Simultaneous Aluminizing and Chromizing of Steels to Form (Fe,Cr)3Al Coatings

A cementation pack involving halide activatorsand elemental Al and Cr powders has been used to achievethe codeposition and diffusion of aluminum and chromiuminto low-alloy steels. A two-step treatment at 925°C and 1150°C yields dense anduniform ferrite coatings of about 400-m thickness,with surface compositions of approximatelyFe₃Al plus several percent Cr. The two stepheating schedule prevents the formation of a blocking chromium carbide atthe substrate surface. An attempt was made to add atrace of Ce to the Al + Cr content of the coating byintroducing Ce oxide into the pack, but there is no evidence that this doping was achieved. Uponcyclic oxidation in air at 700°C, the coated steelexhibits a negligible 0.085 mg/cm² weightgain for 1900 one-hour cycles. Virtually no attack wasobserved on coated steels tested in a simulated boileratmosphere at 500°C for 500 hr. But coatings with asurface composition of only 8 wt.% Al and 6 wt.% Crsuffered limited sulfidation attack in the simulated boiler atmosphere at temperatures higher than500°C for 1000 hr.     

Estimation of Weibull parameters with linear regression method

Abstract

Based on minimum mean square error, a modified probability estimator is proposed by a Monte Carlo simulation for estimating the Weibull parameters with the linear regression method. It is shown that compared with the commonly used estimators, the modified probability estimator gives a more accurate estimation of the Weibull modulus and the same estimation precision of the scale parameter. Furthermore, it is more conservative than the commonly used estimator recommended by previous authors and hence results in a higher safety in reliability predictions.     

Thermomechanical response of 50.7 at.-%Ni-Ti alloy in the pseudoelastic regime

Abstract

The thermomechanical response of 50.7 at.-%Ni-Ti superelastic alloy has been investigated for a range of applied strains and strain rates. Parameters of interest include the critical stresses for nucleation and completion (σ _ms and σ _mf respectively) of the stress induced martensite (SIM) transformation, as well as the magnitude of the endothermal and exothermal reactions as a function of the applied strain and rate of deformation. Novel techniques including infrared thermometry and laser extensometry have been utilised, and are found to be particularly suited to experimentation with nitinol. The transformation temperatures and values of entropy and enthalpy of formation have been determined using differential scanning calorimetry.     

16—THE PREPARATION,FINE STRUCTURE,AND PHYSICAL PROPERTIES OF PARTLY METHYLATED COTTON FABRICS

Cotton fabric has been methylated under a variety of conditions. The extent of methylation has been shown to be dependent on the time of methylation and on the degree of swelling or disruption of structure. The latter is caused by the alkali pretreatment or by the swelling of the partly methylated cellulose. Fabric physical properties have been measured and the influence of the methylation treatment has been examined.     

METAL INERT GAS WELDING OF 2519-T87 HIGH STRENGTH ALUMINUM ALLOY

20 mm thick plates of 2519-T87 high strength aluminum alloy have been welded.The effects of the compositions of filler wires,the heat input and the compositions of shielding gas on the mechanical properties and microstructure of the welded joint have been investigated.The results indicate that finer microstructure,better mechanical properties and higher value of hardness of HAZ can be obtained by using lower heat input.The use of Ar/He mixed shielding gas has several advan- tages over pure Ar shielding gas.With the increase of the proportion of He in the mixed shielding gas, the grain size of the weld metal as well as porosity susceptibility decreases.When the volume ratio of He to Ar reaches 7:3,the porosity and the grain size of weld metal reach the minimum,and the po- rosity can be further reduced by filling some CO_2.     

