摘要:

AbstractThe ferrite grain size of low carbon steel is known to be refined by hot rolling in the austenite phase field at temperatures too low for recrystallisation to occur. The strain thus retained in the austenite increases ferrite nucleation density and in current models of grain refinement it is assumed that each nucleus becomes a grain in the fully transformed microstructure. In this paper it is shown that, in a heavily deformed C–Mn–Nb austenite, ferrite grains impinged, then coarsened during the initial stages of transformation during continuous cooling. The final ferrite grain size was not established until 35% of transformation had occurred. It is suggested, firstly, that ferrite grain refinement due to controlled rolling cannot be modelled simply from observed increases in nucleation density and, secondly, that deformation of austenite has considerably greater potential for grain refinement than is commonly observed, provided that coarsening of the ferrite during transformation can be limited.MST/1722

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.849

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe rate at which microstructural features in H P50 (niobium modified) change with time and temperature has been studied in an attempt to allow evaluation of the thermal history of reformer tubes. The as cast microstructure consists of an austenite matrix and an interdendritic distribution of primary niobium and chromium carbides. After short exposures to service temperatures, a fine distribution of secondary carbides is generated. Samples of HP50 (modified) were aged at a range of temperatures both with and without the application of stress, and the population density of niobium carbide particles in the material was determined. The rate of change of the population of niobium carbide particles in the microstructure was found to vary according to first order rate kinetics, with an activation energy of 473 kJ mol−1. This suggests that a structured programme of microstructural examination of tubes at maintenance shutdowns would yield valuable information on the temperature profile in a reformer furnace.MST/1607

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.855

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe mechanical properties of the rapidly solidified 7000 series powder alloy CW 67 were investigated for various extrusion and heat treatment conditions. The principal aim of the work was to ascertain the optimum processing route for peak aged (T6) material. The highest proof stress in the T6 condition was found to be 572 MN m−2for material extruded at 325°C and aged for 13·5 h at 120°C after solutionising. The ductility of this material was found to be 13·5%. The fracture toughness was measured in two orientations and found to be approximately 21 MN m−3/2in the short transverse direction and 44 MN m−3/2in the longitudinal direction. Degassing and hot compaction was found to improve the fracture toughness of the material substantially.MST/1504

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.862

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe evolution of microstructure and texture in industrially hot rolled AA 3004 aluminium alloy has been studied. Specimens were subjected to isothermal annealing treatments and the recrystallisation kinetics was determined via measurement of changes in hardness at surface and centre locations. Metallographic examination of specimens was performed using electron channelling contrast. With the aid of an image processor, the Mg2Si precipitate count and size distributions were obtained. A significant increase in the number of precipitates was observed to occur after the completion of recrystallisation. Quantitative texture analysis was performed on the tapered specimens to characterise the through thickness effect. The quantity of cube texture component was shown to be higher at the surface than at the centre location and decreased with increasing exit temperature of the hotband.MST/1564

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.869

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractWhen Cr–Mo steel is joined to mild steel and the joint held at an elevated temperature the carbon tends to migrate from the mild steel to the Cr–Mo alloy. This migration is driven by the higher chemical potential of carbon in the mild steel, and leads to a steady accumulation of carbon in the Cr–Mo steel at the weld junction. The present work demonstrates that the carbon enrichment has a strong effect on the sequence of carbides that forms during aging, and the results are rationalised using thermodynamic phase stability calculations. Similar data are reported for other dissimilar metal joints.MST/1611

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.875

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractCrack growth tests have been carried out between 550 and 750°C on AISI 316 and 347 stainless steels containing controlled additions of P, Sn, and S, after a heat affected zone simulating heat treatment. The crack growth rate was found to be controlled by a damage input rate parameter C*. At low values of C*, crack growth was found to be featureless and intergranular in all alloys. Increasing C*led to growth by intergranular microvoid coalescence, and then transgranular microvoid coalescence. Transmission electron microscopy (TEM), scanning TEM, and Auger electron spectroscopy studies showed that, as in the high temperature brittle intergranular fracture mechanism found in ferritic alloys, the slow, brittle fracture mode in austenitic steels is associated with the stress assisted flow of S to grain boundaries ahead of the crack tip. A component of finite strain rate is also required, leading to a greater brittle crack growth resistance in strain aging alloys.MST/1604

