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There are many types of large ductile iron castings, such as: large diesel engine block, large wheel hub, large ball mill end cover, blast furnace cooling wall, large rolling mill frame, large injection molding machine template, large steam turbine bearing seat, hub and base in wind power equipment, and waste slag tank in nuclear power equipment. In addition to these components must meet the mechanical properties specified in the standard, there are some special performance requirements, such as wind power castings require low temperature impact toughness, nuclear slag tank has many additional special acceptance criteria and so on. Therefore, the production of these castings must be carefully considered in advance.

1) What should be considered is how to obtain sound, dense and qualified castings

The technical process of producing large-scale ductile iron castings is basically the same as that of gray iron castings, as long as the selection of scale and sand box design are slightly modified in combination with the characteristics of ductile iron.

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

The common feature of large ductile iron castings is particularly heavy, most require ferrite matrix, mechanical properties must meet the standard data, sometimes plus low temperature impact performance requirements.

1 Specific problems in the production of large ductile iron castings

Due to the slow cooling rate of large ductile iron parts, the eutectic solidification period is as long as several hours, and during this period, the main structure of ductile iron is formed, so a series of problems unique to large section ductile iron or large ductile iron parts appear: small number of ductile graphite, large diameter of ductile graphite, ductile graphite distortion, graphite floating, chemical composition segregation, intergranular carbide and broken graphite (ChunkyGraphite), etc. These problems have been concerned for a long time. Although the formation mechanism is not yet unified, there have been preliminary solutions to specific problems.

Another important problem is how to meet and solve the requirements of low temperature impact toughness? The coincidence of the problem is that the direction and measures to solve these two problems are roughly the same.

2 Ways to solve the unique problems of large ductile iron castings

1) Enhanced cooling to accelerate solidification

There are two generally acceptable claims about the causes of crushed graphite: one is caused by the crushing of spherical graphite; The second is that the stability of the austenite shell is reduced due to heat flow or the segregation of some alloying elements, especially Ce and La, resulting in the change of the growth mode of spheroidal graphite. Regardless of the theory or statement, it is certain that the solidification time of the eutectic stage is too long (I. e. slow cooling) is a direct and objective factor in the formation of crushed graphite. Therefore, no matter what method is adopted, as long as the time of the solidification stage can be shortened, the occurrence of crushed graphite can be effectively prevented.

It is also pointed out in the literature that ductile distortion has a critical cooling rate (0.8 ℃/min)[1]. Graphite distortion is sometimes an abrupt process, so accelerating cooling, shortening the solidification time, especially shortening the solidification time of the eutectic stage, and finding ways to shorten the eutectic solidification stage to less than 2h is a significant effect. There are many measures around this principle: forced cooling; metal type hanging sand; the use of cold iron and so on.

The large thermal conductivity of cold iron, especially the heat storage capacity, is widely considered to be a powerful measure that can be applied. The thermal conductivity of graphite is higher than that of hanging sand cold iron (45 W/m • ℃ and 17 W/m • ℃), but its heat storage capacity is smaller than that of cold iron. If there is a forced cooling condition, graphite is more suitable. For large or extra-large ductile iron castings, forced cooling is still a powerful measure. Generally, air-cooled, mist-cooled or water-cooled devices can be used, and even liquid nitrogen cooling can be used to accelerate the solidification rate of castings. Some data show that when the 20t grade ductile iron spent material container castings solidify, the heat transfer effect is: metal type heat absorption accounts for 58%, graphite and sand type (core part) heat absorption accounts for 3.5, sand type and other device parts heat absorption accounts for 3.5, and water cooling heat conduction accounts for 3.5. It can be seen that the metal mold can conduct more than 50% of the heat of the casting, and the core part of the heat transfer is very small, obviously forced cooling is needed.

2) Improved process technology

(1) careful selection of raw materials

In order to produce high-quality large-scale ductile iron castings, it is worthwhile to select the charge no matter how. The interference elements of raw materials should be as low as possible, and special attention should be paid to the selection of pig iron sources, scrap steel varieties and carburizers.

(2) Chemical composition design

CE should not be too high (4.2~4.3%), if w(C) is 3.6~3.7%,w(Si) should be as low as 1.8~2.0%; In addition, w(Mn)<0.3, w(P) and w(S) should also be strictly limited. Except in special circumstances, alloys are generally not used, so scrap steel must be strictly selected.

W (Si) is low must be achieved, otherwise it is easy to appear broken block graphite, low temperature performance will not meet the requirements, the problem lies in the w(Si) low, and do not appear due to w(Si) low and the ills. The components of Japan 100-ton spent fuel container are: 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) Select duplex smelting

Duplicate smelting can give full play to the characteristics of cupola molten iron into a strong nuclear energy and high thermal efficiency of the electric furnace. Molten iron must be discharged from the furnace at high temperature, and S can be removed when conditions permit. The time in the electric furnace should not be too long. The spheroidization temperature is determined according to the situation and cannot be too high or too low.

