The Criteria for the Transitions Between the Various Oxidation Modes of Binary Solid-Solution Alloys Forming Immiscible Oxides at High Oxidant Pressures

The possible high-temperature corrosion modes ofbinary solid-solution alloys forming two immisciblecompounds by a single oxidant include (1) the exclusivegrowth of external scales of the most-noble component, which may or may not be associated with theinternal oxidation of the most-reactive component, (2)the formation of composite external scales containing amixture of the two compounds, or finally (3) the exclusive growth of the most-stable compound asan external scale. The conditions for the stability ofeach scale structure depend on a number of thermodynamicand kinetics parameters, whose effects are examined quantitatively in this paper. Theconditions for the stability of the various structuresand the criteria for the transitions among them are alsoexamined. The maximum number of possible scale structures is four, but it can reduce to threeand, in some cases, only to two. In particular, theinternal oxidation of the most-reactive component maynot occur if the stabilities of the two oxides are not sufficiently different from eachother.     

Flexible and Scalable Automation Solutions For Scoring Single Nucleotide Polymorphisms Using Rolling Circle Amplification

A single nucleotide polymorphism (SNP) scoring assay that uses ligation-dependent Rolling Circle Amplification (RCA)† was transferred to a series of automated protocols addressing a range of throughput levels. The systems utilised various automation modules consisting of custom-made and offthe-shelf devices. Several system parameters were evaluated to ensure assay integrity and homogeneity. These included reagent carry over, liquid evaporation rates, thermal regulation of reactions and fluorescence reading capabilities.Data analysis software was developed in order to rapidly allocate SNP calls from data generated by the automated system. A modified fuzzy c -means clustering algorithm was employed to separate data points into groups associated with specific genotypes. Data were then presented graphically and within a summary table, which allowed easy and rapid organization and interpretation of data.     

High speed fusion weld bead defects

Abstract

A comprehensive survey of high speed weld bead defects is presented with strong emphasis on the formation of humping and undercutting in autogenous and non-autogenous fusion welding processes. Blowhole and overlap weld defects are also discussed. Although experimental results from previous studies are informative, they do not always reveal the physical mechanisms responsible for the formation of these high speed weld bead defects. In addition, these experimental results do not reveal the complex relationships between welding process parameters and the onset of high speed weld bead defects. Various phenomenological models of humping and undercutting have been proposed that were based on observations of events in different regions within the weld pool or the final weld bead profile. The ability of these models to predict the onset of humping or undercutting has not been satisfactorily demonstrated. Furthermore, the proposed formation mechanisms of these high speed weld bead defects are still being questioned. Recent welding techniques and processes have, however, been shown to be very effective in suppressing humping and undercutting by slowing the backward flow of molten metal in the weld pool. This backward flow of molten weld metal may be the principal physical phenomenon responsible for the formation of humping and undercutting during high speed fusion welding.     

Fracture mechanical investigation of steel grade S890 used in Cleuson–Dixence hydropower plant shaft

Abstract

As a consequence of the accident at the Cleuson–Dixence hydropower plant in December 2000, several investigations were started. In a material qualification programme for the steel S890, as applied in the Cleuson–Dixence plant, strength investigations and small scale fracture mechanic tests in weld metal, heat affected zone and base metal have been performed. Additionally, wide plate tests were performed to verify the results. With the obtained material properties, a fracture mechanics assessment of the critical crack size of the shaft in the area of the pipe rupture was performed using the R6 procedure. This procedure was verified for the given conditions, i.e. material strength and toughness including the welding quality, by the comparison of the calculational and experimental results of the tested wide plates with cracks in base metal and weld metal.     

Modelling of electrically enhanced friction stir welding process using finite element method

Abstract

Conventional friction stir welding (FSW) of high strength and high melting point materials, such as steel and titanium, has the disadvantages of a serious tool wear problem and slow welding speed. A new friction stir welding process for such materials called 'electrically enhanced friction stir welding process (EHFSW)' has been suggested and analysed using finite element modelling. The basic idea of EHFSW is that electric current passes from the welding tool into the workpiece through the contact area in the welding region. Thus it results in more localised heating while welding is in progress and is not simply a preheating process. The temperature distribution in the workpiece during the pin plunge stage and the welding stage of the EHFSW process has been determined. The results show that EHFSW can reduce the plunge force significantly with the help of localised electrical heating during the pin plunge stage, which may imply lower tool wear when compared with conventional FSW. At the same time, in the welding stage, the simulation results indicate that the welding speed of the EHFSW process can be at least two times faster than that of the conventional FSW process. Thus, finite element analysis shows that EHFSW is a promising process and could reduce tool wear while improving the welding speed, especially for high melting/O point materials.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Application of novel heat treatments for aluminium high pressure die castings to industrially produced components

Abstract

High pressure die casting (HPDC) is widely used as a cost effective way to mass produce metal components that are required to have close dimensional tolerances and smooth surface finishes, accounting for ～50% of the aluminium castings produced worldwide. These components are not considered to be heat treatable by conventional means because the high temperatures involved with solution treatment cause surface blistering and dimensional instability. A new heat treatment procedure involving a truncated solution treatment at lower than conventional temperatures alleviates this problem and can significantly improve mechanical properties, in many cases, doubling the 0·2% proof strength after artificial (T6) aging. This may enable current HPDC parts to be redesigned to use less metal while still achieving the required performance. The cost of heat treatment can be easily offset by the reduction in metal content and productivity improvements which result in an overall lower cost of the part. The new process also creates opportunities to substitute for some other cast or wrought products with aluminium HPDC parts of lower weight and lower cost. Application of this heat treatment technology to a range of industrially produced HPDC components is discussed and the cost advantages are briefly considered.     

Homogenisation of heat treatment process in high pressure gas quenching

Abstract

High pressure gas quenching has the advantages of pure convective heat transfer, high levels of control, avoidance of cleaning the quenched parts and low environmental impact. However, typical gas quenching facilities exhibit inhomogeneous flow conditions through the quench load and the parts, resulting in scatter in final properties. The upstream flow profile of the load has been identified as a key factor determining local flow conditions and heat transfer. The intensity of the quenching process is determined by the pressure drop that results from the flow resistance of the quench load, although a significant part of the flow passes between the load and the chamber walls and does not contribute to the quenching process. A simulation of the flow inside a commercial high pressure gas quenching chamber was carried out, using a multiscale model to give faster convergence. An experimental analysis of the flow inside a model quenching chamber through velocity measurements and flow visualisation was also performed. Finally, a quenching run with cylindrical parts in a double-chamber vacuum furnace was used to validate the model results. Various upstream velocity profiles were applied to demonstrate their influence on the quenching result. The multiscale simulation approach and the results of the flow process investigation are reported. Guidelines for gas quenching process optimisation are outlined.     

Microstructure,hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel

Abstract

The present investigation reports on a study that has been taken up to develop an understanding of the electron beam welding characteristics of similar and dissimilar combination of maraging steel and high strength low alloy steel, which are in the hardened condition, i.e. maraging steel, in a solution that was in treated and aged condition, whereas high strength low alloy steel in a quenched and tempered condition before welding. The joint characterisation studies include microstructural examination, microhardness survey across the weldment and measurement of residual stresses. Maraging steel weld metal is under compressive stress rather than tensile stress as observed in low alloy steel welds because the martensite transformation occurs at a relatively low temperature. It has been observed that, in dissimilar metal welds, tensile stress is observed at the fusion boundary of low alloy steel and weld metal, whereas compressive stress is obtained at the location between weld and maraging steel fusion boundary. Dissimilar weldment contains a soft region beside the interface on maraging steel side because of the diffusion of manganese from low alloy steel towards maraging steel. The observed residual stresses, hardness distribution across the similar and dissimilar metal welds are correlated with the observed microstructures.     

Development of microstructure and mechanical properties of as cast Al–Mg–Mn alloy during high temperature ECAP

Abstract

A large scale billet with diameter of 58·5 mm of an as cast Al–Mg–Mn alloy was processed by equal channel angular pressing (ECAP) at 350°C up to six passes. A significant refinement of the grains was observed after six pressings to ～2 μm. And the selected area electron diffraction (SAED) pattern showed that almost all of the grains were separated by boundaries with high angles of misorientation. A banded substructure was not observed during the hot ECAP, and a reasonably equiaxed structure was obtained just after one single pressing. Both the strength and the elongation increased abruptly in a single passage through the die, but thereafter, the increase was more gradual and exhibited a saturation effect after the fourth pressing. The good combination of strength and ductility of the Al–Mg–Mn alloy attained by the hot ECAP appeared to be attractive properties for industrial applications. Moreover, hot ECAP could possibly be used as an alternative step to hot extrusion or hot rolling in industrial processing, to break down an initial coarse as cast structure in a quite large scale billet.