Reg Eadie

Development of a new wear-resistant material: TiC/TiNi composite

In this work, an attempt was made to develop a novel type of wear-resistant composite employing a TiNi alloy matrix reinforced by hard particles. Titanium carbide was chosen as the reinforcing phase because of its high hardness and TiNi alloy as the matrix due to its pseudoelasticity and good toughness. TiC particles may sustain external load, while the TiNi matrix may accommodate deformation, absorb impact energy and retain the hard particles. Such a combination is expected to lead to an enhanced wear resistance, compared to TiNi alloy. As a matter of fact, some efforts were previously made to develop TiNi-matrix composite reinforced by ceramic particles. However, the emphasis of those studies was put on effects of the reinforcing particles on the phase transformation behavior, shape memory effect and some mechanical properties of the composite; no attempt was made to explore the potential benefit of the material for wear application.

Hydride morphology and striation formation during delayed hydride cracking in Zr-2.5% Nb

Abstract These experiments were designed to study hydride formation at the crack tip, acoustic emission (AE), potential drop (PD) and striation formation during DHC (delayed hydride cracking) in Zr-2.5% Nb. The test material was taken from an especially extrude pressure tube, which showed similar strength properties to normal pressure tube material but somewhat coarser microstructure. In testing at K I below 12 MPa √m at both 200 and 250°C very large striations (> 40 μ at 200 and >50 μm at 250°C) were produced. In simultaneous monitoring with acoustic emission and potential drop, both AE and PD jumps were shown to be monolithic. The number of striations on the fracture surface corresponded to the number of monolithic AE/PD jumps. Tapered shaped hydrides with the thick end adjacent to the crack tip were observed. These hydrides grew in size during the incubation period until they reached the striation length and then fractured monolithically. However, when K I was increased beyond about 12 MPa √m for these same specimens, the striation spacing decreased below 30 μ, the monolithic jumping dissolved into more continuous changes in signals, although the smaller striations were still visible on the fracture surface.

Corrosion Fatigue and Near-Neutral pH Stress Corrosion Cracking of Pipeline Steel and the Effect of Hydrogen Sulfide

Abstract Crack advance has been studied in an X-70 pipeline steel, using the compliance technique. The electrolyte used in the study was a very dilute brine bubbled with 10% carbon dioxide (CO2). C...

The effect of annealing on hardness, microstructure and delayed hydride cracking in Zr–2.5Nb pressure tube material

Abstract The effect of annealing at 400 °C on hardness, microstructure and velocity of delayed hydride cracking (DHC) in Zr–2.5 pressure tube material in both the axial and radial directions was investigated. DHC velocity tests were performed at temperatures from 120 to 240 °C on specimens which had been annealed up to 1000 h. The β-phase initially was a thin, continuous film between α-grains providing a path for rapid hydrogen diffusion. Since the β-phase pathways were closely spaced in the axial direction but not in the radial direction, DHC velocity is 1.9 times faster in the axial direction initially. During annealing, the β-phase decomposed to discrete particles, the room temperature hardness dropped 10% and the axial DHC velocity decreased by a factor of five. The radial crack velocity decreased by a factor of three and the ratio of axial to radial DHC velocity decreased to 1.2. The decrease of this ratio corresponded to the large disruption of the β-phase caused by prolonged annealing. The activation energies for DHC velocity in these tests lay between 33 and 47 kJ/mol, suggesting that the existing theories for activation energy are incomplete.

Wear and friction of a new wear-resistant material: TiNi-based composites

The equiatomic TiNi alloy exhibits an excellent wear resistance benefiting from its pseudoelasticity. The wear behavior of TiNi alloy was expected to improve further if hard particles were embedded in the alloy. The hard-phase particles may withstand the external load and the pseudoelastic matrix can absorb impact energy and accommodate a relatively large strain. TiNi-based composites, reinforced, respectively, with TiC and TiN particles were developed by using a vacuum sintering process. Sliding wear of the composites was evaluated. It was demonstrated that the composite exhibited an enhanced wear resistance with a considerable room for further improvement. Friction of the composites against steel under an oil-lubrication condition was also investigated. The correlation between the mechanical properties and wear and friction of the composites is discussed.