The author advocates that large-scale spherical treatment should not use the rush-in method, because the time is too long. At least use the capping method, preferably the special method or the silk feeding method, and feed the silk in a fixed place, even together with the inoculation silk. Do not use commonly used spheroidizing agent, better heavy rare earth spheroidizing agent and light rare earth spheroidizing agent mixed use. If the flushing method is used, 6% w(Mg) and 1.0~1.5w(RE) in the spheroidizing agent can be used. If pig iron is pure, 0.5~1.0w(RE) can also be used. If the wire feeding method is used, a spheroidizing agent with high w(Mg) content can be used, but w(RE) is lower and slightly contains some Ca.

The pouring temperature should be appropriate (1300~1350 ℃), not too high, otherwise the liquid shrinkage is too large; It is advisable to adopt the medium-speed pouring of the dispersed inner runner, and use the high-rigidity casting mold as much as possible to make full use of the graphitization expansion for the self-feeding of nodular cast iron, so as to reduce the burden of the riser and ensure the internal density of the casting.

(4) Pay attention to the problem of pregnancy

Inoculation is one of the most important technological measures. Only by solving this problem can it be possible to ensure low w(Si) content without problems, and to ensure low temperature performance. The problem of gestation is nothing more than the choice of inoculant and gestation treatment methods. Inoculants with long inoculation time can be selected, such as Ba-containing agent (Sr-containing agent is more effective for gray cast iron and Ca is lower), graphite-containing inoculant or RESiFe properly mixed in inoculant.

At present, many enterprises have self-made inoculant. I guess they follow this principle. In short, the breeding "to lag, to be instantaneous", not only the effect is good, and the dose can be greatly reduced. The old method, such as covering during treatment, is very poor, but w(Si) is reduced. The problem now is that if w(Si) is low and the effect is good, the only way out is to change the method. Facts have proved that 2.0% of w(Si) can be achieved. The successful identification is that graphite is smaller and more. Small is more, small spheroidization rate is high, small is not cementite, small segregation degree is light. If large graphite balls can be 200/mm2 or more and the size is 5~6, the spheroidization rate and ferrite amount will naturally not be a problem. In a word, to fight against graphite and strive for small and many graphite, the main means is to deal with it through gestation. W (Si) is low, and there is no free cementite, plasticity and normal temperature, low temperature impact toughness is easy to pass. For large castings, it is easy to carry out large pieces of inoculation in the pouring cup and put a piece of inoculation in the runner. The problem is that there must be a correct concept.

(5) the use of alloys and trace elements

Ni is the only alloying element that can be considered for use in extra-large ductile iron castings because of its unique role. From a technical point of view, w(Ni)<1% is beneficial, but it is not necessary to consider the decision according to the specific situation and from an economic point of view.

Bi and Sb have mature experience in using trace elements in large pieces. it is believed that w(Bi) is added by 0.008~0.010 to make w(RE)/w

The ratio of (Bi)= 1.4 to 1.5 is advantageous for increasing the number of spheres and reducing the risk of occurrence of crushed graphite. Sb can also be used in thick and large parts. Some people think that it will increase the amount of pearlite, but some people use it in ferritic ductile iron. It may be the problem of quantity. There should be no problem with 50ppm. Professor Zhou Jiyang once pointed out that 0.005~0.007% w(Sb) can also inhibit the harmful effects of excessive Ti and RE in molten iron [2].

Although the industry is not unified on the role and mechanism of adding Bi and Sb, a consensus has been formed on the addition of Ni.

(6) Pretreatment is critical

The pretreatment of ductile iron stock solution with graphite pretreatment agent before spheroidization has the positive effect of improving and stabilizing the casting quality [3]. The method is as follows:

After adjusting the composition [pretreatment will increase w(C) by 0.2] → removing S → returning to the electric furnace → adding 0.2~0.25 of pretreatment agent when returning to the electric furnace for 1/4 amount → slightly increasing the temperature to 1470~1480 ℃ after returning all the electric furnace → spheroidizing treatment → inoculation treatment (available Ultraseed)→ pouring.

(7) Use of anti-shrinkage agent QKS

The inventor believes that there is a foreign inclusion of 1 μm in the center of the spheroidal graphite, forming a double-layer core; the inner layer is MgS, CaS(0.5 μm), and the outer layer is MgO, SiO and silicate. Therefore, the inventors added relative amounts of O and S to the inoculant to combine with the metal elements in the inoculant to produce more sulfides and oxides, thereby forming more graphite cores, which produced a ferrosilicon inoculant containing Ca, Ce, S and O. This inoculant can significantly increase the number of graphite spheres, and in the late crystallization of precipitation, late graphitization expansion can effectively offset the late solidification of the shrinkage. In particular, it is more effective for the shrinkage of the local hot section [4]. The experiment points out that for the step test block of 5~40mm, the number of graphite balls is reduced from 300/mm2 to 150/mm2 when using SrSiFe, while the number of graphite balls is not affected by the wall thickness when using Ca-Ce-O-S agent. This is true compared to both BaSiFe and 75SiFe. The shrinkage defect on the hot section of the cross block shows that there are shrinkage holes in the hot section with Ba-containing and Sr-containing inoculants, but not with Ca-Ce-O-S.