Casting process

Li Zengmin 1 Jia Guofang 2 Tan Jianbo 1 Li Lixin 1

(1 Hebei University of Science and Technology, Shijiazhuang 050018, Hebei, China; 2 Handan Xinxing Heavy Machinery Co., Ltd., Handan 056017, Hebei, China)

Abstract: In recent years, the lost foam casting process has developed rapidly, and the scope of application is becoming more and more extensive. This method does not need to start the mold, repair, under the core, close the box, the modeling of the use of non-adhesive dry sand filled plastic foam pattern, in the negative pressure conditions of solid pouring. It greatly simplifies the operation process and improves production efficiency; the casting quality is good and the process yield is high; it not only improves the working environment, but also reduces labor intensity. The many advantages of the lost foam process determine that it has great potential in casting molding and special applications. For example, high manganese steel parts are treated with hot water by casting surplus. The lost foam process solves the crack defects of thin-walled, wear-resistant and heat-resistant castings. In recent years, the author has accumulated some experience in the production practice of lost pattern. In this paper, combined with the characteristics of lost foam casting, the special application of several aspects of the process is briefly described.

Keywords: lost foam casting solid casting water toughening treatment crack defect

　　The Critical Application of lost foam casting process

　　in Foundry

LI Zengmin1 JIA Guofang2 TAN Jianbo1 LI Lixin1

(1.Hebei University of Science and Technology, Shijiazhuang 050018, Hebei;2 Handan Xinxing Heavy Machinery Co., Ltd,Handan 056017 Hebei)

　　Abstract：Over the years，the lost foam casting process has been developing rapidly，its range of application is be expaned day by day. There are not the operations, as pattern drawing, mould mending, core setting, mould closing in the process. The dry sand without any binder is used for fulling the foam pattern when mouling. The pouring of molten metal is made under vacuum. The working operation of foundery practice is simplified greatly, by which the production efficiency is raised, the cast product quality is improved, the yield of casting is high, Not only the operating environment is improved, and the labor intensity is reduced. The numerous advantages of lost foam casting process decides that it has a large potential on casting forming and some critical application, as hot remained water toughening of high mangaanese steel casting, solving the crack defects on thinwall, wear resistant, heat resistant castings by lost foam casting process, and so on. Writers accumulated some experience on the production in lost foam casting process over the years. The critical applications of lost foam casting process were introduced briefly combined with the features of it.

　　Key Words：lost foam casting process, full mould casting, water toughening treatment, crack defects

1 high manganese steel parts using casting residual hot water toughness treatment

The as-cast high manganese steel must be treated with water toughening to achieve the performance required by the application. Due to the rapid cooling rate during casting, the obtained structure is austenite and carbide, and the presence of carbide significantly reduces the strength and toughness of high manganese steel. Water toughening treatment is to reheat the high manganese steel castings to about 1100 ℃ (that is, the austenite area above A3 line), keep it for a certain period of time, so that all carbides are dissolved into austenite, and then quickly immerse the castings in water for rapid cooling, forcing carbides to fail to precipitate, thus obtaining a single austenite structure and making high manganese steel have high toughness.

Water toughening treatment has strict process specifications. The heat preservation time is calculated according to the wall thickness of the casting for 1h every 25mm. The temperature of the casting before entering water should be greater than 950 ℃, and the time from heat preservation to water entering should not exceed 1min. Otherwise, the temperature of the casting cannot be guaranteed due to excessive temperature drop, and the temperature of the water is required not to exceed 40 ℃. The traditional casting method is used to produce high manganese steel. Due to the difficulty of sand cleaning, the castings need to be cleaned in a normal temperature environment. The water toughening treatment can only be carried out by reheating the castings in a centralized furnace, which consumes a lot of fuel and labor.

The lost foam casting uses dry sand to bury the mold, uses micro-vibration and negative pressure physical means to tighten the mold, releases the vacuum after pouring, and the casting is equivalent to cooling in loose dry sand. The process has a very superior sand drop, flip the sand box or directly from the dry sand to dial out the casting, the sand scattered on its own, leaving only a thin coating layer on the surface of the casting, which creates a convenient condition for the rapid water treatment of the casting. We have used this method to mass produce high manganese steel parts such as ball mill lining plate, moistening mill sieve plate, crusher hammer head, etc., with good water toughness effect. According to the composition, wall thickness and weight of different castings, the appropriate sand falling time is mastered by experience to control the temperature before the casting enters the water. Through the test of metallographic, strength and hardness of the castings after water toughening treatment, it is difficult to see the cementite in the tissue, the tensile strength σb ≥ 600MPa, the elongation δ ≥ 20%, and the hardness HBS ≤ 300.

Summary: The high manganese steel castings are produced by the lost-energy process, and the water toughening treatment by using the casting waste heat is convenient, flexible, safe and reliable, and the mechanical properties fully meet the requirements of wear-resistant high manganese steel. It is estimated that this method saves nearly 600 yuan in combustion and labor costs per ton of castings than traditional water-tough treatment, and greatly shortens the construction period, with considerable benefits.