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.883

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe development of heat affected zone microstructures in super duplex stainless steels (nominal composition Fe–25·0Cr–3·5Mo–7·5Ni–O·5Cu–0·25N–0·5W wt-%) has been examined on multipass welds, bead-on-plate welds, and simulated weld samples. Three-dimensional heat flow equations, for different plate thicknesses, have been applied to the grain growth kinetics ofδferrite. The grain growth constants have been rationalised using an equation of the Arrhenius type. The level of agreement between the measuredδferrite grain size and the predicted values is found to be satisfactory. The effect of heat input on theδferrite grain growth has been examined in weld metals and the results indicate that the largeδferrite grain structure can be eliminated using low levels of heat input. Theδ+γ→#x03B4; transformation kinetics and the degree of ferritisation have been studied as functions of the welding variables. The results are discussed in an attempt to quantify a minimum plate thickness and optimum heat input to prevent a high level of ferritisation. Particular emphasis is given to the effect of reheating on the formation of intermetallics, nitrides, and carbides. During the first thermal cycle, the heat input can be sufficient to decompose some of the austenite to a metastable ferritic microstructure. Competition between the reformation of austenite and the nucleation of nitrides and carbides takes place in this supersaturated structure on reheating by subsequent passes. The relative positions of the C curves for formation of austenite and nitrides, and the peak temperature of the second cycle, determine which phases form. The depletion of chromium adjacent to the nitride/δferrite interface is examined using a finite differences model and the implications of the results for the localised corrosion resistance are discussed. The compositions of phases have been measured and the experimental values are compared with a thermodynamic analysis which predicts the equilibrium compositions of both theδandγphases as a function of temperature and chemical composition. Deviation from equilibrium partitioning is observed owing to non-equilibrium cooling conditions, which result in the formation of configurationally frozen structures. Extensive partitioning is observed for high heat input welds but, conversely, low heat input eliminates the partitioning in the heat affected zone. The results are summarised in an attempt to rationalise the relationship between chemical composition, welding conditions, and heat affected zone microstructures.MST/1598

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.896

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe hot cracking susceptibility of welds made on AISI 304 stainless steel modified with from 0·2 to 1·0 wt-%B has been investigated. Varestraint tests showed that the hot cracking susceptibility is high for boron additions of about 0·2%, but is decreased when the boron content is increased to≥0·5%. Steels containing about 0·2%B were found to have a wide solidification temperature range and their high temperature ductility was low compared with boron free AISI 304 steel and the other boron modified steels. Ferrite precipitation was inhibited in the 0·2%B steels and the formation of low melting point grain boundary films was thereby promoted. Increasing the boron content to≥0·6% reduces the coefficient of thermal expansion and narrows the solidification temperature range. In addition, crack refilling was observed, resulting in improved hot ductility and high resistance to hot cracking. It is concluded that in structures where weld restraint forces are not high, hot cracking is not likely to occur if boron additions of>0·6% are made to AISI 304 stainless steel. In T-type and Fisco weld cracking tests, in which the weld restraint forces are close to those experienced by actual structural welds, the boron modified stainless steels show a low hot cracking susceptibility which is not significantly different from that of boron free AISI 304 steel.MST/1548

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.913

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractA new approach for predicting the Young's modulus of two phase composites has been proposed based on a topological transformation and the mean field theory. The new approach has been applied to Co/WCp,Al/SiCp, and glass filled epoxy composites. It is shown that the new theoretical predictions are well within the Hashin and Shtrikman lower and upper bounds (the HS bounds) and are in closer agreement with the experimental results for the corresponding composite systems than both the HS bounds and the predictions of the mean field theory. An advantage of the present approach over other continuum approaches is that it can predict not only the effect of volume fraction of the reinforcing phase, but also the effects of microstructural parameters such as grain shape and phase distribution on the stiffness of composites. It is also shown that the classical linear law of mixtures is a specific case (where the reinforcing phase is continuous and perfectly aligned) of the present approach. In contrast to the classical linear law of mixtures, the present approach can be applied to a two phase composite having any volume fraction, grain shape, and phase distribution. It is shown that in a particulate composite having a given volume fraction of reinforcement, the Young's modulus of the composite increases with increasing contiguity of the constituent phases and this increment is dependent on the stiffness ratio of the constituent phases. Furthermore, the present approach can provide a simple and effective solution to the problem of interaction between particles of the same phase.MST/1587

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.922

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor

摘要:

AbstractThe microstructures of soldered joints made on gold using silver or zinc doped Pb–Sn solder have been studied in the scanning electron microscope before and after accelerated aging. Layers of Au–Sn intermetallic compounds form between the unreacted gold and the solder as a result of the diffusion of gold through the product layers. The reaction kinetics has been studied as a function of temperature for the silver doped system and, in the initial stages, was found to be parabolic with an activation energy of (0·86±0·03) eV. This is the same as for joints made using undoped Pb–Sn solder. As in the undoped system, the growth of the product layers subsequently fell below the t1/2dependence and loss of joint strength occurred only some time after all the gold had been converted.MST/1599b

ISSN:0267-0836    

DOI:10.1179/mst.1992.8.10.930

出版商:Taylor&Francis    

年代:1992

数据来源: Taylor