Process of Electric Furnace Melting Cast Iron and Prevention of Common Defects

In recent years, due to the requirements of environmental protection, the small cupola of most foundry enterprises has been replaced by the small medium frequency induction furnace.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

The process characteristic of smelting cast iron in electric furnace is that the undercooling degree of alloy increases after molten iron remelting, and the superheat temperature is higher, which brings new problems to the quality control of cast iron.

After hot metal remelting in electric furnace, the impurities contained are less than those in cupola, that is to say, the purity is higher, there is no heterogeneous crystal core in its solidification process, and the composition fluctuation and concentration fluctuation during solidification are weaker, which makes the undercooling degree increase (the so-called undercooling degree here refers to the part where the actual temperature of iron carbon alloy with the same composition during solidification exceeds the crystallization temperature along the stable system), The possibility of solidification and crystallization of the alloy along the metastable system is increased, that is, the content of Fe3C (cementite) in the crystallization products is increased. However, the cupola molten iron has a large tendency to develop along the stable system during solidification and crystallization due to its more heterogeneous cores. On the phase diagram of iron carbon alloys, the products of the stable system are ferrite and graphite.

The smelting characteristics of electric furnace cast iron require the casting workers to update their ideas in many aspects such as the composition selection of cast iron, burden ratio, scrap consumption, inoculation process, decarburization and decarburization, sulfur increase and desulfurization, spheroidizing process, creep process, temperature control, and pouring process, and take practical measures to ensure and improve product quality.

1、 Proportioning of cast iron charge for electric furnace and synthetic cast iron

In the foundry industry, it is often said that the composition of the casting material determines the structure, and the structure affects the performance; This sentence is not comprehensive. In production practice, we found that many cast irons have great differences in mechanical properties with the same composition. The quality of molten iron is not only related to its composition, but also closely related to the burden ratio (pig iron consumption, scrap consumption, returned material consumption, alloy addition), melting and tapping temperature, inoculation process, etc. The so-called synthetic cast iron refers to the cast iron material made by carbonization synthesis using more than 50% of scrap steel in the batching. Because it requires a higher melting temperature, it should only be smelted in the electric furnace. At present, synthetic cast iron mainly includes synthetic gray iron and nodular iron.

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

1. Ingredients taboo

(1) . The high proportion of scrap (especially ship plate) should be matched with the high proportion of recycled materials (gating and riser, scrap castings, iron chips), and the amount of scrap added to the synthetic gray iron should not exceed 50%;

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

(3) . The recycled materials exceed 40% (gating and riser, waste castings, iron filings).

2. Optimization combination of ingredients (%)

Composition of pig iron scrap

Proportion A403030 Proportion B304030 Proportion C204040 Proportion D205030

3. Manganese and sulfur content

When the hardness needs to be increased, the manganese content can reach 1.0-1.2%, but it is not required to increase the sulfur content correspondingly (the sulfur content in gray iron shall be analyzed separately).

In order to save costs and use more scrap, a company tried to produce high grade gray cast iron within two months. The amount of scrap steel once reached 60%. For a period of time, in addition to adding scrap steel, the furnace charge and a small amount of iron filings were added back. The initial quality was good, but after a period of time, it was found that the castings had shrinkage cavities, porosity and white hard spots in batches, which continued to become more and more serious.

Cause of this defect: It is preliminarily judged that the high content of MnS in molten iron causes micro shrinkage cavity and porosity of castings, and MnS enrichment forms white hard spots. This is because the high grade gray iron HT300 requires high Mn content (about 1%), and the scrap itself is also high in manganese (16 manganese steel in the ship plate contains 1.6% Mn). However, the accumulation of S in the scrap and the MnS produced by the S and manganese reaction in the recycled iron (including scrap iron) in the furnace charge reaches a certain degree, which will lead to excess, thus producing the above defects.

