Stress Accelerated Grain Boundary Oxidation Research Articles

The influence of oxygen partial pressure on the crack propagation of superalloy under fatigue-creep-environment interaction

We elucidate here the effect of low oxygen partial pressure on the dwell fatigue crack growth rate (DFCGR) of 718Plus alloy at high temperature. In comparison under the air and vacuum conditions with lower oxygen partial pressure, the DFCGR of 718Plus alloy reduced by three orders of magnitude at 704 °C at lower oxygen partial pressure than that of in air condition. By analysis of microstructure, oxide, and crack path under different conditions, it was considered that the oxygen-induced dynamic embrittlement (DE) was the dominant impact mechanism for fatigue crack propagation at high-temperature in 718Plus alloy. Meanwhile, the stress-accelerated grain boundary oxidation (SAGBO) mechanism also played a significant role, especially in the long step-wise period. In addition, the types of oxide formed in creep process were directly related to the oxygen partial pressure and oxidation time.

Effect of stress and atmosphere on grain boundary oxidation of Ni-based superalloy for 700°C A-USC steam turbine

700°C class A-USC (Advanced Ultra Super Critical) thermal power plant is one of the solutions to reduce carbon dioxide (CO2) emission. In order to meet the design requirement, which is capable to be operated under 700°C steam condition, the development of Ni-based superalloys is essential. Fundamental research and demonstration of mock-up plants have been driven by national project in Japan, US, Europe and China, to confirm the reliability of Ni-based superalloys. One issue of superalloys reported by others is the stress accelerated grain boundary oxidation (SAGBO), leading to the deterioration of creep rupture properties during service. We newly manufactured the test apparatus which enables to creep test under high temperature steam environment, and evaluated the grain boundary oxidation behavior of Ni-based rotor material, TOS1X-2, under various stresses and atmosphere. Microstructure observation after creep tested at 750°C for 2,500h, found that thin layer of Cr-rich and Al-rich oxides form at all stress and atmosphere condition. Detail studies revealed that the Al-rich oxide were formed at fine, recrystallized grain boundary, which is driven by initially induced strain by machining. The quantitative analysis of microstructure showed that steam atmosphere slightly accelerates the grain boundary oxidation. On the contrary, effect of applied stress on oxidation depth was not observed. The grain boundary oxidation observed in this study was very small, suggesting that TOS1X-2 exhibits good oxidation and creep rupture properties in 700°C A-USC operating condition.

Impact of the surface quality on the thermal shock performance of beryllium armor tiles for first wall applications

Beryllium will be applied as first wall armor material in ITER. The armor has to sustain high steady state and transient power fluxes. For transient events like edge localized modes, these transient power fluxes rise up to 1.0GWm−2 with a duration of 0.5–0.75ms in the divertor region and a significant fraction of this power flux is deposited on the first wall as well. In the present work, the reference beryllium grade for the ITER first wall application S-65 was prepared with various surface conditions and subjected to transient power fluxes (thermal shocks) with ITER relevant loading parameters. After 1000 thermal shocks, a crucial destruction of the entire loaded area was observed and linked to the stress accelerated grain boundary oxidation (SAGBO)/dynamic embrittlement (DE) effect. Furthermore, the study revealed that the majority of the thermally induced cracks formed between 1 and 10 pulses and then grew wider and deeper with increasing pulse number. The surface quality did not influence the cracking behavior of beryllium in any detectable way. However, the polished surface demonstrated the highest resistance against the observed crucial destruction mechanism.

Investigation of the crack growth behavior of Inconel 718 by high temperature Moiré interferometry

The effect of environment on creep crack growth behaviors of many nickel-base superalloys is a well-documented and serious problem. Stress accelerated grain boundary oxidation (SAGBO) is accepted as the prior mechanism of the environment effect. In this paper, the crack growth behavior of Inconel 718 was investigated by high temperature moire interferometry (HTMI), coupled with SEM/EDAX. Based on the results obtained from this research, the mechanism is proposed to be caused by the segregated Nb, which couples with the oxygen diffusing into the grain boundaries in front of the crack tip and forms an NbO layer on the grain boundaries, thereby causing the brittle elastic cracking behavior.

Completion of CS insert fabrication

The central solenoid (CS) model coil program is in progress with an international collaboration under the frame of the ITER-EDA. The purpose of the CS insert coil is to test the performance of the ITER-CS conductor. The CS insert coil is installed in the bore of the CS model coil and tested at a magnetic flux density of 13 T. The installation work is underway with the inner and outer module of the CS model coil. The superconducting characteristics of the CS conductor, the critical current and the current sharing temperature are evaluated under the operating load. The AC loss characteristics of the conductor are also evaluated under pulsed magnetic field. The fabrication of the CS insert coil was completed on May 1999. The winding tools and the results of the winding of CS insert coil are reported. The heat treatment for Nb/sub 3/Sn processing was performed successfully with no SAGBO (stress accelerated grain boundary oxidation). The procedure of the heat treatment is also reported.

