Effect of Silicon Content on the Corrosion Resistance and Radiation-Induced Embrittlement of Materials for Advanced Heavy Liquid Metal Nuclear Systems

The purpose of this collaborative research project involving the University of Nevada Las Vegas (UNLV), Los Alamos National Laboratory (LANL) and Idaho State University (ISU) is to evaluate the effect of silicon (Si) content on the corrosion behavior and radiation-induced embrittlement of martensitic stainless steels having chemical compositions similar to that of the modified 9Cr-1Mo 2 steel. Recent studies at LANL involving Alloy EP-823 of different Si content have demonstrated that increased Si content in this alloy may enhance the corrosion resistance in molten lead-bismuth-eutectic (LBE). Since very little data exists in the open literature on the beneficial effect of Si content on the corrosion properties, it seems appropriate to initiate a research project to address this technical issue. This proposal is intended to study the effect of Si content not only on the corrosion resistance but also on the radiation-induced embrittlement of martensitic stainless steels. The susceptibility of these alloys with different Si content to stress corrosion cracking, general corrosion and localized corrosion will be evaluated in the molten LBE and aqueous environments of different pH values using state-ofthe- art testing techniques. Testing in the aqueous media is intended to develop baseline data for comparison purpose. Radiation-induced embrittlement of these alloys will initially be studied by irradiating the test specimens with bremmstrahlung gamma radiation from 20-40 MeV electron beams at ISU. These gammas induce (γ, n) reactions in the giant dipole energy region. The principal radiation damage from these irradiations, in turn, stems from the recoiling residual nucleus (with average kinetic energy of approximately 20,000 eV) after the neutrons are emitted. The high penetrability of gammas, whose range is of an order of one meter in steel, ensures that the resulting damage will be uniform over the volume of the sample. The induced activity of these specimens will have very short half-lives (typically minutes) due to the systematics of (slightly) proton-rich nuclei. The resulting radiation-induced hardening can subsequently be evaluated by proper experimental techniques. Later, similar studies can be performed using specimens radiated by neutrons at LANL

Environment-Induced Degradation and Crack-Growth Studies of Candidate Target Materials: Annual Progress Report (May 2003 – May 2004)

As indicated in the original proposal, the primary objective of this task was to evaluate the effect of hydrogen on environment-assisted cracking of candidate target structural materials for applications in spallation-neutron-target (SNT) systems such as accelerator production of tritium (APT) and accelerator transmutation of waste (ATW). The materials selected for evaluation and characterization were martensitic stainless steels including Alloy EP 823, HT-9, and Type 422 stainless steel. The susceptibility to stress corrosion cracking (SCC) of these materials were evaluated in neutral and acidic aqueous environments using smooth and notched tensile specimens under constant-load (CL) and slow-strain-rate (SSR) conditions. Further, the localized corrosion (pitting and crevice) behavior of these alloys was evaluated by electrochemical polarization technique. The extent and morphology of cracking and localized corrosion of the tested specimens were determined by optical microscopy and scanning electron microscopy (SEM). The experimental program proposed in this task was refocused to evaluate the effect of molten lead bismuth-eutectic (LBE) on the corrosion behavior of similar target structural materials in the presence of oxygen. Since the Materials Performance Laboratory (MPL) at UNLV could not accommodate this type of testing, the LBE loop at the Los Alamos National Laboratory (LANL) is currently being used to contain the stressed test specimens to evaluate the SCC and localized corrosion (pitting and crevice) behavior of all three candidate alloys in the molten LBE environment. Since the magnitude of the applied load/stress during these tests could not monitored or controlled (as in conventional SCC experiments) in the LBE environment, the test specimens were self-loaded. Two types of specimen configurations, namely C-ring and U-bend, were used to perform these experiments. The results of SCC testing being conducted at LANL are not yet available. The stress of principal interest in both types of specimen is the circumferential stress. SCC tests using these types of self-loaded specimens have also been conducted at MPL in aqueous environments having neutral and acidic pH values at ambient and elevated temperatures

