Cast iron processing
Features of cast iron processing. Material properties, processing problems and ways to solve them.
One of the most common materials used in industry is cast iron. After all, it has good technological properties, and cast iron itself is a cheap material. Many parts are made of cast iron, such as engine cylinder blocks, pump housings and valves. Cast iron is used where a part of complex shape and sufficient strength is needed.
Cast iron is a multicomponent, iron-carbon alloy with a carbon content above 2%. Alloys that contain less than 2.14% carbon are called steel. It may also contain some manganese (Mn), phosphorus (P) and sulfur (S). Depending on the type of graphite, cast iron is divided into gray cast iron, cast iron with spherical graphite, ductile and alloyed cast iron. Alloying cast iron with nickel, copper, molybdenum and chromium, for example, increases its heat and corrosion resistance, viscosity and strength. Alloying elements are divided into carbide-forming and graphite-forming. Alloying significantly affects the machinability of cast iron.
If we distinguish the main types of cast iron, then these will be:
- White cast iron. In white cast iron, carbon is present mainly in a bound form in the form of cementite (Fe3C). White cast irons have high strength and brittleness, and they are very abrasive, they are difficult to machine, so castings are obtained from them that do not require processing and work under conditions of abrasive wear with dry friction, white cast iron is also used to produce ductile iron.
- Grey cast iron. An alloy of iron, silicon (1.2-3.5%) and carbon. In gray cast iron, carbon is present mainly in the form of graphite inclusions of various sizes and shapes. Grey cast iron has good technological and strength properties, so they have found wide application as a structural material. It is more fragile, but has good thermal conductivity, emits less heat during cutting, absorbs vibration well, therefore it has found application in the production of engines.
- High-strength cast iron. Cast iron with spherical graphite. Carbon is predominantly or completely present in the free state in the form of spherical graphite. It has high strength and satisfactory ductility, can sometimes be used instead of steel. This cast iron has good rigidity, toughness, it is durable, not brittle. But it can not absorb vibrations, low thermal conductivity, as a result of a large heat release.
The main difficulties in processing cast iron are abrasive wear and high temperature. The machinability of cast iron by cutting depends on the chemical composition, physical and mechanical properties and many other factors. Machinability improves with an increase in the graphite content, dispersion and uniformity of the distribution of structural components. Alloying elements create such dispersed and uniform structures, therefore, the machinability of alloyed cast iron is higher than conventional cast iron, of course, provided the hardness is the same. Since, for example, alloying with chromium (Cr) leads to an increase in hardness and deterioration of workability.
The value of the cutting modes of cast iron is influenced by such factors as: the grade of cast iron, the rigidity of the technological system used cutting tool, etc. Therefore, it is necessary to follow the instructions of the tool catalog, carefully taking into account the durability, since the catalogs show cutting modes for maximum performance and minimum durability, which may not meet your goals.
To sum up, then:
- Most cast irons, due to the presence of graphite in the structure, are easily processed by cutting, since graphite allows you to get short “loose” chips and improves the lubrication of the cutting edge.
- For the processing of cast iron, a tool with a negative or a small positive value of the front angle is mainly used.
- Coated tools have greater durability due to the predominance of abrasive wear during processing
- For continuous turning, it is necessary to use coolant, for intermittent turning, it is better not to use coolant.
- The main difficulties in processing are the uneven allowance on the castings, the presence of a casting crust and sand inclusions
- When processing high-strength cast iron, the tool consumption is three times greater than when processing gray cast iron.
- Ductile iron processing requires three times more time, because it is required to underestimate the cutting modes. Accordingly, the tool consumption increases. In this cast iron, graphite in the form of flakes surrounding ferrite, as well as more silicon (Si) than in gray, which determines increased wear of the plates.
Information used: recommendations of manufacturers of carbide tools Kennametal, Dormer, Pramet, Sandvik.