ITER niobium-tin strands reacted under model coil heat-treatment conditions

Heat-treatment conditions were studied for ITER Central Solenoid model coils wound with Nb/sub 3/Sn cables jacketed in Incoloy-908 conduits. The heat treatments recommended by the strand manufacturers had to be modified to meet various requirements for the large coils and to prevent Stress Accelerated Grain Boundary Oxidation (SAGBO) of Incoloy 908. Single-strand tests of ITER wires produced by Vacuumschmelze, IGC, Furukawa and Mitsubishi were carried out prior to the model coil heat treatments to verify the heat-treatment conditions. The modified heat treatments improved the performance of all four wires. Especially, critical currents of Furukawa wire were improved significantly. Test results of the witness samples co-reacted with ITER CS coils indicate that the heat treatments of all 10 layers of the inner module were successfully completed.

高温強度 インコロイ９０８熱処理中の粒界割れ解析

The Incoloy 908 jacket material exhibited intergranular cracking during superconductor reaction heat treatment at 650°C. The mechanism of this intergranular cracking was attributed to stress-accelerated grain boundary oxidation (SAGBO). Incoloy 908 is the jacket material for the cable-in-conduit superconductor for the International Thermonuclear Experimental Reactor (ITER). Fracture surfaces mapped by wave dispersive X-ray (WDX) analyses indicated an accumulation of oxygen at the crack initiation region, but did not show any presence of oxygen around the crack tip. A computational simulation of the conductor manufacturing process indicated the presence of residual surface stresses in the jacket material. Residual surface tensile stresses are created on the jacket outer surface, as a result of plastic deformation during conductor fabrication (known as the jacket compaction process). It has been concluded that the successful heat treatment of the jacket material can be accomplished by:(1) Careful shotpeening on the outer surface of the jacket material to create compressive surface stresses and(2) Evacuation of oxygen by removing oxygen sources such as the oil contaminants and water vapor from the conductor at temperatures below 550°C.

Protection of Ni-base superalloys against stress accelerated grain boundary oxidation (SAGBO) by grain boundary chemistry modification

It is well established that the creep and hold-time fatigue crack propagation rate of many Ni-base superalloys is accelerated by up to two orders of magnitude in air relative to vacuum environment. This effect is associated with the embrittlement of grain boundaries by oxygen in the presence of tensile stresses. In the literature, it is often referred to as stress accelerated grain boundary oxidation (SAGBO). Measures that improve crack growth resistance in air are (1) enlarging the grain size, (2) overaging the microstructure, and (3) modifying the base metal chemistry, for example by addition of grain boundary strengtheners such as boron and zirconium. However, these measures adversely affect other important properties such as yield strength, ultrasonic inspectability or--in the last case--forgeability and weldability. An alternative approach is presented here. Utilizing the beneficial effect of grain boundary strengthening elements such as boron while avoiding their detrimental effect on manufacturability if added to the master melt. The basic idea is to diffuse these elements into the component by a deposition and heat treatment process after manufacturing. As a result, environmentally exposed grain boundaries can be chemically modified for improved resistance against SAGBO. This new approach is described in detail. The beneficial effectmore » on elevated temperature performance is demonstrated using constant strain rate tests at 600 C.« less

Characterization of Incoloy 908 and Avoidance of Stress Accelerated Grain Boundary Oxidation (SAGBO) during ITER Model Coil Fabrication

Incoloy 908, an iron-nickel base superalloy that was developed as a Nb3Sn jacket material for Cable-In-Conduit Conductors, has been selected as the jacket material for the International Thermonuclear Experimental Reactor (ITER) Toroidal Field (TF) and Central Solenoid (CS) coils. It has a coefficient of expansion matching Nb3Sn (to minimise Jc and Tc degradation due to differential contraction after the reaction heat treatment). The alloy exhibits a characteristic of iron-nickel base superalloys: oxygen embrittlement along grain boundaries as a result of heating in an oxygen atmosphere when tensile surface stresses are present. For applications using Incoloy 908, techniques are required to control levels of either oxygen or tensile surface stresses during heat treatment. R & D results performed to develop and qualify such techniques for industrial applications are presented. The work has concentrated on establishing the lowest achievable oxygen levels inside the cable space during the reaction heat treatment and determining the conditions that can be tolerated inside and outside the jacket before SAGBO occurs. The results were applied in the ITER Model Coil programmes, in which about 5.5 km of conductor have been successfully heat treated.

