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Production of coke oven door frame made of HT350 material

Production of coke oven door frames made of HT350 material

Abstract: Domestic coke oven door frame products are mostly made of vermicular graphite cast iron, while export coke oven door frames are mostly made of HT250 or QT450. The coke oven door frame we produced this time is made of HT350 material for the first time. In view of the fact that the wall thickness of the coke oven door frame is more than 200mm, and this material has a serious tendency to shrink. Therefore, we achieve the smooth output of the product through the coordination of the proportion of the furnace charge, the setting of the carbon equivalent, the matching of the pouring temperature, and the feeding of the riser.

Keywords: coke oven door frame; thick and large parts; HT350

The coke oven door frame produced this time is 7.5m long, 0.9m wide, 0.15m~0.3m thick, and weighs 3.5t. It is hollow inside and made of HT350 material. Considering the characteristics of this material, casting is difficult. Therefore, before trial production, we conducted detailed analysis and discussion of each process link and formulated various plans.

Modeling process design

(1) Scale confirmation: The scale of the mold is related to the linear shrinkage of the casting. The linear shrinkage of gray cast iron refers to the solid-state shrinkage that occurs after a complete and continuous solid phase skeleton is formed in the casting in the late solidification stage. It is not only related to the size and deformation of the casting, but also affects the internal stress of the casting and even the occurrence of casting defects such as cold cracking. The linear shrinkage (sand casting) of gray cast iron is generally between 0.5% and 1.3%. In view of the low carbon equivalent of the HT350 grade we produce, we choose a shrinkage of 1.2% based on the final composition of the product.

(2) Slag-retaining net confirmation: The filter is a foam ceramic filter, which has an open porosity of 80% to 90% due to its unique three-dimensional connected curved hole mesh skeleton structure. It can efficiently filter out large inclusions and most tiny suspended inclusions as small as tens of microns in the metal liquid. Our company chooses a filter with a pore size of 10ppi. Based on the filtration capacity of gray cast iron of 2.0kg/cm2 ~ 4.0kg/cm2, the effective filtration area S=1000cm2 is calculated.

(3) Pouring system design: Based on the structural size, weight, and filter cross-sectional area of the product, we designed a technical solution for double-pack pouring and adopted a slag-stop pouring system to solve the problem that this material is prone to slagging.

(4) Feeding riser design: Design a feeding riser at one end of the product where the wall thickness is thicker, and ensure the quality of the product through the heating and feeding effects of the subsequent riser.

Ingredient selection

C: Reducing the carbon content will inevitably reduce the amount of graphite precipitated, thereby reducing the impact of graphite on the metal body. The reduction in carbon content promotes the precipitation of primary austenite, thereby significantly improving the strength of cast iron. The carbon range we chose this time is 2.75% to 2.95%.

Si: Silicon is an element that promotes graphitization and has a solid solution strengthening effect. With an appropriate increase in the amount of silicon in molten iron, the shrinkage tendency is reduced, which is beneficial to improving process performance.

The solid solution strengthening effect of silicon is related to the inoculation method. Large-dose inoculation methods cannot be used. Otherwise, if the amount of silicon is too high, a lot of ferrite will be produced in the body, and the solid solution strengthening of ferrite will not be as high as pearlite. In the process of gestation, it is not advisable to gestate in large doses, but in appropriate amounts. Therefore, we adopted the final plan of 0.4% inoculation volume and 1.7% to 1.9% final silicon.

Mn: In the range of 0.1% to 1.0%, low manganese means high performance, and high manganese means poor performance. However, manganese cannot be reduced too low, otherwise the fluidity of molten iron will be poor and shrinkage will be serious. Therefore, in the end we controlled it between 0.8% and 0.9%.

P: Phosphorus in molten iron has a great influence on microscopic shrinkage. Since phosphorus easily segregates and forms a low-melting phosphorus eutectic at the grain boundary, the melting point of the phosphorus eutectic is 200°C lower than the melting point of iron-carbon-silicon. After the eutectic solidification is completed, there is still a low melting point liquid alloy at the grain boundary, resulting in the formation of microscopic shrinkage porosity. For this reason, we set an upper limit on phosphorus content of 0.075%.