Use of Positron Annihilation Spectroscopy for Stress-Strain Measurements

The purpose of this collaborative research project involving the University of Nevada, Las Vegas (UNLV) and Idaho State University (ISU) is to evaluate the feasibility of determining residual stresses of welded (after pre-straining) engineering materials using a new nondestructive technique based on positron annihilation spectroscopy. The proposed technique is to use γ-rays 2 from a small MeV electron Linac to generate positrons inside the sample via pair production. This method can be used for materials characterization and investigation of defects in thick samples that could not be accomplished by conventional positron techniques or other nondestructive methods. The data generated will be compared to those obtained by other methods such as neutron diffraction (for thin samples only) and ring-core techniques. Materials to be tested in the initial phase will be unirradiated austenitic (Type 304) and martensitic (EP- 823) stainless steels that will be cold-worked and welded prior to the evaluation of their residual stresses. Metallurgical microstructures will also be evaluated. Later, irradiated austenitic materials (Type 316L stainless steel and Alloy 718) may be included in this program

Effect of Silicon Content on the Corrosion Resistance and Radiation-Induced Embrittlement of Materials for Advanced Heavy Liquid Metal Nuclear Systems: Quarterly Progress Report (November 2004 – January 2005)

This proposal is intended to study the effect of Si content not only on the corrosion resistance but also on the radiation-induced embrittlement of martensitic stainless steels. The susceptibility of these alloys with different Si content to stress corrosion cracking, general corrosion and localized corrosion will be evaluated in the molten LBE and aqueous environments of different pH values using state-of-the-art testing techniques. Testing in the aqueous media is intended to develop baseline data for comparison purpose. Radiation-induced embrittlement of these alloys will initially be studied by irradiating the test specimens with bremmstrahlung gamma radiation from 20-40 MeV electron beams at ISU. These gammas induce (γ, n) reactions in the giant dipole energy region. The principal radiation damage from these irradiations, in turn, stems from the recoiling residual nucleus (with average kinetic energy of approximately 20,000 eV) after the neutrons are emitted. The high penetrability of gammas, whose range is of order one meter in steel, ensures that the resulting damage will be uniform over the volume of the sample. The induced activity of these specimens will have very short half-lives (typically minutes) due the systematics of (slightly) proton-rich nuclei. The resulting radiation-induced hardening can subsequently be evaluated by proper experimental techniques. Accomplishments: ● Four experimental heats of martensitic alloys (similar to Mod9Cr-1Mo) with different Si content (0.5, 1.0, 1.5 and 2.0 weight percent) have been melted by vacuum-induction melting (VIM) practice at the Timken Research Laboratory. ● The VIM heats have been processed into round and rectangular bars, followed by thermal treatments. The heat treatments consisted of austenitizing, quenching, tempering and air cooling to develop a fully-tempered martensitic microstructure in all heats. ● Fabrication of test specimens has been initiated. ● Literature review on relevant topics is in progress

Use of Positron Annihilation Spectroscopy for Stress-Strain Measurements: Quarterly Progress Report (December 01, 2004 – February 28, 2005)

The purpose of this collaborative research project involving the University of Nevada Las Vegas (UNLV), the Idaho State University (ISU), and the Los Alamos National Laboratory (LANL) is to evaluate the feasibility of determining residual stresses in cold-worked, plastically-deformed (bent), and welded materials using a nondestructive method based on positron annihilation spectroscopy (PAS). This technique uses γ-rays from a small MeV electron Linac to generate positrons inside the sample via pair production. This method is known to have capabilities of characterizing defects in thick specimens that could not be accomplished by conventional positron technique or other nondestructive methods. The data generated by the PAS method has been compared to those obtained by other methods such as neutron diffraction (ND), X-ray diffraction (for thin specimens), and ring-core (destructive-for thick specimens) techniques. During the initial phase of this task residual stresses induced in experimental heats of austenitic type 304L stainless steel, and martensitic Alloy EP-823 have been determined by X-ray diffraction (XRD), PAS and ring-core (RC) techniques. More recently, residual stress measurements have been performed on Alloy HT-9 subjected to cold deformation and welding using all four techniques. The current testing is focused on the evaluation of residual stresses in irradiated materials (welded/plasticallydeformed), and welded specimens, with and without post-weld-thermal-treatment (PWTT). Measurements of residual stresses in cold-worked and welded specimens of Alloys EP-823 and HT-9 are planned to be performed at the Atomic Energy of Canada Limited (AECL) by using the ND technique. Development of calibration curves using the PAS method are also being planned at ISU involving Alloy HT-9. Transmission electron microscopic (TEM) analyses are also being continued

