摘要:

AbstractSolidification modelling appears to be well developed with regard to simulating macroscopic aspects of the freezing of castings. Therefore, it is most valuable for the identification and solution of feeding problems. One unsolved problem in solidification simulation is that of due allowance having to be made for the heat transfer at the casting/mould interface, where the gap which develops can vary widely over a casting. Any good solidification model will have to take account of this important effect. A further unsolved problem is the redistribution of heat within the solidifying casting by convection in the liquid. Where this predominates, the redistribution of heat affects the location of defects and the effective feeding of castings. Despite excellent progress, it is evident that considerable problems remain with regard to castings. Usually these are filling problems, leading to the incorporation of the surface oxide into the bulk metal by surface turbulence. Computer modelling of this area has only just started. The work being carried out at The University of Birmingham to define good filling regimes is described. As regards simultaneous flow and solidification, modelling is at a relatively primitive stage. The fluidity of pure metals and complex /alloys will not be easy to differentiate accurately. To simulate the phenomenon of remelting in aflowing stream, which the at/hor has termed‘continuous fluidity’, requires accurate modelling. It describes important effects such as the situation in running systems or the gating of a heavy casting through a thin section, in which, despite some early freezing, remelting of the solid allows the flow path to remain open indefinitely. Other aspects of casting, namely, stress, distortion, and the creation of defects, remain on the fringe of current modelling capability and will require new data.MST/1481

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.885

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA method has been developed to refine the microstructure of clean, high strength martensitic steels by partitioning the austenite grain structure with intragranularly nucleated plates of Widmanstätten ferrite. The plates were induced to nucleate heterogeneously on oxide particles present in the steels, by first forming uniform, thin layers of inactive allotriomorphic ferrite at the austenite grain surfaces. This effectively removed the austenite grain boundaries as potential sites for the nucleation of Widmanstätten ferrite, which consequently nucleated intragranularly on impurity oxide particles. The intragranularly nucleated ferrite plates were found to radiate in many directions from each oxide particle, in a manner which subdivided the remaining austenite into fine blocks. Quenching after this partial transformation to allotriomorphic and Widmanstätten ferrite resulted in the martensitic decomposition of the blocks of residual austenite, leading to a significantly refined overall microstructure.MST/1438

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.895

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractA series of C–Mn(–Al) steels containing 0·3 or 1·4 wt-%Mn and increasing volume fractions of AlN was deformed in compression at strain rates from 3×10−2to 3×10−4s−1and temperatures from 850 to 1100°C; the peak strainsεpwere determined. In the lower part of the test temperature range (i.e. below the temperature at which AlN particles lose their effectiveness in restraining grain growth),εpincreased with volume fraction of AlN in low (0;·3%) and high (1·4%) Mn steels. Pinning of the boundaries by the AlN particles also resulted in unusual flow curves, in which the first peak was significantly lower than the second, behaviour that was most marked for the low Mn steels. At the higher temperatures, AlN volume fraction had little effect on the behaviour of any of the steels. In the high Mn steels,εpdecreased with increasing temperature in the conventional manner. In the low Mn steels,εpfirst decreased with temperature; then, at the grain coarsening temperature, there was a dramatic increase inεpfollowed by a further gradual decrease. The unusual behaviour with regard to the change ofεpwith temperature exhibited by the low Mn steels is interpreted in terms of their grain coarsening behaviour, which is markedly different from that observed in high Mn steels.MST/1440

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.904

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effects of sulphur on hot ductility of niobium steels, in which cracking susceptibility on the continuously cast slab surface is highest, have been studied by means of hot tensile testing with particular emphasis on the segregation of sulphur atoms to the matrix/grain boundary Nb(C,N) precipitate interfaces. When low manganese niobium steels are solution treated at high temperature and then deformed at temperatures ranging from the low temperatureγto theγ/αduplex phase regions, two troughs appear in the ductility versus strain rate curve, accompanied by intergranular fracture ofγ, at strain rates of ~1 s−1and 10−3−10−4s−1. The loss of ductility at high and low strain rates is caused by dynamic precipitation of iron rich (Fe,Mn)S and Nb(C,N) particles, respectively, both withinγgrains and on theγgrain boundaries. The dependence on sulphur content is obvious at high strain rates, but it is found that the loss of ductility owing to Nb(C,N) precipitation is also reduced by decreasing the sulphur content to less than 10 ppm. This can be explained by the reduced segregation of sulphur atoms to the grain boundary Nb(C,N) precipitate/matrix interfaces, leading to suppressed decohesion and consequent nucleation of microvoids which result in ductile intergranular fracture ofγ.MST/1425

