Showing papers in "Materials Science and Technology in 1991"

Stress induced transformation to bainite in Fe–Cr–Mo–C pressure vessel steel

Abstract: The kinetics of the bainitic transformation in a polycrystalline Fe–Cr–Mo–C alloy designed for applications in energy generation systems has been studied, with particular attention to the influence of mild tensile stresses on transformation behaviour. The steel was found to exhibit the incomplete reaction phenomenon, in which transformation to bainite stops well before the residual austenite acquires its paraequilibrium carbon concentration. It was found that even in the absence of an applied stress, the growth of bainitic ferrite caused anisotropic changes in specimen dimensions, consistent with the existence of crystallographic texture in its austenitic condition and, significantly, with the nature of the invariant-plane strain shape change that accompanies the growth of bainitic ferrite. Thus, transformation induced plasticity could be detected in fine grained polycrystalline samples, even in the absence of applied stress. The application of an external stress was found to alter radically the t...

Energy criterion for high temperature low cycle fatigue failure

Abstract: High temperature total endurance tests were conducted on three alloys (316L steel at 550°C; 9Cr–1Mo steel at 550°C; Nimonic 101 at 850°C) in low cycle fatigue under continuous cycling. The austenitic steel was observed to cyclically harden, whereas the ferritic steel and the superalloy both cyclically softened. Methods for rationalising evolutionary behaviour were variation of percentage hardening/softening with strain range, cumulative ductility criterion, variation of secant modulus in the hysteresis loops, and cumulative energy criterion. The energy expended per cycle appears to be the most attractive. For a given strain range this was approximately constant at any stage of evolution. More significantly, the cumulative energy at saturation was constant, approximately independent of total strain range, and about 0·5–3·0 J mm−3 according to the material used. The Palmgren–Miner hypothesis (which is widely used in summing fatigue damage) was originally derived on an energy argument and it is propo...

Solidification microstructure of electrodischarge machined surfaces of tool steels

Abstract: Some recent advances made in the understanding of the solidification behaviour of molten metal during electrodischarge machining (EDM) of a range of commonly used tool steels are presented. In general, the recast layers produced by EDM can be classified into three structural types: thick multilayer, intermediate single layer, and thin featureless. The discharge energy affects the average thickness of the recast layer, and hence the distribution of the different types of recast layer formed during EDM. Detailed microstructural studies have shown that during EDM, solidification of the molten metal takes place simultaneously from the top interface with the dielectric and the bottom interface with the underlying metal into the melt, as well as from within the melt towards both interfaces. This leads to the formation of three distinctive sublayers withing each recast layer resolidified from a given pool of molten metal. The effects of steel composition and thermal properties on the solidification micr...

Characteristics of creep deformation in ceramics

Abstract: The creep mechanisms in ceramics are reviewed briefly and then compared with experimental data. It is shown that there are three major types of creep behaviour: a stress exponent close to 5 due to control by dislocation climb and fully ductile behaviour; a stress exponent close to 3 due to control by climb from Bardeen–Herring sources and less than jive interpenetrating independent slip systems; and a stress exponent close to 1 due to diffusion creep. The role of interface reaction control and the transitions from diffusion to power law creep are also examined.MST/1389

Microstructural evolution in nickel during rolling and torsion

Abstract: Microstructural evolution in high purity nickel was investigated by comparing the dislocation substructures formed following torsion deformation with those following rolling. Results from transmission electron microscopy observations and measurements of microdiffraction show a common evolutionary path in rolling and torsion from (i) dislocation tangles to (ii) equiaxed cells bounded by dense dislocation walls (DDWs) which form cell blocks, then to (iii) microbands which, together with DDWs, bound smaller and smaller cell blocks, and, finally, to (iv) subgrains. During this evolution, grains are continually subdivided by the formation of new cell blocks. New cell blocks arise through the formation of new DDWs between cell blocks or of first generation microbands by the subdivision of a single DDW Since the dislocation content of a microband is principally derived from that of the parent DDW, microbands can take various forms, such as a string of small pancake shaped cells in highly recovered DDWs, ...

