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 stave, large rolling mill frame, large injection molding machine template, large steam turbine bearing seat, wheel hub in wind power equipment and Bases and waste slag tanks in nuclear power plants, etc. In addition to meeting the mechanical properties specified in the standard, these components also have some special performance requirements, such as low temperature impact toughness required for wind power castings, and many additional special acceptance criteria for nuclear slag tanks. Therefore, the production of these castings requires careful consideration in advance.
1) What to consider is how to obtain sound, dense, dimensionally acceptable castings
The technical process of producing large-scale ductile iron castings is basically the same as that of gray iron castings, as long as some modifications are made in the selection of scales and the design of sandboxes according to the characteristics of ductile iron.
2) Secondly, do corresponding work according to the common characteristics of large ductile iron castings
The common feature of large ductile iron castings is that they are particularly thick and heavy. Most of them require a ferrite matrix, and their mechanical properties must meet standard data, sometimes with low-temperature impact performance requirements.
1 Unique problems in the production of large ductile iron castings
Due to the slow cooling rate of large ductile iron pieces, the eutectic solidification period is several hours, and the main structure of ductile iron is formed during this period, so a series of problems peculiar to large section ductile iron pieces or large ductile iron pieces appear. : Small amount of nodular ink, large nodular ink diameter, nodular ink distortion, graphite floating, chemical composition segregation, intergranular carbide and chunky graphite (ChunkyGraphite), etc. These problems have long been concerned, and although the formation mechanism is not unified, preliminary solutions have been taken to specific problems.
Another important question is how to meet and solve the requirements of low temperature impact toughness? The coincidence of the problem is that the directions and measures to solve the two major 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 accepted theories about the cause of fragmented graphite: one is caused by the crushing of spheroidal graphite; the other is the decrease in the stability of the austenite shell due to heat flow or the segregation of some alloying elements, especially Ce and La, It is formed by changing the growth pattern of nodular ink. No matter which theory or statement, it is certain that the too long solidification time (ie slow cooling) in the eutectic stage is the direct and objective factor for the formation of fragmented graphite. Therefore, no matter what method is adopted, as long as the time of the solidification stage can be shortened, the appearance of fragmented graphite can be effectively prevented.
It is also pointed out that the nodular distortion has a critical cooling rate (0.8 °C/min) [1]. Graphite distortion is sometimes a mutation process, so accelerated cooling, shortening the solidification time, especially shortening the solidification time of the eutectic stage, to find ways to shorten the eutectic solidification stage to less than 2h has a significant effect. There are many measures around this principle: forced cooling; metal mold hanging sand; the use of cold iron and so on.
The high thermal conductivity of cold iron, especially the strong heat storage capacity, is widely regarded as a powerful measure that can be applied. The thermal conductivity of graphite is higher than that of cold iron (45W/m•°C and 17W/m•°C, respectively), but its heat storage capacity is smaller than that of cold iron. If there is a condition for forced cooling, graphite is more suitable. of. For large or extra large ductile iron castings, forced cooling is still a powerful measure. Generally, air cooling, mist cooling or water cooling 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 waste container castings are solidified, the heat transfer effect is as follows: the heat absorption of the metal mold accounts for 58%, the heat absorption of graphite and sand mold (core part) accounts for 3.5%, and the heat absorption of the sand mold and other devices. It accounts for 3.5%, and water cooling and heat conduction account for 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 has very little heat transfer. Obviously, forced cooling is required.
2) Improve process technology
(1) Carefully select raw materials
In order to produce high-quality large ductile iron castings, it is worthwhile to select the charge no matter what. The interfering elements of raw materials should be as low as possible, and special attention should be paid to the selection of pig iron sources, types of scrap steel, and recarburizers.
(2) Chemical composition design
CE should not be too high (4.2% to 4.3%). For example, if w (C) is selected as 3.6% to 3.7%, w (Si) must be as low as 1.8% to 2.0%; in addition, w (Mn) < 0.3%, w ( P) and w(S) are also strictly limited. Except in special cases, alloys are generally not used, so scrap steel must be strictly selected.
Low w(Si) must be achieved, otherwise fragmented graphite will easily appear, and the low-temperature performance will not meet the requirements. resulting disadvantages. The composition of the 100-ton Japanese spent fuel container 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) Select double smelting
Double smelting can give full play to the characteristics of cupola molten iron nucleation ability and high thermal efficiency of electric furnace. The molten iron must be released at high temperature, and S can be removed when conditions permit, and the time in the electric furnace should not be too long. The spheroidization temperature is determined according to the situation, neither too high nor too low.