Use of Positron Annihilation Spectroscopy for Stress-Strain Measurements: Quarterly Progress Report (September 01 – November 30, 2004)

Accomplishments: ● A technical paper titled “Residual Stress Characterization in Structural Materials by Destructive and Nondestructive Techniques” has been accepted for publication in the Journal of Materials Engineering and Performance, ASM International, Ohio. ● The PAS method has been applied to develop calibration curves for line shape parameters (S and T) using unstressed and stressed (different magnitude) tensile specimens of martensitic stainless steels. These curves will enable the determination of residual stresses in plastically-deformed materials once the magnitude of S or T parameter is determined by the PAS technique. ● The tensile and welded specimens have been irradiated by low energy photon beam at ISU to compare the residual stresses, with and without radiation. ● Measurements of residual stresses have been performed by the PAS technique on both welded and post-weld thermally-treated specimens to evaluate the effect of thermal treatment on the residual stresses generated due to welding. ● Use of transmission electron microscopy (TEM) has been initiated to analyze voids and dislocations in cold-worked/welded materials

Use of Positron Annihilation Spectroscopy for Stress-Strain Measurements: Quarterly Progress Report (June 01 – August 31, 2004)

Highlights of Test Results: • Residual stress measurements by the RC method on cold-worked specimens showed tensile residual stresses in austenitic stainless steel. However, compressive residual stresses were observed in martensitic stainless steel. This difference may be attributed to the difference in metallurgical phases and microstructures resulting from different thermal treatments imparted to them. • Residual stress measurements by both ND and RC techniques on welded specimens showed similar patterns. Welded specimens consisting of similar material showed tensile residual stresses in the vicinity of the fusion line (FL). However, welded specimens consisting of dissimilar materials (austenitic and martensitic stainless steel on opposite side) showed a different pattern, in that residual stresses were compressive near the FL on the martensitic stainless steel side as opposed to tensile residual stresses on austenitic stainless steel side of the same specimen. • The measurements of residual stresses by the PAS technique revealed a reduction in Tparameter with the increased plastic deformation for both austenitic and martensitic stainless steels. A reduced T-parameter indicates enhanced residual stresses in either Alloy. Other Accomplishments: • One graduate student performed PAS measurements on welded specimens consisting of both similar and dissimilar materials at Idaho State University (ISU) during the summer of 2004. Also, tensile test specimens were subjected to the activation process using ISU accelerator to develop calibration curves. Data analyses are ongoing. • Several papers, based on recent data, were presented in technical society meetings. Some of these papers are currently under review for publication in technical journals. • Significant progress has been made to characterize residual stresses in terms of dislocation density by using TEM

Use of Positron Annihilation Spectroscopy for Stress-Strain Measurements: Annual Progress Report (May 2002 – May 2003)

The purpose of this collaborative research project involving the University of Nevada Las Vegas (UNLV) and Idaho State University (ISU) to evaluate the feasibility of determining residual stresses of welded, bent (three-point-bend), and cold-worked engineering materials using a new non-destructive technique based on positron annihilation spectroscopy. The proposed technique is the use γ-rays from a small MeV electron Linear accelerator (LINAC) to generate positrons inside the sample via pair production. This method can be used for materials characterization and investigation of defects in thick samples, which could not be accomplished by conventional positron technique or other non-destructive methods. The data generated will be compared to those obtained by other non-destructive methods such as neutron diffraction and X-ray diffraction (XRD), and a destructive method known as the ring-core technique. Materials that are currently being tested in the experimental program are Austenitic Type 304L stainless steel (SS), Martensitic Alloys EP-823 and HT-9