Introduction:

      In recent years, due to the requirements of environmental protection, most small cupola is replaced by small medium frequency induction furnace, cupola due to the use of coke and other fuels, metallurgical reaction occurs in the smelting process, and the smelting of electric furnace is mainly alloy remelting, metallurgical reaction is not significant.

      The technological characteristic of electric furnace smelting cast iron is that the supercooling degree of alloy increases and the superheating temperature is higher after molten iron remelting, and the quality control of cast iron meets new problems.

      Furnace hot metal after remelting, contain less impurity content in the cupola, which means higher purity, in its lack of the heterogeneous crystallization of the core in the process of solidification, solidification when the constituent fluctuations and the concentration of ups and downs to weaker, which makes the supercooling degree increase (here refers to the so-called supercooling degree of the same composition of the iron carbon alloy solidification actual temperature beyond the stable system that part of the crystallization temperature), The possibility of solidification crystallization of the alloy along the meso-stable system is increased, that is, the content of Fe3C(cementite) in the crystallization products is increased, and cupola molten iron has a great tendency to develop along the stable system during solidification crystallization due to more heterogenous core. On the phase diagram of the fe-c alloy, the crystallization products of the stable system are ferrite and graphite.

      Electric furnace smelting characteristics of cast iron, requirements in the composition of cast iron foundry workers selection, charge ratio, dosage of scrap steel, inoculation process, increase carbon decarburization, resulfurization desulfurization, spheroidizing process, injection vermicularizing process, temperature control, pouring process and so on many aspects need to renew the idea, take practical means to ensure and improve the quality of the product.

      Mr. Xia Zhenhuan, a technical expert, visited "Hundred foundry Forum", and discussed "the characteristics of electric furnace smelting cast iron process and the prevention and treatment of common defects" with netizens and fans.

      Based on 30 years of cupola production and more than 10 years of electric furnace production practice, Mr. Xia Zhenhuan will share his experience and lessons on the above 10 problems with 　Shandong Jinda New Materials Technology Co., Ltd. 

I. Charge ratio of electric cast iron and synthetic cast iron

      In the casting industry, it is often said that the composition of the casting material determines the organization, and the organization about the performance; This is not a complete statement. In our production practice, we found that many cast iron, in the same composition, mechanical properties are quite different. The quality of molten iron is not only related to its composition, but also closely related to the ratio of charge (pig iron dosage, scrap dosage, return material dosage, alloy dosage), melting and oven temperature, inoculation process, etc. The so-called synthetic cast iron refers to the cast iron material made by the method of carburization synthesis with more than 50% scrap steel in the ingredients. Because of the high melting temperature, it is only suitable to be melted in the electric furnace. At present, synthetic cast iron mainly includes synthetic gray iron and ductile iron.

      Through a lot of practice, for high strength gray cast iron such as HT250 and HT300, the left and right strength of scrap steel and pig iron affect the microstructure.

1, ingredients taboo

(1) With high proportion of scrap steel (especially ship plate) and high proportion of return charge (pouring riser, waste casting, iron scrap), the amount of synthetic gray iron scrap should not exceed 50%;

(2) High proportion of scrap steel (especially ship plate) with pig iron with high sulfur and phosphorus content;

(3) The return charge exceeds 40% (pouring riser, waste casting, iron filings).

2. Optimal mix of ingredients (%)

Make up pig iron scrap return charge

Ratio A403030 ratio B304030 ratio C204040 ratio D205030

3. Manganese sulfur content

Manganese content can reach 1.0-1.2% when hardness needs to be increased, but sulfur content is not required to be increased accordingly (for sulfur content in gray iron, separate analysis).

      A company in order to save costs, multi-purpose scrap, in two months trial production synthetic high cast iron, scrap steel dosage was 60%, there was a time in a scrap back to additional burden and small amounts of iron filings, the original quality is good, but after a period of time found a white casting volume shrinkage cavity, shrinkage porosity and hard spots, and constant more and more serious.

      The cause of this defect is that the casting microshrinkage cavity and shrinkage porosity are caused by the high content of MnS in molten iron, and MnS are enriched to form white hard spots. This is due to the high grade of grey iron HT300 components Mn content is higher (1%), combined with high manganese steel scrap itself also (16 manganese steel containing Mn in 1.6%) in the planking, and scrap steel in the S and S in recycled iron (iron) and manganese reaction of MnS in furnace charge accumulation reaches a certain degree, will generate too much, to produce the above defects.