The author advocates that the spheroidizing method of large pieces should not be used because the time is too long. At least use the cover wrapping method, preferably the special method or the silk feeding method. Do not use commonly used spheroidizers. It is better to mix heavy rare earth spheroidizers and light rare earth spheroidizers. If the infusion method is used, w(Mg) 6% and w(RE) 1.0%~1.5% in the nodulizer are enough; if the pig iron is purer, w(RE) 0.5%~1.0% is also acceptable. If the wire feeding method is used, a spheroidizer with a high w (Mg) amount can be used, but the w (RE) should be low, and a little Ca can be used.
The pouring temperature should be appropriate (1300~1350℃), not too high, otherwise the liquid shrinkage will be too large; it is advisable to use a dispersed inner runner for medium-speed pouring, and use a high-rigidity mold as much as possible to make full use of graphitization expansion for self-feeding of ductile iron. , reduce the burden on the riser and ensure the compactness of the casting.
(4) Pay attention to pregnancy problems
Inoculation is one of the more 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 inoculation is nothing more than the choice of inoculant and inoculation treatment method. You can choose an inoculant with a long incubation time, such as a Ba-containing agent (Sr-containing agent is more effective for gray cast iron, and Ca is low), a graphite-containing inoculant, or a proper mixture of RESiFe in the inoculant.
At present, many companies have their own inoculants, and I guess this principle is followed. In a word, "to be delayed, to be instantaneous", not only the effect is good, but also the dose can be greatly reduced. The old method, such as overlay while processing, works poorly, but w(Si) decreases. 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 w(Si) content of 2.0% can be achieved, and the mark of success is that the graphite should be smaller and more. The smaller it is, the more it is, the smaller the spheroidization rate is, the smaller the cementite is, and the smaller the segregation degree is. For large pieces, if the graphite balls are 200/mm2 or more, and the size is 5 to 6, the spheroidization rate and the amount of ferrite will naturally be no problem. In a word, to fight against graphite and strive for small but large amount of graphite, the main means is to inoculate and deal with it. When w(Si) is low, and there is no free cementite, the plasticity and impact toughness at room temperature and low temperature are easy to pass. For large castings, it's a no-brainer to have a large inoculum in the pouring cup and an inoculum in the runner. The problem is having the right concept.
(5) Utilization of alloys and trace elements
The only alloying element that can be considered in extra-large ductile iron castings is Ni, because of its unique role. From a technical point of view, w(Ni) <1% is beneficial, but whether to use it or not depends on the specific situation and from an economic point of view.
Bi and Sb have mature experience in the use of trace elements in large pieces.
The ratio of (Bi) = 1.4 to 1.5 is beneficial to increase the number of balls and reduce the risk of fragmented graphite. Sb can also be used in thick and large pieces. Some people think that it will increase the amount of pearlite, but some people use it in ferrite ductile iron. It may be the problem of the amount. There should be no problem with the amount of 50ppm. Professor Zhou Jiyang once pointed out that the use of w (Sb) 0.005% to 0.007% can also inhibit the harmful effects of excessive Ti and RE in the molten iron [2].
Although the industry has no unified opinions on the role and mechanism of adding Bi and Sb, a consensus has been formed on the addition of Ni.
(6) Preprocessing is very important
Pretreatment of ductile iron stock solution with graphitic pretreatment agent before spheroidization has a positive effect on improving and stabilizing the quality of castings [3]. Methods as below:
After adjusting the ingredients [pretreatment will increase w(C) by 0.2%] → remove S → return to the electric furnace → add 0.2% to 0.25% of the pretreatment agent when 1/4 of the amount → return to the electric furnace and then slightly raise the temperature to 1470～ 1480℃→Spheroidization→Inoculation (Ultraseed available)→Pouring.
(7) Use of anti-cratering agent QKS
The inventor believes that there are foreign inclusions of 1 μm in the center of the nodular ink, 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 a relative amount of O and S to the inoculant, so that it can combine with the metal elements in the inoculant to generate more sulfides and oxides, thereby forming more graphite cores, which results in the formation of more graphite cores. Ferrosilicon inoculants for Ca, Ce, S, O. This inoculant can significantly increase the number of graphite nodules, and precipitate in the later stage of crystallization, and the later graphitization expansion can effectively offset the shrinkage porosity in the later stage of solidification. In particular, it is more effective for shrinkage of local hot joints [4]. The experiment pointed out that for the step test block of 5-40mm, the number of graphite balls was reduced from 300/mm2 to 150/mm2 when SrSiFe was used; while the number of graphite balls was not affected by the wall thickness when Ca-Ce-O-S agent was used. Compared to BaSiFe and 75SiFe both. The shrinkage defect on the hot joint of the cross test block shows that with Ba- and Sr-containing inoculants, shrinkage cavities are found at the hot joints of the cross-section, but not with the Ca-Ce-O-S agent.
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