Lost mold cast iron parts micro-slag slag prevention has a trick

I. Overview

The lost foam casting is to bond the foam patterns similar to the shape and shape of the casting into a pattern cluster, apply the refractory paint and dry it, embed it in the dry quartz sand to vibrate the shape, and pour under the negative pressure to vaporize the model. Liquid metal occupies the position of the model, and a new casting method for forming castings after solidification and cooling, also known as dry sand solid negative pressure casting, known as EPC (Expendable casting proces) abroad, is one of the more advanced casting processes.

China's lost foam casting began in the 1980s. Based on the foreign lost foam casting technology, it has experienced decades of research and development, and gradually formed a lost foam casting technology and production pattern with distinctive Chinese characteristics. Although the road has experienced twists and turns, but with the unremitting efforts of many scientific researchers, the development of lost foam casting technology is extremely obvious, especially before and after 2009, due to the realization of the lost foam equipment technology and the localization of raw and auxiliary materials, and Formed a series of production and production and supply of one-stop production pattern, thereby accelerating the application of lost foam casting process in China's foundry industry. According to incomplete statistics, by 2009, the output of China's lost foam castings has reached 427,000 tons, and the largest single-piece lost-die castings have reached 12 tons. Although the lost foam casting has been developed rapidly, due to the closed technology of various manufacturers and the failure to form a unified industry standard like traditional casting technology, each of the lost foam manufacturers has its own characteristics, which also limits the disappearance to a certain extent. The development of die casting has a certain gap with the production level of lost foam in foreign countries.

Although the lost foam casting has reduced the processes of sand mixing, demoulding, trimming, lower core, and boxing compared with the conventional sand casting, the quality problems caused by the process to the workpiece are avoided, and the size of the casting is greatly improved. Accuracy and reduced surface roughness; compared with precision casting, the production process and production environment are relatively simple and low in cost, but due to the casting of solid molds in lost foam casting, complex physical reactions and chemical reactions exist in the filling process. Like castings produced by other processes, lost foam castings will inevitably produce various types of casting defects. Among them, micro-slag inclusions are found in the interior of the casting base, which is difficult to visually detect. Second, through more complicated inspection methods and damage to castings. It was found that the micro-slag inclusion of the lost foam casting became a problem of lost foam casting.

Second, the existence form of microscopic slag in lost foam cast iron parts

The slag inclusion is a kind of casting defect, commonly known as slag eye. The slag inclusion of the traditional casting process casting is generally formed because the smelting molten iron is not pure. In addition to the imperfection of molten iron to form slag inclusions, the lost mold process castings may also cause slag inclusions in the castings due to imperfect processes and residuals in the casting process, and the slag inclusions are generally concentrated under the skin of the castings and excessively rounded corners. The shape has a line shape, a sheet shape, a block shape, etc., and the color is black. Since the slag inclusion on the non-machined surface is difficult to find, it is also harmful to the quality of the casting.

Our company used the lost foam process to produce the IIIB type iron seat. Since the iron seat is a non-processed casting, in order to ensure the safety of railway transportation, we have done a lot of destructive tests, cutting all parts of the iron seat and found iron. The connection between the tail and the working surface is close to the edge part A (see the drawing). It is not a smooth and clean metal surface, but an irregular black object. It is judged as slag inclusion by microscopic observation. Because the slag inclusion is extremely small, if it is difficult to find without careful observation, the proportion under the microscope is about 10%.

Slag inclusion

According to the use condition of the iron seat, here is the main force part during the use of the iron seat. Due to the existence of the slag inclusion defect, the mechanical properties of the slag inclusion are greatly reduced, so that the safety of the line cannot be guaranteed. The slag defect becomes an important determinant of whether the iron seat is produced by the lost foam process.

Third, the loss of mold micro-slag analysis

The main processes of the lost foam casting process are: the production of pattern clusters (pattern making → pattern combination → painting coating → model cluster drying), buried mold, melting and casting. According to the mechanism of slag inclusion formation, we believe that there are three main reasons for the slag inclusion defect: First, the slag inside the molten iron cannot be cleaned after smelting, especially the smelting of ductile iron, after the spheroidization treatment is completed, The residual part of the spheroidization reaction remains in the molten iron; the second is that during the casting process, the process control is not strict, the scum on the surface of the molten iron enters into the mold with the molten iron; the third is the filling process unique to the lost foam process, the high temperature A strong chemical reaction occurs between the molten iron and the pattern. The heat of the molten iron is used to vaporize the sample, and the gas generated by the gasification of the sample is extracted by a vacuum pump. If the gasification of the sample is incomplete, the gasification of the sample is generated. The gas cannot be pumped out of the mold in time, and the gas pressure generated in the mold will leave the mold residue under the surface of the casting, which will cause the slag inclusion defect in the casting (the mold residue defect in the lost foam industry).

