Heat Transfer during solidification of hot dip aluminizing coating

Hot dip aluminizing coating is one of the most effective methods of surface protection for steels and is gradually gaining popularity. Although the pulling speed is one of the most important parameters to control the coating thickness of aluminizing products, however, there are few publications on the mathematical modeling of pulling speed during the hot dip process. In order to describe the correlation among the pulling speed, coating thickness and solidification time, the principle of mass and heat transfer during the aluminizing process is investigated in this paper. The mathematical models are based on Navier-Stokes equation and heat transfer analysis. Experiments using the self-designed equipment are carried out to validate the mathematical models. Specifically, aluminum melt is purified at 730 ℃. The Cook-Norteman method is used for the pretreatment of Q235 steel plates. The temperature of hot dip
aluminizing is set to 690 and ℃ the dipping time is set to 3 min. A direct current motor with stepless speed variation is used to adjust the pulling speed. The temperature change of the coating is recorded by an infrared thermometer, and the coating thickness is measured by using image analysis. The validate experiment results indicate that the coating thickness is proportional to the square root of pulling
speed for the Q235 steel plate, and that there is a linear relationship between coating thickness and solidification time when the pulling speed is lower than 0.11 m/s. The prediction of the proposed model fits well with the experimental observations of the coating thickness.

1 Introduction

Hot dip aluminizing steel has a higher corrosion resistance and more desirable mechanical properties compared with hot dip galvanizing steel. The principle of hot dip aluminizing is that the pretreated steel plates are dipped into the molten aluminum alloys at a certain temperature for a suitable time. Aluminum atoms diffuse and react with Iron atoms to form a composite coating of Fe–Al compound and aluminum alloy that has strong bonding force with the matrix to satisfies the requirement of protecting and strengthening the surface. In short, hot dip steel material is a kind of composite material
with comprehensive properties and of low cost. Currently, such techniques as Sendzimir, Non-oxidizing reducing, Non-oxidizing and Cook-Norteman are usually employed for hot dip aluminizing, through which large-scale productions can be realized due to their high efficiency of production, stable quality of products and less pollution. Among the four technologies, Sendzimir, Non-oxidizing reducing and Non-oxidizing are characterized by complex processes, expensive equipment and high cost. Nowadays, the Cook-Norteman method becomes widely used owing to the advantages of flexible processes, low cost and environment friendly.

For hot dip aluminizing process, the coating thickness is an important criterion to evaluate the coating quality and plays a key role in determining the properties of the coating. How to control the coating thickness during the hot dip process is therefore considered crucial in guaranteeing an excellent coating quality. As we already know, there is a close coupling correlation among the coating thickness, pulling speed and solidification time. Therefore, in order to control the hot dip process and improve the coating quality, it is necessary to build up a mathematical model that can describe this correlation. In this paper, the mathematical model of coating thickness and pulling speed is derived from the Navier-Stokes equation. The heat transfer during coating solidification is analyzed, and the relationship of coating thickness and solidification time is established. The experiments of hot dip aluminizing Q235 steel plates based on Cook-Norteman method are carried out with a self-made equipment. The real temperature and
thickness coating are measured accordingly. The theoretical derivations are illustrated and confirmed by the experiments.

2 Mathematical Model

2.2 Heat transfer during the solidification of coating Since the aluminum coating is very thin, it can be taken as parallel fluid flowing on the flat surface of plated pieces. Then it can be analyzed from x direction. The schematic diagrams of coating-substrate are presented in Fig. 2 and the temperature distribution is shown in Fig. 3.

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