Tianjiao Wang

Evaluation of distortion of castings

Abstract Displacement results are usually used as the criterion for distortion of castings during casting and heat treatment processes. However, the displacement results consist of both contraction and distortion. Contraction is a kind of uniform volume change, which can be solved by enlarging the castings by a contraction coefficient. However, the actual distortion has to be deleted by giving inverse distortion or by mechanical correction method. In this paper, algorithms are presented to evaluate the distortion of castings. One method is to subtract the uniform contraction from the displacement results. The other is to calculate the deflection angle between the normal direction of the discretised surface triangle of the original casting shape in stereolithography format and that of the simulated casting shape. The third one is the application of actual machining allowance in the evaluation of distortion. These methods are validated by three case studies, including a heavy hydroturbine blade casting.

Observation of the Mold-Filling Process of a Large Hydro-Turbine Guide Vane Casting

The mold-filling process has a determining effect on the quality of castings, and it has always been a hot but difficult research topic. The authors developed a wireless monitoring system for the mold-filling process of castings based on a contact time method and an observation system based on heat-resistant high-speed cameras. By using these two systems, the filling process of a turbine guide vane casting with a stepped gating system was investigated. The filling profile of the casting was recorded, and the filling time of nine typical positions was acquired. These results show that at the beginning, the liquid steel flowed out from the top ingate, which was designed to be the last to fill. The numerical simulation of the filling of the guide vane was performed, and the outflow from the top ingate were predicted. Finally, the gating system of the casting was improved with enlarged sprue. The new design features bigger sprue to ingate ratio; therefore, it could avoid the overflow from the top ingate and realize stable filling.

Deformation Prediction of a Heavy Hydro Turbine Blade During the Casting Process with Consideration of Martensitic Transformation

Heavy hydro turbine castings are made of martensitic stainless steel, which undergoes martensitic transformation during the casting process. Therefore, both residual stress and deformation are affected not only by uneven cooling but also by martensitic transformation. In this paper, a coupled thermo-martensitic phase transformation–stress model was established and it was implemented by further development with ABAQUS, which also incorporated the thermal and mechanical boundaries, and the contact pair between the casting and mold. The system was applied to the analysis of a heavy hydro blade casting. Results of stress, displacement, and martensite phase fraction were obtained. It is found that martensitic transformation has a significant effect on the stress and deformation results. The displacement in the normal direction of local areas was calculated to represent deformation in the x, y, and z directions. The deformation of the blade casting occurred mainly at the two thin corners with 18 and 22xa0mm in opposite tendency. The simulated results were compared with the measured machining allowance, and they are basically in agreement.

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.

Numerical simulation of the macrostructure evolution of a heavy steel ingot

The macroscopic grain structure of a 234 t steel ingot was simulated by using cellular automaton and finite element method provided by ProCAST. The growing speed coefficient of Kurz–Giovanola–Trivedi model was modified to reflect the growing process of grains. Chilled fine grains, columnar grains and coarse equiaxed grains were obtained from surface to core of the ingot, which were roughly in agreement with the measured ones. The influences of the nucleation and growth parameters were investigated. It was found that the relationship between nucleation and growth determines the macrostructure profile. An index, the ratio of growing speed over the nucleated sites at certain undercooling was proposed to estimate the macrostructure tendency.

Reply to Discussion of “Observation of the Mold Filling Process of a Large Hydro-Turbine Guide Vane Casting”