2 to solve the thin wall wear-resistant, heat-resistant casting crack defects

The cyclone is the main anti-wear casting on the multi-tube dust removal equipment. The structure is tubular, with an inner diameter of Φ230mm, a length of 950mm, and a wall thickness of 12mm. The material is wear-resistant cast iron (KMTCr26), which is characterized by poor toughness, large shrinkage and easy cold cracking. At first, CO2 water glass sand was used for molding, split pattern parting and horizontal pouring, and the crack rate of the casting was as high as 40%. In addition, metallurgical rotary kiln castings, including push head (ZG30Cr24Ni7SiN), kiln head iron protection, kiln tail iron protection (ZG30Cr26Ni20), etc. These castings have complex structure, with an outline size of about 1600mm × 600mm × 800mm and an average wall thickness of 20mm. They are all made of high chromium and nickel heat-resistant alloy steel and have a great tendency of hot cracking. The original process uses resin sand group core molding, the casting crack rate is as high as 100%.

When the above-mentioned castings are cast in sand, the common problem is that cracks are very easy to appear, and when the cracks of the cyclone castings are serious, the cracks are broken along the axial direction. Even if the crack is small, the material can not be welded and can only be scrapped. Most of the castings of push head and iron guard have cracks at the junction of rib walls. When the cracks are serious, the width is 10mm, the length is 400mm, and the rejection rate is as high as 30%. The casting with small crack can be delivered after welding and patching, but the working temperature of the heat-resistant steel casting is ≥ 800 ℃, and the crack will be triggered or even disconnected in a very short time.

In order to solve the crack problem of the above castings, under the condition of sand casting process, the following process improvements were carried out: limestone 70# sand core; Straw rope in the core; Loosen the box in advance and sand fall early; Inserting iron nails (internal cold iron) at the junction of the rib wall and other measures, but have not received good results. At the beginning of last year, it was decided to use the lost foam process to produce such castings, and the crack problem was effectively solved. In production, the release time of the vacuum degree is mainly controlled. When the surface of the casting is solidified to a certain extent, the vacuum will be released, so that the casting can shrink freely in the dry sand and minimize the casting stress. Cyclone belongs to thin-walled high chromium cast iron, which needs to be controlled to release the vacuum immediately after the mold is full. For thin-walled heat-resistant steel castings such as push head and guard iron, stop the pump 1~2min after pouring to release the vacuum. All the above measures have achieved good results: the crack defects of cyclone castings are all eliminated; The crack rate of push head and iron guard castings is reduced to 10%, and then the carbon content of molten steel is controlled at the lower limit of the standard, and the crack defects are completely solved.

Summary: Lost foam casting uses dry sand to bury the mold. After removing the negative pressure, the mold has good concession, which is incomparable to other sand casting using binder hardening. This provides a very effective solution for solving the crack defects of high alloy thin-walled parts.

3 Sand casting part of the use of real mold, simplify the difficulty of large casting production.

The base, workbench, column and beam are the main components of the 8m vertical lathe. The beam is made of HT250 and the rest are HT200. Gray cast iron has excellent shock absorption and wear resistance, and is widely used in the body of large-scale processing equipment. The outline size of the base and worktable is DN8m × 0.8m, and the average wall thickness is 30mm ~ 50mm. The outline size of the beam is 9.5m × 1.2m × 0.8m, and the main wall thickness is 30mm. The outline size of the column is 9m × 5m × 0.5m, and the main wall thickness is 30mm. The single weight of the above castings is above 12t, which is a large non-standard casting, and the production nature is often a single piece and a small batch.

The above-mentioned castings are large in volume and complex in structure, and casting production is very difficult. If solid casting is adopted, it is difficult to ensure the success of casting because many parts are not easy to fill with molding sand, and it is easy to cause deformation of the shape when molding, brushing, drying and embedding. For traditional sand casting, around how to make the cavity, it is necessary to consider multi-parting, multi-unpacking and multi-design sand cores. A large number of tooling, wooden molds and core boxes need to be made. Modeling, core-laying and box-closing are all very troublesome, with heavy workload and long production cycle. For similar large castings, the company adopts sand casting as the main method, and uses the advantages of convenient and flexible white mold to combine the two, which greatly simplifies the casting process and achieves good results.

The local complex structure on the large casting is divided to make a solid white mold separately, and the molding is embedded around the main wood mold as a live block, which can realize the purpose of less boxes and less cores, thus simplifying the traditional casting process. Whether the local selection of real shape on the casting should be analyzed for different casting structures. As long as it is convenient to fill sand and locate, the local embedded solid pattern living block will not affect the casting quality, but also simplify the modeling operation to a great extent. At this time, the local embedded solid pattern can be considered. For structures such as ribs, local protrusions and even one other local structure on the casting, embedded live blocks with solid patterns can be considered. This method requires that it is best to choose self-hardening sand that does not need to be dried, and the real pattern should be coated with anti-sticking sand coating in advance.

Summary: For a single piece and a small batch of large castings, sand casting is the main method for production, and local real casting is adopted, which can realize less unpacking and less core lowering, and greatly simplify the difficulty of modeling. It can shorten the production cycle, improve the production efficiency and reduce the production cost. And can reduce the casting size deviation caused by wrong box and wrong core, so as to achieve the goal of getting twice the result with half the effort.

　　Concluding remarks

The use of lost foam casting can achieve metal composite casting, the preparation of special coatings can be surface alloyed castings, the use of high-density foam pattern production of refractory shell, can achieve medium and large precision casting. It is believed that with the rapid development of the lost foam process, its application scope is more and more extensive.

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　　Introduction to the author:

Li Zengmin (1957-), male, Hebei Xinle, professor of Hebei University of Science and Technology, chairman of Hebei Foundry Society, deputy secretary-general of the Real Casting Professional Committee of China Foundry Association. Mainly engaged in lost foam casting principle, technology and equipment research and technical services, teaching and other work.

Telephone:13803337215，Email：zengmin9000@163.com