Synthesis of nitrogen ceramic povvders by carbothermal reduction and nitridation Part 1 Silicon nitride

Abstract: Silicon nitride powders of high α content were prepared from mixtures of very fine amorphous silica and carbon powders in flowing nitrogen at 1400–1500°C. The higher the reaction temperature within this range the lower the yield of silicon nitride in spite of a higher rate of SiO generation. The overall reaction rate increased with increasing nitrogen flowrate. An increase of carbon content of the starting mixtures increased the reaction rate by increasing the rate of SiO formation as well as the rate of Si3N4 nucleation. It is thought that the effect of the carbon particle size on the reaction rate is more significant than that of the specific surface area. The amount of silicon carbide formed and the α/β ratio in the reaction products were strongly affected by a change of the source of carbon used in the starting mixtures.MST/1346a

Effect of titanium additions on austenite grain growth kinetics of medium carbon V–Nb steels containing 0·008–0·018%N

Abstract: An investigation has been carried out into the effects of multiple additions of the microalloying elements V, Ti, and Nb on the austenite grain growth characteristics of 0·4C–1·6Mn (mass-%) steels. It has been shown that simultaneous additions of V, Ti, and Nb inhibit grain growth and increase the grain coarsening temperature markedly. In both V and Ti steels, increased Nb contents refine further the austenite grain size. Using a thermodynamic model, good agreement has been obtained between the measured and calculated volume fractions of carbonitride particles at the solidus temperature, although such particles are too large for effective grain boundary pinning. Afeature of interest is that increased Nb contents in Ti–V steels decrease the size of the larger Ti rich carbonitride particles and inhibit their formation during solidification of the steel. The stability of the carbonitrides increases with increasing Ti content especially. The N dissolved in the austenite decreases with simultaneous ad...

Aging at 400–600°C of submerged arc welds of 22Cr–3Mo–8Ni duplex stainless steel and its effect on toughness and microstructure

Abstract: Submerged arc welds of a 22Cr–3Mo–8Ni (wt-%) duplex stainless steel were aged in the temperature range 400–600°C to simulate stress relieving conditions of mild steel Particular attention was paid to the relationship between toughness and microstructure It was concluded that sufficient toughness could be obtained on aging in the range 500–550°C provided that the aging time did not exceed 10 h At temperatures >550°C, rather rapid embrittlement occurred as a result of the precipitation of essentially R phase and to some extent π phase At ~≤500°C, spinodal decomposition of ferrite caused embrittlement, albeit more slowly than the precipitate induced embrittlement Precipitation of the Mo rich phases Rand π was found to cause depletion of Mo in solid solution in ferrite This offers a possible explanation for the decrease in pitting corrosion resistance observed in previous workMST/1426

Development of fracture toughness of ultrahigh strength low alloy steels for aircraft and aerospace applications

Abstract: Recently, ultrahigh strength low alloy steels, e.g. AISI 4340 and 300M, have been used increasingly for critical structural aircraft and aerospace applications. These steels can be employed successfully at yield strengths of ≥1400 MN m−2 but their use has often been limited in commercial practice because of low fracture toughness compared with other types of ultrahigh strength steel. The results of studies carried out over the past two decades to improve the fracture toughness are presented. Particular emphasis is placed on improvements obtained by microstructural control via thermal and thermomechanical treatments, sulphide inclusions, and new alloying design. The major metallurgical factors controlling fracture toughness are discussed for each of these techniques.MST/1413

Cell and band structures in cold rolled polycrystalline copper

Abstract: The effect of plastic strain on the deformation microstructure has been investigated in polycrystalline copper rolled at room temperature to 5, 10, 20, and 30% reduction in thickness (equivalent strain 0·06–0·42). Results from transmission electron microscopy (TEM) observations show that dense dislocation walls (DDWs) and cells develop during the initial stages of cold rolling. Grains having a high density of DDWs are described as high wall density (HWD) structures, and grains having a low density of DDWs are described as low wall density (LWD) structures. These structures are characterised by cell size, misorientation across the cell walls, and the crystallographic orientation of the grains in which they appear. The DDWs in the HWD structures have special characteristics, extending along several cells and having a misorientation across them greater than that across ordinary cell boundaries at the same strain. The DDWs appear to have a macroscopically determined orientation. Analysis of their crys...

Influence of titanium on hot ductility of as cast steels

Abstract: The influence on hot ductility of a small addition (0·02%) of titanium to C–Mn–Al–Nb steel and 0·05/0·15%C–Mn–Al steels has been examined over the temperature range 700–1100°C and at a strain rate ...

