Knowledge

There are many types of large ductile iron castings, such as large diesel engine blocks, large wheel hubs, large ball mill end covers, blast furnace cooling walls, large steel rolling mill frames, large injection molding machine templates, large turbine bearing housings, wind turbine hubs and bases, and nuclear waste tanks, etc. In addition to meeting the mechanical properties specified in the standards, these components also have some special performance requirements, such as low-temperature impact toughness for wind power castings and many additional special acceptance standards for nuclear waste tanks, etc. Therefore, when producing these castings, thorough consideration must be given in advance.

1) The first consideration is how to obtain sound, dense, and dimensionally qualified castings.

The technical process for producing large ductile iron castings is basically the same as that for gray iron castings. Only slight modifications are needed in aspects such as shrinkage allowance selection and sand box design, taking into account the characteristics of ductile iron.

2) Secondly, corresponding work should be done in response to the common characteristics of large ductile iron castings.

The common characteristics of large ductile iron castings are that they are particularly heavy, most require a ferritic matrix, and their mechanical properties must meet the standard data. Sometimes, additional requirements such as low-temperature impact performance are also imposed.

 

1. Special problems in the production of large ductile iron castings

Due to the slow cooling rate of large ductile iron castings, the eutectic solidification period can last for several hours, during which the main microstructure of ductile iron needs to form. As a result, a series of problems specific to large-section ductile iron or large ductile iron castings have emerged: low number of nodules, large nodule diameter, nodule distortion, graphite floatation, chemical composition segregation, intergranular carbides, and chunky graphite. These problems have long been recognized, although the formation mechanisms are not yet fully understood, initial solutions to specific issues have been developed.

Another important issue is how to meet and address the requirements for low-temperature impact toughness? The coincidence lies in the fact that the directions and measures to solve these two major problems are largely the same.     

 

2. Approaches to Solving Special Problems of Large Ductile Iron Castings

1) Intensifying Cooling to Accelerate Solidification

There are two generally accepted explanations for the formation of fragmented graphite: one is the fragmentation of spherical graphite; the other is the reduction in the stability of the austenite shell due to thermal flow or the segregation of certain alloy elements, especially Ce and La, which leads to a change in the growth pattern of graphite spheres. Regardless of which theory or explanation is correct, it is certain that a long eutectic solidification time (i.e., slow cooling) is the direct and objective factor for the formation of fragmented graphite. Therefore, any method that can shorten the solidification time can effectively prevent the appearance of fragmented graphite.

Some literature also points out that there is a critical cooling rate for the distortion of graphite spheres (0.8 ℃/min) [1]. Graphite distortion is sometimes a sudden process. Therefore, accelerating cooling, shortening the solidification time, especially the eutectic solidification time, and trying to shorten the eutectic solidification stage to within 2 hours can have a significant effect. There are many measures around this principle: forced cooling; sand coating on metal molds; using chill irons, etc.

Chill irons have a high thermal conductivity and strong heat storage capacity, and are widely recognized as a powerful measure that can be applied. The thermal conductivity of graphite is higher than that of sand-coated chill irons (45 W/m•℃ and 17 W/m•℃ respectively), but its heat storage capacity is smaller than that of chill irons. If forced cooling conditions are available, using graphite is more appropriate. For large or extra-large ductile iron castings, forced cooling remains a powerful measure. Generally, air cooling, mist cooling, or water cooling devices can be used, and even liquid nitrogen cooling can be adopted to accelerate the solidification speed of the casting. Data shows that during the solidification of a 20-ton class ductile iron spent fuel container casting, the heat transfer effect is as follows: the metal mold absorbs 58% of the heat, graphite and sand molds (core parts) absorb 3.5%, sand molds and other devices absorb 3.5%, and water cooling conducts 3.5%. It can be seen that the metal mold can conduct more than 50% of the heat of the casting, while the core part conducts very little heat. Clearly, forced cooling is necessary.

 

2) Improving Process Technology

 

(1) Carefully Selecting Raw Materials

To produce high-quality large ductile iron castings, it is worthwhile to carefully select the furnace materials. The interfering elements in the raw materials should be as low as possible, especially paying attention to the source of pig iron, the type of scrap steel, and the selection of carbon additives.

 

(2) Chemical Composition Design

CE should not be too high (4.2% to 4.3%). If w(C) is selected as 3.6% to 3.7%, w(Si) must be reduced to 1.8% to 2.0%; in addition, w(Mn) < 0.3%, and w(P) and w(S) should also be strictly limited. Except in special cases, no alloying is generally used, so scrap steel must be strictly selected.

Low w(Si) is a must, otherwise fragmented graphite is likely to occur, and the low-temperature performance may not meet the requirements. The problem lies in the need to keep w(Si) low while avoiding the disadvantages caused by low w(Si). The composition of a 100-ton class spent fuel container in Japan is: w(C) 3.6%, w(Si) 2.01%, w(Mn) 0.27%, w(P) 0.025%, w(S) 0.004%, w(Ni) 0.78%, w(Mg) 0.065%.

