Acoustic technique to discriminate boiling state during quenching

Abstract

In quench hardening, it is important to determine the boiling state (film, nucleate or convection), in order to control the cooling process. However, the boiling state changes with time and with position on the specimen. A new method developed to discriminate the boiling states is described. High frequency induction heating was applied to stabilise the boiling state and the sound of boiling was monitored with an underwater microphone. The results were evaluated by using fast Fourier transform spectroscopy. For the results obtained using high frequency induction heating and various quenching methods, the boiling states could be discriminated by matching the sound frequency of the sample. Consequently, it was possible to discriminate the boiling states from the frequency observed in nucleate boiling, which has a characteristic frequency band and high intensity compared with film and convection states.     

Prediction of aluminium concentration in molten zinc pot of continuous hot dip galvanising line

Abstract

The strict control of aluminium concentration in a galvanising pot is extremely important to meet the high surface quality of galvanised steels that has been demanded by the industry. The present study proposes a mathematical model for the prediction of aluminium concentration in a galvanising pot of a continuous hot dip galvanising line. It is assumed that aluminium in the molten zinc pot is consumed as coating layer, dross, and inhibition layer. The quantities of aluminium consumed as dross and inhibition layer in the molten zinc pot are evaluated using the results of physical model experiments available in the literature. The operation conditions of the continuous hot dip galvanising line are online collected and then used as input data for the model. It is found that dross is a main source of the preferential consumption of aluminium in the molten zinc pot. The predicted pot aluminium concentration follows the trend of experimentally measured values rather well during both galvanising and galvannealing operations.     

Effect of exciting current of high frequency electromagnetic field on initial solidification of steel during electromagnetic continuous casting

Abstract

A three-dimensional finite element model of the electromagnetic field and temperature field of electromagnetic continuous casting (EMCC) process was developed. The aim was to investigate the effects of induction heat of high frequency electromagnetic field on the early solidification process of molten steel in mould under various conditions of exciting current parameters. The results show that the induction heat has significant effects on the early solidification process, which appear as increasing the billet surface temperature, thinning the initial solidified shell and lowering the starting point of the initial solidification. The increases in exciting current frequency and density make the effects of induction heat on solidification process increase remarkably. Especially, with the exciting current frequency increase, the early solidification process and shell growth become non-uniform in billet circumferential direction. Furthermore, if the exciting current density exceeds a certain value, there occurs a high temperature region in the top of molten steel column, and then the initial solidification rate is greatly decreased. As a conclusion, the effects of induction heat on initial solidification process must be considered when the exciting current frequency and density are adjusted during the electromagnetic continuous casting process.     

Electrical and thermal properties of carbon nanotubes/PMMA composites induced by low magnetic fields

Abstract

Ni particles supported on carbon nanotubes (CNTs) were dispersed in a polymethyl methacrylate (PMMA) matrix by solution blending and then cast onto an electrode to get composite films under low magnetic fields. The orientation of CNTs in the films was characterised by scanning electron microscope and optical microscope. Multimeter and high resistance meter were used to study the electrical behaviour of the nanocomposites. The glass transition temperature T _g of PMMA was determined by differential scanning calorimetry. The results show that the alignment of the CNTs dispersed in the PMMA was achieved under a low magnetic strength below 0·5 T. Because of the ferromagnetism of Ni particles, the magnetic alignment of CNTs susceptibly changed. The magnetic alignment units in this work were rod-like CNTs aggregates instead of single CNTs, which took part in the buildup of a specific CNTs network structure in PMMA matrix. The network structure played a key role in significantly improving electrical conductivity and T _g of the nanocomposites.     

Induction welding and heat treatment of steel pipes: evolution of crystallographic texture detrimental to toughness

Abstract

Steel welding using induction heating to produce pipelines is found to have lower toughness at the weld junction than the base material, even after a heat treatment which reaustenitises the weld zone. Detailed crystallographic characterisation indicates that the poor toughness is due to the crystallographically coarse grains present after welding; the coarse scale is not visible using just optical microscopy. The post-weld heat treatment does not improve the situation at the weld junction, because the detrimental crystallographic characteristics are reproduced on cooling.     