In order to reduce the content of MnS in molten iron, it is generally adjusted by adding a certain amount of high-quality new iron (low S and low Mn). In addition, improving the inoculation effect can refine MnS and weaken 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 recycled material is only about 1230 degrees, using more scrap increases the power consumption, increases the supercooling tendency of molten iron, and also absorbs a large amount of nitrogen. Generally speaking, the synthetic cast iron process is not suitable for gray cast iron, but more suitable for ductile iron

2、 On Sulphur Increasing of Grey Cast Iron in Electric Furnace

As already mentioned, compared with cupola melting, the medium frequency induction furnace melting process of cast iron has the advantages of high melting temperature, but it has many disadvantages, mainly in three aspects: first, the hot metal has a large tendency of supercooling, which is very easy to produce D and E type graphite that affects the mechanical properties of materials; Second, the molten iron is pure and there are few heterogeneous crystal cores, which leads to poor inoculation effect. Under the same composition conditions, the casting strength is low and the iron is hard; Third, the shrinkage tendency is large. When the manganese content in high grade gray cast iron is high, it is easy to produce micro shrinkage cavity and porosity.

For the above problems, the countermeasures are:

1. Increase a high temperature holding time at the later stage of melting to make the iron crystal grains melted by various furnace charges as uniform as possible, especially to refine graphite;

2. An appropriate amount of foreign heterogeneous core (such as sulfide) is added to strengthen the inoculation effect and promote the formation of A-type graphite;

3. Control the content and proportion of sulfur and manganese in high-grade gray cast iron, and control the proportion of recycled materials to reach the appropriate composition.

These measures are different for castings with different structures, and should be mastered in practice.

One day, a company smelted 6 heats of gray iron HT300 hot metal in an electric furnace and cast hydraulic valves G03, G02 and other products. After dissecting the internal organization, it found that a large area of micro shrinkage cavities, porosity and cracks were found. A total of 830 pieces were scrapped (see the attached figure). Test the Brinell hardness HBS241, chemical composition C3.27, Si1.78, Mn0.83, S0.087, P0.04. Pearlite 98%, E-shaped graphite 80% (A-type 20%), graphite length 5 grades. According to the research and analysis of relevant personnel, there should be a problem with the molten iron material.

The results of chemical composition analysis seem normal for general thin wall HT300 castings, but there is a problem with hydraulic valve castings (with thick walls). Cause of this defect: It is preliminarily judged that the micro shrinkage cavity, shrinkage porosity and shrinkage crack of castings are caused by the high content of MnS in molten iron, that is, the content of S and Mn in molten iron exceeds the applicable range of castings (the composition of different castings is different).

As a certain amount of S increasing agent is added in smelting, the content of S and Mn in molten iron accumulates to a certain extent, which will cause the content of S in molten iron to exceed the requirements of normal solidification and crystallization of the casting itself, resulting in such defects. Countermeasures: Stop adding S increasing agent, adjust Mn content, ensure normal content of five elements in HT300 gray iron, and eliminate all defects after adjustment.

It is correct in theory that a certain amount of MnS can be formed by adding an S increasing agent into the hot metal of electric furnace gray iron to serve as a heterogeneous core to improve the inoculation effect. However, in recent years, most literature and data say that it is appropriate to control the S content of high-grade gray iron of electric furnace at 0.05-0.10%. However, many factories have proved that when the Mn content is about 1%, if the S content in the casting composition analysis exceeds 0.05%, the casting starts to produce shrinkage defects, When S content exceeds 0.07%, batch shrinkage will occur. How to explain this phenomenon?

There are two forms of S in gray cast iron, one is the simple substance, the other is the combined MnS. The sulfur in gray iron, which plays the role of the crystal core, is mainly the combined MnS. Our current testing methods (whether chemical analysis or spectral analysis) can only analyze the S in the simple substance state in castings and molten iron, while the S in the combined state (MnS) cannot be analyzed. When the elemental S content exceeds 0.05%, the S content 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, during the solidification process of molten iron, some brown MnS powder foam is produced while CO or CO2 is separated, forming the reaction gas shrinkage cavity of iron slag. As long as certain conditions are met, this kind of gas 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 of electric furnace, which comes from:

