Determination of lower bound fracture toughness of a high strength low alloy steel welded joint

Abstract

The use of high strength low alloy steels for high performance structures (e.g. pressure vessels and pipelines) requires high strength consumables to produce an overmatched welded joint. This globally overmatched multipass welded joint contains two significantly different microstructures, as-welded and reheated. In this paper, the influence of weld metal microstructure on fracture behaviour is estimated in comparison with the fracture behaviour of composite microstructures (as-welded and reheated). The lower bound of fracture toughness for different microstructures was evaluated by using the modified Weibull distribution. The results, obtained using specimens with crack front through the thickness, indicated low fracture toughness, caused by strength mismatching interaction along the crack front. In the case of through thickness specimens, at least one local brittle microstructure is incorporated in the process zone at the vicinity of the crack tip. Hence, unstable fracture occurred with small, or without, stable crack propagation. Despite the fact that the differences between the impact toughness of a weld metal and the that of base metal are insignificant, the fracture toughness of a weld metal can be significantly lower.     

Resistance and friction stir spot welding of DP600: a comparative study

Abstract

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Friction stir spot welding (FSSW) was invented as a novel method to spot welding sheet metal and has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn coated DP600 AHSS (1·2 mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process. Results show a correlation found among microstructure, failure loads, energy requirements and bonded area for both spot welding processes.     

Study on resistance welding of metallic sandwich panel

Abstract

Inner structured and bonded (ISB) panel, a type of metallic sandwich panel, consists of two thin skin plates bonded to a micropatterned inner structure. It has an overall thickness of 1–3 mm and attractive properties, such as ultralight weight, high efficiency in stiffness to weight and strength to weight ratios. In this paper, a new joining process using resistance welding is developed for joining ISB panels. Electrodes with curved surface are applied to join the two skin plates and micropatterned inner structure, while two stoppers are used to keep a gap between the electrodes during welding. Owing to this gap, the inner structure shape is maintained without damage. Current, welding time and electrode gap were selected as the process parameters. This process is simpler and more suitable for mass production than previous processes for joining the panel. Appropriate welding conditions were studied by measuring the tensile load with respect to variations in welding time and electrode gap.     

Corrosion and protection characteristics of phosphate coatings: effect of yttrium oxide as additive

Abstract

The corrosion and protection characteristics of phosphate coatings formed in a phosphating solution containing mainly ZnO, H₃PO₄ and NaF, using Y₂O₃ as an additive, were investigated through SEM, polarisation curves and EIS diagrams. The results show that the corrosion protection of phosphate coatings has been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and causing the corrosion current to decrease. The protection ability of phosphate coatings depends mainly on their barrier performance. The phosphate coatings formed in the phosphating solution with 10 and 20 mg L^–1 Y₂O₃ have finer crystal structures and smaller porosity; therefore, they exhibit better corrosion resistance and adhesion properties than those without Y₂O₃ and with 40 mg L^–1 Y₂O₃.     

Erosive abrasive wear behaviour of stainless steel surface produced by plasma transferred arc hardfacing

Abstract

Erosive abrasive wear is caused by high speed impact of particles entrained in a fluid system on the surfaces of components such as boilers and furnaces. Erosive abrasive wear in boilers results from the impact of hard particles such as ash or clinker entrained in flue gases and can lead to serious damage. The life of boiler and furnace components encountering erosive abrasive wear in service, which are most commonly fabricated from carbon steels, can be improved by hardfacing with a wear resistant material. The effects of wear parameters such as particle size, flux and velocity on the erosive abrasive wear behaviour of a stainless steel surface produced by the plasma transferred arc hardfacing have been investigated using an experimental design approach. The wear resistance of the stainless steel surface was found to be twice that of the carbon steel substrate.     

Textured surfaces in sliding boundary lubricated contacts – mechanisms,possibilities and limitations

Abstract

The mechanisms of friction and wear in boundary lubrication are complex with influences from the surface roughness and hardness of surfaces, the lubricant and the wear products. Introduction of a texture on either surface can influence several important parameters. Wear particles can be collected or produced by the surface texture. A lubricating film can suffer or gain and the lubrication regime might change. This paper presents an overview of the tribological effects and important parameters of textured surfaces in sliding boundary lubricated contact, based on the experience of the authors and on published results. Examples of successful and less successful textured contacts are given and some recommendations regarding size, orientation and textured area fraction are presented.     

Biocompatible alumina ceramic for total hip replacements

Abstract

Their resistance to wear and biocompatibility make ceramics ideal materials for medical applications, such as implants. For over 30 years, pure alumina has been the dominant material for ceramic hip prostheses. Interest in alumina hip prostheses continues to grow, due to the relatively short life of polymer/metal prostheses, mainly resulting from osteolysis and aseptic loosening caused by polymer wear debris. Since its introduction by Boutin in the 1970s, substantial improvements have been achieved in the microstructure of medical grade alumina by improving purity and processing to give complete densification and fine, uniform grain sizes. A brief review is given of the types of alumina used in total hip replacement, the development of medical grade alumina, and methods of in vivo and in vitro investigation of alumina prostheses, with a focus on current knowledge of the damage observed on alumina prostheses. Particular attention is paid to wear mechanisms and the influence of materials properties on wear behaviour. A region of relatively severe wear, known as stripe wear, is widely observed on retrieved alumina hip prostheses. This type of wear can now be replicated in vitro in joint simulators by the introduction of a 'microseparation' motion during the test cycle. Finally, the future of ceramic hip prostheses and development of the next generation of ceramics for hip prostheses is discussed.     

Tribologic behaviour and suspension stability of iron and copper nanoparticles in rapeseed and mineral oils

Abstract

Metal particles, suspended as wear debris or as additives, have a major influence on lubrication technology. They are known mostly for negative roles, but even iron nanoparticles, which are likely to pass through most filters, have not yet been quantitatively evaluated for their effects on friction and wear. In this study iron and copper nanoparticle suspensions were formulated in high sulphur paraffinic mineral oils and food grade rapeseed oil. The suspension stabilisation mechanism based on steric repulsion appeared more effective than ionic repulsion principle. Iron nanoparticle suspensions were investigated using four ball antiwear tribotester. Iron nanoparticles did not show statistically significant effects on wear or friction in mineral oil suspension. However, addition of surfactants improved the tribological performance. Wear, friction and sample temperature data along with microscopy evaluation suggested that formation of protective films in the friction zone was the most likely reason for reduction of wear, average friction and the duration of break-in regime.     

Surfactant dithiophosphates lubricating oil additives

Abstract

This paper presents a research on the tribological properties and the action mechanisms of surfactant lubricating oil additives, polyalkoxy glycol ammonium salts and O,O,S-triesters of polyalkoxy glycol dithiophosphoric acid. To understand the physicochemical properties of surfactants, polyalkoxy glycol dithiophosphoric acid behaviour both in solution and at metal surfaces are identified and discussed. The STM adsorption on steel surface, four-ball and ball disc tests of friction of water, synthetic and mineral oil solutions studies of these additives are presented. Friction and wear properties are dependent on tribological parameters, chemical structure of the additives and their surface active behaviour in tribological systems. The relationships are presented on the example of additives with different molecular structures and masses.     

Abrasive wear behaviour of sintered composite austenitic stainless steels having γ-Fe and M23C6 phases

Abstract

Using powder metallurgy, composites of austenitic stainless steel were produced along with unreinforced stainless steel mixed with titanium, cobalt and molybdenum particles. Wear resistance of the materials was measured by a two body pin on disc wear tester. SiC abrasive papers of 80 and 220 mesh sizes were used as abrasive media. Wear tests were performed under loads of 10, 20 and 30 N at room temperature. The abrasive wear measurements showed that the softer, unreinforced austenitic stainless steel exhibited higher mass loss than the composites. Furthermore, the abrasive wear resistance of the reinforced austenitic stainless steel composites increased with increasing FeTi, FeMo, or Co volume content. In addition, the wear rate against the 80 grade SiC abrasive paper increased more than against the 220 grade SiC abrasive paper.