Tianyou Huang

The effect of ultrasonic processing on solidification microstructure and heat transfer in stainless steel melt

The heat transfer in the ultrasonic processing of stainless steel melt is studied in this thesis. The temperature field is simulated when the metal melt is treated with and without ultrasound. In order to avoid the erosion of high temperature melt, ultrasound was introduced from the bottom of melt. It is found that the temperature of melt apparently increases when processed with ultrasound, and the greater the ultrasonic power is, the higher the melt temperature will be; ultrasonic processing can reduce the temperature gradient, leading to more uniform temperature distribution in the melt. The solidification speed is obviously brought down due to the introduction of ultrasound during solidification, with the increasing of ultrasonic power, the melt temperature rises and the solidification speed decreases; as without ultrasound, the interface of solid and mushy zone is arc-shaped, so is the interface of liquid and mushy zone, with ultrasound, the interface of solid and mushy zone is still arc-shaped, but the interface of liquid and mushy zone is almost flat. The simulation results of temperature field are verified in experiment, which also indicates that the dendrite growth direction is in accord with thermal flux direction. The effect of ultrasonic treatment, which improves with the increase of treating power, is in a limited area due to the attenuation of ultrasound.

The comparison of ultrasonic effects in different metal melts

The effect of ultrasonic treatment of the melts is mainly ultrasonic streaming and cavitation. In this paper, the ultrasonic streaming in water, aluminum and steel melts was numerically simulated and compared. And the simulated results of streaming in water were validated by experimental results. In the experiment, the ultrasonic booster was immersed vertically into water, the ultrasonic streaming phenomenon was observed by high-speed CCD (Charge-coupled Device) system, then the streaming velocity and streamlines were obtained. The cavitation area and threshold in aluminum and steel melts were compared. The results show that the effective streaming and cavitation area in steel melt is smaller than that in aluminum melt, and far smaller than that in water. A symmetrical vortex forms both in water and aluminum melt by the drive of downward ultrasonic streaming caused by the booster tip. However, in steel melt, a double-vortex structure, including a vortex in the upper part and a vortex with reverse cycling in the lower part appears in the flow field. As a result, inclusions and air bubbles may be trapped in steel melt. The density and viscosity of the fluids are the main factors influencing ultrasonic streaming and cavitation. The results provide references for the application of ultrasonic treatment in metal melts.

Stress analysis and deformation prediction of a heavy hydraulic turbine blade casting during casting and heat treatment

Abstract Owing to their large and curved shape, blade castings, a key component for heavy hydro turbines, are susceptible to deformation during casting and heat treatment. In the present paper, the stress analysis of a blade casting during both casting and heat treatment is performed. The coupled thermo-stress and thermo-phase transformation stress models are used for casting and heat treatment respectively. Machining allowance distribution is used as the deformation criterion and an algorithm of inverse deformation determination is presented. The mechanical properties of the martensitic stainless steel ZG0Cr13Ni4Mo (13Cr–5Ni–1Mo) at different temperatures are measured under as cast and heat treated status. Finally, the inverse deformation of the blade during both casting and heat treatment processes is obtained, and a series of sections of the blade casting with inverse deformation design are given for pattern making. The calculated deformation results are compared to the measured one, and they are basically in agreement.

Thermal stresses in a cylinder block casting due to coupled thermal and mechanical effects

Thermal stress in castings results from nonuniform cooling. The thermal stress and the deformation can change the casting and mold contact conditions which then alter the heat transfer between the casting and the mold. The contact element method was used to study the interaction between a sand mold and a casting. The contact status was then fed back to the heat transfer analysis between the sand mold and the casting to re-evaluate the heat transfer coefficient based on the gap size or pressure between surfaces. The thermal and mechanical phenomena are then coupled in two directions. The method was applied to analyze stress in a stress frame specimen casting and a cylinder block. The results are more accurate than without consideration of the contact effects on the heat transfer.