      In order to reduce the MnS content in molten iron, a certain amount of high-quality new pig iron (low S and low Mn) is generally added to adjust, in addition to improve the inoculation effect, MnS can be refined, reduce its adverse effects.

      When the amount of scrap is too large, because the melting point of scrap is about 1530 degrees, while the melting point of pig iron and returnable charge is only about 1230 degrees, the use of scrap increases the power consumption, increases the undercooling tendency of molten iron, and absorbs a large amount of nitrogen. Generally speaking, the synthetic cast iron process is not suitable for gray cast iron, but more suitable for nodular iron

      Two, on the electric furnace gray cast iron sulfur problem

      As has been said before, compared with cupola smelting process, medium frequency induction furnace smelting cast iron, in addition to the advantages of high melting temperature, but there are many shortcomings, there are mainly three aspects of the problem: first, the undercooling tendency of hot iron is large, easy to produce D, E type graphite affect the mechanical properties of the material; Second, the hot iron is pure, and the heterogeneous crystal core is less, leading to poor inoculation effect. Under the condition of the same composition, the casting strength is low and the iron is hard;              Third, the tendency of shrinkage is larger. When the manganese content in high grade gray cast iron is higher, it is easy to produce microscopic shrinkage cavity and shrinkage porosity.

To address the above problems, the following measures should be taken:

1, in the late melting to increase a high temperature holding time, as far as possible to make all kinds of charge molten iron grains uniform, especially refined graphite;

2, increase the amount of foreign heterogenous core (such as sulfide), strengthen the inoculation effect, promote the formation of type A graphite;

3, control the sulfur, manganese content and proportion of high grade gray cast iron, control the proportion of return charge, to achieve the appropriate composition

These measures, for different structure of casting products are different, need to master in practice.

      A certain company smelted iron HT300 in furnace 6 and cast hydraulic valves G03, G02 and other products in an electric furnace on a certain day. A large area of microscopic shrinkage cavity, shrinkage cavity and shrinkage crack were found after the internal tissue dissection, and a total of 830 were scrapped (see the attached picture). Brinell hardness HBS241, chemical composition C3.27, Si1.78, Mn0.83, S0.087, P0.04. Pearlite 98%, e-shaped graphite up to 80% (A type 20%), graphite length 5 grade. According to relevant personnel research and analysis, should be the hot metal material out of the problem.

      The results of chemical analysis appear to be normal for normal thin-walled HT300 castings, however there is a problem for hydraulic valve castings (with thicker walls). The cause of this defect: the preliminary judgment is that the casting microshrinkage cavity, shrinkage porosity and shrinkage crack are caused by the high content of MnS in the molten iron, that is to say, the CONTENT of S and Mn in the molten iron exceeds the suitable range of the casting (there are differences in the components of different castings).

      Due to the addition of a certain amount of s-increasing agent in smelting, the content of S and Mn in molten iron accumulates to a certain extent, which will lead to the amount of S in molten iron exceeding the requirements of normal solidification and crystallization of the casting itself, resulting in such defects. Countermeasures: stop adding S agent, adjust the content of Mn, ensure the normal content of five elements of HT300 gray iron, after adjustment, all the defects are eliminated.

      Increased S agent by joining in the molten iron furnace ash to form a certain amount of MnS, as heterogeneous core, improve the effect of inoculation, which from the theory is correct, but most of the literature in recent years, the electric furnace high grade of grey iron S content should be controlled in 0.05 0.10% more appropriate, however, many factories of practice has proved that when Mn content is about 1%, If the content of S in the casting composition analysis exceeds 0.05%, the casting begins to produce shrinkage cavity defects. When the content of S exceeds 0.07%, the batch shrinkage cavity will occur. How to explain this phenomenon?

