Stainless Steel – Machinability

Machinability Graphs

The chart shows the position of Stainless steel vs the other material groups

Bar Chart - Machinebility of stainless steel vs other material groups

The chart shows the machinability of the most popular stainless alloys

Bar Chart - Machinability of popular stainless steel alloys

What is stainless steel?

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).

types of 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%

On which stainless steel group do you want to learn?

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.

Main Features of Austenitic Stainless Steel:

  • Corrosion resistance: Excellent.
  • Heat Treatable: No.
  • Magnetic: No.
  • Chromium content: 16-20.0%
  • Nickel content: 6-15%
  • Molybdenum content: 2-4%
  • Typical max Hardness: 180 HB
  • Typical Tensile Strength: 90 [Kpsi]
  • Popular materials: 303, 304, and 316.
  • Typical parts: Valves and fasteners in a chemically harsh environment, Marine, Medical.

Machinability of Austenitic Stainless-Steel 300 series (303/304/316)

Main Problems:
Best Practice:
  • Use TiAlN PVD grades or thin-layer CVD grades.
  • Use a good supply of coolant directed to the cutting edge.
  • Avoid machining at a constant depth of cut to reduce the risk of Vg (Notch Wear).
Main Materials:
  • SAE 303 (Din X10CrNiS18-10) is considered a “Free-Cutting” material and is the easiest to machine Austenitic Stainless Steel. This is achieved by adding Sulfur and Selenium to 304. However, it comes with the “price” of lower corrosion resistance.
  • SAE 304 (Din X5CrNi18-10) is the most popular and versatile Stainless Steel type. It has good corrosion resistance and still maintains fair machinability. It is easier to machine and cheaper compared to 316.
  • SAE 316 (Din X5CrNiMo17-12-2) is the most popular stainless steel for harsh environments. The main difference between 316 and 304 stainless steel is that 316 contains an increased amount of molybdenum. This additive gives 316 very good heat and corrosion resistance. However, it is the most difficult to machine among the commonly used stainless steels.
Cutting Speeds Recommendations for 300 Series
SAE Machinability Turning Milling
303 75% 920 SFM
280 mm/min
460 SFM
140 mm/min
304 40% 600 SFM 180 mm/min 330 SFM 100 mm/min
316 36% 500 SFM 150 mm/min 260 SFM 80 mm/min

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.

Main Features of Martensitic Stainless Steel:

  • 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.

Main Features of Ferritic Stainless Steel:

  • 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.

Machinability of Ferritic/Martensitic Stainless-Steel 400 series

Martensitic/Ferritic Stainless is on the border between ISO P and ISO M materials. It can be machined with carbide grades for both Alloy steel and Stainless steel. Typical wear is usually flank and crater (Like in alloy steel), with an occasional build-up edge. Machinability is better when compared to Austenitic stainless and is in the range of alloy steels. Grades with the suffix F (Like 430F/420F) are freecut materials with higher Sulfur (S) content and less Molybdenum (Mo). This tweak increases the machinability but results in lower corrosion resistance. Grades with the suffix C (like 440C), have higher Carbon (C) content, which increases the strength and hardness after heat treatment.

Cutting Speeds Recommendations for 400 Series
SAE Machinability Turning Milling
430F 75% 920 SFM
280 mm/min
460 SFM
140 mm/min
410 54% 660 SFM
200 mm/min
330 SFM
100 mm/min
440 40% 530 SFM
160 mm/min
260 SFM
80 mm/min

PH Series Stainless Steel

Main Features of Precipitation-hardening Stainless:

  • Corrosion resistance: Good.
  • Heat Treatable: Yes.
  • Magnetic: Yes.
  • Chromium content: 14-17%
  • Nickel content: 4-10%
  • Molybdenum content: 1.5-2.5%
  • Typical max Hardness: 400 HB (After heat treatment)
  • Typical Tensile Strength: 200 [Kpsi]
  • Popular materials: 17-4PH (AISI 630)
  • Typical parts: Aerospace and Oil & Gas components.

A group of stainless steel alloys with good corrosion resistance that can be heat treated to provide tensile strengths of up to 3 times more than 304/316 grades. The addition of copper, aluminum, and titanium enables to achieves precipitation hardening. They are used in the oil & gas and aerospace industries, where a combination of strength and corrosion resistance is critical. SAE 17-4PH (Din X5CrNiCuNb174 / AISI 630), is the most popular in this family, with 45% machinability in the annealed state (Similar to 304), but much lower after heat treatment.

Designation convention: Cr-Ni PH, for example, 17-4 PH, has 17% chromium and 4% nickel. (See more examples in below chart)

Precipitation-hardening stainless steels can be divided into 3 main groups:

Group Alloy Cr Ni
Martensitic 15-5 PH 15% 5%

17-4 PH
(Alloy 630)

17% 5%
Austenitic - Martensitic 15-7 PH 15% 7%
17-7 PH 17% 7%
Austenitic 17-10PH 17% 10%
  • Alloy A286, with 26% nickel content, is classified as a PH alloy by some sources. We classify it as a nickel-based superalloy.

PH stainless steel alloys are available in two conditions – annealed (condition A) or tempered (condition C). The annealed alloys have a hardness of 20-30 HRC and are relatively easy to machine. After machining, parts can be age-hardened to Rockwell 32-42 HRC. Tempered (condition C) are delivered with a hardness of up to 43 HRC alloys can be hardened to above 50 HRC. Pay attention to the condition and hardness when determining the cutting speed.

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. 

Duplex alloys lose their strength and corrosion resistance at temperatures above 570°F (300°C), limiting their application range!

Duplex alloys are divided into two main categories:

  • Alloys that are designed for highly corrosive environments but with less focus on their strength are nicknamed “lean” or “standard”.
  • Alloys designed for Increased strength and mildly corrosive environments are nicknamed “Super-Duplex” or “Hyper-Duplex”.

These categories are identified by the pitting resistance equivalence number (PREn), calculated based on the chemical composition:

\( \large PERn\,=\,\text{%Cr}\,+\,3.3\,\times\,\text{%Mo}\,+\,16\,\times\,\text{%N} \)
\( \small PERn\,=\,\text{%Cr}\,+\,3.3\,\times\,\text{%Mo}\,+\,16\,\times\,\text{%N} \)
  • Lean: PERn Less than 32
  • Standard: PERn between 32-39
  • Supper-Duplex: PERn between 40-45
  • Hyper-Duplex: PERn Above 45

Main Stainless Steel Duplex Alloys

Material PERn Category Machinability Cr Description
Duplex 1803 (F51) 34 Stabdard 28% 22% The original 22% Cr duplex stainless steel alloy.
Duplex 2205
(F60 / F51)
36 Stabdard 28% 22.5% An improved version of F51 (improved pitting corrosion resistance). Dual-certified as F60/F51 (S31803/S32205).
Duplex 2760 (F55) 43 Supper-Duplex 16% 25% A super duplex stainless steel based upon a 25% Cr composition, but with an addition of tungsten.
Duplex 2707 (A276) 47 Hyper-Duplex 10% 29% Extremely high mechanical strength and high resistance to stress corrosion cracking in chloride environments.

Machinability Table

The below table lists the machinability ratings of all the major stainless steel alloys.

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