Materials Glossary
ISO 513 sorts every workpiece material a machinist is likely to touch into six groups — P, M, K, N, S, and H — based on how the material actually behaves in the cut. This page focuses on what defines each group's machining character: how the chip forms, the hardness range you're dealing with, and how the material handles heat. For how these groups map to picking an actual insert grade, see our Carbide Grades page.
ISO 513 exists because "steel" or "stainless steel" alone doesn't tell a toolmaker enough to publish a reliable speed, feed, or edge geometry. The standard groups materials by how they actually cut — chip type, hardness, heat behavior — so a color and a letter on a package can stand in for a whole set of machining characteristics. Each group gets a code letter and a color, both used across catalogs, inserts, and grade charts industry-wide.
The largest and most common group, from unalloyed carbon steel to high-alloy tool steel. Chip formation is generally a continuous, ribbon-like chip that's straightforward to control with a chipbreaker. Hardness and machinability vary a lot within the group depending on carbon and alloy content, but as a rule steel doesn't run especially hot in the cut and behaves predictably.
Alloyed with chromium (minimum ~12%) and often nickel and molybdenum, in ferritic, martensitic, austenitic, or duplex structures. Austenitic grades in particular are gummy and tend to smear rather than shear cleanly, producing long, stringy chips. Stainless work-hardens readily and holds heat at the cutting edge, which drives notch wear and built-up edge if speeds and feeds aren't dialed in.
An iron-carbon-silicon material where graphite forms in flakes or nodules. Unlike the ductile groups, cast iron produces short, brittle, powdery chips that break on their own rather than curling away — which is part of why it's often considered comparatively easy to machine. The graphite and silicon content is abrasive to the cutting edge even though the chip itself is soft.
Aluminum, copper, brass, and similar soft, non-ferrous metals. These typically produce long, continuous chips and allow much higher cutting speeds than any ferrous group, since the material shears easily and generates comparatively little heat. High-silicon aluminum is an exception — it's abrasive despite the group's generally easy-cutting reputation.
Heat-resistant superalloys and titanium alloys. Chips form under high shear with a thin, highly-loaded contact zone against the tool, and the material holds its strength and generates concentrated heat right at the edge instead of dissipating it. Low thermal conductivity and rapid work hardening make this the toughest group to cut — see our HRSA page for the full picture.
Steel hardened into roughly the 45–65 HRC range, along with chilled cast iron. Chips are short and segmented, hardness alone makes the material abrasive to the edge, and cutting generates significant localized heat. This group typically calls for hard, wear-resistant tooling built for hard-part turning rather than conventional roughing grades.