fe-A1 nanograin clusters. Tensile strength was found to be proportional to the sample’s density squared. For full-dense compacts, tensile strength of nanocrystalline compacts approaching 1 GPa was obtained.

Mössbauer study and thermodynamic modeling of Fe–C–N alloy

Abstract Mossbauer spectroscopy and Monte Carlo computer simulation have been combined to understand the reason for the solid-solution stability of Fe–0.93 C–0.91 N alloy (mass%). Interstitial concentrations in austenite and ferrite were determined on the basis of X-ray diffraction measurements of the lattice dilatation. The hyperfine structure of Mossbauer spectra was analyzed to identify different atomic configurations in solid solutions and determine their fractions. Thereafter Monte Carlo simulation of the interstitial distribution in ferritic and austenitic solid solutions was performed, and values of the interstitial–interstitial interaction energies were obtained for the first and second coordination spheres in austenite and the first to the fourth coordination spheres in ferrite. Simulation shows that in both austenitic and ferritic phases the interaction of interstitial atoms is characterized by a strong repulsion within the first two coordination spheres. Experimental data and simulated interstitial distributions are consistent and complementary. It is concluded that the absence of interstitial clusters prevents carbide and nitride precipitates and causes the higher thermodynamic stability of Fe–C–N solid solutions as compared with Fe–C and Fe–N ones.

Nanostructure characterization of mechanical alloyed and consolidated iron alloys

High-energy ball milling (which is readily adoptable to commercial application) was used to develop sufficient quantities of nanostructured material to produce compacts capable of being measured for macroscopic properties. Characterization of the ball-milled powders show that grain boundary properties play a significant role in the overall properties of the milled powder. Nearly full-dense compacts were produced by hot-pressing. Characterization of the strength properties of these compacts show that there was little influence of hardness, density, or alloy composition on the failure properties. The range of failure stress was large and when fitted to a Weibull distribution suggest that failure was the result of flaws or cracks resulting from the hot-pressing. Hardness data, commonly used to evaluate the strength of nanostructured materials, showed no correlation to tensile strength, but correlated highly to compression maximum stress.

Initial stages of coal slag interaction with high chromia sesquioxide refractories

Slagging coal gasifiers operate at temperatures as high as 1650°C in a reducing environment, requiring combustion chambers to be lined with refractories. The liner materials of choice are semi-porous high chromia refractories. Recently, a new series of high-chromia aluminia sesquioxide refractories have been developed. Both long term and short term tests are being conducted to evaluate the performance of these materials. In this study, the initial stage of slag-refractory interactions was analyzed. Samples of gasifier slag were compacted and placed upon the surface of these new chromia refractories and the temperature was raised consistent with start-up operating conditions of commercial gasifiers. The slag was completely molten by the time the furnace achieved a temperature consistent with gasifier operation conditions: 1350°C. Measurement of the slag contact angle, slag spread along the slag-refractory interface, and the loss of slag due to slag infusion into the refractory were monitored by camera. Analysis suggests a single phenomenon with an activation energy of approximately 54 kcal may be the controlling factor. Cross-section analysis of the sample and analysis of slag chemistry indicate that slag infusion preceded the slag-refractory interface front movement and that the iron component of the slag was becoming concentrated at the slag-refractory interface leading to the formation of a chromium-iron spinel phase. Results of these short term tests are critical in characterizing and understanding the results long term slag-refractory interactions.

Fractal characterization of wear-erosion surfaces

Wear erosion is a complex phenomenon resulting in highly distorted and deformed surface morphologies. Most wear surface features have been described only qualitatively. In this study wear surfaces features were quantified using fractal analysis. The ability to assign numerical values to wear-erosion surfaces makes possible mathematical expressions that will enable wear mechanisms to be predicted and understood. Surface characterization came from wear-erosion experiments that included varying the erosive materials, the impact velocity, and the impact angle. Seven fractal analytical techniques were applied to micrograph images of wear-erosion surfaces. Fourier analysis was the most promising. Fractal values obtained were consistent with visual observations and provided a unique wear-erosion parameter unrelated to wear rate.

Wear Evaluation of High Interstitial Stainless Steels

A new series of high nitrogen-carbon manganese stainless steel alloys are studied for their wear resistance. High nitrogen and carbon concentrations were obtained by melting elemental iron-chromium-manganese (several with minor alloy additions of nickel, silicon, and molybdenum) in a nitrogen atmosphere and adding elemental graphite. The improvement in material properties (hardness and strength) with increasing nitrogen and carbon interstitial concentration was consistent with previously reported improvements in similar material properties alloyed with nitrogen only. Wear tests included: scratch, pin-on-disk, sand-rubber-wheel, impeller, and jet erosion. Additions of interstitial nitrogen and carbon as well as interstitial nitrogen and carbide precipitates were found to greatly improve material properties. In general, with increasing nitrogen and carbon concentrations, strength, hardness, and wear resistance increased.

Oxides reactions with a High-chrome sesquioxide refractory

In slagging coal-gasifier systems, the combination of oxides present as impurities in coal and combustion temperatures that can exceed 1650°C restrict the use of liner materials in the coal combustion chambers to refractories. In this study, the slag-refractory interactions of a new high chrome sesquioxide refractory was characterized. High-temperature cup tests showed that the molten oxides infused into the refractory and that the sesquioxide refractory reacts with the oxides in a manner similar to spinel phase refractories. Studies of the coal slags individual oxide components showed CaO reacts with the chrome refractory to form a low melting Ca(CrO2)2. FeO reacts with the sesquioxide to form a interface layer of (Cr,Fe)3O4 spinel phase. Results of this study now make it possible to design studies for improving corrosion resistance to increase refractory life.

High carbon-nitrogen iron alloys

A new processing technique produces high carbon–high nitrogen iron alloys by melting iron-carbon steels in a hot isostatic pressing (HIP) furnace with nitrogen as the pressurizing gas. Furnace cooling O-1 tool steel with enhanced nitrogen concentrations resulted in the retention of the austenite phase without formation of carbide and nitride precipitates. The duplex austenite/ferrite structure has enhanced hardness, strength, and wear resistance.