Residual stress of a jacket material for ITER superconducting coil

The jacket material, Incoloy 908, of the superconducting coil for International Thermonuclear Experimental Reactor (ITER) has a possibility of fracture due to stress-accelerated grain boundary oxidation (SAGBO) in the region of tensile residual stress above 200 MPa. In order to avoid the SAGBO fracture, a depth dependence of the strain in the jacket sample was obtained from the (1 1 1) plane spacing measured with the neutron diffractometer, RESA, installed at JRR-3M in JAERI. A residual stress distribution was calculated from the strain. The result shows that the tensile residual stress exceeds 200 MPa of the SAGBO condition in some areas inside the jacket.

Epsilon and Eta Phases Precipitated in an Fe-38Ni-13Co-4.7Nb1.5Ti-0.4Si Superalloy.

An Fe-38Ni-13Co-4.7Nb-1.5Ti-0.5Si alloy is a controlled low thermal expansion superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures by an intensional Si addition. The precipitation of second phases, e phase, η phase and so on, in this superalloy is investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffractometry. A time-temperature-transformation diagram for the e phase is presented.The crystal structure and the lattice parameters of the e phase and the orientation relationship with the alloy matrix g are determined. It is shown that the η phase is not a stable phase as well as γ' phase, but cellular η phase is the most stable precipitation phase in this alloy.

The potential for SAGBO cracking in a Nb3Sn wind-and-react solenoid using a CICC with Incoloy 908 jacket and glass fabric insulation

The 45-T Hybrid Magnet System being constructed for the National High Magnetic Field Laboratory (NHMFL) in collaboration with the Francis Bitter National Laboratory is composed of a resistive insert and superconducting outsert. Two of the outsert solenoids use Nb3Sn and will be wound with cable-in-conduit conductors (CICCs) using the wind-and-react technique. A candidate for the jacket material is Incoloy 908, but an obstacle to this choice is its sensitivity to Stress Accelerated Grain Boundary Oxidation (SAGBO). The conditions for SAGBO-residual stress, elevated temperature, and exposure to oxygen (even in extremely low levels)-are difficult to avoid in coils fabricated by the wind-and-react process. The glass fabric insulation, applied during winding, is at high temperature, both a source of oxygen (primarily in the form of absorbed water vapor) and, being highly compacted by the winding, an impediment to its removal. In attempt to solve this problem NHMFL has developed a pre-baking procedure in vacuum at 730 C, to be applied to the glass fabric before to insulate the coil. In order to test the efficiency of pre-baking and to check the existence of possible other features that also lead to SAGBO, a dummy coil, wound with a CIC conductor jacketed with Incoloy 908, has been manufactured. The coil has been heat treated in very high purity helium atmosphere. The results are reported.

Precipitation and Growth of .GAMMA.' Phase in an Fe-38Ni-13Co-4.7Nb Superalloy.

An Fe-38Ni-13Co-4.7Nb base superalloy (alloy 909) is the latest low thermal expansion chromium-free superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures. This investigation is carried out to elucidate the relation between the age-hardening and the nucleation and growth behavior of γ' precipitates in alloy 909 by micro-Vickers hardness test and transmission electron microscopy. The hardness of alloy 909 measured at aging temperatures is ca. HV100 lower than that measured at room temperature. The hardness of specimen aged at 893-1033 K for durations up to 720 ks closely relates to the mean size of γ' precipitates. The growth kinetics of the γ' precipitates in e phase free region is explained by Lifshitz-Slyozov-Wagner's theory of volume diffusion controlled growth at 943-1033 K. The activation energy for the growth of γ' precipitates is estimated to be 254 kJ/mol which is nearly equal to those of diffusion of Ti or Fe atoms in γ-iron or nickel.

Materials selection and processing issues pertinent to the fabrication of a large magnet by the insulate-wind-react-impregnate technique

The 45 T hybrid magnet system under construction for the National High Magnetic Field Laboratory will contain three superconducting coils using cable-in-conduit conductor technology. Two of these will use Nb 3Sn wire and will be fabricated by the insulate-wind-react-impregnate technique. Critical issues are the selection of the fabric with which the conductor will be wrapped before winding, removal of the fabric sizing (which may leave carbon residue), suppression of oxygen in the heat-treatment vessel (because of the sensitivity of the Incoloy 908 conduit to cracking due to stress-accelerated grain-boundary oxidation) and full impregnation of the interstices of the windings for good structural and electrical insulation integrity. The challenge in this task is that all requirements must be met simultaneously. We report on the test programme to verify that the selected materials and processes will be successful when applied to fabrication of the final coils.