S: Too low sulfur in gray cast iron is harmful. The shape of graphite is poor, and graphite easily grows into harmful thick flake graphite. Since the carbon atoms in the molten iron are more likely to be enriched on the coarse flake graphite, a large number of graphite crystal nuclei cannot grow into flake graphite. As a result, the molten iron in the later stage of solidification cannot be effectively graphitized and causes severe shrinkage, and the effect after incubation is also not good. When the sulfur content is less than 0.05%, sulfur increasing treatment must be carried out, otherwise the inoculation effect will be poor.

Sulfur is an element that hinders graphitization. However, it is precisely when the nucleation ability of molten iron is very good and the incubation is done well, it is sulfur that prevents graphite from growing into thick flakes, thus giving other graphite nuclei the opportunity to grow into flake graphite. The purpose of refining the graphite, increasing its quantity and evenly distributing it is achieved.

However, too much sulfur in gray cast iron is also harmful. After all, sulfur is an element that hinders graphitization. Too much sulfur will increase white spots.

Adding sulfur can improve the performance of gray cast iron because sulfur refines the graphite morphology and simultaneously refines the eutectic clusters. As the sulfur content increases, the length of the graphite becomes shorter, the ends become blunt, and the shape becomes curved, thus improving the properties of the cast iron. The sulfur range we finally selected is between 0.06% and 0.08%.

Confirmation of charge ratio

All-scrap smelting: The use of all-scrap carbonization process in electric furnace smelting is an important measure to improve material performance. As carburization promotes graphite crystal nuclei, the graphite is refined. The distribution of graphite is more uniform. Evenly distributed and appropriately refined graphite is obviously very beneficial to improving the strength of gray cast iron. At the same time, the shrinkage tendency of the molten iron is reduced. Therefore, the molten iron smelted by the advanced scrap steel carburization process has a smaller shrinkage tendency and has higher material performance than the molten iron smelted by using a large amount of pig iron without carburizing agent.

From a technical point of view, it is unscientific to use more pig iron. First, the strength performance will be affected. Many people can accept this and have experience in this regard. However, if more pig iron is used than scrap steel, the shrinkage tendency of molten iron is greater, I believe many people will not agree with this point of view.

Why does excessive use of pig iron reduce performance and increase shrinkage?

There are many coarse hypereutectic graphites in pig iron. This coarse graphite is hereditary. If the melting temperature is low, the coarse graphite will not be easily eliminated. Coarse graphite structure is more likely to be inherited. Coarse graphite makes it easier for carbon atoms in molten iron to concentrate on the coarse graphite, weakening the effect of graphitization precipitation that should have occurred in other parts during the solidification process. Therefore, the local shrinkage tendency during the solidification process of molten iron increases, and the coarse graphite inevitably reduces the performance of the material.

However, in view of the fact that smelting all scrap steel will seriously increase power consumption and prolong the smelting time, comprehensive consideration, we chose the furnace charge ratio of 20% to 30% pig iron and 70% to 80% scrap steel.

Selection of carburizing agent: For electric furnace melting, it is recommended to use the all-scrap steel process, which requires solving the carburizing technology, and the carburizing agent becomes the most important link in the carburizing technology. The quality of carburization determines the quality of molten iron. Whether the carburization process can achieve a good graphitization effect, reduce the shrinkage tendency of molten iron, and improve material properties mainly depends on the carburizing agent.

The carburizing agent must be selected through high-temperature graphitization treatment. Only after high-temperature graphitization treatment can carbon atoms transition from the original disordered arrangement state to the ordered arrangement state of flake graphite, and flake graphite can become the best core for graphite nucleation and promote graphitization. After high-temperature graphitization treatment, the nitrogen and hydrogen content in the carburizer can be reduced to prevent pores in the casting.

If the selected carburizer has not undergone high-temperature graphitization treatment, the nucleation ability of graphite will be greatly reduced, and the graphitization ability will be weakened. Even if the same amount of carbon is reached, the results will be completely different. An important indicator of a good carburizing agent is that S is less than 0.03%.

