Research and Development of Large Diameter Centrifugal Casting Ductile Iron Pipe

1. Production scale selection Our factory originally only had two water-cooled metal centrifugal casting machines, the maximum specification is DN700, and the annual production of DN100-DN700 ductile iron pipes is about 50,000 tons. According to market surveys, our factory found that the demand for large-caliber tube pipes above DN800 is quite large. Therefore, we designed a DN800-DN1200 water-cooled metal centrifugal casting machine in 1997. However, because we were a joint venture at that time, foreign investors temporarily Do not plan to invest again, so the matter is stranded. In 1999, we designed the original DN400-DN700 centrifuge by calculating the flow of cooling water to make it reach the capacity of producing DN800 bulbs, and the effect was good. Even so, it still cannot meet the needs of DN1000 and above bulbs. Therefore, our factory researched and decided to develop large-caliber bulbs in two phases. In the first phase, first install DN1000-DN1600 pipes, (including T-type interface DN1000, DN1200, DN1400, K-type interface DN1000, DN1200, DN1400, DN1600), and continue to install DN1800-DN2200 pipe after the success of DN1000-DN1600 pipe, annual production The capacity will eventually reach more than 50,000 tons.

Second, the choice of centrifuge Centrifugal casting machine is the key equipment for the production of centrifugal cast pipe. Nowadays, centrifugal casting pipes have the following two production methods: one is the water-cooled metal type centrifugal casting method; the other is the hot mold method and the centrifugal casting method. The hot mold method can be divided into the coating metal centrifugal casting pipe method and the resin sand centrifugal casting pipe method. The advantages of the water-cooled metal centrifugal cast pipe method are: the outer surface of the cast pipe is of high quality, the productivity is high, and the level of mechanization and automation is high. However, the disadvantages are: higher precision requirements for the control system, low tube mold life, high temperature graphitization annealing for cast pipes, relatively high cost of cast pipes, and relatively large initial investment. The advantages of the hot mold method are: the cost of the cast pipe is low, and the cast pipe does not need to be subjected to high temperature graphitization annealing.

The disadvantage is that the productivity is not high, and the surface quality is slightly poor. According to our factory’s expected annual output of 20,000 tons in the first phase, our factory chooses the coating metal centrifugal casting method and newly installs a DN1000-DN1600 coating thermal mold centrifuge. Since the tube mold of the large-caliber centrifuge has weighed 20-40 tons, a fixed rotating form of the tube mold is used to ensure the stable operation of the centrifuge. At the same time, three production stations are set up to improve production efficiency. The three production stations are respectively; paint spraying station, centrifugal pouring station, and pipe-drawing station. The paint spraying station has a fixed rotating mechanism and a paint spraying trolley with a long handle. Before spraying paint, preheat the tube mold to 220-260℃ and then start spraying. The thickness of the paint after spraying is 1-1. 5 mm. Use a crane to transfer the sprayed paint pipe to the next station, and centrifuge the pouring station. The centrifugal pouring station is also provided with a fixed rotating mechanism and a pouring trolley. The fixed rotating mechanism of the pouring station can make the tube mold rotate on it at high speed.

The casting trolley is provided with a walking mechanism, a fan-shaped bag tilting device, a casting launder, and an inoculant adding device. 3-5 minutes after the molten iron is poured into the pipe mold, spray water mist into the pipe mold to strengthen cooling. After the temperature of the cast pipe in the pipe mold reaches 700°C or lower, the rotation of the pipe mold is stopped and the pipe mold is hoisted to the next station, and the pipe is pulled out. This station is equipped with a pipe trolley. After the pipe is dialed at this station, it is lifted to the annealing furnace by a crane, and the casting is finished. 3. Hot metal smelting Our factory has prepared a 15t/h water-cooled long-lasting cupola without lining. Every time the furnace is built, it can be opened day and night for 30 days, which can ensure the continuous production of large pipes. During the opening of the 15-ton cupola, the excess molten iron can be transferred to the original production one (the production one produces DN100-800). At the same time, there is a 45-ton electric furnace with a power of 2500KW. The molten iron melted in the cupola is sent to the 45-ton electric furnace by a transfer trolley, where it continues to heat up, adjust its composition and store it. Fourth, making the socket sand core In order to form the internal shape of the pipe socket part, a sand core needs to be placed at the socket end of the pipe mold, that is, one sand core is consumed every time a pipe is cast. According to the daily output of 30 to 60 pieces, our factory selects self-hardening resin sand to produce sand cores by hand molding, and designs two sets of core boxes according to each specification and prepares special cast iron core bones.

