Quality control measures of large section ductile iron castings

Since ductile castings came out in 1947, it has gone through a process of roughly ten years. A new type of metal composite material has been put into industrial production, it has been developed rapidly, the proportion of production has been continuously increasing. The survey shows that the global production of ductile iron in 2005 has reached 19.59 million tons, the production of ductile iron in my country has reached 5.83 million tons. The American magazine "ModernCast2ing" predicts that 2004 to 2008, the growth rate of global ductile iron casting production will reach 3.8%, which is higher than the average annual growth rate of global metal casting production ranks second. One of the key reasons why the production volume of ductile iron can develop rapidly is that with the development of manufacturing, thick section ductile iron has been used more more widely in metallurgy, machinery, transportation, nuclear fuel storage transportation, the demand is rapid. increase.

It is generally believed that ductile iron castings with a wall thickness of more than 100mm are called large section ductile iron castings. Large-section ductile iron parts have large size, large weight, large wall thickness, large heat capacity during precision casting, slow solidification. It is very easy to cause spheroidization inoculation recession, which will cause changes in the structure of castings, especially in castings. The heart is more serious. The specific performance is that the graphite balls are thick, the number of graphite balls is reduced, the graphite floats, the graphite balls are deformed, resulting in a variety of non-spherical graphites, mainly pseudo-flakes, worms, bursts fragments. At the same time, due to the redistribution of solute elements during solidification, there will still be a series of problems such as serious element segregation, intergranular carbides, white mouth. As a result, the material mechanical properties of ductile castings deteriorate, especially the elongation. plasticity is obviously reduced, which restricts the popularization application of large-section ductile castings. With the rapid development of the manufacturing industry, the internal external quality requirements of large-scale ductile iron are getting higher higher, the precision casting staff are facing new challenges opportunities.

In view of the problems in the production of large-section ductile iron, the precision casting staff put forward a variety of quality control measures, which are introduced in detail the following aspects.

1 Speed up cooling

One of the key processing measures for precision casting of large-scale ductile iron castings is to accelerate the refrigeration of the castings. Through various cooling methods, the solidification time is shortened the quality of large-section ductile iron castings is effectively controlled. Some scientific research results believe that the solidification time of molten iron should be controlled within 2h. For large-section ductile iron castings, various molding sands are difficult to meet the demand for rapid cooling. The conventional method to increase the cooling speed is to add cold iron, forced refrigeration, cold iron forced refrigeration·1383·FOUNDRYTECHNOLOGYVol.28No.10Oct.2007 in conjunction with the application.

(1) Kallang iron

Cold iron has high thermal conductivity strong heat storage capacity. It is a widely used enhanced refrigeration material. The cold iron working face should be processed before the first application, there must be no pores, blisters other defects. When repeated use, the working face must be polished dry, the number of cold iron applications should be controlled according to the actual situation. Experiments show that chilled iron can only work within a certain range. When the wall thickness of the casting is too large, it still cannot eliminate the fragmented graphite.

(2) Forced cooling

Forced cooling refers to the use of mandatory measures such as water cooling, water vapor cooling air cooling to strengthen the solidification of castings, reduce the solidification time, ensure the required result structure. But when adopting, we must pay attention to the safety factor of forced water cooling.

(3) Cold iron forced cooling

In order to better utilize the cooling effect of cold iron, especially the cooling effect of graphite cold iron, forced cooling measures can be adopted on the inner outer surface of the cold iron. The more commonly used cooling media are air, atomized water, liquid oxygen water. Which media is used for forced cooling depends on specific conditions. For example, in the foundry processing plant of the Second Machine Tool Enterprise Group in Qiqihar City, in the process of developing large-section high-elongation ductile iron production manufacturing, a vent pipe is placed in the middle of the cold iron, the cold iron is placed in position connected to the pipe during modeling. When pouring, the compressed air is continuously supplied in the pipeline, which obviously promotes the cooling rate of the casting ensures the graphite shape, which has achieved an excellent effect.

(4) Add suspending agent

When pouring, the suspending agent is evenly distributed in the molten iron plays the role of "internal cooling iron". The usual suspension casting process should consider the particle size, specific gravity, composition other factors of the medium, the temperature, fluidity of the molten iron, the performance of the suspension agent used. The addition amount usually does exceed 5%, but for thick large cross-section balls Iron, due to the relatively long time in the liquid state, the addition amount can be increased to 10% on the basis of selecting a suitable miniature cold iron. Experiments have proved that the white iron structure iron pellets with a particle size of 5-7mm a composition close to the chemical composition of the large-section ductile iron simulation sample are added as miniature cold iron to the molten metal, which can reasonably inhibit the spheroidization decline of large-section ductile iron. , Improve the uniformity of the organization the distribution of impurity elements, improve the mechanical properties

2 Strictly control the chemical composition

The manufacturing regulations for large section ductile iron parts strictly control the chemical composition.

(1) Carbon equivalent

High carbon equivalent is easy to cause graphite floating, but too low is easy to produce defects such as shrinkage porosity. the perspective of improving the casting performance of molten iron, try to make the carbon equivalent of molten iron close to the eutectic point. Due to the influence of spheroidizing elements, the eutectic point of ductile iron shifts to the right to 4.6%～4.7%. Some people think that it is more appropriate to control the carbon equivalent between 4.1% 4.3%. Usually control w(C) at 3.4%～3.7%. For example, according to the literature], in the trial production process of «600mm×1500mm large section ductile iron parts, w(C) is chosen to be 3.5%～3.7%, which also achieved excellent effects. .