      There are two forms of S in gray cast iron, one is elemental and the other is MnS in the combined state. The sulfur that plays the role of crystallization core in gray iron is mainly MnS in the combined state. Our current assay means (whether chemical analysis or spectral analysis) can only analyze S in the elemental state of castings and molten iron. However, S in the combined state (MnS) is undetectable. When the content of elemental S exceeds 0.05%, the content of S in the combined state is relatively high. At this time, in the molten iron:

MnO+FeS=MnS+FeO, FeO+C=Fe+CO, or 2FeO+C=2Fe+CO₂

      At this time, in the solidification process of molten iron, part of brown MnS powder is produced when CO or CO2 is precipitated, forming reaction gas shrinkage cavity of iron slag. As long as certain conditions are met, this kind of shrinkage cavity occurs not only in the molten iron of electric furnace but also in the molten iron of cupola. In fact, we have added some sulfur in the melting process, which comes from:

1. The sulfur and phosphorus content in the casting system is much higher than that in the casting.

2, the sulfur in pig iron, the sulfur content in general pig iron is not high, and we buy ordinary pig iron above carry different degrees of slag (garbage), we will not test, but these garbage contains high sulfur and phosphorus, will be brought into the furnace;

3, scrap steel and pig iron and other charge rust, iron oxide content is higher, into the hot iron will increase the sulfur absorption rate. In this case, if we add iron sulfide to increase S, it would be too much. In the actual production of high grade gray cast iron, it is advisable to control the elemental S in molten iron between 0.03 and 0.05%.

Three, electric furnace high grade gray iron breeding and metamorphic treatment

      Regarding the inoculation process of high grade gray iron (HT300 as an example), the traditional inoculation amount is 0.3-0.4% of the amount of iron treated (mainly produced by cucua). In recent years, with the popularity of electric furnace, the inoculation amount has gradually increased, and the latest information recommends 0.5-0.6%. Through long-term practice, I choose the inoculation amount at about 0.8%. The strength, hardness and machining properties of the castings are improved, and the internal defects of the castings are greatly reduced.

      A company manufacturing high grade of solenoid valve, the technical requirements of the casting hardness greater than HB200, strength is greater than 300 n/was, this product main wall thickness more than 50 mm, through many experiments, at the same time of increasing an inoculation quantity, take the second stream inoculation, to eliminate the defect of the thick wall bring organization bulky, improves the casting density, guaranteed the product quality.

      As for the secondary flow inoculation of molten iron, uniform inoculating agent with particle size of 0.2-0.7mm is added before pouring, which is more suitable for thick parts, but increases the shrinkage performance of molten iron when used for small parts.

      At one time, the surface of some products of a company showed white bright spots and high hardness, and the tool slipped. After analysis, it turned out that the inoculant block was too large, and the capacity of hot iron package was not suitable, resulting in the inoculant failed to melt completely when pouring hot iron, and the local silicon concentration of the casting formed hardening phase; The same defect will occur when the secondary flow inoculation is carried out at low temperature of molten iron.

      There is a factory specializing in the production of HT300 grey iron hydraulic parts, casting a KP pump body, casting wall thickness of about 30mm, according to HT300 experience ingredients, hot iron composition: C3.0-3.1%, Si1.7-1.8%, Mn0.95-1.05%, P0.05%, S0.04%, casting body anatomical tensile strength of 300N/mm2, but continuous multiple batches of products near the gate shrinkage and shrinkage crack, no matter how to adjust the pouring system, but no effect.

      There is no other way but to improve the carbon equivalent and reduce the strength. When the strength is adjusted to C3.2-3.3% and Si1.8-2.0%, the defects disappear. However, after the pressure test of the products after processing, most of the products have expansion and leakage, and the tensile strength of the ontology test is not qualified, causing the OEM to return the products in batches. Associated with having a group of similar pump body before, due to listen to others' advice, S with pyrite, molten iron containing S, when more than 0.07%, the casting shrinkage of large area, accumulate a large amount of waste, in order to deal with this batch of scrap, according to the principle of rare earth desulfurization, when to join this kind of waste, the inoculation in the process of adding a small amount of rare earth magnesium ferrosilicon (about 0.2%), effectively reduce the sulfur content, The problem of shrinkage cavity is solved.

      In view of the shrinkage and cracking of KP pump at that time, although the original molten iron was not high in sulfur, a small amount of rare earth magnesium ferrosilicon (about 0.2%) was also added during gestation, and the ideal result was achieved, and the shrinkage cavity problem was completely solved. Analysis of the mechanism of the cast iron to produce trap, mainly the gases in the molten iron (including oxygen, nitrogen, hydrogen, etc.) of these gases in the late coagulation precipitation, molten iron can't added, the defects, and rare earth magnesium ferrosilicon, as a kind of grey iron inoculant (is also a kind of nucleating agent), is a good removal of gas, hot air content are greatly reduced, the defects are eliminated.

                                                           ----------   Shandong Jinda New Materials Technology Co., Ltd.