common problems and solutions of electric furnace smelting

There are many problems in the work of electric furnaces. The following is only an introduction to some common problems.

1. The element burn is too large

The burning loss of Si, Mn, Cr and other easily oxidizable elements in the induction furnace is mostly 3% to 5%. Over-value burning loss and fluctuations in the chemical composition of cast iron will inevitably cause a series of structure and performance problems. Large element burnout usually occurs when the melting time is too long and slagging protection is not paid attention to. If the amount of scrap steel is large, there are many light and thin materials, the charge will be watery and rusty, and the problem will be aggravated.

The way to avoid excessive element burning is:

(1) The charge should be as clean as possible, the shape should not be branched, and the size should not be too large or thin.

(2) Put an end to racking materials and create all conditions that can melt quickly.

(3) Slag should be generated in time in the early stage of smelting, and slag will be covered under high temperature in the later stage. Give full play to the protective effect of slag.

(4) If the factory has chips to be used, some can be laid on the bottom of the furnace, and some are added to the molten pool in batches.

2. High oxygen in molten iron

The induction furnace does not have the oxidizing atmosphere of the cupola, and because the [O] and [FeO] in the molten iron react with [C], the Fe is protected by C, and there is not much dissolved oxygen in the molten iron. However, in the later stage of smelting, in order to promote the absorption of the C-increasing agent, the electric frequency is often lowered to strengthen the agitation of the molten pool. If the "hump" is too high and the frequency modulation time is too long, the probability of contact between the molten iron and the atmosphere will increase, and the dissociated O ions will enter the molten iron. Adding materials in the later stage of smelting without baking will also increase [O] and [H].

The way to prevent high O is:

(1) Do not over-tune the frequency in the later stage of smelting.

(2) Do not use wet materials and tools later.

(3) The overheating temperature should not be too high, and avoid keeping the temperature for a long time under high temperature.

3. After machining the casting, crack-like pores are found

Fissured pores are characteristic of N pores. When [N] exceeds the limit, it is easy to occur, and there are many non-metallic inclusions in the molten iron, and the probability of occurrence is higher. "Illnesses are imported from the mouth", so the amount of electric arc furnace scrap must be limited. The [N] of electric arc furnace scrap is high, while that of converter scrap is not. It is also necessary to prevent the mixing of waste alloy steel materials with high N content, such as high manganese steel, heat-resistant high chromium ferritic steel and chromium manganese nitrogen steel, and austenitic steel. The N content of different C-increasing agents varies greatly, and the N content of calcined petroleum coke is much higher than that of artificial graphite C-increasing agents. SiC contains less N than artificial graphite, so it is safe to use it. If you find that the amount of N in the molten iron is high, you should make a prompt decision and use Ti (Fe), Al, Zr (Fe), etc. for solid N treatment.

It must be noted that crack-like pores must of course be prevented, but it is not that the lower the N, the better. For gray cast iron, N can shorten the length of graphite and have a tendency to make the ends of graphite smooth. N dissolving in solid solution can promote the refinement of pearlite and increase the number of pearlite. N also has a fertilizing effect and promotes graphitization. Therefore, soluble N has utility value for gray cast iron.

4. Poor inoculation effect of gray cast iron

In some factories, the effect of gray cast iron inoculation is not good, even adding more inoculants will not help. This is related to the low content of O and S in the molten iron of the induction furnace.

The basic solution is to use FeS-based S-increasing agent to increase S to 0.07%～0.10%. A few factories also add O additives in the later stage, such as sponge iron or sintered iron or cutting chips to make [O] above 30ppm. Some factories use oxygen-sulfur inoculants. These methods are nothing more than generating oxysulfides, which act as the core of graphite. It must be pointed out that in low-S molten iron, the formed graphite is relatively easy to disappear. This is probably one of the reasons why low-S gray cast iron cannot be conceived.

5. Few graphite balls and uneven ball diameter

There is spherical graphite, indicating that the amount of residual Mg is no problem. The fault lies in poor gestation or fertility decline. If the casting does not produce white mouth, it means that the fertility gap is not large. From the perspective of smelting, artificial graphite C-increasing agent should be used to control the graphite core. Industry insiders recommend that SiC is better than artificial graphite. It is also pointed out that SiC is light, easy to float, and the surface is blocked by SiO2 film, which affects dissolution. Therefore, it is better to add SiC during the charging period. If the method of feeding the silk is used, the inoculation must be carried out slightly after the spheroidization is completed, so as to avoid the accelerated decline of the inoculation.

People often ask what is the appropriate number of graphite balls. In the nodular cast iron standard, the number of graphite balls is not specified. Calculated based on the graphite size of grade 6, the approximate number of spheres>150/mm2. ADI's process control stipulates that the number of graphite balls is not less than 100/mm2.

6. The impact toughness of ductile iron is low

QTD800-10R in ferritic nodular cast iron and austempered nodular cast iron (when requested by the buyer) has regulations on the minimum impact energy of V-notch specimens. If the impact energy fails to meet the requirements, it must be scrapped. It is critical to determine their chemical composition. Taking QT350-22L and QT400-18L in ferritic ductile iron as examples, low P and Mn are required, and the amount of Si is accordingly limited. From a smelting perspective, should:

(1) Select carbon scrap steel and use a certain amount of high purity pig iron for casting.

(2) According to the above tips, do the four aspects of quality control of molten iron.

(3) Do the pre-treatment of the furnace well. This can prevent problems before they happen.

7. Elephant feet

The so-called elephant feet refers to the bottom of the inner cavity of the crucible, which has a ring that is recessed inward and looks like an elephant foot. It is caused by the overheating of the molten pool at the bottom of the stack during the melting period, causing the SiO2 in the furnace lining to react with the C in the molten iron, causing serious corrosion of the furnace lining. The furnace has a large capacity, and the hydrostatic pressure on the bottom of the furnace is large, and elephant feet are more likely to appear.

The countermeasures are as follows:

(1) Do not add materials randomly, do not use large materials, do not use whole waste castings.

(2) Work harder to help the charge to go down.

(3) If elephant feet tend to be found, repair them in time.

The above is about the problems in smelting and the corresponding solutions. Anyang Huatuo Metallurgy has specialized in foundry materials for more than 10 years. I hope that the content shared above will be helpful to you. If you have any other questions, please feel free to consult.