(2)Si

Silicon only has the effect of promoting graphitization increasing the content of ferrite, but also has the effect of promoting spheroidization decline, leading to low temperature brittleness, promoting the formation of fragmented graphite. Most studies believe that the amount of silicon in thick section ductile iron should be limited to 1.8% to 2.2%.

(3) Mn

Manganese is an element that promotes the formation of carbides is prone to segregation, which must be controlled. Generally, the manganese content in thick section ductile iron should be controlled within 0.1% to 0.4%, the lower limit is taken for ferritic ductile iron, the upper limit is taken for pearlite ductile iron.

(4)P

Phosphorus is easy to accumulate in the grain boundary during the solidification process, causing the brittleness of the casting reducing the toughness. Therefore, the content of phosphorus should be as low as possible. The very good condition is below 0.02%.

(5)S

Sulfur is a globally recognized hazardous element should be as low as possible. Certain measures of pre-desulfurization of molten iron must be adopted to reduce the sulfur content of molten iron. However, some studies have shown that in order to ensure good spheroidization, the original molten iron w(S) should be too low.

3 Add a small amount of alloying elements

A large number of studies have shown that in large-section ductile iron, adding bismuth antimony in a certain ratio with rare earth elements can effectively improve the graphite morphology, increase the number of graphite balls, reduce eliminate the variable graphite in the core of large-section ductile iron castings. Improve the performance of ductile iron.

1) Bi

Many references think that a reasonable combination of a small amount of bismuth an appropriate amount of rare earth can significantly increase the spheroidization rate of large section ductile iron increase the number of graphite balls. Studies have shown that adding 0.002% Bi using inoculation treatment can significantly increase the number of graphite balls. According to references, in large section ductile iron, adding 0.005% Bi can effectively refine graphite, increase the spheroidization rate the number of graphite balls, improve the spheroidization level, prevent reduce the formation of special-shaped graphite. After adding Bi, the amount of ferrite in the ductile iron matrix increases, so it can be used to produce high-toughness ductile iron castings based on ferrite matrix.

(2)Sb

In large-section ductile iron, antimony has the same effect on the spheroidization rate the number of graphite balls as bismuth. Scientific research has confirmed that adding Sb with a mass fraction of (20×10-6～50×60-6) to molten iron, even if w(Si) is as high as 2.5%, there will be no fragmented graphite. Unlike bismuth, antimony strongly promotes the formation of pearlite in the matrix. After adding a small amount of antimony, the amount of pearlite in the matrix increased significantly, the strength hardness increased significantly. Therefore, it can be used in the production of high-strength ductile iron castings based on pearlite matrix.

4 Suitable spheroidization inoculation treatment

The spheroidizing agent mainly used for large section ductile iron has no essential difference with the commonly used spheroidizing agent. References] think that the amount of spheroidizing agent used to treat large section ductile iron should be more than that of ordinary grade ductile iron, but this will bring some side effects. The spheroidizing agent needs to be added with nucleation anti-decay trace elements. The spheroidization ability of heavy rare earth is stronger than that of light rare earth, it is resistant to decline.

"Foundry Technology" 10/2007 Li Xiaojuan, etc.: The quality control measures for large-section ductile iron castings are strong, but the cost is higher than the use of light rare earths. According to my country's raw material resource conditions smelting conditions, the use of light heavy rare earths is Reasonable choice. But if the material requirements are particularly high, the reasonable use of ordinary rare earth spheroidizing agent alone can also meet the requirements. For example, FAW Wuxi Diesel Engine Factory used light rare earth plus antimony spheroidizing agent in the production test of high-quality injection molding machine template type thick large section ductile iron parts, achieved good results. When spheroidizing, the detonation time, reaction speed reaction time should be controlled. The spheroidizing treatment adopts the flushing method, which is a simple, flexible economical method, it is also widely used abroad.

Scientific reasonable inoculation treatment is the key reasonable effective way to increase the number of graphite balls. Therefore, the current production of ductile iron attaches great importance to the development design of instant inoculation technology. The choice of inoculant should be matched with the corresponding spheroidizing agent. The inoculant should also contain anti-decay elements. According to references, the addition of strontium barium has a greater effect on strengthening inoculation slowing down spheroidization, long-acting inoculants are suitable for thick large pieces. It is generally believed that multi-level high-dose inoculation is beneficial to improve the quality of ordinary ductile cast iron, but the reference] believes that multi-level high-dose inoculation only can eliminate the fragmented graphite, but promotes the production of fragmented graphite. The development trend in recent years is to reduce the amount of gestation, reduce the number of gestation, the gestation time is as short as possible (ie instant gestation). The inoculation method should be based on the principle of adjacent pouring, the effect of using in-mold inoculation, sprue cup inoculation silk inoculation flow inoculation is good.

In addition, to adopt a scientific reasonable pouring process, the molten iron should be filled smoothly into the mold, the pouring temperature is suitable for too high.

The solidification crystallization characteristics of large-section ductile iron put forward unique requirements for its casting process. In order to obtain large-section castings with accurate size, internal compactness, good spheroidization, graphite grain refinement, it is based on the existing product quality control measures. Sexually, continuous discussions are still needed.