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German ductile iron smelting and processing technology

Sources:本站 | Release date: 2017-11-07 | Browsing volume:
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In 1977, 64% of the ductile iron produced in Germany was melted by an induction furnace, 15% by a hot cupola, 2.3% by a cold cupola, and the rest directly from hot metal in blast furnaces and other furnaces.
German ductile iron hot metal temperature in Shanghai is generally above 1520 ℃. Therefore, the double melting method is widely used in ductile iron production, mainly used to adjust the chemical composition and temperature of the cupola and high retaining hot metal.
According to the author in Germany inspection, even with direct-reading spectrometer monitoring, cold wind cupola melting ductile iron quality is also very large, so cupola melting ductile iron is being replaced by the electric furnace. For example, THYSSEN FWH Foundry is equipped with a 65t, two 35t and a 8t industrial furnaces and produces over 180t heavy castings.

First, ductile iron
GGG-40 cast ductile iron accounts for about 70% of the total German ductile iron production. In order to ensure the as-cast performance, it is far from enough to regard the low manganese content as the quality mark. More importantly, the content of the disturbing spheroidizing element and the stabilized pearlite element is low and stable. The composition of German-made ductile iron for pigs in Germany is shown in Table 1. The trace elements and their contents are not more than the following quantities.
The production of large sections of ductile iron should be specially intended to interfere with the spherulites Ti, Pb, Sb and Bi in the charge and promote the contents of B, V and Cr and the stable pearlite elements Mn, Gu and Sn.
In Germany, the proportion of pig iron dropped from 32% in 1960 to 16% in 1987 in contrast to the charge-to-charge ratio, while the scrap level increased from 18% to 39%.

Second, the chemical composition of choice
Table 2 gives an approximation of the chemical composition of the ferrous iron. The number in parentheses is the possible alloying element content.
German ductile iron carbon equivalent is generally 4.2 ~ 4.3%. When the casting modulus [M] is small, the minimum carbon equivalent of the as-cast carbides can be determined by this empirical formula:
In order to reduce the required overheating temperature and reduce the hot metal shrinkage tendency, the highest possible carbon and low silicon should be used for the same carbon equivalent. So ordinary iron content of 5i does not exceed 2.6%; special ductile iron content of GGG-35.3 and GGG-40.3 does not exceed 2.100 / ae. As the Si% xSc (eutectic) product increases No more than 3.0}, the number of graphite balls and the propensity to produce ferrite also increase
In the usual chemical composition of GGG-4o, if the content of manganese is less than 0.7%, the amount of pearlite in the as-cast ferrite ductile iron is not mainly from manganese, I} is determined by trace elements tin, chromium and residual magnesium. The following empirical formula regarding the effect of accompanying elements and interfering elements on the above-mentioned number of spherical pearlite:
The amount of pearlite (%) = -2.45 + 768. [% Sn] -5 "2" (% Si +106. (% Mg) -142. (% CO) +138. (% Cr) +549 % N) +101. (% Al) +51.6 (% P)
German ferritic ductile iron content of manganese is generally 0.1-0.2%, pearlite ductile iron is 0.3 --- 0.4 Ordinary ductile iron content of less than 0,08%, ductile iron ductility is required low At 0.05%. The original hot metal sulfur content of less than 0.02% of the general requirements, or to desulfurization.
MG residue depends on the retention time of molten iron and casting wall thickness, generally 0.02 - 0.08%, to offset the interference elements.Most nodulizers contain about 1,0% of rare earth elements.A variety of interference and accompanying elements The maximum allowable content depends on the casting wall thickness, if less than the content given in Table 3, you can not add rare earth elements