Influences of porosity on mechanical and wear performance of pseudoelastic TiNi-matrix composites

Pores usually exist in sintered tribo-composites and may strongly affect the performance of the composites. In this work, the influence of pores on the wear behavior of sintered pseudoelastic TiNi-matrix tribocomposites was investigated. In particular, changes in the density of pores and corresponding variations in wear resistance of the composites were studied. It was demonstrated that mechanical properties and the wear resistance of the composites were strongly affected by voids. The wear resistance was enhanced when the density of pores was reduced by using wax to enhance the compaction during pressing. It was also interesting to observe that pores are sealed during the wear process. This results in improving wear resistance during wear, especially under high loads. Some other contributing mechanisms are also discussed.

Metallographic and Fractographic Observations of Hydrides during Delayed Hydride Cracking in Zr-2.5% Nb Alloy

Abstract Potential drop measurements, optical microscopy, and scanning electron microscopy were performed to study the mechanism of delayed hydride cracking (DHC), the relation of the fracture to the hydride morphology, and the fractography of the DHC mechanism. The material used in this study was taken from modified extrusions of the material used to manufacture Zr-2.5% Nb pressure tubes. The material was electrolytically hydrided to approximately 60μg/g before testing. Cracking tests were carried out at 250°C with an applied KI of 12MPa m . The number of potential “jumps” was strongly correlated to the number of striations on the fracture surface. The results indicate that the DHC process occurs in these samples in an intermittent fashion. Brittle fracture is the operating fracture mechanism for the hydrides that cover most of the fracture surface, but there are some regions of ductile fracture both within the fracture and at the striations.

An SEM study of β-phase decomposition during the annealing of Zr-2.5% Nb alloy

Abstract The effect of annealing on the decomposition of the β-phase in Zr-2.5% Nb pressure tube material was studied by SEM (scanning electron microscopy). Annealing was carried out at 400°C for 0, 24 and 1000 h. Previous to annealing, the specimens were electrolytically hydrided, the final hydrogen amount being about 60 μg/g. Long time annealing causes the β-phase ligaments to decompose into smaller particles. Hydrides are formed along the β-phase ligaments and they become shorter as the lengths of the ligaments decrease with annealing. The SEM technique proved to be useful in studying the decomposition of the β-phase ligaments.

Corrosion of X-65 Pipeline Steel Under a Simulated Cathodic Protection Shielding Coating Disbondment

Pipeline corrosion is a common problem for polyethylene (PE) tape-coated oil and gas transmission pipes buried under ground. It is often seen under cathodic protection (CP) shielding coating disbondments, where a wrinkle filled with ground water could be formed. This study investigates corrosion of X-65 pipeline steel under a simulated CP shielding coating disbondment with a gap size of 1 cm. Results showed that the corrosion rate of pipeline steel under the CP shielding coating disbondments is a function of carbon dioxide (CO2) and level of CP at the holiday of the coating disbondment, and could be significantly different to that in bulk environments.

Environmental effects on near-neutral pH stress corrosion cracking in pipelines

Publisher Summary Stress corrosion crack (SCC) growth mechanisms in pipeline steels exposed to near-neutral pH environments are not well understood, although cracking is generally thought to result from some combination of crack-tip dissolution and hydrogen embrittlement. This chapter discusses how electrochemical factors in the environment influence the dissolution and cracking rates in near-neutral pH SCC highlighting the factors that influence the tendency of cracks to become dormant. The synthetic near-neutral pH soil solutions are designed on the basis of compositions of electrolytes extracted with distilled water from soils removed from near-neutral pH SCC sites. The corrosion resistance of X-65 steel in various synthetic solutions is determined using weight loss coupons of ∼ 15 × 15 × 1 mm suspended in test solution. The compact toughness specimens are machined from the X-65 pipe with the crack plane normal to the circumferential (hoop) direction. Each specimen is polished to produce a scratch-free surface prior to tests. Specimens are then prefatigued in air to produce a sharp crack tip from the machined notch in accordance with ASTM E647. A possible explanation for SCC morphology in the NOVA Trapped Water solution might be related to creep deformation at the crack tip. The pipeline steels are susceptible to creep deformation at room temperature, which can produce several percent strain under a constant stress above the yield strength.