Variation in as cast and aged structures of centrifugally cast heat resistant steel solidified in an electromagnetic field and under different cooling conditions

Abstract

The influences of an applied electromagnetic field and cooling conditions on the as cast and aged structures of centrifugally cast heat resistant HK40 steel tubes were investigated. The results reveal that both the electromagnetic field and the cooling ability of the casting mould have significant effects on the macrostructure morphology and the volume fraction and distribution of eutectic carbides, as well as on the homogeneity of secondary precipitation caused by high temperature aging treatment. The differences in the structures are attributed to the different solidification processes of the cast tubes. It is indicated that a reasonable combination of electromagnetic field and cooling conditions during solidification is greatly beneficial, leading to an improvement of creep resistance of cast tubes.     

Deformation and fracture behaviour of porous FVS0812 aluminium alloy prepared by spray deposition

Abstract

The deformation and fracture behaviour of a porous FVS0812 aluminium alloy prepared by spray deposition was investigated using compression testing. Rapid densification of the porous alloy was observed before a height reduction of ~50%. The optimum strain rate and deformation temperature to achieve maximum density were determined. Microstructural evolution involved the following stages: breakup of boundaries between adjacent particles; plastic deformation of particles; and collapse of large pores into various smaller pores and their disappearance. A strain hardening phenomenon occurred at all strains when compression was carried out at at 573 and 673 K, while a recovery mechanism came into operation at 773 K and a true strain of 0.2. The critical strain for the occurrence of strain softening in the porous alloy was much higher than that for the fully dense alloy. The fracture criteria curve of the porous alloy was a straight line, with a slope of ~0.45268 at high temperature and ~0.47636 at room temperature, almost parallel to that of the fully dense alloy in homogeneous compression.     

Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Effects of homogenisation treatment on microstructure and hot ductility of aluminium alloy 6063

Abstract

Several homogenisation treatments were applied to direct chill (DC) cast ingots of aluminium alloy 6063, in order to analyse the resulting microstructures developed from these diverse conditions and their effects on the hot ductility of this alloy. Imaging was performed using scanning electron microscopy (SEM) and a focused ion beam (FIB) instrument. These techniques identified variations in distribution and morphology of second phase particles (AlFeSi and Mg₂Si). FIB results for the various AlFeSi particles correctly identify their shapes in three dimensions (3D). The particles were identified by energy dispersive spectroscopy (EDS) in the SEM, and by X-ray diffraction (XRD) for bulk samples. Hot tensile testing (HTT) was conducted between 470 and 600°C to asses the hot ductility for each condition. The inferior ductility of as cast samples was due to the poor bond strength of the β AlFeSi phase at the grain boundaries. Homogenised samples, which contain α AlFeSi, exhibited improved ductility. Samples that were water quenched following homogenisation were absent of Mg₂Si precipitates, when these elements remained in solid solution. These exhibited the highest ductility.     

Thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys

Abstract

The thermal stability of electrodeposited nanocrystalline nickel and iron–nickel alloys has been studied using TEM, X-ray diffraction, and atom probe analysis. All of the as deposited materials were purely fcc and had grain sizes of 10–20 nm. Heat treatment of nanocrystalline nickel in the range 190–320°C resulted in abnormal grain growth with an activation energy of 122±15 kJ mol^-1. Abnormal grain growth in Fe–50 at.-%Ni was only observed at 400°C but not at 220 or 300°C, where grain growth was very slow. In Fe–33 at.-%Ni, room temperature aging resulted in the formation of large grained areas (～1 µm), some of which transformed to bcc. In heat treated nickel specimens, some evidence of sulphur and carbon enrichment was found at grain boundaries.     

Very strong low temperature bainite

Abstract

Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2.5 GPa together with an ability to flow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographic details are reported of the very fine bainitic microstructure associated with the incredibly low transformation temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than ~ 10^-17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation.     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Determination of potency factor of cobalt for estimation of nickel equivalent in 16Cr–2Ni martensitic stainless steel

Abstract

Cobalt is considered to be a potential substitute for nickel in 16Cr–2Ni martensitic stainless steel. However, the percentage of cobalt that can substitute 1% of nickel remains to be determined. A computer program was developed for accurate prediction of δ ferrite content in Schaeffler type diagrams. The δ ferrite content in the steels was measured by metallography and estimated using the computation technique for various Schaeffler type diagrams. The percentage error in the experimentally measured and computed δ ferrite contents was ±1·0%. From the present study, the potency factor of cobalt was computed to be 0·64 that of nickel in the estimation of nickel equivalent.     