Third, the ball of treatment
Germany improved tundish processing ductile iron process, the absorption rate of magnesium up to 60%, 80% reduction of flue gas generated by combustion of magnesium, so its application is more extensive.
Bell method widely used in the production of large ductile iron pieces, such as the FWH plant on the use of this method of production. The method has the advantages of stable treatment of molten iron, high magnesium absorption rate, increasing silicon content of only 0.2-0.5% due to strong spheronization reaction, which is favorable for reactant floating. However, the heat loss during processing is relatively large, which takes a long time. The nodulizer used in the bell method is supplied in mass form by weight, and its chemical composition is shown in Table 4.
The compositions of nodulizers suitable for the flotation method are shown in Table 5, where the rare earths of VL63 (s) and VL53 (s) are very high for the treatment of hot metal with special metallurgical properties.
Spheroidization must be intended to the Italian original hot metal water content, with the titanium content increases, to achieve complete spheroidization of the Mg content also increased. When Ti is more than 0.05%, spheroidal graphite formation will be strongly hindered and the number of graphite balls will be reduced. The most harmful is the titanium will reduce the critical value of other interfering elements, thereby increasing its harmfulness. However, the adverse effect can be counteracted by the cerium contained in the nodulizer.

Fourth, pregnant treatment
Common German ductile iron inoculants in addition to ordinary FeSi, FeSi-Zr and FeSi-Mn-Zr is also widely used. The chemical composition of common inoculants in Table 6
ZL80 is a widely used, inoculation slow decline inoculant, a different melting point can be selected. ZIRCOGRAF melting speed, can be used for a variety of breeding process.
Ductile inoculant aluminum content should be less than 1%, particle size does not exceed 8mm. Suitable incubation temperature 1400-1480 ℃ from the incubation process to the completion of the casting is generally not more than} 5 minutes. Many births most suitable for the production of ductile iron, for which small and medium-sized single-piece production of castings are used pouring basin inoculation, the small machine-shaped castings}} l type or with the flow inoculation. Pregnancy block buried in the sprue basin or sprue bottom, easy to operate. The average volume of ladle during pregnancy is 0.4 '-0.5%. If it is necessary to carry out secondary gestation, the maximum volume of ladle should be 0.3%. The pouring volume is 0.1-0.2% Large, but will result in "over birth" defects, such as graphite floating, shrinkage shrinkage and so on.

Five, cast and heat treatment
The two types of ductile iron, GGG-35. 3 and GGG-80, have implications for large castings, and the former must be ferritic-annealed. The remaining grades of ductile iron can be cast, but also by heat treatment methods
Casting ductile iron production, we must consider the type of castings in the cooling rate, strict control of chemical composition, to avoid casting serious segregation of components.
Production of ferrite ductile iron, especially to ensure that the impact of tough ductile iron. Must use special pig iron containing trace elements and low manganese. GGG-40.3 requires ferrite grains are small and uniform, so the charge and smelting technology strict requirements; if the casting wall thickness difference or pearlite body volume exceeds 10%. Usually also heat treatment.
Adjusting the properties of ductile iron GGG-50 in ferritic and pearlite matrix is much more complex than that of GGG-40 or GGG-70 and GGG-80 because of minor variations in chemical composition and process parameters resulting in matrix organization Make a difference. It is particularly difficult to get the same amount of pearlite at different wall thicknesses of large castings. Therefore, in the case of design requirements, GGG-40 or GGG-60 should be used instead of GGG-50.
Base to pearlite-based GGG-60 --- as-cast method with the production. GGG-7D requires a flaky pearlite matrix. For thick-walled castings, this structure can be formed in the as-cast state only by adding stabilized pearlite elements. The heat treatment method to produce GGG-70 above brands, also subject to casting wall thickness limit, in order to ensure the performance of castings, and sometimes also need to join alloying elements.

Sixth, alloying
In order to cast ductile iron GGG-60 to GGG-80, it is necessary to add alloying element copper or tin, which is a stable pearlite, or to use a higher manganese content. In actual production, copper is usually added. The amount of copper necessary to obtain the pearlite matrix depends on the wall thickness of the casting and the manganese content of the starting molten iron. Increasing the amount of manganese reduces the amount of copper added, but increases the tendency of carbides. The addition of copper to the castings also reduces the effects of wall thickness sensitivity.
The effect of tin on the generation of pearlite is about 10 times stronger than that of copper, and the tendency of the combination of tin and manganese to form carbides is stronger than that of copper and manganese. If the tin content required to achieve all the pearlite structure is exceeded, the tensile strength of the ductile iron decreases instead, which is due to the segregation of tin at the grain boundaries. It is often added to stabilize the weak pearlite to produce nickel
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