What is the key to the strength of cast iron motors

About Cast Iron

To talk about gray cast iron, one must first understand cast iron. Cast iron is divided into white cast iron and gray cast iron according to the different forms of carbon.

A small amount of carbon in white cast iron exists outside the ferrite, while the vast majority exists in the form of Fe3C (cementite) in cast iron. The fracture surface is silver white, hard and brittle, difficult to cut, and the casting that cannot be machined in some areas is actually similar to white cast iron.

Grey cast iron exists entirely or mostly in the form of free carbon (graphite), with a gray fracture surface, making it the most widely used type of cast iron.

Overview of Grey Cast Iron

The structure of cast iron includes a metal matrix and graphite. Different degrees of graphitization result in three different structures of gray cast iron.

The carbon in ferritic gray cast iron is entirely formed into graphite, which is composed of a ferritic matrix and graphite.

Ferrite pearlite gray cast iron is composed of pearlite, ferrite matrix, and graphite.

Pearlite gray cast iron is composed of a pearlite matrix and graphite.

Pearlite gray cast iron has the highest strength and is also widely used; Ferritic gray cast iron has low strength and is rarely used.

The grade of gray cast iron is represented by HT, and the suffix number represents the minimum tensile strength. Such as HT100, HT150, HT200, etc.

From a microscopic perspective, gray cast iron is actually a steel structure mixed with flake graphite.

Magical graphite

The presence of graphite endows cast iron with excellent machinability, friction reduction, and shock absorption properties. But graphite itself has almost zero strength, which is equivalent to many small cracks in the cast iron matrix structure, cutting off the matrix structure. In addition, the tip of graphite causes stress concentration, greatly reducing the mechanical strength of cast iron. The degree of reduction is related to the shape, size, and distribution of graphite. Therefore, various methods are widely used in modern casting production to change the shape, size, and distribution of graphite, reduce its harmful effects, and greatly improve the mechanical properties of cast iron. The cultivation of cast iron, malleable iron, and ductile iron is the result of this effort.

Before pouring liquid cast iron, inoculants or modifiers such as silicon iron and silicon calcium alloy are added to the cast iron to become the core of graphite crystallization during solidification, obtaining small and evenly distributed graphite flakes and improving the mechanical properties of cast iron. To ensure the formation of pearlite matrix in cast iron, the carbon and silicon content should be less than 2.5%.

Chemical composition and its influence of gray cast iron

Carbon and silicon are elements that promote graphitization. The higher the content of carbon and silicon, the more graphite there is in cast iron, and the less cementite there is. To ensure the presence of a certain amount of graphite in cast iron and prevent the formation of white cast iron, a certain amount of carbon and silicon must be present. The carbon content in gray cast iron is 3.2-4%, and the silicon content is 1-3%.

Sulfur hinders graphitization, reduces fluidity, and increases thermal brittleness, and must be controlled below 0.15%. Manganese also hinders graphitization, but the effect is not significant. It can form MnS with sulfur, reducing the harmful effects of sulfur. The manganese content in cast iron is 0.6-1.3%.

Phosphorus has little effect on graphitization, but it can change the fluidity of cast iron. Excessive phosphorus content can make castings brittle, generally controlled below 0.3%.