Stainless Steel is the 2nd most popular materials group (After Steel) in machine shops. Machinability of Stainless Steel ranges from free-cutting grades like 430F and 303 that have machinability of 75%, and to up challenging grades like 316 with Machinability of 36%.

Stainless Steel Machinability Graphs

Bar Chart - Machinebility of stainless steel vs other material groups
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)[20] with a minimum of 10.5% Chromium[1]; most grades will have additional alloying elements[2] such as nickel (Ni)[3] and molybdenum (Mo[4]).

Why does stainless steel have good corrosion resistance?

Chromium (Cr) in combination with oxygen (O) creates a thin film layer of Cr2O3 on the surface of the steel, which provides non-corrosive property to the material. This layer blocks the oxygen’s diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal.

What are the main types of Stainless Steel?

Property Austenitic Martensitic Feritic PH Duplex
Corrosion Resistance Excelent Fair Good Good Excelent++
Magnetic? No Yes Yes No No
Heat Tratable? No Yes No Yes No
Machinabilty 35-75% 40-75% 40-75% 40-50% 20-30%
Avg Hardness [HB] 180 Max 600 200 Max 400 280
Avg Strengh [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

This, most popular family of Stainless Steels characterized by 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 the machine. It lacks in strength and hardness[21] compared to other types of Stainless Steel.

Main Features of Austenitic Stainless Steel:

  • Corrosion resistance: Excellent.
  • Heat Treatable: No.
  • Magnetic: No.
  • Chromium[1] content: 16-20.0%
  • Nickel[3] content: 6-15%
  • Molybdenum content: 2-4%
  • Typical max Hardness: 180 HB[5]
  • 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[9] grades or thin layer CVD[10] 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[4]. This additive makes gives 316 very good heat and corrosion resistance. However, it is the most difficult to machine among the commonly used stainless steels.
Cutting Speeds Recoendations for 300 Series
SAE Machinability Turning[22] Milling[23]
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 and therefore poses 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[3] 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, Other parts that require more strength but less critical in terms of corrosion resistance.

Ferritic Stainless Steel

Ferritic stainless steel materials have a Chromium[1] content of up to 18% with almost no nickel. They have better corrosion resistance than Martensitic grades but less compared to the Austenitic grades. 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[21]: 200 HB.
  • Typical Tensile Strength: 100 [Kpsi].
  • Popular grades: 409 / 430.

Machinability of Ferritic/Martensitic Stainless-Steel 400 series

Martensitic/Ferritic Stainless is on the border between ISO P[24] and ISO M[11] materials. It can be machined with carbide grades[25] for both Alloy steel[12] 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[13] materials, with higher Sulfur (S) content and less Molybdenum (Mo[4]). This tweak increases the machinability but results in lower corrosion resistance. Grades with the suffix C (like 440C), have higher Carbon (C)[14] content, which increases the strength and hardness after heat treatment.

Cutting Speeds Recoendations for 400 Series
SAE Machinability (%)[15] 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

A sub-group of Austenitic stainless steels that can be heat treated to provide tensile strengths of up to 3 times more than 304/316 grades. They are used in the oil and gas and aerospace industries where a combination of strength and corrosion resistance is critical. Precipitation hardening is achieved by the addition of copper, aluminum, and titanium[16]. SAE 17-4PH (Din X5CrNiCuNb174), is the most popular in this family with a machinability of 45% in the annealed state (Similar to 304), but much lower after heat treatment.

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 compared to standard austenitic stainless 304 or 316. They can have Chromium (Cr) content of up to 30% and Nickel (Ni)[3] up to 9%. General machining guidelines are like 316 with about 20% lower cutting speeds[17] and more attention to clamping stability.

Machinability Table for Stainless Steel

Page Glossary Terms
1. Chromium (Cr) ( Chromium ) Chromium added to carbon steel in percentages greater than 11% creates Stainless Steel. At this percentage and greater (When combined with Nickel), the corrosion resistance of steel vastly increases, and oxidation of the iron is prevented. Chromium also helps to improve mechanical properties, even in smaller amounts. It will increase the steel’s strength, hardness, and ability to be heat treated.
2. Alloying elements for machining ( alloying elements ) Alloying element is a chemical element added to the primary substance of the material (in most cases ferrous) to tweak and enhance mechanical, metallurgical, and physical properties to suit different engineering needs.
3. nickel (Ni). Nickel is one of the most important alloying element in the machining world. It is added in various quantities to many materials having a major effect on their properties. Its presence in high quantity creates materials that are very hard to machine.
4. molybdenum (Mo. Molybdenum, like chromium, effects the corrosion resistance of steel. Molybdenum also increases the hardenability, toughness, and tensile strength of steel. The hardenability is increased by lowering the required quench rate during heat-treatment. Molybdenum also decreased the risk of pitting (PRE) by improving resistance to chloride.
5. Brinell scale [HB] ( HB ) One of the most common units used for listing the hardness of steel materials. the test is done with a 10 mm steel ball pressed with 3000 Kgf (6,614 Lbf). Common values for machined materials range from 100 HB for very soft materials up to 650 HB for heat-treated steels.
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