What is M material group?
Despite being replaced by ferritic stainless steel for non-critical applications, austenitic stainless steel is the most common choice when corrosion is a concern. There is a wide range of austenitic grades optimized to resist corrosion in various environments. Increased contents of Cr, Mo and N tend to improve corrosion resistance but at the same time reduce machinability. Alloying additions of Ti may contribute to abrasive wear of tools.
Duplex stainless steels consist of approximately equal amounts of ferrite and austenite. They combine corrosion resistance with a higher strength compared to the austenitic grades, which means that less material can be used and weight can be reduced. They also contain less nickel than corresponding austenitic grades which is often a cost benefit.
The austenitic and duplex stainless steels cannot be hardened by quenching like carbon steels but instead they harden by deformation. Cold formed parts, sheared edges and machined or shot-blasted surfaces are therefore considerably harder than the bulk material. Additions of sulphur (exceeding 0.020%) may increase the machinability of these materials considerably but may also affect other properties like corrosion resistance, weldability and hot ductility. For this reason bar material is more often treated for improved machinability than e.g. plate material. Stainless steels with improved machinability often bear brand names like Sanmac, Prodec or Ugima.
Often used in applications that place a limited demand on corrosion resistance. The ferritic material is relatively low-cost due to the limited Ni content. Examples of applications are:
- shafts for pumps,
- steam and water turbines,
- hot water heaters,
- pulp and food processing industries, due to lower requirements on corrosion resistance.
ISO M Pentagram
Seco classifies the machinability of materials based on 5 important properties: abrasiveness, ductility, strain hardening, thermal conductivity and hardness.
ABRASIVENESS, is defined as variations in hardness caused by alloying elements that are able to form hard carbide, oxide and intermetallic particles. This results in excessive wear on the cutting edges. Some examples of highly abrasive materials are Ni‑ alloys and carbon fibre reinforced plastics.
DUCTILITY, which results in adhesion and built‑up edge, refers to a high elongation at fracture for a material. This is one of the key difficulties in machining aluminiums and titanium alloys.
STRAIN HARDENING occurs when cutting hardens a surface in comparison to the bulk material. This is a well‑known challenge to overcome when machining Ni‑based alloys.
THERMAL CONDUCTIVITY refers to heat conduction of the material. The lower the thermal conductivity of a workpiece material, the more the heat will concentrate on the cutting edge, which results in excessive cutting edge temperatures.
HARDNESS is a material’s resistance to deformation. The higher the hardness, the greater the force needed to deform the material. High hardness results in high heat generation as well.
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Basic guidelines for machining ISO M materials, e.g. stainless steel machining:
- High thermal loads and hard surface scales are your main concern (leading to complex flank and crater wear, notch wear, plastic deformation)
- Use big depth of cut and high feed
- Use cutting speed to balance tool life with economic considerations on the process, but avoid build-up edge window of cutting speeds
- Use dedicated carbide grades and appropriate cutting geometry to balance with selected feed
- Rich emulsion (8% – 12%) cooling is advised, JETSTREAM gives excellent results
ISO M Tab extract
Inline Content - Survey
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