Showing papers by "Arnold R. Marder published in 2005"

Microstructural characterization of a double-sided friction stir weld on a superaustenitic stainless steel

Abstract: The microstructure of AL-6XN plates joined via a double-sided friction stir weld has been investigated. The microstructural zones that develop during friction stir welding (FSW) reflect decreasing strains and less severe thermal cycles with increasing distance from the weld centerline. The nugget, located around the centerline, has a refined structure of equiaxed grains as a result of the extreme strain and temperatures experienced during welding. Several features are seen within the nugget, one of the most prominent being a steady stream of tungsten inclusions created by accelerated tool wear. The heat-affected zone consists of a mixture of relatively large austenite grains and smaller recrystallized grains present at grain boundaries. These fine grains were shown to be austenite and no evidence of sigma phase in this region was apparent. The thermal mechanical-affected zone, located between the nugget and heat-affected zone, shows a microstructural transition from the completely refined structure to a structure very similar to the base metal. Of particular importance is that, unlike fusion welding, microsegregation has been avoided during FSW. Due to the changing microstructure from base metal to the weld zone, there are corresponding changes in hardness. Moving toward the centerline from the base metal, hardness increases due to refinement of the microstructure.

Gaseous corrosion resistance of Fe-Al-based alloys containing Cr additions Part I: Kinetic results

Abstract: Iron–aluminum-based weld overlay claddings have recently been considered as corrosion resistant coatings. In this particular study, 10 alloys were exposed at 500 °C to three different corrosive environments ranging from a highly sulfidizing to an oxidizing atmosphere. Corrosion kinetics were determined for exposed alloys and cross-sectional microscopy was used to relate the corrosion products to the corrosion rates. It was found that critical aluminum and chromium concentrations were required to prevent rapid corrosion kinetics during 100 h of exposure. Titanium was found to help the corrosion resistance of a Fe–Al–Cr alloy when exposed to a highly sulfidizing environment, but had an insignificant effect on the corrosion kinetics of alloys exposed to a mixed oxidizing/sulfidizing and an oxidizing atmosphere. The total aluminum and chromium concentration was taken as the alloy content factor ( ϕ = at% Al + at% Cr) in order to numerically represent alloys based on the alloying elements. Critical alloying content values were found for each environment based on the corrosion kinetics. It was determined that 19 at% Al and 1 at% Cr was required to prevent rapid corrosion kinetics in all three corrosive environments.

Gaseous corrosion resistance of Fe-Al based alloys containing Cr additions. Part II. Scale morphology

Abstract: Iron–aluminum based weld overlay claddings are currently being considered as corrosion resistant coatings for corrosion protection at 500 °C in aggressive sulfidizing and oxidizing atmospheres. These alloys rely on the formation of a thin passive Al2O3 layer for their corrosion protection. Once the alloy can no longer maintain the protective oxide scale, passive layer breakdown can occur, where non-protective nodules form at random locations along the surface of the sample. These nodules can continue to grow together and eventually overgrow the protective oxide rendering the sample no longer corrosion resistant. In this current study, 10 iron–aluminum based alloys containing chromium and titanium additions were exposed to three high-temperature corrosive environments at 500 °C for 100 h. The passive layer breakdown phenomenon was observed in all three environments on several non-protective alloys. The extent of nodule growth was measured and used to identify corrosion resistant alloy compositions. It was found that aluminum and chromium additions help to form and maintain a protective oxide layer and critical alloying contents were required to prevent passive layer breakdown during 100 h of exposure. Critical alloying values necessary to prevent nodule formation were compared to critical alloying levels required to prevent rapid corrosion kinetics. While corrosion kinetics are helpful in determining protective alloy compositions, the corrosion scale morphology must be characterized to ensure that the alloy is completely protective during the exposure time.

Optimization of Superaustenitic Stainless Steel Filler Metals for Welding Advanced Double Hull Combatant Ships

Abstract: : The performance of welds on superaustenitic stainless steels (SASS) such as AL-6XN has been improved through the use of a new microstructutal development sequence. Thermodynamic calculations were used to isolate the range of possible filler metal compositions, the results of which were confirmed experimentally through alloy construction and EPMA analysis. HED welds on the alloys were used to induce the massive transformation, which is capable of producing fully-austenitic welds with uniform distributions of Mo. SASS welds using such a filler metal should display improved corrosion resistance and weldability over current Ni-base filler metal solutions. The tensile, fracture toughness, and fatigue properties of AL-6XN plate material in the short-transverse (S-T) orientation were studied. The presence of brittle second-phase particles in the form of microstructutal packets has been shown to be detrimental to the mechanical properties of AL-6XN.