What Is The Primary Factor Affecting The Machinability Of Materials?
First of all, the chemical composition of steel is very important. The higher the alloy composition of steel, the harder it is to process. As the carbon content increases, the metal cutting performance decreases.
Second, the structure of the steel is also very important for metal cutting performance. Different structures include: forged, cast, extruded, rolled and machined. Forgings and castings have very difficult to machine surfaces.
Third, hardness is an important factor affecting metal cutting performance. The general rule is that the harder the steel, the harder it is to process. High speed steel (HSS) can be used to process materials up to 330-400HB; high speed steel + titanium nitride (TiN) coatings can process materials up to 45HRC; for materials with hardness 65-70HRC, Carbide, ceramic, cermet and cubic boron nitride (CBN) are used.
Fourth, non-metallic inclusions generally have an adverse effect on tool life. For example, Al2O3 (alumina), which is a pure ceramic, has a strong abrasiveness.
Fifth, the last one is residual stress, which can cause metal cutting performance problems. It is often recommended to perform a stress relief process after roughing.
In general, the higher the hardness and strength of cast iron, the lower the metal cutting performance and the lower the expected life of the insert and tool. Most types of metal cast iron used in metal cutting production generally perform well. Metal cutting performance is related to structure, and harder pearlitic cast iron is more difficult to process. Flake graphite cast iron and malleable cast iron have excellent cutting properties, while ductile iron is quite bad.
The main types of wear encountered when machining cast iron are abrasion, bonding and diffusion wear. Abrasive is mainly produced by carbides, sand inclusions and hard cast skin. Bond wear with built-up edge occurs at low cutting temperatures and cutting speeds. The ferrite portion of cast iron is the easiest to weld to the insert, but this can be overcome by increasing the cutting speed and temperature.
On the other hand, diffusion wear is temperature dependent and occurs at high cutting speeds, especially when using high strength cast iron grades. These grades have high resistance to deformation and result in high temperatures. This wear is related to the interaction between the cast iron and the tool, which allows some cast irons to be machined at high speeds using ceramic or cubic boron nitride (CBN) tools for good tool life and surface quality.
Typical tool properties typically required for machining cast iron are high heat hardness and chemical stability. However, it is also related to the process, workpiece and cutting conditions, requiring the cutting edge to have toughness, heat fatigue wear and edge strength.
The degree of satisfaction with cutting cast iron depends on how the wear of the cutting edge develops. Rapid bluntness means hot cracks and nicks that cause premature fracture of the cutting edge, damage to the workpiece, poor surface quality, excessive waviness, and the like. Normal flank wear, balance and sharp cutting edges are just what you need to do.