The supporting equipment also includes a rotor sand mixer and 2 simple sand core heating furnaces. The production process of the sand core is: put the weighed quartz sand into the sand mixer, then start the sand mixer and slowly pour in the weighed curing agent and continue mixing for 30 seconds, and then weigh it again Pour in the resin and continue mixing for 30 seconds before sanding. After the sand is produced, fill the end face of the core bone with resin, then close the core box and continue to smash the remaining part. After the sand core is tamped, the core box is hoisted on the drying oven for 15 minutes, and then the mold is opened to take the core. Finally, the finished sand core is brushed with graphite paint, ignited and dried, and then it can be directly sent to the centrifugal station for use. After adopting the above process, the sand core can be used and made now, and the continuity of production is ensured. Five, annealing furnace There is almost no cementite produced by chilling in the matrix of the tube produced by the coating metal type, so there is no need to perform high-temperature graphitization annealing, but only low-temperature graphitization annealing is required to reduce the content of pearlite in the matrix. Therefore, our factory has newly installed two car-type annealing furnaces. The fuel is liquefied. The two furnaces can return 30-60 pipes per day, which can basically meet the needs of production continuity. 6. Finishing equipment Our factory is equipped with grinding socket, grinding inner wall, cutting ring chamfering, hydraulic test, cement coating lining, on-site metallurgy and other equipment in the finishing process, which can meet the overall repair and inspection of the cast pipe. After the cast pipe is coated with cement, it is put into the cement curing tank by crane for curing. The two ends of each pipe are sealed with canvas and inserted into the steam pipe and injected with steam. Keep the temperature in the pipe at 40-60℃ for curing 6－ 8 hours. After curing, the cast pipe cement lining can be lifted out of the curing tank and sent to the painting line. The painting line has a single-station zinc spraying machine and a two-station surface resin paint spraying machine. The surface of the painted pipe is sprayed with marks and trademarks. At this point, the entire production process is completed. 7. Application of large diameter pipe: Since the large pipe production line was put into production, the product has been in short supply, and good economic benefits have been achieved. Products have been successively applied to key water delivery projects in some large and medium-sized cities such as Wuxi, Yangzhou, Changshu, Ningbo, Qingdao, Yantai, Xiamen, Wuhan, and Shijiazhuang.

Through use, we found that the following can be beneficial to the installation of the tube: 1. Wooden supports must be placed on the socket before leaving the factory to prevent the elliptical deformation of the socket of the cast tube during transportation and storage. After the two pipes are installed and connected correctly, the socket wooden brace can be removed. Premature bracing can easily cause installation difficulties. 2. The socket end of the large pipe must be chamfered correctly, the bevel and the end face must be smooth, and the chamfer is too small or not chamfered, which will easily affect the installation. Therefore, before the large-scale installation, be sure to check whether the chamfer of the socket meets the installation requirements. 3. In order to facilitate the installation of the pipeline, a layer of grease should be applied to the sealing surface of the pipeline and the rubber ring before installation. The grease must not contain any toxic ingredients and should have good lubricating properties, and it should not pollute the conveying medium. We have discovered through practical use of this that the use of animal oil is more conducive to the installation of large-caliber bulbs of DN800 and above because of its spherical granular shape, compared with vegetable oil. Since our current large-diameter pipe output cannot meet the demand of the order, we are currently installing a new single-station pipe casting machine that can produce DN1000-DN2200 T or K large pipes. Core equipment, finishing equipment, etc.) for capacity expansion and transformation. It is expected to be put into production at the end of August this year. After the second phase of the project is put into production, the production scale of our factory’s big pipes will be doubled and the maximum production specification can reach DN2200.