Importance to treat the gray iron melted by electric induction furnace
This company is a large state-owned enterprise, the production of many varieties of gray iron castings, the previous use of Cupola melting equipment, the production of gray iron castings have been very stable quality.
In order to further implement the national requirements for Energy Conservation and emission reduction, optimize the environment, and achieve high efficiency and better development prospects for enterprises, high efficiency and low power consumption medium frequency induction furnace is used to replace Cupola as melting equipment to produce gray iron.
After the electric induction furnace was put into production, the following problems appeared in the production process for a period of time:
(1) cracks appeared in the cover plate of the metallurgical tractor.
(2) the appearance of chill on ball mould.
(3) the gap of the rear cover of the molten iron vehicle is serious, and some of them have shrinkage defects.
After the casting quality problems occurred, the relevant technical personnel of the company analyzed and concluded that after smelting in an electric induction furnace, there was basically no change in the raw materials used for smelting, and the raw materials were sampled for inspection Found that the quality indicators are within the prescribed limits. Therefore, the quality problems caused by raw materials are basically eliminated.
By metallographic examination of the casting with defects, it was found that the casting with cracks was all e-type graphite, and the casting with white defects was mainly d-type graphite Chemical composition analysis of these gray iron castings showed that the average wS of molten iron was about 0.028 %. It is pointed out in the literature that the number of the eutectic group is very small when the sulfur content is low, i. e. the nucleation degree is very small. With the increase of sulfur content, the number of EUTECTIC group increases rapidly. When wS reaches about 0.05 %, the increase of EUTECTIC group tends to slow down. The results show that the effect of conventional inoculation is not obvious when the WS of induction furnace is below 0.05 %, and the white tendency of castings increases when wS ≤0.03 %. The main reason is that the content of sulfur and sulfide is low, the quantity of crystal nucleus is reduced, the nucleation ability is low, the size of white mouth is increased, the type graphite is decreased, the type D and type E graphite is increased. Because of the strong Directivity, small mechanical strength and impact toughness of e-type graphite, the casting is easy to crack. The main reason for the low sulfur content is the long holding time at a high temperature. In the process of Cupola melting, the content of sulfur in coke is high, the liquid iron and coke contact directly, the molten iron increases sulfur, but in the process of electric furnace melting, due to the low content of sulfur in raw material pig iron and scrap, there is no sulfur increase On the contrary, in the smelting process, the molten slag formed with other elements to form Sulfides, which floated on the surface of the molten iron. Because of the melting characteristics of the molten iron in the electric furnace, the high-temperature holding time is longer. As nucleated grains, the sulfur compounds melt in large quantities during the holding period, which leads to the decrease of the crystal nucleus of sulfide and the nucleation ability of graphite And with the continuous extension of the holding time of Molten Iron, undercooling continued to increase, increasing the depth of white.
In order to overcome the low sulfur content and low graphite nucleation ability, our company used sulfur increasing agent in electric furnace smelting, which increased the liquid iron wS from 0.028% to 0.06% ~ 0.08% , and increased the graphite nucleation ability At this point all the metallographic structure of the casting is a type graphite or mainly a type graphite.
In order to avoid the appearance of e-type graphite, the silicon-barium long-acting inoculant was introduced and used instead of traditional inoculant 75SiFe to increase the particle size of silicon dioxide, which provided the necessary conditions for the formation of a-type graphite At the same time, the addition of Barium in ferrosilicon prolongs the effective incubation time and restrains the formation of e-type graphite.
For the castings with serious white defects, the amount of inoculant was increased, and 2% clean non-rust primary iron was added in the ladle before molten iron was produced, which effectively increased the graphite particles and eliminated the white defects.
In addition, in the production of gray iron, it is found that there are differences in the characteristics between the molten iron of electric induction furnace and Cupola,such as easy control of temperature and composition, high purity of Molten Iron; But also has the bad characteristic, like the white mouth tendency is big, the contraction tendency is big. Therefore, in the production of gray iron castings in our company, in addition to the requirements to achieve charge purity, to remove pig iron, scrap surface rust, but also the carbon equivalent slightly higher than the Molten Iron Cupola melting, and melting operation to fast out Reducing the holding time of molten iron in furnace, especially outside the high temperature holding time, the treatment of molten iron is also very important. In the production process, the control of WS of molten iron is 0.06% ー0.08 %, for example, W / S ≤0.03% In addition to inoculant, 2% primary iron should be added to increase the quality of graphite and eliminate the defects of thin-wall gray iron castings. After the above-mentioned technology was applied in the production process, the cracks in the casting such as the cover plate of the metallurgical tractor, the white defects in the cast iron mould and the back cover of the liquid iron car were basically eliminated, and the graphite shape in the microstructure was mainly a type The content of pearlite is above 90% . Due to the stability of the composition and the improvement of graphite morphology, the porosity tendency of the castings was also improved, and the mechanical properties were improved.