Analysis of causes of defects in gray cast iron and preventive measures
I. Main factors affecting the mechanical properties of gray cast iron:
Process and metallurgical factors: mainly cooling rate, overheating treatment of molten iron, inoculation treatment, furnace charge characteristics, etc.
(1) About the influence of cooling rate
Cast iron is a material that is very sensitive to cooling rate. The thick and thin wall parts of the same casting may have very different structures inside and outside, which is commonly known as structural inhomogeneity. Because the graphitization process depends to a large extent on the cooling rate. There are many factors that affect the cooling rate of castings: casting wall thickness and weight, type of casting material, pouring head and weight, etc. Since the wall thickness, weight and structure of the casting depend on the working conditions and cannot be changed at will, their influence on the structure should be considered when selecting the chemical composition.
(2) About the influence of inoculation treatment of molten iron
Inoculation treatment is to add inoculant to the molten iron before it enters the casting cavity to change the metallurgical state of the molten iron, thereby improving the microstructure and properties of the cast iron. For grey cast iron, the purpose of inoculation treatment is to obtain the structure of type A graphite, pearlite matrix, and fine eutectic clusters, as well as to reduce the tendency of white cast iron in thin walls or corners of the casting and the sensitivity to the wall thickness of the casting; for malleable cast iron, it is to shorten the short annealing cycle, increase the allowable wall thickness of the casting and improve the structure of the organization; for ductile iron, it is to reduce the tendency of white cast iron in the casting, increase the spheroidization rate and improve the roundness of graphite.
(3) The influence of molten iron overheating treatment.
Increasing the superheating temperature of molten iron can:
① Increase the content of combined carbon and reduce the carbon content of graphite accordingly
② Refine the graphite and form dendritic graphite
③ Eliminate the "hereditary" of cast iron
④ Improve the uniformity of the organization on the cross section of the casting
⑤ Favorable to the shrinkage compensation of the casting. Similarly, the insulation of molten iron has a similar effect to the overheating of molten iron.
(4) Influence of charge characteristics
In actual production, it is often found that when the metal charge is changed (for example, using pig iron from different origins or changing the charge ratio, etc.), the chemical composition seems to remain unchanged, and the cast iron has different structures and properties. This shows that the properties of the raw materials directly affect the properties of the cast iron smelted with it, which is called the "heredity" of cast iron. Therefore, increasing the temperature of the molten iron and using a variety of iron charge ingredients can eliminate this "heredity" and improve the structure and properties of cast iron.
In summary, the process factors and metallurgical factors of cast iron have a great influence on the mechanical properties of cast iron. Therefore, the control of these influencing factors should not be ignored.
2. Gray cast iron cannot be heat treated to meet the grade requirements
Generally speaking, heat treatment can greatly improve the structure and properties of casting alloys, but under the condition of gray cast iron, the role that heat treatment can play is relatively small. In gray cast iron, graphite has a great influence on the performance of cast iron, and any heat treatment method cannot change the morphology and distribution of graphite. Therefore, it is not possible to effectively improve the performance of gray cast iron to meet the grade requirements through heat treatment.
However, there are many ways to improve the mechanical properties of gray cast iron, such as reasonable selection of chemical composition, change of charge composition, overheating of molten iron, inoculation, trace or low alloying, etc., all of which can achieve good results.
3. Precautions for producing high-grade gray cast iron (inoculated cast iron)
When producing high-grade gray cast iron (generally HT200 and above), in order to obtain high mechanical properties, the amount of graphite and the length of graphite must be reduced as much as possible. The traditional method is to reduce the carbon and silicon content of molten iron and increase the condensation rate of molten iron, but when the amplitude is slightly larger, D-type supercooled graphite and white cast will appear, which will reduce the mechanical properties of gray cast iron.
