About the Anomalous Properties of Blast Iron Without Graphite Emissions

Abstract

The development of modern materials science provides for the constant search for new materials with higher properties. Expensive iron-nickel alloys, the so-called Invars, are currently widely used as such materials. Speaking about the structure and properties of cast iron, the influence of gases (hydrogen, nitrogen and oxygen) on the formation of its structure and properties is most often ignored. In the practice of obtaining cast iron castings, to impart the desired properties to cast iron, the treatment of the melt with various substances is often used. In addition to affecting the chemical composition of the melt, its properties can be changed using other factors. One of the simplest in implementation and efficiency can be a change in the temperature of the melt and residence time at this temperature. To study the effect of thermal cycling on the linear expansion and microstructure of cast iron, two variants of melt processing were carried out. The first one consisted in heating to 1300 °С, holding for 15 minutes and cooling until a hard crust was obtained. The second one is heated to 1550 °С, held for 15 minutes and cooled to a temperature of 1350 °С. Both processes were repeated many times. The performed processing of the melt completely removes the precipitation of free graphite from the cast iron structure. Metallographic analysis of dilatometric samples made it possible to establish that thermal cycling leads to the formation of ledeburite. The effect of heat treatment on the linear expansion and microstructure of blast furnace iron after pretreatment of the melt was studied. The most effective was the chemical-thermal treatment (carburizing in the environment of the Bonduzhsky carburizer) followed by quenching. Carrying out carburizing in the environment of the Bonduzhsky carburizer leads to an increase in the values of the linear expansion coefficient in the region of low and high test temperatures. However, subsequent quenching allows one to obtain rather low values of the coefficient of linear expansion: 3,1·10–6, deg–1 at a temperature of 150 °С and 1,8·10–6, deg–1 at a temperature of 400 °С. Comparing the coefficient of linear expansion of blast furnace iron without precipitation of graphite with the coefficients of linear expansion of Invars in different temperature ranges, it can be seen that the coefficient of linear expansion after preliminary processing takes on rather low values.