What is H material group?
There is a direct correspondence between some of the ISO P and ISO H groups. Depending on the material’s condition, the material may belong to an ISO P group or an ISO H group. Materials from P2 & P3 may be found in H3 after hardening, P4 & P5 may be found in H5 and P6 and P7 may be found in H7 after hardening.
Hardened steel is used to make metal objects that are subject to high levels of force or abuse. For example, some automotive parts are made from this material, particularly those used for trucking. Hand tools and drill bits often consist of this type of steel, as do many knives.
One of the primary advantages to hardened steel is its added resistance against wear and abrasion. This material can withstand frequent abuse and heavy loads without damage or failure. It is also better able to resist rust and corrosion than standard steel products.
ISO H 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 H materials, e.g. hardened steel machining:
- Very high mechanical loads are your main concern (leading to flank and crater wear and breakage of cutting edges)
- Use small to medium depth of cut and feed
- Use cutting speed to balance tool life with economic considerations on the process
- Use specific carbide grades or PCBN cutting materials and strong cutting geometry
- Emulsion (4% – 6%) cooling is advised
ISO H Tab extract
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