Dynamic measurement of temperature field and deformation in hydraulic turbine blade at cooling stage of heat treatment

The hydraulic turbine blade is a kind of large,complex and curved casting.It is susceptible to significant warping deformation in the heat treatment process because of no-uniform cooling and phase transformation,which is a serious problem for the dimension control of the blade.A non-contact temperature field and dynamic displacement measurement system is established with the infrared thermal imaging system used for temperature measurement and special devices for the deformation measurement.It is adopted to investigate the temperature distribution and deformation behavior of a blade at the cooling stages of normalizing and tempering in production.The results reveal the temperature evolutions and dynamic deformation of the blade in heat treatment.The deformations of the blade mainly occur at the cooling stage of normalizing owing to the effect of uneven cooling and phase transformation.While,it is small during tempering in which only uneven cooling effect exists.This research and the results are of great significance to the exploration and study of the deformation behavior of hydraulic turbine blades.And the established system is useful for temperature fields and deformation measurement for parts during heat treatment.

Improved thermal stress analysis for castings

Abstract Three algorithms are presented to address three problems in stress analysis of castings: difficulties for enmeshment of complicated casting into finite element meshes, coupling of thermal and stress analysis, and determination of inverse deformation, respectively. An algorithm of conversion of finite difference meshes into finite element meshes is achieved by reformatting the data of meshes. An algorithm of the heat transfer coefficient at the casting/mould interface is presented and the coupling of thermal and stress analysis is realised with the feedback of stress and deformation to heat transfer. Machining allowance is applied as a criterion for deformation evaluation of castings. And insufficient machining allowance is transformed to inverse deformation which is fed back to the original casting design for recalculation of stress and deformation. Therefore, the design of casting with appropriate inverse deformation is obtained. Case studies about a cylinder block and a hydro turbine blade casting are illustrated.

A New Method for Ultrasonic Treatment on the Melt of Steel

Ultrasonic treatment of the melt metals is a hot research topic; however, it is hard to introduce ultrasound into liquid steel because of the requirement of resistance to high temperature and erosion in the melt. In this paper, a new method, unit piece method (UPM), is proposed to introduce ultrasound into liquid steel. The metal to be treated is cast together with amplitude transformer as free end. During the experiment the transformer is placed upward and its free end is melt by induction coils. Then the amplitude transformer powered by ultrasound acts on the melt pool. The effect of ultrasound on the melt of 304 stainless steel is studied. The microstructure of the treated area is significantly modified and equi-axed grains are obtained. The effect of power of ultrasound is discussed. This method is proved to be effective.

Water analogue experimental study on the filling process of a steam turbine housing casting

Mould filling is of determining effect on the quality of castings. Numerical simulation is widely used for the study of mould filling, but the result is necessary for validation. Water analogue experiment used to be a main way for mould filling research before, but it was hard to simulate the mould filling of real castings. In this paper, a new water analogue system for real castings was established. The casting cavity was shaped by assembling transparent organic glass slices cut by laser, with each slice exactly representing the geometry of each casting section. Therefore, the shape of the mould cavity is guaranteed. The system was applied to study the filling process of a heavy steam turbine housing casting weighing 70.3 tons. Its filling process was observed and analysed. It is found that there is alternating flow in the gating system because of the height difference of the ingates. That also resulted into the uneven filling during the process. Therefore, the design was improved and it was proved to be more effective through experimental study. This water analogue method is proved to be applicable for research on the filling process of castings.

Experimental study on the filling process of the lost foam casting process of a cast iron plate

Lost foam casting owns a series of advantages, however, EPS pattern burns while the liquid metal fills, which makes the filling process complicated and directly leads to defects in castings. A measurement system of the liquid metal filling based on a modified contact time method (MCTM) was established. The filling of measured points is illustrated by a LED pattern, and the filling time of all the points is obtained by the analysis of the recorded movie. This method is free of limitation of signal acquisition channels. The system was applied into the experimental study of the filling of a vertical plate-shaped specimen of cast iron. The filling of one hundred positions was measured, and the filling profiles and filling velocity during filling process were obtained, which reflected the features of its filling process. The influence of some parameters including the metastatic head, the density of the EPS pattern, the thickness of coating and the permeability of sand mould on the filling process were investigated.