Stainless Steel Types

A short video explaining the difference between Austenitic, Martensitic, Ferritic, and Duplex Stainless steel

Property Austenitic Martensitic Feritic PH Duplex
Corrosion Resistance Excelent Fair Good Good Excelent++
Magnetic? No Yes Yes No No
Heat Treatable? No Yes No Yes No
Machinability 35-75% 40-75% 40-75% 40-50% 20-30%
Hardness ~180 Max 600 ~200 Max 400 ~280
Strength [Kpsi] ~90 ~120 ~100 ~200 ~250
Cr 16-20% 11-14% 11-18% 14-17% 18-30%
Ni 6-15% 0-2% 0-1% 4-8% 4-7%
Mo 2-4% - 0-1.2% 1.5-2.5% 0-5%

Austenitic Stainless Steel

Austenitic is the most popular family of Stainless steel and is characterized by high Chromium content, up to 20%, with the addition of Nickel of up to 15%. Due to the high nickel content, It has better corrosion resistance, but it is harder to machine. It lacks strength and hardness compared to other types of Stainless Steel. Most alloys in this series have low carbon content, below 0.1%. This makes them ductile; therefore, chip control and BUE are significant concerns for machinists. Alloys with the suffix “L” (For example 304L / 316L), have minimal carbon content (Usually 0.03%), which makes them even more problematic for machining.

Martensitic Stainless Steel

It is the second group in terms of popularity, characterized by Chromium content of up to 14% with almost no nickel. This group of alloys can be heat-treated and hardened, therefore providing higher strength. However, it has corrosion resistance only in atmospheric conditions and cannot be used in harsh environments.

  • Corrosion Resistance: Moderate.
  • Magnetic: Yes.
  • Heat Treatable: Yes.
  • Chromium content: 11-14%
  • Nickel content: 0-2%
  • Molybdenum content: None.
  • Typical max Hardness: 600 HB (After heat treatment).
  • Typical Tensile Strength: 120 [Kpsi].
  • Popular materilas: SAE 420 / 440.
  • Typical parts: Razor blades, Surgical instruments, and other parts that require higher strength but are less critical in terms of corrosion resistance.

Ferritic Stainless Steel

Ferritic stainless steel materials have a Chromium content of up to 18% with almost no nickel. They have better corrosion resistance than Martensitic grades but less than Austenitic ones. It cannot be hardened by heat treatments.

  • Corrosion Resistance: Good – Moderate.
  • Heat Treatable: No.
  • Magnetic: Yes
  • Chromium content: 11-18%
  • Nickel content: 0-1%
  • Molybdenum content: 0-1.25%.
  • Typical max Hardness: 200 HB.
  • Typical Tensile Strength: 100 [Kpsi].
  • Popular grades: 409 / 430.
  • Typical parts: Auto exhausts, grills, coffee machine parts, and other household appliances.
  • Corrosion Resistance: Good – Moderate.
  • Heat Treatable: No.
  • Magnetic: Yes
  • Chromium content: 11-18%
  • Nickel content: 0-1%
  • Molybdenum content: 0-1.25%.
  • Typical max Hardness: 200 HB.
  • Typical Tensile Strength: 100 [Kpsi].
  • Popular grades: 409 / 430.
  • Typical parts: Auto exhausts, grills, coffee machine parts, and other household appliances.

Duplex Stainless Steel

  • Corrosion resistance: Excellent++.
  • Heat Treatable: No.
  • Magnetic: No.
  • Chromium content: 18-30.0%
  • Nickel content: 4-7%
  • Molybdenum content: 0-5%
  • Typical max Hardness: 280 HB
  • Typical Tensile Strength: 150 [Kpsi]
  • Popular materials: F51 (2205) and A276 (2707).
  • Typical parts: Paper production equipment, Desalination of seawater, and Oil & Gas parts.

This sub-group is called duplex since these materials have a two-phase Austenitic-Ferritic structure. They benefit from the advantages of both austenitic and ferritic properties, leading to increased strength, higher toughness, and broader corrosion resistance. They provide higher corrosion resistance and tensile strength than standard austenitic stainless 304 or 316. Chromium (Cr) content can reach 30% (Much higher than austenitic alloys) and Nickel (Ni) 9% (Lower than austenitic alloys). General machining guidelines are like 316, with about 20% lower machinability and more attention to clamping stability. Commercially, they are cheaper than austenitic stainless steels due to their lower nickel content. 

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