Key measures for gray cast iron production (Qufu Longxiang Smelting and Casting Accessories Co., LTD.)

How to improve the internal and appearance quality of castings, improve the technical content of castings, and cope with the market competition is the subject faced by some domestic manufacturers. Every link in the production of castings has an important impact on quality and can not be ignored. Some technical measures summarized in my actual work are summarized as follows for reference.

First, the rigidity of the sand core and sand mold

After the sand casting, the swelling force caused by the static pressure or solidification of the iron liquid often leads to the movement of the mold wall and the collapse of the sand core, which will produce internal shrinkage holes and surface shrinkage of the casting. Therefore, in order to make the casting size stable, the mold should be tightened to the maximum extent. In order to save molding materials, hollow sand core is widely used in core making, which is lighter than the solid core, so the heat capacity is small, and the solidification rate is slow, which will lead to the expansion of sand mold or the collapse of sand core. In addition, liquid iron may penetrate into the empty part through cracks on the core head or sand core, which will also cause defects in the casting. In order to improve the rigidity of the hollow sand core, it can be filled with wet sand or water glass sand. The shell core can also be made into two halves, its internal reinforcement, after the core can be bonded to obtain a hard sand core.

Second, the correct choice of pouring temperature

1. Defects may be formed when the pouring temperature is too low

(1) manganese sulfide pores This kind of pores are located below the skin of the casting and are mostly above, often revealed after processing, and the diameter of the pores is about 2~6mm. Sometimes the hole contains a small amount of slag, metallographic studies show that this defect is caused by MnS segregation and slag mixing, due to the low pouring temperature, while the liquid iron contains high Mn and S content. In order to prevent this defect, when the cupola is used to melt iron, the S can be continuously removed by air flow in the ladle of the porous material, and the S can be reduced to 0.06% ~ 0.08%. Such S content and appropriate Mn content (0.5% ~ 0.65%) can significantly improve the purity of liquid iron, so as to effectively prevent such defects.

(2) liquid slag inclusion

After processing, a single small hole will be found under the skin of the casting, and the diameter of the hole is generally 1~3mm. In some cases there are only one or two small holes. Metallographic studies showed that these pores appeared with a small amount of liquid slag inclusion, but no segregation of S was found there. The research shows that this defect is related to the pouring temperature, when the pouring temperature is higher than 1380℃, this defect is not found in the casting, so the pouring temperature should be controlled at 1380-1420 ℃. It is worth mentioning that changing the design of the pouring system has not eliminated this defect, so this defect can be considered to be due to the low pouring temperature and the formation of the iron liquid in a micro-reducing atmosphere.

(3) Pores caused by gas in the sand core

Porosity and porosity are often caused by poor core exhaust. Because the sand core is mostly hardened in the core box during core making, the number of exhaust holes in the sand core is often insufficient. In order to form an air vent, additional holes can be drilled after the core has hardened. The test shows that the improvement of the core ventilation system can make the casting temperature more adjustable.

The common reason for the low pouring temperature is that before pouring, the iron liquid is transported and stays in the open ladle for a long time and dissipates heat. The use of a ladle cover with insulation material can significantly reduce heat loss.

2. The pouring temperature is too high

Too high pouring temperature will cause sand mold expansion, especially for castings with complex sand cores. When the pouring temperature is ≥1420℃, the waste will increase, and when the pouring temperature is 1460℃, the waste will reach 50%. In production, the temperature of liquid iron can be controlled better by using induction furnace.

Third, the formation of crystal nuclei

(1) The influence of gestation

Inoculation treatment sometimes also increases casting defects, because of the strong inoculation and rapid nucleation of cast iron, the tendency to form carbide increases. Therefore, it is recommended that the amount of inoculant can prevent white mouth when inoculation treatment, and the crystal nucleus in the sound casting is much less than the casting with shrinkage holes.

(2) The role of sulfur

Due to the low S content in most scrap steel, only cast iron with low S content (≤ 0.05%) can be obtained when melting scrap steel in electric furnace. This kind of cast iron does not work for many inoculants, because the incubation decay is very fast, so when the scrap steel is melted in the electric furnace, it often produces white holes in the casting. Therefore, sometimes a carburizer with a very high sulfur content is used, which can make the final ws≥ 0.05% to ensure the inoculant effect.

(3) Heat preservation and superheating temperature of liquid iron

In recent years, people tend to use electric furnaces to melt and hold liquid iron, but increasing the superheating temperature and increasing the holding time will reduce the formation of crystal nuclei, so there is a risk of white holes. Taking into account economic and material properties, the temperature should be as low as possible when holding heat for a long time.

4. Casting size error caused by sand core

When the sand core is heated, it first expands and then produces plastic deformation. This change caused by high temperature is related to the decomposition of the molding sand and the binder used.

(1) The effect of hardening temperature and hardening time The expansion and thermoplasticity of resin sand are related to the hardening temperature and time. If the sand core has a long hardening time, its primary and secondary expansion will be large, which may cause deformation problems. While the hardening time is short, the sand core expansion is small and the decomposition is fast. Therefore, strict control of hardening time and temperature is very important for the manufacture of high-temperature performance sand cores.

(2) The influence of sand core coating Most of the sand core surface to brush paint, the test found that the depth of refractory paint into the surface of the sand core has a great impact on the deformation of the sand core, when coated with the coating containing surfactants, because of the large penetration depth to prevent secondary expansion.

V. Conclusion

The methods summarized in this paper are very important for the production of gray cast iron. In the technical control, the first is the application of metallographic inspection method to identify defects. Secondly, with the help of chemical analysis to test, with the correct identification method, it is easier to find measures to prevent defects.