Ajit K. Roy

Cracking of titanium alloys under cathodic applied potential

The slow-strain-rate test technique was used to evaluate the susceptibility of Ti Gr-7 and Ti Gr-12 to hydrogen-induced-cracking and/or stress corrosion cracking. Ti Gr-7 and Ti Gr-12 are two candidate container materials for the multi-barrier package for nuclear waste. The tests were done in a deaerated 90 degrees C acidic brine (pH approximately 2.7) containing 5 weight percent (wt. %) NaCl using a strain rate of 3.3x10(-6)s(-1). Before being tested in the acidic brine, specimens of each alloy were pulled inside the test chamber in the dry condition at ambient temperature. Then, while in the test solution, specimens were strained under different cathodic controlled potentials. These controlled potentials were selected based on the corrosion potential measured in the test solution before the specimens were strained. Results indicate that the times to failure (TTF) for Ti Gr-12 were much shorter than those for Ti Gr-7. Furthermore, as the applied potential became more cathodic, Ti Gr-12 showed reduced ductility in terms of percent reduction in area and true fracture stress (sigma(f)). In addition, the TTF and percent elongation reached the minimum values when Ti Gr-12 was tested under an impressed potential of -1162mV. However, for Ti Gr-7, all these ductility parameters were not significantly influenced by the changes in applied potential. In general, the results of hydrogen analysis by secondary ion mass spectrometry showed increased hydrogen concentration at more cathodic controlled potentials. Optical microscopy and scanning electron microscopy were used to evaluate the morphology of cracking both at the primary fracture face and the secondary cracks along the gage section of the broken tensile specimen. Transgranular secondary cracks were observed in both alloys possibly resulting from the formation of brittle titanium hydrides due to cathodic charging. The primary fracture face was characterized by dimpled microstructure indicating ductile failure.

Tensile Properties of Martensitic Stainless Steels at Elevated Temperatures

Tensile properties of quenched and tempered martensitic alloys EP-823, HT-9, and 422 were evaluated at temperatures ranging from ambient to 600 °C as a function of three different tempering times. The results indicated that the yield strength, ultimate tensile strength, and the failure strength were gradually reduced with increasing temperature. The ductility parameters were enhanced at elevated temperatures due to increased plastic flow. However, the tempering time did not significantly influence these properties. The evaluation of the fracture surfaces by scanning electron microscopy revealed reduced cracking and dimpled microstructures, indicating enhanced ductility at higher testing temperatures.

Residual stress characterization in structural materials by destructive and nondestructive techniques

Transmutation of nuclear waste is currently being considered to transform long-lived isotopes to species with relatively short half-lives and reduced radioactivity through capture and decay of minor actinides and fission products. This process is intended for geologic disposal of spent nuclear fuels for shorter durations in the proposed Yucca Mountain repository. The molten lead-bismuth-eutectic will be used as a target and coolant during transmutation, which will be contained in a subsystem vessel made from materials such as austenitic (304L) and martensitic (EP-823 and HT-9) stainless steels. The structural materials used in this vessel will be subjected to welding operations and plastic deformation during fabrication. The resultant residual stresses cannot be totally eliminated even by stress-relief operations. Destructive and nondestructive techniques have been used to evaluate residual stresses in the welded and cold-worked specimens. Results indicate that tensile residual stresses were generated at the fusion line of the welded specimens made from either austenitic or martensitic stainless steel, with reduced stresses away from this region. The magnitude of residual stress in the cold-worked specimens was enhanced at intermediate cold-reduction levels, showing tensile residual stresses in the austenitic material while exhibiting compressive stresses in the martensitic alloys. Comparative analyses of the resultant data obtained by different techniques revealed consistent stress patterns.

Environment-Assisted Cracking of Structural Materials under Different Loading Conditions

Abstract Significant efforts are ongoing, nationally and internationally, to reduce the radioactivity of spent nuclear fuel (SNF) and high-level waste (HLW) for their disposal in a potential reposi...