1. The content of sulfur and phosphorus in the gating system is much higher than that in the casting due to the re melting gating system;

2. The sulfur content in pig iron is not high in general pig iron, while the ordinary pig iron we buy carries slag (garbage) of different levels, which we will not test, but these garbage contains high sulfur and phosphorus, which will be brought into the furnace;

3. The iron rust and iron oxide content of scrap, pig iron and other furnace materials are high, and the sulfur absorption rate will be increased when they enter the molten iron. Under such circumstances, it would be too much if we added iron sulfide to increase S. In actual production of high-grade gray iron castings, it is appropriate to control the elemental substance S in molten iron between 0.03-0.05%.

3、 Inoculation and Modification Treatment of High Grade Grey Iron in Electric Furnace

With regard to 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 in cupola production). In recent years, with the popularity of electric furnaces, the inoculation amount has gradually increased, and the latest data recommends 0.5-0.6%. Through long-term practice, I have selected an inoculation amount of about 0.8%, achieving a comprehensive improvement in strength, hardness and cutting performance, and greatly reducing the internal defects of castings after processing.

A company produces a high-grade solenoid valve. The technical requirements are that the hardness of the casting is greater than HB200, the strength is greater than 300N/mm2, and the main wall thickness of the product is more than 50mm. Through many tests, while increasing the primary inoculation amount, the secondary stream inoculation is adopted to eliminate the defect of coarse structure caused by the thick wall, improve the density of the casting, and ensure the product quality.

With regard to the secondary stream inoculation of molten iron, a homogeneous inoculant with a particle size of 0.2-0.7mm is added before pouring, which is more suitable for thick pieces, but it increases the shrinkage property of molten iron when used for small pieces.

There was a time when some products of a company showed white bright spots on the surface with high hardness after processing, and the tool slipped. According to analysis, it was that the inoculant block was too large, which was not suitable for the capacity of the molten iron ladle, resulting in that the inoculant could not be completely melted during molten iron pouring, and the local silicon content of the casting was enriched to form a hardening phase; When the hot metal temperature is low for secondary stream inoculation, the same defect will be produced.

A factory specializing in producing HT300 gray iron hydraulic parts poured a KP pump body with a wall thickness of about 30mm. According to HT300’s experience ingredients, molten iron ingredients: C3.0-3.1%, Si 1.7-1.8%, Mn 0.95-1.05%, P0.05%, S0.04%. The tensile resistance of the casting body reached 300N/mm2. However, shrinkage and cracking occurred near the inner gate for several consecutive batches of products. No matter how the pouring system was adjusted, it was ineffective,

There is no way but to increase the carbon equivalent and reduce the strength. When the carbon equivalent is adjusted to C3.2-3.3% and Si 1.8-2.0%, the defects disappear. However, after the pressure test of the products after processing, most of them have expansion and leakage, and the body test tensile is also unqualified, causing the OEM to return the products in batches. It is associated with a number of similar pump bodies in the past. After listening to others’ suggestions, we increased S with iron sulfur. When the content of S in molten iron was above 0.07%, the castings were shrunk in a large area and a large number of waste products were accumulated. In order to deal with this batch of waste products, according to the principle of rare earth desulfurization, when adding such waste products, a small amount of rare earth magnesium ferrosilicon (about 0.2%) was added during the incubation process, effectively reducing the sulfur content and solving the problem of shrinkage.

In view of the shrinkage depression and crack of KP pump at that time, although the sulfur content of the original molten iron was not high, a small amount of rare earth magnesium ferrosilicon (about 0.2%) was also added during inoculation, which also achieved ideal results, and the shrinkage problem was completely solved. According to the analysis of its mechanism, the shrinkage of cast iron is mainly caused by the gases (including oxygen, nitrogen, hydrogen, etc.) in molten iron. When these gases precipitate in the late solidification period, molten iron cannot be supplemented, resulting in defects. However, as a gray iron modifier (also an inoculant), rare earth magnesium ferrosilicon is good at removing gases. The gas content of molten iron is greatly reduced, so the defects are eliminated.