Nitrogen addition to iron powder by mechanical alloying

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 bctFe phase did form. Lattice expansion calculations indicate that the sum of the interstitial bcc-Fe and bctFe 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.     

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.     

Effect of laser surface melted zirconium alloys on microstructure and corrosion resistance

Zirconium alloys were laser surface melted (LSM) using a continuous wave CO₂ laser at energy densities of 4,7 and 10 kJ cm^–2. LSM samples examined using SEM and optical microscopy exhibited resolidified regions with several different microstructures, including ultrafine martensite. Corrosion performance was obtained by steam autoclave tests and immersion tests in 10% FeCl₃ at room temperature. Coarser microstructures performed better than fine microstructures in autoclave tests, while fine microstructures performed better than coarse microstructures in 10% FeCl₃ immersion tests. Accelerated corrosion in the autoclave and immersion tests was observed to occur near the laser beam overlap region. The surface chemistry was examined for alloy segregation using secondary iron mass spectroscopy. Tin and iron alloy elements segregated near the periphery of each melt pool. Segregated regions containing increased iron concentrations associated with each laser pass were responsible for accelerated corrosion.     

Microstructural and failure characteristics of metal-lntermetallic layered sheet composites

A processing technique for the fabrication of layered metal-intermetallic composites is presented, in which a self-propagating, high-temperature synthesis reaction (SHS) was initiated at the interface between dissimilar elemental metal foils. The resultant composite microstructure consisted of a fully dense, well-bonded metal-intermetallic layered composite. In this United States Bureau of Mines study, metal (Fe, Ni, or Ti) foils were reacted with Al foils to produce metal-metal aluminide layered composites. Tensile tests conducted at room temperature revealed that composites could be designed to behave in a high-strength and high-toughness manner by altering the thicknesses of the starting elemental foils. Failure characteristics revealed that the processes that govern ductilevs brittle behavior of the composites occur early in the fracture.     

High-pressure nitrogen gas alloying of Fe-Cr-Ni alloys

The melting of Fe-Cr-Ni alloys under 200 MPa nitrogen pressure has shown that nitrogen significantly improves the mechanical properties. Tensile strengths of these high-nitrogen alloys were found to be proportional to interstitial nitrogen concentration. The tensile strengths of nitrogen-alloyed steels could be significantly increased over comparable carbon-alloyed steels. The increase in tensile strength was found to be proportional to the square root of the interstitial nitrogen concentration indicating that the strengthening may be controlled by thermal effects.     

Inherent oxidation protection of Fe-5Cr-15Ni-2Si-4.5Mo

The oxidation mechanism of Fe-5Cr-15Ni-2Si-4.5Mo alloy was investigated in order to determine the role of Si and Mo in providing oxidation resistance. It was determined that the oxidation protection in the temperature range 750–950°C resulted from formation of a continuous oxide sublayer of SiO ₂ (or possibly Fe ₂ SiO ₄).Molybdenum formed an intermetallic Fe ₂ Mo _1–x Si _x that eventually diffused out into the grain boundaries and formed a protective barrier to the oxidation process. The mechanism behind the improved oxidation is the formation of a SiO ₂ layer at the metal-oxide interface that retards the outward diffusion of Fe. It was also established that the oxidation mechanism was controlled by an activation energy equal to that of Fe ³⁺ ions diffusing through SiO ₂.     

Oxidation study of Fe-8Cr-10Ni-(0-3)Si-(0-6)Mo alloys

Mechanisms are proposed to explain the oxidation rate behavior of Fe-8Cr-10Ni alloys to which varying amounts of either Si (0–3%) or Mo (0–6%), or both have been added. The formation and breakdown of a silica sublayer cause significant changes in the oxidation mechanism. The formation of the silica depends on preformation of a Cr₂O₃ outer layer. The addition of Mo enhances the oxidation protection of Fe-Si alloys by producing an Fe-Mo-Si precipitate in the base metal.     

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.     

Finite element analysis of a laser-processed fracture specimen

Finite element analysis was used to simulate the static failure of a AISI 4140, three-point bend, Charpy specimen. Non-linear finite element models (FEM) were constructed to represent standard Charpy, fatigue-precracked Charpy, and laser-processed Charpy specimens. For the laser-processed Charpy FEM, a strain-based failure criterion was used to simulate crack propagation through the 0.5 mm thick laser-processed zone. For comparison, a 0.5 mm long crack was used in the fatigue-precracked FEM and similarly loaded. Results showed that the numerically calculated load for crack initation through this zone compared favorably to that reported in earlier experiments. Furthermore, after the crack had propagated through the laser-processed zone within the FEM-comparison of plastic strain contours for this model and that for a fatigue-precracked model showed that similar patterns exist around the crack tip. These results indicate that laser-processing and fatigue-precracking should provide a similar basis for fracture toughness measurements.