Cracking of martensitic alloy EP-823 under controlled potential

The susceptibility of martensitic Alloy EP-823 to stress corrosion cracking was evaluated with and without an applied cathodic potential using the slow-strain-rate (SSR) testing technique. The magnitude of the applied potential was based on the corrosion potential determined by cyclic polarization. The cracking susceptibility in an acidic environment at different temperatures was expressed in terms of the true failure stress (σf), time to failure (TTF), and ductility parameters, including percent elongation (%El) and percent reduction in area (%RA). The data indicate that the magnitudes of σf, TTF, %El, and %RA were reduced due to cathodic charging. The scanning electron microscopic evaluations of the primary fracture surface of the specimens used in SSR testing revealed a combination of ductile and brittle failures. Further, the secondary cracks at the gauge section of these specimens were characterized by branching.

Temperature and Load Ratio Effects on Crack-Growth Behavior of Austenitic Superalloys

The role of temperature and load ratio (R) on the crack propagation rate (da/dN) of Alloys 276 and 617 under cyclic loading was investigated. The results indicate that the rate of cracking was gradually enhanced with increasing temperature when the R value was kept constant. However, the temperature effect was more pronounced at 100―150°C. Both alloys exhibited maximum da/dN values at a load range of 4.5 kN that corresponds to an R value of 0.1. The number of cycles to failure for Alloy 276 was relatively higher compared with that of Alloy 617, indicating its slower crack-growth rate. Fractographic evaluation of the broken specimen surface revealed combined fatigue and ductile failures.

Doppler Broadening Analysis of Steel Specimens Using Accelerator Based In Situ Pair Production

Positron Annihilation Spectroscopy (PAS) techniques can be utilized as a sensitive probe of defects in materials. Studying these microscopic defects is very important for a number of industries in order to predict material failure or structural integrity. We have been developing gamma‐induced pair‐production techniques to produce positrons in thick samples (∼4–40 g/cm2, or ∼0.5–5 cm in steel). These techniques are called ‘Accelerator‐based Gamma‐induced Positron Annihilation Spectroscopy’ (AG‐PAS). We have begun testing the capabilities of this technique for imaging of defect densities in thick structural materials. As a first step, a linear accelerator (LINAC) was employed to produce photon beams by stopping 15 MeV electrons in a 1 mm thick tungsten converter. The accelerator is capable of operating with 30–60 ns pulse width, up to 200 mA peak current at 1 kHz repetition rate. The highly collimated bremsstrahlung beam impinged upon our steel tensile specimens, after traveling through a 1.2 m thick concre...

Tensile Deformation of Alloy 617 at Different Temperatures

Austenitic Alloy 617 has been identified as a candidate structural material in hydrogen generation using nuclear heat. A temperature of 950°C has been recommended to achieve the maximum efficiency in hydrogen generation involving numerous chemical reactions. The tensile properties of this alloy have been determined at temperatures ranging from ambient to 1000°C. Simultaneously, the fracture toughness of this alloy is being evaluated using fracture-mechanics-based compact tension specimen. This paper presents the results of tensile testing of this alloy as a function of temperature. The results of fractographic evaluations using scanning electron microscopy are also included.Copyright

Metallurgical and Corrosion Studies of Modified T91 Grade Steel

Modified 9Cr-1Mo steels containing different Silicon (Si) content have been tested for evaluation of their metallurgical and corrosion properties. The results of tensile testing indicated reduced failure strain up to 400°C followed by its enhancement at a higher temperature. Stress corrosion cracking tests at constant load did not show any failure in any materials. However, reduced ductility and true failure stress were noted in slow strain rate (SSR) testing, indicating increased cracking susceptibility at elevated temperatures. The corrosion potential became more active with increasing temperature in polarization studies. A combination of intergranular and cleavage failures was observed in specimens tested by the SSR technique.

Mechanical Properties and Cracking Behavior of High-Temperature Heat-Exchanger Materials

The structural materials selected for high-temperature heat-exchanger applications are expected to withstand very severe operating conditions including elevated temperatures and aggressive chemical species during hydrogen generation using nuclear power. Three different cycles namely sulfur-iodine, calcium-bromine and high temperature electrolysis have been identified for hydrogen generation. Three different structural materials namely Alloy C-22, Alloy C-276 and Waspaloy have been tested to evaluate their high-temperature tensile properties and stress corrosion cracking (SCC) resistance in an acidic solution. The data indicate that all three alloys are capable of maintaining appreciably high tensile strength upto a temperature of 600°C. The results of SCC testing indicate that all three materials are highly resistant to cracking in an acidic solution retaining much of their ductility and time to failure in the tested environment. Fractographic evaluation by scanning electron microscopy revealed dimple microstructure indicating significant ductility in all three alloys.Copyright