Since the metal slag and the sample residue are very similar in color, they are all black, and the parts remaining in the casting to form slag are the same. Therefore, it is difficult to distinguish whether the slag defect of the lost foam casting is from the color and the part is the metal slag or the pattern residue. However, since the metal slag is generally bulky, it can be seen by the naked eye, and the proportion formed is small, and the slag inclusion volume of the IIIB type iron seat is small and the proportion is large, so we have established the premise of strict smelting and casting process. Next, the main solution is to solve the pattern residue.

Fourth, micro-slag slag prevention measures

1. Strict molten iron smelting and treatment process

(1) In order to reduce the excessive slag caused by the corrosion of raw materials during the smelting process, we strictly stipulate the surface cleaning standards before the use of raw materials, and use the vibration, shot blasting, heating and other treatment methods to rust and oil the surface of the raw materials. The impurities are cleaned.

(2) In the later stage of smelting, the slag generated by the smelting is removed as much as possible by using the slag-removing agent. At the same time, before the molten iron is discharged, the temperature of the molten iron is raised to about 100 ° C, and the slag is cleaned 2 to 3 times with a slag-removing agent. Then, the temperature of the molten iron is lowered to the temperature required for the furnace to be discharged.

(3) The spheroidizing package is cleaned, and the spheroidizing agent and the inoculant used for the spheroidizing treatment are baked before use, and the iron scraps used for covering are heated to remove the oil before use.

(4) After the spheroidization reaction is completed, it is allowed to stand for 3 to 5 minutes, and the slag is removed by the poly slag for 2 to 3 times.

2. Casting process

(1) The pouring cup is made of water glass sand hardened, and other ways of pouring the cup are strictly prohibited.

(2) During the pouring process, a special person is arranged to stop the slag.

(3) The molten iron must be filled with the pouring cup when pouring, and the molten iron is required to be continuous and the flow rate is stable.

(4) Through manual control, the vacuum degree in the sandbox during pouring is guaranteed to be within a certain range, and the fluctuation is about 0.05MP.

3. Pattern production process control

(1) Under the premise of ensuring the strength of the pattern, by adjusting the foaming time and the steam pressure, the particle size after foaming of the mold material is increased, and the density of the pattern (white mold) is minimized. Through a number of tests, we controlled the density of the IIIB type iron seat pattern to 0.018 to 0.026 g/cm ³ .

(2) Under the premise of ensuring the requirements of the sprue strength, the density of the sprue pattern (white mold) is reduced as much as possible. Through the test, we controlled the density of the sprue pattern to 0.016 to 0.022 g/cm ³ .

(3) By adjusting the particle size and proportion of the refractory material in the coating, under the premise of ensuring the surface roughness of the casting, the gas permeability of the coating is maximized, and the gas generated by the pattern in the casting process is smoothly discharged.

(4) When the pattern is combined, the purpose of ensuring the strength of the pattern cluster is achieved by using a mosaic method in the case of using a small amount of the adhesive.

(5) Control the particle size ratio of the filling sand, strengthen the dust removal of the filling sand, ensure the gas permeability of the filling sand during the pouring process, and discharge the gas generated by the pattern.

Fifth, the implementation effect

For the type IIIB iron seat tested in the first furnace, we carried out all destructive tests, and carried out cutting inspection on the important parts of the workpiece. No slag defects were found after the inspection. Subsequently, according to the control measures of each process in the first furnace test process, we continuously tested 5 furnaces, and cut and inspect the castings in each sand box with a sampling ratio of 50%, and sampled 960 pieces of workpieces. The sampling results showed that there were two pieces of IIIB type iron seat hole with slag inclusion defects (visible to the naked eye), and the IIIB type iron seat was overly rounded (forced part), and no slag inclusion defects were found by microscopic examination. According to the precautionary measures formulated during the test, we revised and supplemented the lost foam casting process of the IIIB type iron seat and entered the mass production.