At present, the traditional casting technology of riser plus allowance is generally used in the production of large cast steel gears in China. It has the advantages of convenient molding and low operation technology. It is a relatively mature process only for the casting process. However, in order to remove large-scale subsidies, the combustion and melting methods such as oxygen acetylene and electric arc are generally used to make the local temperature rise abruptly, and then the matrix is cooled rapidly under the action of heat conduction and air absorption, which is easy to produce large thermal stress, martensitic brittle structure and cracks, resulting in high rate of repair and rejection, resulting in large economic losses. In order to solve this problem, the foundry technology of sand coated external cold iron was designed. After production and application, the effect is remarkable.
Selection of materials, equipment and methods
The typical gear is selected, and its parts are shown in Figure 1. The material is ZG310-570, which weighs about 5T. The technological parameters of cold iron are calculated theoretically, and the external cold iron and sand layer are designed according to the modification of parts and operation. The production test adopts 10t electric arc furnace smelting, water glass surface dry modeling, bottom pouring, oxygen acetylene flame cutting riser, heavy oil reverberatory furnace stress relief annealing, A-type display pulse reflection ultrasonic flaw detection.
Theoretical calculation and modification design of sand laying external chill process
The theoretical calculation and modification design of the sand coated outer chill are carried out with the outer diameter, flange thickness and flange height of the gear shown in Figure 1. Considering the feeding effect of riser and the sequential solidification of casting, the shape of outer chill is selected as “convex” shape. According to the Japanese steel casting manufacturing standard, the sanding amount of the external cold iron is 8-10 mm, but considering that the sanding is too thin and easy to fall off, not easy to compact and increase the operation difficulty, the sanding amount is increased to 10-12 mm, and the thickness of the external cold iron is correspondingly increased to make up for the loss of heat conduction speed caused by the thickening of sanding amount. In order to facilitate the hoisting, removal and reuse of the sand coated external cold iron, it is divided into 6 sections, which are combined for use with a clearance of 10-15mm. In order to further reduce the technical difficulty of sand laying operation, a number of hanging sand grooves are set on the external cold iron, with the width of 20 mm, depth of 10 mm and spacing of 30 mm. In order to make the gas in the mold discharge smoothly when it is stimulated by the thermal expansion of molten steel, 2-3 10 mm air outlets are arranged in each sand hanging tank.
