Group M materials include stainless and heat resistant steels.
When machining this type of material shear type chips are generated and the cutting edge is subject to severe impact and vibration.
Machinability of workpiece materials
The features of stainless steel include adhesiveness, high work hardening properties and low thermal conductivity.
Stainless steel has relatively poor machinability because of its high cutting resistance and the heat generated during machining, which causes obvious tool damage such as chipping and crater wear. Stainless steel also has high affinity to tool materials, lead to problems such as welding and crater wear. In addition, oxidation damage and notch wear are also typical types of tool damage seen when machining stainless steel. Notch wear can also cause burrs on the machined surface.
Engine valves, an example
of Group M work.
When machining group M materials, it is necessary to select a sharp cutting edge tool in combination with a high heat resistance tool grade. This is to reduce cutting heat and prevent associated problems.
Furthermore, to prevent tool notching and burrs on the machined surface, it would be better to choose a cutting edge with a large rake angle that provides additional sharpness to reduce the cutting resistance and temperatures.
When the chips break the cutting edge is subject to severe vibrations. This means cutting edge strength also has to be taken into consideration when selecting cutting tools. As mentioned, there is a need to try to reduce the cutting resistance. In order to do that a chip breaker that does not forcefully break the chips was used. However, with recent developments such as near net shape workpieces, the machining allowance has reduced significantly. This therefore means that effective chip control becomes an issue and a chip breaker that easily breaks chips needs to be selected.
Types of stainless steel
Stainless steels are classified into various types according to their composition and features. General classification is based on the alloy elements and structure. There are five main groups of stainless steels. Austenitic, ferritic, martensitic and duplex and precipitation-hardened are both derived from austenitic.
Among stainless steels, precipitation-hardened stainless steels with a high tensile strength are generally said to have the poorest machinability. These are followed in degree of difficulty to cut by duplex and austenitic stainless steels.
Martensitic stainless steels and ferritic stainless steels are not so difficult-to-cut materials, even when compared to general steel. It is considered that among stainless steels in the same group, a higher carbon content allows easier machining.
Heat resistance steel
Heat resistant steel is material with an increased content of heat-resistant elements such as nickel. This means that the thermal conductivity is low and allowances should be taken for the extra heat generated during machining. Materials whose composition contains more than 50% of alloyed elements such as nickel are called heat-resistant alloys or superalloys and are is classified into the S classification group, not M.