Development of process for thermomechanical treatment of ultrahigh strength steel prepared by electroslag refining

Abstract

Thermomechanical treatment (TMT) is the simultaneous use of work hardening, and grain refinement along with solid solution and precipitation strengthening. In this investigation, four alloys, with a base composition of 0·28%C, 1·0%Mn, 4·2%Cr, 1·0%Mo, 0·34%V, were prepared by electroslag refining (ESR) and by addition of small amounts of Ti and Nb and by increasing Cr and V to 4·8 and 0·48% respectively. In two of the alloys a yield strength in excess of 1550 MPa was obtained in the as cast quenched and tempered condition. Attempts were made to further increase the yield strength by thermomechanical treatment. The process parameters for thermomechanical treatment were optimised by adopting procedures such as calculation of stability of precipitates, hot compression test, determination of cooling rates in different coolants, and modelling of TTT and CCT diagrams. The process involved prerolling of the ESR ingot to a bar at 1200°C, followed by hot rolling in two passes starting from 950°C and finishing at 850°C with equal deformation of 25% in each pass to convert the bar into plates. These were immediately cooled in one of the cooling media: air, polymer–water solution (1 : 1·5) and oil. Yield strength in excess of 1750 MPa was obtained in oil cooled specimens of the alloy with titanium addition and that where Cr and V were increased. The niobium added specimen gave strengths, similar to that obtained for the base alloy, in spite of the fact that the as cast alloy had shown very high strengths, presumably because of the high soaking temperatures and grain growth. Air cooling gave the lowest strengths and oil cooling the highest.     

Effect of ZnO coating of whiskers on thermal expansion properties of aluminium borate whisker reinforced aluminium composites

Abstract

The pure aluminium composites reinforced by ZnO coated aluminium borate whiskers were fabricated by squeeze casting. The microstructure of the composite was observed using an optical microscope and the thermal expansion behaviours of the composites were investigated in the range from 50 to 400°C. In addition, the effects of heat treatment and thermal cycling on the thermal expansion behaviours of the composite were also investigated. The results show that the coefficient of thermal expansion of as cast composites decreases with the ZnO coating content increasing. However, heat treatment time and thermal cycling lead to an increase in the CTE of the composite.     

Investigation of tensile strength of hydroxyapatite with various porosities by diametral strength test

Abstract

It is appropriate to administer the diametral test to biomedical materials used in dental applications because stresses formed on dental implants are similar to those that formed in this test. To show this similarity, an experimental study of diametral strength testing of hydroxyapatite was performed. The influence of porosity on hydroxyapatite was investigated experimentally to determine how the diametral strength was affected. Hydroxyapatite was air sintered at 1100°C for 1 h with porosities ranging from 1 to 32%. The results indicated that hydroxyapatite with improved densification had higher diametral strength values. X-ray diffraction analysis showed that sintered samples were pure hydroxyapatite.     

A Method of Magnesium Treatment for Producing Gravity-diecast Ductile Iron

Equipment for the in-stream treatment of ductile iron has been adapted for gravity-diecasting. The treatment unit consists of a pouring basin, reaction chamber, and a reservoir for holding treated iron. The device is highly mobile in so much that it can be transferred from furnace to successive dies and back again, allowing consecutive pouring. The authors have studied the effect of reaction chamber design and nodularising-alloy type on the microstructure of cast specimens, using three types of chamber design and three types of nodulariser at different temperatures. The results indicate that a carbide-free structure with a high nodule count of up to 1500 n/mm² within the as-cast structure of a diecast rod specimen of 30 mm dia. can be achieved using this treatment equipment, when employing suitable nodularising alloy and process chamber design.     

14—EQUIVALENT TWISTS IN TYRE CORDS

This paper describes an empirical method for choosing a twist level for a new tyre cord that will ‘match’ the twist in an existing successful cord, i.e., one that will give the same twist angle. The technique is based on published cord geometry and is shown to give reliable results. No knowledge of densities or packing factors is required, and the results are independent of the number of yarns in the cord. The method offers the possibility of calculating twist angles and related properties for cords at any twist once a sufficiently reliable value of one cord constant has been determined. A table of matched twists calculated in this way is given.