Heat flow model for pulsed welding

Abstract: An analytical model of the heat flow during pulsed welding is constructed by first developing a solution for the temperature distribution due to an instantaneous, stationary, Gaussian heat source. This solution is first extended to treat the case of a moving heat source for a single pulse and then extended to treat the case of pulsed welding by summing the temperature changes due to a series of pulses. Test gas tungsten arc welds in mild steel are used to confirm the validity of the model.MST/1371

Influence of SiC particle distribution and prestraining on fatigue crack growth rates in aluminium AA 6061–SiC composite material

Abstract: The fatigue crack growth properties of an aluminium AA 6061 alloy containing 15 vol.-% particulate SiC and of the corresponding matrix alloy with the same grain size were investigated. The composit...

Influence of sulphur and niobium on hot ductility of as cast steels

Abstract: The influence of S and Nb on the hot ductility of as cast C–Mn–Al steel has been examined. Tensile test samples were cast in situ, cooled at 100 K min−1 to test temperatures in the range 750–1000°C, and strained to failure at a strain rate of 2×10−3s−1. Both S and Nb caused the hot ductility trough to deepen and widen, an addition of 0·03%Nb giving a deterioration in ductility similar to that obtained by increasing the S content from 0·002 to 0·016%. Grain size was coarse and similar for all steels; thus, the changes in ductility could be related to changes in precipitate and inclusion particle distribution. The addition of Nb resulted in the presence of a coarse Nb(C,N) eutectic and a fine matrix and boundary precipitation of Nb(C,N). The addition of S resulted in the precipitation of coarse sulphides at the interdendritic boundaries, as well as the presence of a fine precipitation of Fe oxysulphides similar in size and distribution to the Nb(C,N) precipitation. The apparent similarities in preci...

Carbothermic reduction of ilmenite and rutile as means of production of iron based Ti(O,C) metal matrix composites

Abstract: An assessment has been made of the viability of producing Fe–Ti(O,C) metal matrix composites by the one step carbo thermic reduction of iron-ilmenite and iron-rutile mixtures with coal. Pelletised mixtures of iron, rutile or ilmenite, and collie coal were heated in a tube furnace to temperatures between 1300 and 1600°C under flowing argon. The microstructures of reacted pellets were examined using optical and scanning electron microscopy and the phases present were characterised by X-ray diffraction. Excellent dispersions of the Ti(O,C) phase in the iron matrix were obtained at temperatures above 1450°C. The addition of fluxes of group II halides or BaSo4 to the reduction mixture promoted separation of the Ti(O,C) productfrom the iron. Preliminary experiments on the scale-up of the reduction procedures are reported.MST/1361

Microstructural refinement of duplex stainless steels

Abstract: The experimental results of an investigation of a chemical (alloying) and a physical (deformation) method for refining the microstructure of duplex stainless steels are described. The chemical technique involves alloying with copper. When 3 wt-%Cu is added to a 30Cr–8Ni steel, it was found that the copper precipitation (which is more rapid than that of austenite) takes place with a multitwinned morphology. The subsequent ferrite–austenite transformation nucleated on the finely dispersed copper particles results in a fine, homogeneously distributed, twinned austenite morphology. Copper particles also helped the refinement of the austenite by pinning the interphase interfaces. Another way of refining the microstructure is through cold deformation. As quenched fully ferritic steel with 30%Cr and 8%Ni was cold rolled by various amounts and aged at temperatures below 1000°C. On aging, partial recovery preceded the austenite transformation and nucleation occurred mainly on subgrain boundaries, thus resu...

Interlayer selection and thermal stresses in brazed Si3N4-steel joints

Abstract: The influence of Cu, Kovar, Mo, and W interlayers on the magnitude and distribution of thermal stresses and on the tensile strength of brazed Si3N4 − steel joints is examined using a combination offinite element modelling (FEM) calculations and direct experiment. The FEM takes into account plastic flow and changes of mechanical properties of the interlayer, the steel substrate, and the filler metal as temperature decreases following the brazing operation. Joints made using low yield strength/high ductility interlayers, such as Cu, have lower thermal stresses and higher strengths than those made using low thermal expansivity/high yield strength interlayers, such as Mo or W. A composite interlayer comprising Cu and W will produce the lowest thermal stresses during brazing. Increasing the thickness of the interlayer decreases the thermal stresses produced during brazing, since the rigid restraint effect due to the high yield strength/high elastic modulus steel substrate is reduced.MST/1442

Influence of extrusion process parameters and texture on mechanical properties of Al–Li extrusions

