PMPA Craftsman Cribsheet #139: ISO Material Groups, Hardened Materials

Published May 1, 2025

By David Wynn, Technical Services Manager, PMPA

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Hardened materials are very difficult to machine. Hardness in materials means “resistance to penetration.” Finding tools that are harder than the material being machined becomes increasingly challenging as hardness levels increase. Coming from the older high strength steel (HSS) perspective, these are grind-only hardnesses. The advent of new technologies has made hard machining more accessible and increased its popularity. Hardened materials are typically classified into four categories, which are then divided by the material hardness range. The same material can often be hardened (then tempered back) to multiple levels of hardness, allowing it to be placed in multiple categories on this list. ISO material groups is what we use for identification of material we are cutting with carbide inserts.

Machinability is essentially meaningless for these categories of materials. There are tool geometries and grades of carbide that achieve high surface feet per minute (sfm) in hard materials. It is all about cutting parameters. For instance, one company makes a milling cutter that can achieve 375 sfm at 0.0002-inch feed rate at a 5% of diameter (of the mill) with depth of cut in materials of 65-70 HRC.