Confirmation of melting parameters

Selection of melting temperature: For electric furnace melting, overheating the molten iron temperature to above 1500 is very harmful. If the temperature is above 1500℃ for too long, the carbon in the molten iron will be severely burned and the number of crystal nuclei will be reduced, resulting in an increased shrinkage tendency of the molten iron and a serious whitening tendency.

It is not recommended that the molten iron temperature be higher than 1500°C when smelting gray cast iron in an electric furnace. Even if a large amount of pig iron is used for melting, in order to eliminate the graphite inheritance of the pig iron, after the temperature of the molten iron reaches above 1500°C for a short time, it should be quickly cooled to below 1500°C before being discharged. For molten iron that is not coming out of the furnace immediately and needs to be kept warm, the temperature should be lowered to about 1400°C to keep it warm. The high temperature we chose for this melting is below 1500°C.

Determination of pouring temperature: high temperature comes out of the oven, low temperature pouring is one-sided. When the molten iron comes out of the furnace at a high temperature, the quality of the molten iron is good. After solidification of the molten iron, the molten iron can be improved and refined. At the same time, due to the high superheat of the molten iron and the low viscosity of the molten iron, sufficient sedation time can easily cause gas to escape and the slag to float, which can effectively improve the quality of castings. However, the temperature of the furnace is too high, and the molten iron needs to stay for a period of time to cool down. This process greatly reduces the inoculation effect and is also very harmful. Therefore, we deal with the amount of molten iron and the condition of the pouring ladle according to the pouring temperature of the product, and finally select a furnace temperature of 1470°C ~ 1480°C.

Breeding and pouring

Inoculation treatment is to add inoculant to the molten iron before the molten iron enters the mold cavity. Its purpose is to promote graphitization and reduce the tendency of whitening. Improve the mechanical properties and other properties of cast iron. Improve section sensitivity. Improve the shape and distribution of graphite, reduce the formation of supercooled graphite and symbiotic ferrite, obtain medium-sized A-type graphite, and appropriately increase the number of eutectic clusters.

Each inoculant has its optimal addition ratio. If the inoculant is not added enough, white or numb tissue will easily appear on the casting. If too much inoculant is added, the graphite in the casting will become coarse. In order to avoid inoculation decline, the molten iron should be poured as soon as possible after inoculation treatment, usually within 10 minutes. For thick and large castings, the inoculation amount should be increased by 20% to 50%. However, too much inoculant will not bring greater inoculation effect, but will waste the inoculant, lower the temperature of the molten iron, and increase the shrinkage of the casting and the defects of pores and slag inclusions. It is generally recommended that the mass fraction of silicon brought into the molten iron during incubation should not exceed 0.3%.

Due to the large degree of oxidation of molten iron in China, the dosage of inoculants used mostly exceeds this value. The higher the grade required for gray cast iron, the lower the carbon equivalent of the selected molten iron, and the greater the amount of inoculant that needs to be added.

Rare earth elements can neutralize many impurity elements in gray cast iron and purify molten iron. In particular, they react strongly with sulfur to form the core of graphite nucleation, which significantly affects the performance of the material. Therefore, the role of rare earth elements in cast iron is beneficial, especially when using molten iron with high carbon equivalent to produce relatively thick castings. Adding rare earth elements can refine graphite and improve material properties.

Therefore, in the end we chose the incubation method of a small amount of rare earth elements combined with an appropriate amount of barium-containing long-acting inoculants.

Project effects

According to this plan, we conducted three batches of experiments to verify the effect.

Table 1 Three groups of experimental performance results

Product Number Tensile Strength
1 250
2 280
3 275

Use an attached cast test bar with a diameter of 50mm for production testing, as shown in product number 1.2.3.

The product performance and metallography are qualified, the product appearance quality is good, and there are no detailed casting defects. After inspection by the user, it meets the requirements.

Figure 1 Product No. 1 Metallography

Figure 2 Product No. 2 Metallography

Figure 3 Product No. 3 Metallography

Summary

Through tracking and testing throughout the production process, we have improved the production process and operating procedures of the HT350 material to ensure the stability of subsequent product quality. We will continue to optimize and improve in the future to further improve product quality.

Coking Equipment Manufacturer
coke ovens
coke oven door

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