Effects of Cr,Mn, Si,Cu and Zr on microstructure and mechanical properties of high carbon Fe–16Al alloy

Abstract

Effects of alloying elements Cr, Mn, Si, Cu and Zr on the microstructure and mechanical properties of Fe₃Al (Fe–16Al) based alloy containing ～0·5 wt-%C have been investigated. Six alloys were prepared by a combination of air induction melting with flux cover and electroslag refining (ESR). ESR ingots were hot forged and hot rolled at 1373 K and were further characterised with respect to microstructure and mechanical properties. The base alloy and the alloys containing Cr, Mn, Si and Cu exhibit a two phase microstructure of Fe₃AlC_0·5 precipitates in Fe₃Al matrix whereas the alloy containing Zr exhibits a three phase microstructure, the additional phase being Zr rich carbide precipitates. Cr and Mn have high solubility in Fe₃AlC_0·5 precipitates as compared to Fe₃Al matrix whereas Cu and Si have very high solubility in Fe₃Al matrix compared to Fe₃AlC_0·5 precipitate and Zr has very low solubility in both Fe₃Al matrix and Fe₃AlC_0·5 precipitate. No significant improvement in room and high temperature (at 873 K) strengths was observed by addition of these alloying elements. Furthermore, it was observed that addition of these alloying elements has resulted in poor room and high temperature ductility. Addition of Cr, Mn, Si and Cu has resulted in marginal improvement in creep life, whereas Zr improved the creep life significantly from 22·3 to 117 h.     

Influence of duplex melting process on melt quality and solidification of grey cast iron

Abstract

The quality of molten iron produced by two melting processes, the medium frequency induction furnace melting process and the shortcut duplex melting process with blast and medium frequency induction furnaces, has been studied comparatively. The shortcut melting process contributes to a higher undercooling tendency of base iron, and the extent of this tendency depends on other processing parameters such as ratio of charges, fluctuation in composition, superheating temperature and holding time, etc. When the shortcut melting process is used in the production of grey cast iron, there are no essential differences, with subsequent proper inoculation, in the microstructure and mechanical properties of castings compared with the conventional medium frequency induction melting process. Inoculation is a very effective method to alleviate and/or eliminate the negative effect when the shortcut melting process is used.     

Aluminium foaming using a CO2 laser

Abstract

This paper explores the possibility of laser assisted aluminium foaming from a foamable precursor material. A preliminary study of the experimental results shows a unidirectional and localised expansion of the foam. A pore size gradient and a density gradient exist in the structure as the processing conditions change. The foam has large pores and lower density for slow processing speeds, in contrast to the case of fast processing speed where there is a small pore size but higher density. On the top surface as the melt collapses, there is a closure of pores.     

Short cycle hardening and tempering behaviour of low alloy tool steel 60WCrV7

Abstract

The effect of short heat treatment cycles, typical of induction or laser hardening, on the low alloy steel 60WCrV7, was simulated using a quench dilatometer. The heat treated material was examined using optical and scanning electron microscopy, microhardness testing, X-ray diffraction and microimpact testing. The effects of variation in austenitising time and temperature as well as tempering parameters on the hardness and toughness obtained are reported, and the metallurgical reasons for this behaviour are discussed. The results provide conclusions for optimum rapid cycle heat treatments and reveal significant potential for strongly reduced heat treatment cycles, which may have applications in areas besides surface hardening.     

The position control of induction motors using a binary disturbance observer

A control approach for the robust position control of an induction motor based on the binary disturbance observer is described. The binary controller with the binary disturbance observer is implemented for the position control of the induction machine subjected to load disturbances and realizes continuous control. The binary disturbance observer is used to eliminate the chattering problem of a sliding mode disturbance observer. In order to eliminate the steady-state error, the binary disturbance observer with an integral augmented switching hyperplane is proposed. The robustness is achieved, and continuous control is realized by employing the proposed observer without the chattering problem and the steady-state error. The effectiveness of the proposed observer is confirmed by the comparative experimental results.