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ISO K

What is the ISO K Material Group?

The ISO K material group consists mainly of cast irons, which are metals that, in addition to iron, contain five key alloying elements: carbon (C), silicon (Si), manganese (Mn), phosphorus (P), and sulfur (S). Of these, carbon, silicon, and manganese have the greatest impact on the tensile strength (UTS) and Brinell hardness (HB) of cast iron. Higher contents of carbon and silicon typically result in higher UTS and HB values. Compared to steel, cast irons have lower strength and ductility, but they provide significant advantages in many applications, such as cost-effectiveness, excellent damping properties, and wear resistance.

Why are K Materials Important?

Cast irons offer many advantages over other metal alloys. Their low cost in large production runs, ability to be cast into complex shapes, and excellent damping properties make them highly popular for various industrial applications. Cast irons are also known for their good wear resistance, making them well-suited for components that are exposed to wear and corrosion.

Key Features of the K Material Group

Cast irons can be divided into subgroups based on their microstructure and the form of graphite:

Microstructure and Graphite Shape:

  • Lamelar (flake graphite): The flake graphite structure gives gray cast iron excellent machinability and good vibration-damping properties.
  • Vermicular (compact): The compact graphite structure provides better strength and toughness compared to the flake structure, but with slightly reduced machinability.
  • Nodular (spheroidal graphite): The nodular graphite structure (ductile cast iron) offers the best mechanical strength and toughness, but is harder to machine than flake graphite cast iron.

Matrix Type:

  • Austenitic: Offers better ductility and corrosion resistance.
  • Martensitic: Hard and wear-resistant.
  • Malleable: Good ductility, strength, and machinability.

Examples of the Most Common Cast Irons in the ISO K Material Group

Gray Cast Iron (GCI):

  • Properties: Good machinability, excellent vibration damping, moderate strength.
  • Applications: Engine blocks, machine frames, brake discs.

Ductile Cast Iron (NCI):

  • Properties: High strength and toughness, excellent wear resistance, moderate machinability.
  • Applications: Cylinder blocks, gearbox housings, gears.

Austempered Ductile Iron (ADI):

  • Properties: Excellent strength and toughness, good wear resistance, more difficult to machine.
  • Applications: Machine parts, shafts, bearings.

Industry Segments and Component Applications

As members of the ISO K material group, cast irons offer a wide range of applications, particularly due to their excellent castability, shape retention, and wear resistance. Here are some key industry segments and example applications:

Automotive Industry

Applications:

  • Cylinder blocks: Good thermal conductivity and wear resistance.
  • Cylinder heads: Corrosion resistance and good machinability.
  • Gearbox housings: Excellent wear resistance and mechanical strength.

Machine Building

Applications:

  • Machine frames and components: Good wear resistance and vibration damping.
  • Machine tool parts: Good machinability and dimensional stability.
  • Brake discs: Good wear and heat resistance.

Motor Manufacturing

Applications:

  • Cylinder blocks: Good wear resistance and strength.
  • Cylinder heads: Heat resistance and corrosion resistance.

Other Industry Segments:

  • Pipelines and water systems
  • Pipes and fittings: Corrosion and deformation resistance.

Marine Industry

  • Marine engines and pump parts: Corrosion resistance and machinability.

Machinability and Weldability

Machinability: Cast irons are renowned for their good machinability, especially gray cast iron with flake graphite, which offers excellent chip-breaking capabilities. Ductile cast irons and austempered cast irons require more attention to machining parameters and tool selection.

Weldability: Cast irons are generally less weldable compared to steels. Weldability can be improved with alloying elements and preheating. For example, ductile cast iron is more weldable than gray cast iron.

Tips for Machining ISO K Materials

  • High tribological loads: High tribological loads are the main concern (leading to flank and crater wear). Use durable tools to reduce wear.
  • Appropriate depth of cut and feed: Use high depths of cut and feed rates to achieve the best machining efficiency and tool life.
  • Balanced cutting speed: Balance cutting speed for optimal tool life and process cost-efficiency.
  • Use of specific carbide grades: Employ thick and wear-resistant carbide grades (in mass production, PCBN tool materials may be the best option). Unchipped inserts are the basic choice.
  • Cooling: Dry machining is possible, but emulsion cooling should be used for health reasons (graphite dust).

Main Material Traits

Cast irons are Fe-C (iron–carbon) alloys with a relatively high amount of silicon (1–3 % Si). Their carbon content is usually above 2 %. Chromium (Cr), molybdenum (Mo), and vanadium (V) form carbides that increase the strength and hardness of the iron but decrease its machinability.

Cast irons are categorized into five main types:

  • Gray cast iron (GCI)
  • Malleable cast iron (MCI)
  • Ductile cast iron (NCI)
  • Compacted graphite iron (CGI)
  • Austempered ductile iron (ADI)

General Machinability

Cast irons tend to form short chips, and chip control is typically good in most conditions. The specific cutting force ranges from 790–1350 N/mm. High cutting speeds result in abrasive wear, particularly with irons that contain sand inclusions. The mechanical properties and graphite content of ductile, compacted, and austempered irons require special attention compared to standard gray iron. Often, cast irons are machined dry, but wet machining is also possible—mainly to control carbon and iron dust.

The Effect of Hardness

The effect of hardness on cast iron machinability follows the usual rules. For example, austempered irons (ADI), compacted graphite irons, and ductile irons can have hardness up to 300–400 HB. Malleable cast irons and gray iron generally have a hardness of 200–250 HB. White cast iron can have a hardness above 500 HB, where the carbon is not free but is present as cementite (Fe3C). White irons are extremely abrasive and difficult to machine.

Types and Their Machinability

Malleable Cast Iron K 1.1–1.2 and Gray Cast Iron K 2.1–2.3:

  • Definition: Malleable cast iron is made from a white iron-like structure by heat treatment, resulting in a structure composed of ferrite, pearlite, and temper carbon. Gray cast iron contains flake graphite.
  • Most common workpieces: From malleable cast iron – shaft bearings, wheel rims, pipe fittings; from gray cast iron – engine blocks, compressors.
  • Machinability: Both have good brittleness, and machinability is especially excellent in gray cast iron.

Ductile Cast Iron K 3.1–3.5:

  • Definition: Ductile cast iron contains spheroidal graphite inclusions that improve strength and toughness.
  • Most common workpieces: Crankshafts, tandem shafts, turbocharger housings.
  • Machinability: Strong tendency for built-up edge formation, especially in softer grades. In harder grades, abrasive wear is more likely.

Compacted Graphite Iron K 4.1–4.2:

  • Definition: Compacted graphite iron features good strength-to-weight characteristics and moderately good machinability. Absorbs less engine vibration than gray or ductile iron.
  • Most common workpieces: Engine manufacturing, cylinder heads, brake discs.
  • Machinability: Machinability lies between that of gray and ductile iron. Use suitable tool geometries for the best results.

Austempered Ductile Iron K 5.1–5.3:

  • Definition: Austempered iron is heat treated for high strength, toughness, and fatigue resistance.
  • Most common workpieces: Automotive industry – suspension and transmission components.
  • Machinability: Tool life is shorter compared to ductile iron. Work hardening generates high cutting forces.

Summary

Cast irons classified under the ISO K material group are widely used due to their many advantages. They offer affordable cost, good wear resistance, and excellent vibration damping, which makes them popular in numerous industrial applications. Key uses include engine blocks, machine frames, brake discs, and piping. Machinists and machine shops need to be familiar with the specific features and machining requirements of these materials to ensure optimal performance and high-quality end results. The machining of cast irons demands the use of proper tools and cutting parameters for successful outcomes.