摘要:

AbstractThe thermodynamic driving force necessary to stimulate the nucleation of acicular ferrite and bainite is investigated for a series of high strength weld metals, using a combination of dilatometry, scanning electron microscopy, and thermodynamic calculations. The results indicate that the nucleation of acicular ferrite and bainite can be represented by the same thermodynamic model. It therefore appears that the nucleation mechanism of acicular ferrite on non-metallic inclusions in weld metals is essentially similar to that of bainite at the austenite grain boundaries. Metallographic observations confirm the notion that acicular ferrite is essentially intragranularly nucleated bainite.MST/1897

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.353

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe thermal stability of three stainless steel conversion coatings for high temperature applications (e.g. photothermal conversion catalysis) are investigated. The thermal oxidation in air up to 1000°C of Fe–17Cr, Fe–18Cr–1·3Al, and Fe–22Cr–5Al coatings (all wt-%) are compared. This study has revealed a critical temperature below which the coating thickness is preserved; the critical temperature increases and the thermal oxidation of the conversion coating decreases with higher chromium and aluminium content. This is attributed to the difference in the substitution ratio ofγlacunar phase (additionally oxidised substituted magnetite), which is the main component of the conversion coatings. The thermal stability of this phase is higher when it is richer in chromium or aluminium. Higher contents of these elements raise the temperature of formation of chromite (FeCr2O4) and alumina, the occurrence of which causes thickening of the coating during thermal treatment.MST/1891

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.359

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe various types of void formed in wide gap brazed joints of C1023 nickel base superalloy produced using prepacks of nickel base braze mixes have been investigated and systematically categorised. Of particular interest are interfacial and interstitial voids and unwetted pockets, which are features frequently found in joints brazed using such a technique. Examination of brazed joints produced under a wide range of conditions revealed that during heating to the brazing temperature, the braze mix partially sinters together, causing the prepack to shrink towards the centreline, leaving two channels next to the joint faying surfaces. At the same time, relatively large pockets of free space are created within the partially sintered mass of prepack. At the brazing temperature, the filler metal deposited at the gap mouth becomes molten and this molten filler is drawn into the gap preferentially through the fine capillary paths in the partially sintered mass of prepack. The relatively large pockets of free space in the prepacks as well as the channels adjacent to the joint faying surfaces are bypassed by the molten filler metal owing to their lack of capillarity. As a result, the various brazing defects described above will be formed should solidification occur before these empty spaces are filled. The effects of materials and process variables on the formation of various brazing defects are briefly discussed.MST/1925

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.364

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractA recently developed continuous casting simulator and the‘Wumsi’hot deformation simulator have been used to carry out laboratory simulation tests to determine the as cast microstructure and the recrystallisation behaviour of a Nb–V microalloyed steel during the process of direct charging. By variation of the initial specimen thickness (between 25 and 60 mm) different values of total strainΦΣcould be imposed to improve the coarse as cast microstructure. For a series of deformation schedules the total strain was divided systematically into two components: an austenite grain refining strainΦγGF(above the recrystallisation stop temperature TRS) and an austenite strengthening strainΦγS(below TRS). After hot deformation slow and accelerated cooling with simulated coiling were employed. It was found that a total strainΦΣ>1·4 is required to ensure mechanical properties that were comparable or even superior to those found using the conventional cold charging process. The coarse as cast austenite microstructure can be refined significantly whenΦγGF=0·3–0·6. The austenite strengthening strainΦγSrepresents the dominant component of the total strain if a satisfactory toughness is to be achieved. Strength properties are less sensitive to the applied strain.MST/1872

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.370

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe hot deformation characteristics of IN 600 nickel alloy are studied using hot compression testing in the temperature range 850–1200°C and strain rate range 0·001–100 s−l. A processing map for hot working is developed on the basis of the data obtained, using the principles of dynamic materials modelling. The map exhibits a single domain with a peak efficiency of power dissipation of 48% occurring at 1200°C and 0·2 s−1, at which the material undergoes dynamic recrystallisation (DRX). These are the optimum conditions for hot working of IN 600. At strain rates higher than 1 s−1, the material exhibits flow localisation and its microstructure consists of localised bands of fine recrystallised grains. The presence of iron in the Ni–Cr alloy narrows the DRX domain owing to a higher temperature required for carbide dissolution, which is essential for the occurrence of DRX. The efficiency of DRX in Ni–Cr is, however, enhanced by iron addition.MST/1856

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.377

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractThe residual stresses remaining after each individual production phase in two separate manufacturing processes involving production of round rods from Cr–V spring steel JUSČ.4830 (BS 735 A 50, AISI 6150) are discussed. The first manufacturing process comprises annealing, straightening, grinding stage 1, and grinding stage 2, and the second consists of annealing, straightening, grinding stage 1, and roll milling. The aim of the investigation was the assessment of both manufacturing processes with regard to the residual stress distribution produced. The measurements were carried out via electrochemical removal and on line measurement of strain using strain gauges. The specimens for the measurements were randomly selected from the normal production process. This caused considerable spread, and an improved representation of the results obtained was achieved via a statistical treatment. A characteristic feature of both production processes is the occurrence of compressive residual stresses in the surface and in the subsurface layers to a depth of 0·5 mm. Roll milling, in the second manufacturing process, is carried out between two roll mills with displaced axes and the residual stress standard deviation is increased because of the spiral movement of the spring steel rod between the two roll mills, and the random specimen location along the round rod. The first production process proved to be more desirable because it provides smaller scatter of residual stress values along the rods.MST/1871

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.384

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractTo date there have been few studies of the effects of liquid metals on fatigue crack propagation. In this paper, the effects of liquid gallium on the room temperature fatigue crack propagation characteristics of a leaded brass are examined. In particular, the effects of load ratio and cyclic frequency are studied. Previous models for the effect of liquid metal on fatigue crack propagation are discussed in the context of this work and a new model for fatigue crack propagation in liquid metals is proposed.MST/1889

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.389

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractAll boundary systems of the Zr–Mo–Pd system show at least one eutectic. This indicates the existence of low melting temperature (~1000°C) eutectic alloys in this ternary system, and is therefore directly related to the possibility of glass formation. Some amorphous and microcrystalline ternary samples were prepared via rapid solidification (~106K s−1) in a splat cooling apparatus with a radio frequency levitation coil and a high velocity two piston arrangement driven by solenoids. To obtain the starting material, homogeneous Zr–Mo–Pd samples of differing composition were prepared via arc melting techniques. Some amorphous alloys have also been obtained in the Zr–Pd–(Si,B) systems.MST/1662

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.397

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractExperiments were carried out to study the temperature dependence of the interfacial shear strength in SCS-6 fibre reinforced Ti–24Al–11Nb (at.-%) composites. The fibre pushout technique was used with a diamond indenter and a high temperature microhardness tester. The interfacial shear strength was found to decrease with increasing temperature.MST/1893

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.399

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

AbstractAn accelerated aging response, detected in AA 6061 based metal matrix composites, in age hardening curves was examined via microstructural observations. The influence of vacancies on the precipitation processes of AA 6061 alloy and its composites was also considered. In the composites, after peak aging, a higher density of dislocations and a higher density of intermediateβ′precipitates was observed. It is thought that the dislocation assisted nucleation processes reduce the apparent activation energy for precipitation, and a higher nucleation density of precipitates in the composites, compared with that in the AA 6061 alloy, leads to an earlier attainment of the peak aging condition.MST/1906

ISSN:0267-0836    

DOI:10.1179/mst.1994.10.5.406

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor