ISO Material Group

What is ISO Material

There is a wide variety of materials used in the metal cutting and CNC cutting industry, based on the ISO standards, the workpiece materials can be divided into six groups. Each class of metal or alloy is indicated by a code and color. The ISO classification divides the workpiece materials into six categories, mainly based on the dominant physical properties of the different materials. These properties are determined by the alloying elements, heat treatment, etc.

Workpiece material groups are used to help determine the correct cutting tool and starting values for machining conditions in particular applications. The material category can be identified by the letter or color.




Stainless Steel


Cast Iron


Non-ferrous metals


HRSA and Titanium


Hardened Steel

ISO Material Groups - Workpiece Material Groups

– P (Steel): the most common material group, ranging from unalloyed to high alloyed material, including steel castings, ferritic and martensitic stainless steels. Machinability is normally good but differs depending on material hardness and content.

– M (Stainless Steel): stainless steel are materials alloyed with a minimum of 12% chromium, other alloys can be nickel and molybdenum. Different conditions such as ferritic, martensitic, austenitic, and austenitic-ferritic, make this a large family. They all expose cutting edges to a great deal of heat, notch wear and built-up-edge.

– K (Cast Iron): cast iron is a short-chipping type of material. Grey cast iron (GCI) and malleable cast iron (MCI) are quite easy to machine, while others including nodular cast irons (NCI), compact cast irons (CGI) and austempered cast irons (ADI) are more difficult. All cast irons contain silicon carbide, which is very abrasive to the cutting edge.

– N (Non-Ferrous): non-ferrous metals are softer types of metals such as aluminum, copper, brass, etc. Aluminum with a silicon content of 13% is very abrasive. Generally, high cutting speeds and long tool life can be expected.

– S (HRSA and Titanium): HRSA refers to heat-resistant superalloys, including a great number of high-alloyed iron, nickel, cobalt, and titanium-based materials. They are sticky, create built-up-edge, work hardening and generate heat. They are difficult to cut and have a short tool life.

– H (Hardened Steel): this group covers steels with a hardness between 45-65 HRC and also chilled cast iron around 400-600 HB. The hardness makes them difficult to machine. The materials generate heat during cutting and are abrasive to cutting edges.

ISO Material Classification Chart

Just dividing the material into six different groups does not provide enough information to select the correct tool and cutting parameters. Therefore, materials can be further subdivided into the following subgroups based on their structure, composition, etc.


Steel is Iron (Fe), with the addition of 0.1 – 2.5 wt. % of Carbon (C). Besides carbon, steel may also contain many other alloying elements up to a total content of around 20%. Pure Iron is very soft. Iron becomes steel by “playing” with the mix and amount of the different alloying elements.

Steel Sub-Groups:

  • Carbon Steel: Carbon steel is the most simple and cheapest form of steel. It is usually defined as steel made of Iron (Fe) with small Carbon (C) addition but without any other alloying elements. The subgrouping is done according to the carbon content. —machinability 50%-80%.
  • Free-Cut Steel:Free-Cutting Steel is a nickname for carbon steel with an increased amount of Sulfur, Lead, and Phosphorus for the sole purpose of improving their machinability. —Machinability 70%-170%.
  • Low Alloy Steel: There is no scientific definition, but in practice, low allow steels are carbon steels with additional alloying elements (on top of the carbon and Manganese) of up to 5%. These elements are added to improve strength, toughness, corrosion resistance, wear resistance, and hardenability. —Machinability 40%-75%.
  • High Alloy Steel (Tool Steel): High alloy steels can contain 5-20% content of alloying elements. They are called tool steels since they are mostly used to manufacture tools for cutting, pressing, extruding, and coining. —Maintainability 20%-50%.


Stainless steels, as their name suggests, are a group of steel alloys with a shiny appearance and good corrosion resistance. The base element (70-80%) is Iron (Fe) with a minimum of 10.5% Chromium; most grades will have additional alloying elements such as nickel (Ni) and molybdenum (Mo).

Stainless Steel Sub-Groups:

  • Austenitic Stainless Steel: High Chromium content, up to 20% with the addition of Nickel up to 15%. Due to the high nickel content, It has much better corrosion resistance, but it is the hardest to machine. —Machinability 35%-75%.
  • Martensitic / Ferritic Stainless Steel: Characterized by Chromium content up to 18% with almost no nickel. It is the only group of stainless steel that can be heat-treated and therefore is the best. However, it has the least resistance to corrosion.—Machinability 40%-75%.
  • Duplex Stainless Steel: This sub-group is called Duplex since these materials have a two-phase Austenitic – Ferritic structure. They are designed to provide higher corrosion resistance and tensile strength. They can have Chromium (Cr) content up to 30% and Nickel (Ni) up to 9%. —Machinability 20%-30%.


Cast Iron is a group of Iron- Carbon-Silicone materials with 1.8-4% carbon (C) and 1-3% of Silicon (Si) content. The silicon pushes part of the carbon out of the solution, forming graphite flakes. The material is brittle at room temperature but has a low melting point and thus has excellent castability. Each subgroup contains different alloys at a wide range of hardness. The hardness has a major effect on machinability, and therefore we compare all the subgroups at 230 HB.

Cast Iron Sub-Groups:

  • Grey Cast Iron: Characterized by its graphitic flakes that give the material its grey appearance. It has lower toughness than steel; however, it has good resistance to plastic deformation and is therefore widely used for housing components.—Machinability 105% (at 230 HB).
  • Nodular cast iron: The graphite is formed in round nodules, hence the name nodular. Unlike grey cast iron, it is a tough material with good fatigue resistance. Its mechanical properties are equivalent to plain carbon Steel. —Machinability 95% (at 230 HB).
  • Malleable cast iron: Formed in a slow and long heat treatment process (Several days), yielding a much tougher material, with lower sensitivity to cracking and better impact resistance (compared to grey and nodular iron). —Machinability 85% (at 230 HB).


Superalloys are a group of materials engineered to have very high strength and superb corrosion resistance. Some of them also preserve these properties at very high temperatures and chemically hostile environments. They are mainly used in jet engines, turbines, oil&gas equipment, and medical implants.

Super Alloys (HRSA) Sub-Groups:

  • Nickel (Ni) based superalloys: The main feature is excellent strength across a wide temperature range combined with good corrosion resistance. HRSA’s are used mainly for aircraft jet-engine parts and in the oil & gas industry. —Maintainability 10%-40%.
  • Iron-based superalloys: A more economical alternative to nickel-based alloys. They provide the same advantages but to a lesser extent and at a lower price. They are used mostly on less critical components that still require heat resistance properties. —Maintainability 25%-50%.
  • Cobalt-based superalloys: Excellent in their wear resistance and chemical stability in harsh and hot conditions. Therefore, they are mainly used in valves and fittings in an acidic environment and medical implants such as artificial hip joints. It is the most difficult to machine superalloy sub-group. —Maintainability 6%-20%
  • Titanium-based superalloys: Excellent strength-to-weight ratio. On the one hand, it is almost as light as aluminum, and on the other hand, it has a higher strength than most steel alloys. On top of that, it has superb corrosion resistance. This combination makes it popular in aerospace components and medical implants. —Maintainability 15%-20%.

ISO Class Subgroup Material
P-Steel P1 Free Cutting steel and structural steel
P2 Carbon steel and low alloy steel
P3 Medium alloy and heat treated steel
P4 High alloy steel
P5 Tool steel
P6 High tensile strength steel
H-Hardened Steel H1 Hardened steel
H2 Hardened bearing steel
H3 Hardened tool steel
H4 Hardened martensitic stainless steel
H5 Hardened white cast iron
M-Stainless Steel M1 Ferritic stainless steel
M2 Austenitic stainless steel(good machinability)
M3 Austenitic stainless steel(medium machinability)
M4 Martensitic stainless steel
M5 PH stainless steel
S-Heat resistant super alloy & titanium S1 Heat resistant super alloys (good machinability)
S2 Heat resistant super alloys (medium machinability)
S3 Heat resistant super alloys(low machinability)
S4 Low alloy titanium(good machinability)
S5 High alloy titanium(medium machinability)
K-Cast iron K1 Grey cast iran
K2 Nodular cast iron
K3 Austenitic cast iron
K4 ADI cast iron
N-Non Ferrous N1 Aluminum alloys < 12% Si
N2 Aluminum alloys > 12%Si
N3 Copper
N4 Bronze and brass
N5 Plastic materials
N6 Fiber and composites