Dynamic deformation and fracture properties of simulated weld heat affected zone of 9Cr-1Mo steel from instrumented impact tests

Abstract

Dynamic deformation characteristics such as dynamic yield stress and dynamic strain hardening exponent along with dynamic elastic-plastic fracture toughness for the different microstructural regions of heat affected zone (HAZ) of a nuclear grade 9Cr-1Mo steel have been evaluated by instrumented Charpy tests. Isothermal heat treatment at different temperatures was used to simulate the different microstructural regions in the HAZ of this steel, namely the over tempered base metal, intercritical, fine prior austenitic grained martensitic, and coarse prior austenitic grained martensitic regions. Effects of interparticle spacing on the dynamic deformation and fracture properties have been studied. It has been observed that the dynamic yield stress and the dynamic fracture toughness follow power law relationships with the interparticle spacing whereas the strain hardening exponent follows a linear fit.     

Analysis of constitutive behaviours and processing map of 7020 aluminium alloy

Abstract

The authors present a study on the hot formability of 7020 aluminium alloy. Isothermal hot compression tests of solid cylindrical specimens were performed in the temperature range of 300–550°C and the strain rate range of 0·001–10 s^–1. Stress–strain curves obtained from the experiment data are fitted using the Sellars–Tegart constitutive equation to obtain the constitutive parameters. Using the dynamic material model, the authors develop a processing map based on the flow stress data. The map shows that the parameters suitable for hot working are a temperature range of 450–550°C and a strain rate range of 0·001–0·1 s^–1. This parameter range is where the efficiency of power dissipation is above 27% and where dynamic recrystallisation occurs. Unstable regions to be avoided in hot forming are deduced from an instability condition. The processing map is validated by comparing the microstructures of deformed compression specimens.     

Modelling correlation between hot working parameters and flow stress of IN625 alloy using neural network

Abstract

In this work, an optimum multilayer perceptron neural network is developed to model the correlation between hot working parameters (temperature, strain rate and strain) and flow stress of IN625 alloy. Three variations of standard back propagation algorithm (Broyden, Fletcher, Goldfarb and Shanno quasi-Newton, Levenberg–Marquardt and Bayesian) are applied to train the model. The results show that, in this case, the best performance, minimum error and shortest converging time are achieved by the Levenberg–Marquardt training algorithm. Comparing the predicted values and the experimental values reveals that a well trained network is capable of accurately calculating the flow stress of the alloy as a function of the processing parameters. Sensitivity analysis revealed that temperature has the largest effect on the flow stress of the alloy being in good agreement with the metallurgical fundamentals.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

Modelling hot deformation of Inconel 718 using state variables

Abstract

A dislocation density based state variable model has been developed to describe the characteristic flow stress behaviour during hot deformation of polycrystalline superalloy Inconel 718. Model equations have been formulated to describe the role of the evolving microstructures on the macroscopic flow stress response to deformation. Following a peak in the flow stress associated with strain hardening, the model utilises mechanisms associated with dynamic recovery and recrystallisation to explain the gradual decrease in flow stress with continued deformation. Incorporation of these microstructure based state variables also enables prediction of microstructures associated with a range of hot deformation conditions. Model flow stress predictions have been validated against isothermal uniaxial compression tests conducted over a range of temperatures and strain rates relevant to industrial forging conditions.     

Hot cracking in tungsten inert gas welding of magnesium alloy AZ91D

Abstract

Plates of 3–5 mm in thickness were extracted from an AZ91D ingot and then butt joints of the plates were produced using tungsten inert gas (TIG) welding method. The TIG arc was also used to deposit welding beads on some of the thin plates. No cracking was found in the butt joints. However, hot cracking was always observed to propagate from the heat affected zone (HAZ) under the welding bead into the weld metal right after a welding bead was deposited on the thin plate. Metallographic and fractographic evidence was obtained to show that the hot cracking is 'liquation cracking' in the partially melted HAZ under the high thermal stresses. In the butt joints, the weld metal has the finest grains, highest strength and best ductility, and the HAZ was found to be the 'weakest link'.     

Residual stresses on flat specimens of different kinds of grey and nodular irons after laser surface remelting

Abstract

Laser surface remelting is one of the best procedures for surface modification of ferrite–pearlite nodular irons. Using this procedure on cast irons it is possible to achieve a very high wear resistance, which can be compared with the wear resistance of surface hardened heat treatable steels. The properties of the modified layer depend on the microstructure before heat treatment and on the amount of energy input transferred into the surface layer of the specimen. The research work has focused on the study of residual stresses after laser surface remelting of different kinds of grey and nodular irons. The identification of residual stresses was performed using the relaxation method, which involved measuring specimen strain. The results of the measured residual stresses confirm that the stresses strongly depend on laser surface remelting conditions. The measurement of residual stresses is very important in exacting dynamically loaded machine parts, which have been subjected to different kinds of heat treatment. In designing parts designers very frequently demand the presence of compressive residual stresses after heat treatment and finish grinding of the surface, since this increases the fatigue strength of the material and reduces the danger of fracture.     

Microstructural characteristics of gas tungsten arc synthesised Fe-Cr-Si-C coating

Abstract

The gas tungsten arc (GTA) method was used to synthesise Fe-Cr-Si-C alloy coatings, and processing effects on the coating were investigated experimentally. Coatings were developed on an AISI type 1040 steel substrate. Four different regions were obtained in the surface coating; and in these regions either a hypoeutectic or a hypereutectic microstructure was found. The hypoeutectic microstructure consisted of primary dendrites of austenite (γ) phase and eutectic M₇C₃ (M=Cr,Fe) carbides. On the other hand, the hypereutectic microstructure consisted of M₇C₃ primary carbides and eutectic. A hypoeutectic or hypereutectic microstructure was determined by the combination of particularly carbon concentration, solidification rate, and extent of substrate melting. The higher hardness of the hypereutectic microstructure is attributed especially to the formation of M₇C₃ primary carbides. The lower hardness of the hypoeutectic microstructure is related to three effective parameters: first, the presence of γ phase in the primary dendrites; second, excessive dilution from the base material; and third, relatively low concentrations of chromium and carbon.     

Oxidation behaviour of Ti6Al4V titanium alloy in oxygen

Abstract

The isothermal oxidation behaviour of two phase (α + β) titanium base alloy Ti6Al4V (coupons) has been studied at 1050, 1150, 1250, and 1340 K in O₂ gas at atmospheric pressure for 2, 4, 6, 8, and 12 h. Investigations on kinetic behaviour followed by the metallographic examination of oxidised scale morphology was carried out. Thermogravimetric data (weight gain v time) exhibited parabolic behaviour. Below 1250 K, the rate of oxidation substantially decreased after 8 h exposure, however, at 1340 K the oxidation rate was markedly high over the whole 12 h period. Parabolic rate constants were 0.234×10^-7, 3.67×10^-7, 10.72×10 ^-7, and 31.17×10^-7 kg² m^-4 s^-1 at 1050, 1150, 1250, and 1340 K respectively. The effective activation energy of oxidation was 88 kJ mol^-1. The instantaneous rate constant k _i exhibited a marked deviation from parabolic behaviour at high temperatures e.g. 1150, 1250, and 1340 K, however, k _i at lower temperature (1050 K) remained broadly unchanged with time exhibiting no deviation from parabolic behaviour. Metallographic observation of the sample coupons treated at 1340 K revealed an identical oxide scale morphology with increased thickness over the time.     

Development and validation of a processing map for AFNOR 7020 aluminium alloy

Abstract

A simple instability condition is applied to delineate the regions of unstable metal flow in the processing map utilising flow stress data for AFNOR 7020 aluminium alloy, which was fitted in a constitutive equation. Unstable regions identified from the simple instability condition at different strain levels were superimposed in the processing map. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening effect on plastic instability during hot deformation of the material. The processing map has been developed and validated by comparing microstructural observations of deformed compression specimens.     

Comparison of simulated and experimental strain ratio R for different grain shapes in sheet metal

Abstract

Predicting mechanical properties by means of a simple indicator is of great importance to sheet metal forming. An important parameter characterising the formability of a rolled sheet is the plastic strain ratio R which is strongly determined by the texture. The angular variation of R value in the rolling plane has been calculated from the orientation distribution function using the Bunge method. The following grain interaction models have been tested: two Taylor full constraint models ({hkl}〈111〉 and {110}〈111〉 plus {112}〈111〉), three relaxed constraints models (RC₄ , RC₃ , RC₂ ), and the Sachs–Kochendörfer model. The shapes of the grains were investigated by means of the secant method which allowed the spatial parameters of the two- dimensional structure image to be measured. The comparison of simulated and experimental data has proved that in the case of aluminium killed steels, the relaxed constraints model RC₄ is the best predictor of the plastic strain ratio. Good results were also obtained using the Sachs-Kochendörfer model.