KEY POINTS IN SOLIDIFICATION PROCESS OF GRAY CAST IRON AND NODULAR CAST IRON (4)
- Morphology of primary AUSTENITE DENDRITE
The Crystal Lattice of austenite is face-centered cubic. When the AUSTENITE is nucleated and grown directly from the liquid iron, the AUSTENITE is stable only when the surface energy is minimum and octahedral crystal is formed. Then, because the concentration gradient of solute in the liquid iron at the edge of the ARRIS and angles of the crystal is large and easy to diffuse, and the growth rate of the ARRIS and angles is higher than that of the plane, the primary dendrite is formed, on the basis of which the secondary Dendrite is grown, and then the tertiary dendrite is grown Commonly known as AUSTENITE dendrites.
In practice, the primary austenite dendrite can grow into columnar or equiaxed crystal because of temperature difference, composition segregation, and heat flow. Columnar Crystals nucleate on the mold wall and grow in the opposite direction to the heat flow. The Nuclei are formed in the equiaxed crystalline liquid iron and grow in the direction of heat flow.
The AUSTENITE DENDRITE in cast iron is also characterized by imperfection and asymmetry, and the growth state of each dendrite and each part of a branch is different. In addition, there are also damaged and missing parts under the action of heat flow.
The main parameters describing the characteristics of Austenite dendrites are the spacing of Secondary Dendrites, the average length of Dendrites, the number and orientation of dendrites.
- Effect of primary AUSTENITE DENDRITE ON PROPERTIES OF CAST IRON
For Gray cast iron, the amount of primary austenite dendrite is an important factor that affects the mechanical properties. The volume fraction of Dendrite in cast iron increases, and the strength of cast iron increases.
For Ductile Iron, the number and spacing of primary austenite dendrites have an important influence on the shape, size, and distribution of graphite spheres. For example, if the DENDRITE spacing is large, there can be large graphite spheres between dendrites, and if the dendrite spacing is small, only small graphite spheres can be produced, because part of the graphite spheres is precipitated in the molten iron between dendrites. Therefore, it is necessary to control the quantity and morphology of primary austenite to control the quality of ductile iron better.
- Control of primary AUSTENITE DENDRITE
Many factors affect the quantity and morphology of primary austenite Dendrite in cast iron, such as chemical composition and temperature of molten iron, the cooling rate of molten iron in the mold, undercooling degree, the effect of inoculation and so on.
The carbon equivalent of cast iron is an important factor affecting the amount of primary austenite dendrite. The amount of AUSTENITE dendrite decreases with the increase of carbon equivalent. Under the same carbon equivalent condition, increasing Si / C ratio (increasing SI content and decreasing C content) can increase the number of primary austenite dendrites.
The amount and morphology of primary austenite are affected by the temperature of the molten iron, the holding time of the molten iron at high temperature, the cooling rate after pouring, and the supercooling degree during solidification These parameters are often determined by a variety of process requirements, by changing these parameters to control the degree of freedom of primary austenite Dendrite is not large.
In the past ten years, the research on primary Austenite Dendrite has been strengthened gradually, and the effect of inoculation on primary Austenite Dendrite has been paid more and more attention.
From the viewpoint of unidirectional nucleation, the primary graphite in hypereutectic cast iron can be used as heterogeneous nucleation of primary austenite Dendrite, and so it is. In hypoeutectic cast iron, the addition of crystalline graphite powder can be used as heterogeneous crystal nucleus of Austenite Dendrite, but the effect of fine graphite powder is difficult to control and stabilize because it is easy to dissolve in Molten Iron.
The addition of pure iron powder, as the homogeneous crystal nucleus of Austenite dendrite precipitation, should be the most effective, the problem is that the pure iron powder is easy to melt into the liquid iron, difficult to control.
Some research work in Japan shows that the primary austenite dendrites are easy to nucleation and precipitation when Fine powder-quartz or-cristobalite is added into liquid iron.
At present, the most widely used inoculation processes in foundry industry focus on the nucleation of graphite during the eutectic transformation of cast iron. How to improve it, so that we can not only control the eutectic change, but also control the precipitation of primary Austenite, is a subject worthy of serious study.
So far, our knowledge of primary austenite is not enough, and the degree of freedom of control is indeed not great. One reason for less research on primary austenite is that it is difficult to distinguish primary austenite dendrite from EUTECTIC austenite in cast iron after eutectoid transformation.
To understand the structure of initial solidification, liquid quenching is widely used in the early stage, and in recent years, the method of direct isothermal quenching from high temperature after solidification is widely used. However, it is not convenient to use these two methods in the process research of production enterprises. The “color metallographic technique” proposed by the professor Dalian University of Technology can show the solidification structure of cast iron by conventional metallographic means, which may be very suitable for the study of primary austenite dendrites.