High-strength gray cast iron smelting technology knowledge explanation

This paper introduces how to obtain high-strength gray cast iron smelting technology under the conditions of higher carbon equivalent and better machinability in the electric furnace smelting process, and how to control the trace elements of the material.
1 Factors affecting material properties

1.1 Effect of carbon equivalent on material properties

    The main factors that determine the properties of gray cast iron are the graphite morphology and the properties of the metal matrix. When the carbon equivalent (CE=C+1/3Si) is higher, the amount of graphite increases, and the shape of graphite deteriorates when the incubation conditions are not good or there are trace harmful elements. Such graphite reduces the effective area of ​​the metal matrix that can bear the load, and produces stress concentration when bearing the load, so that the strength of the metal matrix cannot be exerted normally, thereby reducing the strength of the cast iron. Among the materials, pearlite has good strength and hardness, while ferrite is softer and lower in strength. When the amount of C and Si increases, the amount of pearlite decreases and the amount of ferrite increases. Therefore, the increase of carbon equivalent will affect the tensile strength of cast iron castings and the hardness of casting bodies in terms of graphite shape and matrix structure. In the smelting process control, the control of carbon equivalent is a very important factor to solve the material properties.

1.2 Influence of alloying elements on material properties

    The alloying elements in gray cast iron mainly refer to Mn, Cr, Cu, Sn, Mo and other elements that promote the formation of pearlite. The content of these elements will directly affect the content of pearlite. At the same time, due to the addition of alloying elements, it is refined to a certain extent. The amount of ferrite in the matrix is ​​reduced or even disappeared, and the pearlite is refined to a certain extent, and the ferrite in it is solid solution strengthened due to a certain amount of alloying elements, so that the cast iron always has a higher strength properties. In the control of the smelting process, the control of the alloy is also an important means.

1.3 Influence of charge ratio on materials

    In the past, we have always insisted that as long as the chemical composition meets the requirements of the specification, it should be possible to obtain materials that meet the standard mechanical properties. In fact, this view only sees the conventional chemical composition, ignoring the role of some alloying and harmful elements in it. effect. For example, pig iron is the main source of Ti, so the amount of pig iron used will directly affect the content of Ti in the material and have a great impact on the mechanical properties of the material. Also scrap steel is the source of many alloying elements, so the effect of scrap steel content on the mechanical properties of cast iron is very direct. In the early days of the electric furnace being put into use, we have been using the charge ratio of the cupola furnace (pig iron: 25~35%, scrap steel: 30~35%). As a result, the mechanical properties (tensile strength) of the material were very low. When we realized that the scrap steel When the amount of scrap used will affect the performance of cast iron, the amount of scrap is adjusted in time, and the problem is quickly solved. Therefore, scrap is a very important control parameter in the melting control process. Therefore, the charge ratio has a direct impact on the mechanical properties of cast iron materials, and is the focus of smelting control.

1.4 Influence of trace elements on material properties

    In the past, we only paid attention to the influence of the five conventional elements on the cast iron material in the smelting process, while the role of other trace elements was only a qualitative understanding, but they were rarely analyzed and discussed quantitatively. In recent years, due to the development of casting technology. With progress, the smelting equipment is constantly updated, and the cupola has been gradually replaced by the electric furnace. Of course, electric furnace smelting has the incomparable advantages of cupola furnace, but electric furnace smelting also loses some advantages of cupola smelting, so the influence of some trace elements on cast iron is also reflected. Because the metallurgical reaction in the cupola is very strong, the charge is in a strong oxidizing atmosphere, most of which are oxidized and discharged together with the slag, and only a small part will remain in the molten iron, so some have an adverse effect on the casting. The trace elements pass through the metallurgical process of the cupola and generally do not adversely affect the formation of cast iron. In the smelting process of cupola, nitrogen in coke and nitrogen (N2) in air will dissolve into molten iron in the form of atoms at high temperature, so that the nitrogen content in molten iron is relatively high.

    According to statistics, since the electric furnace was put into operation, the number of scraps due to high lead content and the scrapped molten iron due to too high lead content cannot be adjusted, and the number of unqualified products due to insufficient nitrogen content is also quite high, causing the company great economic loss.

    On the basis of our years of experience and theory in electric furnace smelting, I believe that the key trace elements in the electric furnace smelting process are mainly N, Pb, and Ti. The influence of these elements on gray cast iron mainly includes the following aspects:

lead

    When the lead content in the molten iron is high (> 20PPm), especially in the interaction with the high hydrogen content, it is easy to form Widmanner's graphite in the castings with thick and large sections. This is because the resin sand has good thermal insulation performance, and the molten iron The cooling in the mold is slower, (the tendency is more obvious for thick and large sections) the molten iron is in the liquid state for a long time, and the molten iron solidification is closer to the solidification conditions in the equilibrium state due to the action of lead and hydrogen. When this type of casting is solidified and continues to cool, the carbon in the austenite will be precipitated and become secondary graphite in the solid state. Under normal conditions, secondary graphite only thickens the eutectic graphite sheet, which does not have a great impact on the mechanical properties. However, when the nitrogen and hydrogen content is high, the surface energy of the graphite on the same crystal plane of the austenite will be reduced, so that the secondary graphite will grow along a certain crystal plane of the austenite and extend into the metal matrix. Observed under a microscope, Many small graphite flakes like burrs grow on the side of the flake graphite flakes, commonly known as graphite long hairs, which is the reason for the formation of Widmanners graphite. Aluminum in cast iron can promote the absorption of hydrogen in the molten iron and increase its hydrogen content. Therefore, aluminum also has an indirect effect on the formation of Widmanner's graphite.