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.915

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThis paper presents a study of the microstructural changes that occur during warm rolling of a sintered tungsten ingot on a Kocks mill. The material was subjected to between one and four rolling passes. The results show that during warm rolling the grain structure undergoes several dynamic processes. High angle boundaries are formed and then migrate. As the grains grow, they are subdivided by low angle boundaries that are formed by dislocations generated during rolling. These low angle boundaries eventually transform into high angle grain boundaries and in turn begin to migrate. At the same time, potassium containing bubbles that are present in the investigated lamp quality tungsten become narrower and longer. The narrowing that occurs is greater than would be predicted from a constant volume criterion because the density of the material increases during rolling. When the rolled rod is annealed, abnormal grain growth occurs. The grain size depends highly on the amount of work to which the material has been subjected and the variations in grain size across the rod diameter can be explained in terms of the amount of deformation that the material has undergone. Also, during the anneal the potassium containing ellipsoids spheroidise. In most examples spheres form by contraction of the ellipsoid to form a single sphere. However, in the rod subjected to four rolling passes some ellipsoids have a sufficiently high aspect ratio for break-up into two spheres to occur. This behaviour is well explained by the Rayleigh model for break-up of a cylindrical fluid.MST/1430

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.923

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe influence of extrusion process parameters on the mechanical properties of differently shaped AA 8090 and AA 8091 extrusions was investigated. Strength was shown to be influenced less by extrusion temperature and ratio than by the extrusion aspect ratio (width/thickness), which determines texture; the strength decreases considerably with increasing extrusion aspect ratio. On the basis of a quantitative texture analysis, the strength decrease can be calculated extremely accurately by the Sachs or the Taylor model depending on the homogeneity of slip distribution. Th.e influence of width/thickness ratio can be generalized and the strength in different areas of complex extrusions can be predicted.MST/1406

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.937

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractThe effect of zinc content on the processing map for hot deformation ofβbrass has been studied in the temperature range 550–800°C and strain rate range 0·001–100 s−1in brasses containing 44, 47, 51 wt-%Zn. The three types ofβbrass exhibit superplastic deformation at temperatures of 550–800°,≦750°C, and 700–800°C, respectively, and at strain rates of<1 s−1. In the 47%Znβbrass, superplasticity is curtailed at 800°Cowing to abnormal grain growth caused by anomalous diffusion. In all threeβbrasses, superplastic deformation is dependent on the diffusion coefficient. At temperatures less than 700°C and in the strain rate range 0·01−1 s−1, the 51%Znβbrass exhibits a domain of dynamic recrystallisation caused by the presence ofγphase particles in the matrix. At higher strain rates (10 and 100 s−1), the brasses become microstructurally unstable, which is manifested by oscillations in the stress–strain curves and by adiabatic shear bands and strain markings in the micrographs.MST/1421

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.947

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractI t has been common practice to use mould withdrawal unidirectional solidification to produce single crystal castings. To grow single crystals successfully, it is important to control several solidification parameters, such as the morphology of the solidification front (solid/liquid interface), thermal gradient, and growth rate during solidification. It is the aim of this study to develop a solidification model that can predict such solidification parameters for various design and operating conditions. The solidification phenomena in the process modelled are basically controlled by two heat transfer mechanisms: conduction and radiation. A set of heat transfer equations and boundary conditions were employed to describe mathematically the heat transfer phenomena. Then the finite difference method was used numerically to solve these equations for specified boundary conditions to obtain the temperature distribution and temperature variation in the casting. The solidification parameters can subsequently be deduced from these temperature data. Several thin plate castings were tested using the model developed. The following design and operating conditions were evaluated: susceptor temperature (power input), withdrawal speed, changes of cross-sectional area in the casting, and geometrical arrangement of the casting tree.MST/1422

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.954

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

AbstractIntrinsic and extrinsic factors that influence the mechanical properties of as welded weldments in an Al–Cu–Li alloy, 2090, have been investigated. Electron beam (EB) and gas tungsten arc (GTA) welds were produced in 2090–T3 and 2090–T8. The tensile properties of as welded and solutionised and aged weldments at 293 and 77 K are presented. The results show that differences in thermomechanical processing between the base metal and the fusion zone lead to inhomogeneities in the weldment microstructure and, consequently, differences in properties. In the as welded condition, the EB and GTA fusion zones lack the strengthening precipitates present in the base metal. As a result, increased base metal constraint on the as welded fusion zone confines the plastic deformation to within the weld and therefore the measured mechanical properties of the weldment are those of the weld material. After equivalent thermal processing, the properties of the EB and GTA weldments are as good as, or better than, those of the base metal. In the post-weld solutionised and overaged condition, the base metal lacks the volume fraction and homogeneity of strengthening precipitates found in the EB and GT Afusion zones, and as a result failure occurs in the base metal.MST/1405

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.965

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

ISSN:0267-0836    

DOI:10.1179/mst.1991.7.10.973

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor