Effect of treatment variables on size refinement by phosphide inoculants of primary silicon in hypereutectic Al–Si alloys

Abstract

The effects of phosphorus containing inoculant identity/process history, addition level, addition temperature, and contact time on number density N _A of primary silicon particles in small volumes of cast Al–20 wt-%Si are reported. Inoculation replaces the coarse, branched primary silicon otherwise obtained in the upper part of these castings with a uniform distribution of small polyhedral primary silicon particles. The inoculants tested increased in effectiveness in the sequence: die pressed and heat treated Al–Fe–P; die pressed Al–Fe–P; extruded Al–Cu–P; and Al–Fe–P prepared by a proprietary route. This last inoculant gave a maximum N _A at 200 ppm addition level in this volume of melt for a contact time of 10 min at 800°C, and at 10 min contact time for an addition level of 100 ppm at this temperature. An addition temperature of 850°C produced a small reduction in N _A, compared with 750 or 800°C. The significance of these findings is discussed in the context of previously published work and possible mechanisms leading to less effective inoculation at high addition levels and extended contact times.     

Development of heavy steel plate for Mayflower Resolution,special purpose vessel for erection of offshore wind towers

Abstract

Special problems necessitate special solutions! Installation vessels for the erection of offshore wind towers are subject to extremely demanding design and structural specifications. Such projects are made possible only by the use of high strength, fine grained structural steels possessing good toughness properties even at extremely low temperatures; in addition, such steels must also offer good workability. Such steel plate material exhibits mechanical properties greatly superior to those possessed by conventional shipbuilding plate. This article focuses on the material for such an installation vessel and the underlying steel development work performed at AG der Dillinger Hüttenwerke.     

Effect of activating flux and shielding gas on microstructure of TIG welds in austenitic stainless steel

Abstract

The aim of this research is to study the effect of an activating flux, two shielding gases (100%Ar and 50%Ar z 50%He) and a range of weld currents on the microstructure of autogeneous A-TIG welds on an austenitic stainless steel. Metallographic, Mössbauer, X-ray diffraction and magnetic permeability methods were used in the study to evaluate ferrite content in the welds. The increase in welding current coarsened the microstructure and increased the retained ferrite content in welds made with and without flux. The activating flux increases the ferrite content and changes the distribution of ferrite in the welds. The influence of flux on ferrite content is less significant in Ar/He than in Ar shield welds. The process of filling steel samples, currently used in the Mössbauer method, drastically changes the microstructure of the parent and melted austenitic stainless steels.     

Investigation of iron powder friction on tungsten carbide tool wall

Abstract

In the powder shaping process, friction is often considered as one of the main limiting factors since it can be the cause of bad green density distributions as well as ejection defects. Therefore, knowledge of this is important for better controlling the compaction stage. This paper makes an attempt to bring out the different process parameters (density, normal stress, sliding velocity, temperature, and displacement) which influence the friction between a standard iron powder and a tool in tungsten carbide. With this view, a sliding piece device has been built and is described. Several tests have been performed; from the results emerge a decrease of the friction coefficient with increasing density or normal stress, conversely the velocity increases the friction while temperature has no significant role. In this work, particular attention is given to the amplitude of the displacement showing that it can highly affect friction and wear of green parts. An attempt is made at explaining the observed phenomena and results are correlated with an analytical expression.     

Study on copper precipitation during continuous heating and cooling of HSLA steels using electrical resistivity

Abstract

Electrical resistivity technique was used to study the phase transformation and copper precipitation during continuous heating and cooling of three Cu bearing high strength low alloy (HSLA) steels. Dilatation measurements were performed to compare the results with the resistivity. During heating, the dilatation plot revealed Ac₁ and Ac₃ temperature, while resistivity measurements indicated precipitation of copper in the range of 370–550°C. A method was demonstrated to estimate the amount of copper precipitation during continuous heating. During continuous cooling, the austenite transformation temperatures could be derived from resistivity, which compare well with dilatometry. A hysteresis between heating and cooling curve was noted possibly owing to formation of bainite during cooling. Non-isothermal kinetic analysis of dilatation data during continuous cooling yields an activation energy of 62 kJ mol^?1, which could be related to the formation of bainite, whereas higher activation energy of 237 kJ mol^?1 obtained from resistivity data may correspond to the diffusion of Cu in iron, associated with the copper precipitation during austenite transformation.     

Prediction of surface hardness after ferritic nitrocarburising of steels using artificial neural networks

Abstract

A model for prediction of the surface hardness of structural steels after nitrocarburising has been designed. The model is based on a backpropagation feedforward artificial neural network. The performance of the model was checked, using data from the published literature. Good correspondence between predicted from the artificial neural network (ANN) and experimental data was observed. The effects of the chromium, manganese, and nickel contents in the steels on the hardness were studied. Using the model the surface hardnesses after nitrocarburising for some commonly used steels are also predicted. The ANN model has been designed with the aim of optimising the application of nitrocarburising technology.     

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.     

Investigation of cracks in original material of Cleuson-Dixence shaft

Abstract

After the big accident in the Cleuson-Dixence Hydropower Plant in December, 2000, several investigations were performed. This contribution summarises the efforts undertaken to clarify the mechanism of the catastrophic failure by the investigation of original material taken from the shaft near the zone of the accident. By extensive non-destructive testing, four plates were removed by flame cutting and tested in the laboratory again. From these results some specimens were taken for further investigations using light microscope and scanning electron microscope. It was found that the investigated cracks are caused by the hydrogen induced cold cracking mechanism after welding, which is also called delayed cracking. By applying a proper pre- and post-weld heat treatment, these cracks can be avoided and by a diligent use of non-destructive testing methods, the required quality of the welding can be assured.     

Comparison of intensity of high temperature surface damage for 20MoCrS4 steel with varying parameters of dynamical heating

Abstract

When implementing new progressive forming technologies and thermomechanical treatments, it is necessary to pay attention to side effects, which negatively influence the semiproduct quality. During heating to the forming temperature in hot or warm forming operations, semiproducts are exposed to high temperatures in direct contact with corrosion environment for a certain period of time. Using protective atmospheres in mass production is unsuitable for economical reasons. To minimise the influence of heating on the surface degradation, it is necessary to reach the desired temperature and uniform temperature field as quickly as possible, yet in compliance with physical properties of the heated material and with the technology in use. The experiments were focused on investigation of various temperatures and heating times simulating conditions of dynamical and classical processes. The comparison of the influence of heating temperature and time on the iron scale occurrence intensity as well as the influence on the surface layers of the semiproducts was performed.     

Thermophysics characteristics and densification of powder metallurgy composites

Abstract

An analytical densification model describing the final stages of hot pressing and sintering has been developed and found to be consistent with empirical findings. The behaviour of composite powders for the matrices of diamond tools has been studied under hot pressing conditions. Differential scanning calorimetry was used to determine the heat capacity at constant pressure C _p of pure Co, 663Cu, and composite iron- and cobalt based powders (also containing WC, Ni and 663Cu). The relationship between C _p and composite densification has been analysed, and it has been found that optimised rare earth additions to the iron based composite powders can produce C _p characteristics close or equivalent to that of pure Co powders. This modified composite powder has been used to hot press diamond drill and saw bits that show good properties. Employing a densification regime guided by the dynamic model has been found radically to improve stability in service (bend strength, hardness, impact, ductility and porosity).