Adding an appropriate amount of ferrosilicon-based iron alloy particles to the molten iron in front of the furnace or before pouring is called inoculation. Inoculation provides a large number of nucleation particles in the molten iron for graphite to nucleate. Effective inoculation will promote the precipitation of graphite, thereby eliminating white cast iron, refining flake graphite and transforming supercooled graphite into non-directional uniformly distributed graphite (type A graphite). It can not only greatly improve the comprehensive mechanical properties, but also improve the uniformity of the cast structure, and reduce the difference in mechanical properties caused by uneven wall thickness and different cooling rates between the corners and the core of the casting. Therefore, inoculation of molten iron is an indispensable technology for producing high-grade gray cast iron (inoculated cast iron).
In order to make inoculation effective, the requirements of inoculation on the original iron liquid must be met, that is, the original iron liquid should have a lower carbon and silicon content, or the original iron liquid should have a lower carbon equivalent. The lower the carbon equivalent, the better the inoculation effect and the higher the strength of the gray cast iron. On the contrary, the higher the carbon equivalent, the poorer the inoculation effect. Since silicon can be adjusted by adding inoculants, when considering the carbon equivalent of the original iron liquid, the carbon is always maintained at about 2.8%~3.2%, and the silicon is maintained slightly below the critical value that can significantly promote graphitization. Then, the inoculant is added to make the silicon content exceed the critical value to obtain the effect of inoculation. In addition, the wall thickness and cooling rate of the casting also affect the structure of the inoculated cast iron parts, and they should also be considered when selecting the chemical composition. Generally, the carbon and silicon content of thick parts are taken at the lower limit, and the upper limit is taken for thin parts.
In addition to neutralizing the influence of sulfur, the role of manganese in high-grade gray cast iron (inoculated cast iron) has a special requirement, that is, with the help of it, the gray cast iron can obtain pearlite structure, so the manganese content of high-grade gray cast iron (inoculated cast iron) is generally higher. Sulfur can weaken the graphitization effect of the inoculant, so sulfur is often limited to less than 0.12%. In recent years, some people also believe that in order to obtain a good inoculation effect, the sulfur content of the original iron liquid cannot be too low. Therefore, in the case that the sulfur content in the molten iron gradually decreases in the future, how to determine the sulfur content in the inoculated cast iron seems to be a problem worthy of attention.
The phosphorus content is generally limited to less than 0.15% based on the requirements of mechanical properties, but some machine tool gray cast iron parts (often made of inoculated cast iron) need to be wear-resistant, and the phosphorus content can be increased to about 0.3%~0.5% or higher.
4. Four prohibitions when using inoculants for gray cast iron inoculation
The inoculants used for gray cast iron can be classified according to function, main elements, shape, etc. There are four taboos when using inoculants for gray cast iron inoculation:
First, the inoculant must be dried before use.
Second, it is forbidden to use pure silicon or pure ferrosilicon as inoculants
Third, the inoculant cannot be added hastily, and the method must be particular.
Fourth, the particle size of the inoculant
Five. Possible defects of gray iron castings
In the production of gray iron castings, common casting defects include: pores, unqualified composition and performance, hot cracks and cold cracks, shrinkage and shrinkage, slag eyes and iron beans, cold shut and insufficient pouring, sand eyes and sand inclusions, meaty and misaligned, deformation, etc. Usually, the causes of these defects are not only the problems of molding and core making, but sometimes there are many production process problems such as smelting and pouring, sand quality, sand drop cleaning, etc. Therefore, specific analysis is required in order to take appropriate and reasonable measures to solve them.
In the production of gray iron castings, common casting defects include: pores, unsuitable composition and performance, hot cracks and cold cracks, shrinkage and shrinkage, slag eyes and iron beans, cold shut and insufficient pouring, sand eyes and sand inclusions, meaty and misaligned, deformation, etc. Usually, the causes of these defects are not only the problems of molding and core making, but sometimes there are many production process problems such as smelting and pouring, sand quality, sand drop cleaning, etc. Therefore, specific analysis is required in order to take appropriate and reasonable measures to solve them.