Abstract: The 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

Solidification sequences in stainless steel dissimilar alloy welds

Abstract: A series of laser and gas tungsten arc welds traversing stainless steels of different chemical compositions has been studied, to elucidate the role of austenite or ferrite nucleation and cooling rate on solidification behaviour. It has been found that a steel with a high CrEQ/NiEQ ratio can be induced to solidify as metastable austenite by initiating the weld in a steel with a lower CrEQ/NiEQ ratio in which the thermodynamically stable solidification mode is austenitic. The austenite dendrites are then found to continue growth across the weld junction into the undiluted regions of the high CrEQ/NiEQ ratio material. By providing austenite particles in this way, nucleation is rendered unnecessary and it is found that solidification to metastable austenite can be induced at cooling rates significantly lower than previously encountered. The results of these and other experiments in which the welding speed was changed during the experiment are interpreted to yield new information about the mechanisms o...

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

Abstract: A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2 intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al13Cr2 primary intermetallic particles. The solidification of these powders is discussed in terms of so...

Properties and microstructure of WC–TiC–Co and WC–TiC–MO2C–Co(Ni) cemented carbides

Abstract: The present study is an attempt to observe the changes in microstructure and properties of modified WC–10Co cemented carbides from the viewpoint of the distinctive role played by modified binder phase. Introduction of TiC into WC–10Co cemented carbide results in microstructural non-uniformity, i.e. a wide range of grain size distribution, which in turn gives rise to a drastic drop in values of transverse rupture strength and toughness. The modification of binder and carbide phases by incorporating, respectively, nickel and M02C improves the microstructural uniformity, which ensures better mechanical properties. The present findings have been interpreted in terms of various quantitative microstructural parameters, with particular attention being given to the wettability factor.MST/1363

Modelling thermal and microstructural evolution on runout table of hot strip mill

Abstract: A mathematical model has been developed to predict the thermal and phase transformation response of a 0·34 and a 0·05 wt-%C steel during cooling on the runout table of a hot strip mill. The model incorporates the cooling characteristics of laminar water bar sprays, the austenite–ferrite plus pearlite phase transformation kinetics as a function of the austenite grain size, and the heat of transformation. Overall heat transfer coefficients for the laminar water banks were determined from data obtained from in-plant trials carried out at the Stelco Lake Erie Works (LEW) hot strip mill. Isothermal and continuous cooling diametral dilatometer tests were performed on a Gleeble 1500 thermomechanical simulator at temperatures and cooling rates that simulate LEW hot strip mill conditions. The isothermal data were used to establish the phase transformation kinetics as afunction of austenite grain size and temperature. The continuous cooling results were used to obtain the relationship between cooling rate, ...

Constitution of Ti–Al–C alloys in temperature range 1250–750°C

Abstract: An investigation has been made of the solid state constitution of the titanium rich portion of the Ti–Al–C system; partial isothermal sections have been established at 1250, 1050, and 750°C by means of electron microscopy (including energy dispersive X-ray analysis) and X-ray diffraction. In addition, a schematic liquidus projection has been deduced based on the solid state and as cast structures. The carbide phases present in the range studied are TiC, Ti3AlC, and Ti2AlC.MST/1305

Erosion–corrosion of preoxidised Incoloy 800H in fluidised bed environments: effects of temperature, velocity, and exposure time

Abstract: The effects of preoxidation as a potential protective measure for alloys exposed to erosion-corrosion have been evaluated for Incoloy 800H in laboratory simulated fluidised bed conditions. The performance of the specimens after exposure to such environments was estimated from weight change data, electron microscopy, and X-ray diffraction patterns. The results showed that, in 'corrosion dominated' conditions, i.e. at relatively high temperatures and low velocities, preoxidation was successful in reducing the erosion-corrosion of the underlying alloy. However, when the velocity was increased and the temperature decreased, preoxidation afforded only some short term protection to the alloy. In these more 'erosion-corrosion dominated' regimes, preoxidation delayed the incubation period for erosion-corrosion of the alloy, but, once the scale was removed, the erosion-corrosion rates were similar to those of the non-preoxidised alloys. Comparisons are made between the results of the present and other erosion-corrosion studies, to explain the distinctive pattern of alloy wastage in these environments.

Influence of isothermal solidification on microstructural development in Ni–Si–B filler metals

Abstract: An examination is reported of the microstructure of joints between nickel substrates fabricated using Ni–Si–B filler metal (AWS classification BNi-3). This investigation was undertaken because the presence of residual boride and silicide phases after joining can affect the subsequent mechanical properties of the joint. The microstructures developed in the joints are compared with those formed in similarly heat treated filler metal. Various models of the transient liquid phase process are also considered and predicted times to complete isothermal solidification are compared with experimental values. An initial examination of the influence on filler metal microstructure of a ternary addition (chromium) to the substrate is included.MST/1506

Effects of processing parameters on aluminide morphology in aluminium grain refining master alloys

Abstract: An investigation has been made of the effects of processing parameters on the microstructure of Al–Ti and Al–Zr alloys produced by the reduction of potassium fluorotitanate and potassium fluorozirconate by aluminium. Blocky aluminides (TiAl3 and ZrAl3 respectively) are produced when the reduction is performed in the liquid+solid phase region and flaky aluminides are produced when it is performed in the liquid phase region. The use of Na2TiF6 rather than K2TiF6 has no apparent effect on the alloy structure. The microstructure and the size of aluminide blocks or flakes produced are further influenced by the subsequent cooling rate experienced by the alloy. Post-fabrication heat treatment of Al–5Ti–0·2B (wt-%) alloys (produced by combined reduction of potassium fluqrotitanate and potassium fluoroborate by aluminium) causes boride particles to precipitate on the aluminide surfaces. These borides grow in size and decrease in number with heat treatment time.MST/1355

Structure, mechanical properties, and stress corrosion behaviour of high strength spray deposited 7000 series aluminium alloy

Abstract: A 7000 series high strength aluminium alloy was produced by spray deposition via the Osprey process. This alloy, designated AlZn11MgCu, exhibits an excellent combination of fracture toughness and strength in both the peak aged T6 and overaged T7 conditions. The yield strength and fracture toughness at room temperature are 700 MN m−2 and about 40 MN m−3/2 in the T6 temper, and 500 MN m−2 and 75 MN m−3/2 in the T7 temper, respectively. The exceptional high fracture toughness is attributed to the absence of coarse constituents and oxides. The microstructure is homogeneous, free of segregation, and has a fine grain size. The hardening precipitates in the T6 temper are mainly η′, and in the T7 temper they are η (MgZn2), resulting in a different fracture behaviour; in the T6 temper the fracture mode is microshearing and in the T7 temper the fracture mode is ductile failure. Both T6 and T7 conditions show a stress corrosion crack growth rate higher than that of comparable alloys, 7075-T651 and 7075-T7351...

Influence of aging condition on tensile and fatigue fracture behaviour of aluminium alloy 6063

Abstract: An Al–Mg–Si alloy 6063 was heat treated in the underaged, peak aged, and overaged conditions. The microstructures of the alloy in the different aging conditions were examined using transmission electron microscopy. Tensile and fatigue tests were carried out and the resulting fractures were studied using scanning electron microscopy. It has been found that the alloy shows different tensile and fatigue properties and different modes of fracture under different aging conditions. The results have been explained in terms of slip distribution and grain boundary strength.MST/1188

Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

Abstract: The effects of reheating temperature and of the strain and temperature of roughing deformation passes on the kinetics of strain induced precipitation after a finishing deformation pass have been investigated for several niobium high strength low alloy (HSLA) steels. Strain induced precipitation was detected via its strengthening effect on a second finishing deformation carried out either by experimental rolling or by plane strain compression tests. Precipitates were also observed using thin foil electron microscopy. Decreasing reheating/roughing temperature and increasing roughing strain were found to significantly accelerate precipitation – this acceleration is attributed to clustering of niobium in solution.MST/1409

Synthesis of nitrogen ceramic povvders by carbothermal reduction and nitridation Part 2 Silicon aluminium oxynitride (sialon)

Abstract: Silicon aluminium oxynitride (β′ sialon) powders have been prepared by reacting kaolin–carbon mixtures at 1400–1500°C in flowing nitrogen. The kaolin/carbon ratio in the starting mixtures must be accurately controlled to obtain single phase β′ powders. The overall reaction rate increased with increasing nitrogen flowrate. However, if the nitrogen flowrate was too high, the amount of 15Rand AlN phases in the reaction products increased. A change of carbon source in the starting mixtures affected the phase composition as well as the morphology of the product powders. It was found that the evaporation of SiO is a most important factor in the reaction, strongly affecting the phase composition of the reaction products.MST/1346b