We appreciated Professor John Campbell’s comments very much. The mold filling of liquid metals is very important, especially as the first step of making a casting. Therefore, the filling system design is a very serious work. The melt filling process has been a 1000year mystery because it cannot be directly observed because of the mold obstacle. So, researchers have resorted to water analogy system. However, the analog results have been always doubted. In recent years, the X-ray observation method unveiled the filling process for specimens or small castings in laboratories. But, still some problems need to be solved such as dealing with the complicated and large castings. In our paper, we highlighted the effective direct observation system of mold filling and its successful application in a 10 t heavy steel casting, i.e., the using of high temperatureresisted camera to directly observe the filling process of heavy steel castings weighing in tons, which is affordable and the liquid steel flow can be directly watched at real time. It is a very valuable method for foundries, especially for the production of heavy castings, the direct observation means optimization of filling system and finally reducing waste and improving casting quality. Up to now, this method have been used in seven new heavy castings in the foundry of Harbin Electric Machinery Co., Ltd., and some observation results approve the experience, while some are in contradiction with the experience. As to the gating system design related to my paper, I agree with Professor John Campbell about the avoidance of the free fall of the melt in air or in the sprue to prevent inclusions and turbulence. However, in foundries, the filling system design is by theories with the consideration of the factors such as experience, cost, pouring ladle, and easy-going with on-site workers. Years ago I watched a bottom nozzle of a ladle was hardly opened till the end of filling of a heavy steel casting. Thus, in foundries, all these factors have to be considered, especially, in the production of heavy steel castings. Counter-gravity filling by using pressurized furnaces or pumps has been successfully applied in the production of aluminum castings; however, it is still hard to be adopted in steel castings up to date. The viewpoint about steady fluid flow in mold filling is easy to understand, but its importance in foundrymen minds varies. I will surely recommend these filling system design ideas and principles to foundries and assure them of their significance and effectiveness.

Measurement and Observation of the Filling Process of Steel Castings

The filling process is significant for the quality of castings, and it has always been a hot but hard topic. The author develops a wireless measurement system for the filling process of casting based on contact time method and an observation system for the filling process of casting based on high speed camcorders working under high temperature. By using these two systems, the filling process of a turbine blade and a hub casting were measured and observed. And the filling situation of the castings with bottom gating system was obtained and the filling time of a number of typical positions was recorded. The result showed that liquid steel flows through the forward ingates at first. The velocity of the liquid steel in the mold was obtained by the calculation of the filling time. The study also shows that these two systems operate conveniently and reliably. They are effective tools for monitoring of the filling process of castings and future optimizing of gating system, and have broad prospect in casting production.

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.

Distortion Behavior of a Heavy Hydro Turbine Blade Casting During Forced Air Cooling in Normalizing Treatment Process

Distortion behavior of blade castings in heat treatment process determines their geometrical accuracy, and improper control of it may result in additional repair, shape righting, or even rejection. This article presents a novel approach for discovering the distortion behavior of heavy blade castings during heat treatment process in production. Real-time measurements of distortion and temperature field of a heavy hydro turbine blade casting weighted 17xa0ton during forced air cooling in normalizing treatment process were carried out by using deformation measurement instruments and an infrared thermal imaging camera. The distortion processes of the typical locations of blade and the temperature field of the blade were obtained. One corner on the blade outlet edge side exhibits variation of distortion with two peaks and a valley. The range reaches 97xa0mm and the final distortion value is 76xa0mm. The maximum temperature difference on blade surface reaches 460xa0°C after 80xa0min of cooling. Influences of thermal stress and phase transformation stress on the distortion of the blade were elucidated and discussed. The results are of great significance for the understanding and control of the distortion behavior of hydro turbine blades in heat treatment.

Through process numerical simulation of a heavy hydro turbine blade casting

Heavy Hydro turbine castings, made of martensitic stainless steel, of spatially twisted shape, are susceptible to deformation during both casting and heat treatment processes. During production, uneven heating and cooling, phase transformation, stress and stain are responsible for the deformation of castings. However, the current research most focus on casting process or heat treatment process or both but separately, martensitic phase transformation is usually neglected in casting process. In this paper, a coupled thermo-martensitic phase transformation-stress model is established. And it is implemented by secondary development of ABAQUS, which deals the finite element model change and incorporates multi thermal and mechanical boundaries during casting and heat treatment processes as well, such as the contact pair between casting and mold during casting process. The stress analysis of a heavy hydro blade casting was performed by this system. The stress, displacement and phase are obtained. The effect of martensitic phase transformation on the stress and deformation is discussed. The simulated deformation is compared with measured results, and their agreement is improved by the through process simulation.