 

(3) Choosing Double Melting

Double melting can fully utilize the strong nucleation ability of cupola molten iron and the high thermal efficiency of electric furnaces. The molten iron must be poured at a high temperature, and sulfur can be removed if conditions permit. The time in the electric furnace should not be too long. The spheroidization temperature should be determined according to the situation, neither too high nor too low. The author suggests that for large-sized parts, the injection method should not be used for spheroidization treatment due to the long processing time. At least the covering method should be adopted, and the special method or wire feeding method is preferred. The wire feeding should be done at a fixed location, and even the feeding of inoculation wire can be considered. The spheroidizing agent should not be the commonly used one. It is better to use a mixture of heavy rare earth and light rare earth spheroidizing agents. If the injection method is adopted, the spheroidizing agent should contain 6% w(Mg) and 1.0% to 1.5% w(RE); if the pig iron is relatively pure, 0.5% to 1.0% w(RE) is also acceptable. If the wire feeding method is used, a spheroidizing agent with a high w(Mg) content can be used, but the w(RE) should be low, and a small amount of Ca can be added.

 

The pouring temperature should be appropriate (1300 to 1350 ℃), not too high, otherwise the liquid shrinkage will be too large. It is advisable to use a dispersed inner gate for medium-speed pouring and to use a high-stiffness mold as much as possible to fully utilize the expansion during graphitization for self-compensation of ductile iron, reducing the burden on risers and ensuring the internal density of the castings.

 

(4) Pay attention to inoculation issues

Inoculation is one of the most important technological measures. Only by solving this problem can both low w(Si) content and no problems be guaranteed, and the low-temperature performance can also be ensured. The issue of inoculation is essentially the selection of inoculation agents and inoculation methods. Inoculation agents with a long inoculation effect can be chosen, such as Ba-containing agents (Sr-containing agents are more effective for gray cast iron and have a lower Ca content), graphite-containing inoculation agents, or a proper amount of RESiFe can be mixed in the inoculation agent.

 

Currently, many enterprises have their own homemade inoculation agents, and I guess they all follow this principle. In summary, inoculation "should be delayed and instantaneous", not only is the effect good, but the dosage can also be greatly reduced. The old method of covering during treatment has a poor effect, but the w(Si) content is reduced. The current problem is to achieve both low w(Si) content and good effect, and the only solution is to change the method. Facts have proved that a w(Si) content of 2.0% can be achieved. The sign of success is that the graphite should be small and numerous. The smaller it is, the more there will be, the higher the spheroidization rate, the less free cementite, and the lighter the segregation. For large parts, if the graphite spheres can reach 200/mm2 or more, with a size of 5 to 6 grades, the spheroidization rate and ferrite content will naturally not be a problem. In short, fight against graphite, strive for small and numerous graphite, and the main means is through inoculation treatment. With a low w(Si) content and no free cementite, the plasticity and impact toughness at room temperature and low temperature will be easily passed. For large castings, it is easy to perform large block inoculation in the pouring cup and place an inoculation block in the runner, but the key is to have the correct concept.

 

(5) Utilization of alloys and trace elements

Among the alloy elements that can be considered for use in extra-large ductile iron castings, only Ni has a unique role. From a technical perspective, w(Ni) < 1% is beneficial, but whether to use it or not should be determined based on specific circumstances and economic considerations.

 

The trace elements that have mature application experience in large castings are Bi and Sb. It is believed that adding w(Bi) 0.008% to 0.010% and maintaining a w(RE)/w(Bi) ratio of 1.4 to 1.5 is beneficial for increasing the number of spheres and reducing the risk of fragmented graphite. Sb can also be used in thick and large parts. Some people think it will increase the amount of pearlite, but others have used it in ferritic ductile iron, possibly due to the amount. Using 50 ppm should not be a problem. Professor Zhou Jiyang once pointed out that using w(Sb) 0.005% to 0.007% can also suppress the harmful effects when there is an excessive amount of Ti and RE in the molten iron [2]. Although the industry has not reached a consensus on the role and mechanism of adding Bi and Sb, there is a consensus on the addition of Ni.

 

(6) The pretreatment effect is crucial

Pretreating the original liquid of ductile iron with a graphitizing pretreatment agent before spheroidization has a positive effect on improving and stabilizing the quality of castings [3]. The method is as follows:

After adjusting the composition (pretreatment will increase w(C) by 0.2%) → desulfurization → pour back into the electric furnace → add 0.2% to 0.25% of the pretreatment agent when 1/4 of the volume is left → slightly increase the temperature to 1470 to 1480 ℃ after pouring back into the electric furnace → spheroidization treatment → inoculation treatment (Ultraseed can be used) → pouring.

 

(7) The use of anti-shrinkage agent QKS

The inventor believes that there is a 1 μm foreign inclusion at the center of the spheroidal graphite, forming a double-layer core; the inner layer is MgS and CaS (0.5 μm), and the outer layer is MgO, SiO and silicate. Therefore, the inventor added a certain amount of O and S to the inoculant, so that they could combine with the metal elements in the inoculant to produce more sulfides and oxides, thereby forming more graphite cores. This led to the development of a silicon-iron inoculant containing Ca, Ce, S and O. This inoculant can significantly increase the number of graphite spheres, and it precipitates in the later stage of crystallization. The graphite expansion in the later stage can effectively counteract the shrinkage porosity in the later stage of solidification. Especially for the shrinkage porosity in local hot spots, it is more effective [4]. Experiments show that for 5 to 40 mm step test blocks, the number of graphite spheres decreases from 300/mm² to 150/mm² when using SrSiFe; while using the Ca-Ce-O-S agent, the number of graphite spheres is not affected by the wall thickness. The same is true when compared with BaSiFe and 75SiFe. The shrinkage defects at the hot spots of the cross test blocks show that there are shrinkage cavities at the cross-section hot spots when using Ba and Sr inoculants, but there are no shrinkage cavities when using the Ca-Ce-O-S agent.

 

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