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)[19] 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)[1] 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[1] content, up to 20% with the addition of Nickel[3] up to 15%. Due to the high nickel[3] content, It has much better corrosion resistance, but it is the hardest to the machine. It lacks in strength and hardness[20] 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[4] content: 2-4%
  • Typical max Hardness[20]: 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:
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[21] Milling[22]
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[1] content of up to 14% with almost no nickel[3]. 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[1] content: 11-14%
  • Nickel[3] content: 0-2%
  • Molybdenum[4] content: None.
  • Typical max Hardness[20]: 600 HB[5] 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[3]. 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:

Machinability of Ferritic/Martensitic Stainless-Steel 400 series

Martensitic/Ferritic Stainless is on the border between ISO P[23] and ISO M[11] materials. It can be machined with carbide grades[24] for both Alloy steel[12] and Stainless steel. Typical wear is usually flank and crater (Like in alloy steel[12]), with an occasional build-up edge[6]. 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)[25] content, which increases the strength and hardness[20] after heat treatment.

Cutting Speeds Recoendations for 400 Series
SAE Machinability (%)[14] Turning[21] Milling[22]
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[15]. 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)[1] content of up to 30% and Nickel (Ni)[3] up to 9%. General machining guidelines are like 316 with about 20% lower cutting speeds[16] 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.
6. Built-Up Edge (Bue) ( Build up Edge ) Built-Up Edge (Nicknamed BUE) is a wear mechanism caused by the welding of chips to the insert body. It can occur when machining any raw material but is more common when machining sticky materials, such as low carbon steel, austenitic stainless steel, and aluminum.
7. cutting edge. Cutting edge refers to the "Micro-Geometry" of the cross-section at the tip of the inserts that engages with the workpiece material. Although the length of the cross-section could be less than 1 mm it has an enormous effect on the performance.
8. Notch Wear (Vg). Notch wear is a wear mechanism that forms on the flank and rake of a turning insert cutting edge at the "Depth of Cut Line" when machining austenitic stainless steel and superalloys
9. PVD (Physical Vapor Deposition) ( PVD ) In PVD, the coating layer is spattered on the substrate and does not form a chemical bond with it. Therefore, the adhesion is lower, but the process induces compressive residual stress that improves the overall toughness of the carbide insert. PVD is good for creating thin coatings between 1 to 8 microns. PVD coated inserts need to operate at lower cutting speeds when compared with CVD, however, they are tougher, have a smoother surface (less friction), and can be applied also on sharper edges (small honing and ground inserts).
10. CVD (Chemical Vapor Deposition) ( CVD ) As its name suggests, in this process the coating forms a chemical bond with the substrate. Therefore, the adhesion to the substrate is very strong. With CVD it is possible to create a thick coating of 5 to 25 microns. Due to its thicker layer, CVD provides excellent heat insulation and enables achieving higher cutting speeds compared with PVD. The downside is more sensitivity to cracks and fractures.
11. Stainless Steel ( ISO M ) Stainless steels (ISO M), 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).
12. Alloy steel. There is no scientific definition, but in practice, alloyy 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 the strength, toughness, corrosion resistance, wear resistance, hardenability, and the steel’s hot hardness.
13. Free-Machining Steel ( freecut ) Free-Cutting Steel is a nickname for carbon steel with additional alloying elements for the sole purpose of improving their machinability and chip control. They are also nicknamed Free-Cut or Free-Cutting materials.
14. Machinability (%). Machinability is the ease with which a metal can be machined. It is represented in percentage relative to a reference metal. A smaller value means the metal is harder to machine. Very difficult to machine materials can rate 10-20%, while very easy to machine material can reach 200-400%
15. Titanium (Ti) ( titanium ) Titanium is a chemical element with the symbol Ti. Titanium alloy is usually made from about 88% of Ti with alloying elements, mostly vanadium (V) and aluminum (Al). What makes it a unique and useful metal are several properties not found together in other materials. It has an 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.
16. Cutting Speed [Vc] ( cutting speeds ) In machining, the words "Speed", "Cutting Speed", "SFM" and "Surface Speed" all refer to the relative velocity between the tip of the cutting edge and the workpiece. The definition is the same for all machining operations turning, milling, etc. Opposed to feedrate which has a different definition for different applications (...)
17. ISO Material Groups in Machining ( Material Groups ) In the machining industry, workpiece materials are divided into groups. Classifying correctly the material group gives a good starting point to choose the correct grade and initial cutting speed.
18. Machinability ( mr ) Machinability is the ease with which a metal can be machined. It is represented in percentage relative to a reference metal. A smaller value means the metal is harder to machine. Very difficult to machine materials can rate 10-20%, while very easy to machine material can reach 200-400%
19. Iron (Fe). Iron is a chemical element with the symbol Fe (from Latin: Ferrum). It is, by mass, the most common element on Earth, forming much of Earth's core. Iron alloys, such as steel, stainless steel, and cast iron, are the most common industrial metals because of their mechanical characteristics and economic cost. Humans started to master iron as helpful material for making tools in Eurasia as early as 2000 BCE. In some regions, iron tools and weapons began to displace copper only around 1200 BCE.
20. hardness. Hardness is a measurement of the resistance to localized plastic deformation caused by force or abrasion. Materials with high hardness would generally be stronger and more wear-resistant, but on the other hand, more brittle and sensitive to fracture.
21. Turning. Turning is a machining process that cuts materials (usually metal) by moving a cutter along a workpiece while the workpiece rotates. The cutter is typically made of steel holder with an indexable insert made from carbide. Due to its fundamentals, turning operation produces only round, symmetrical parts.
22. Milling. Milling is a machining process that cuts materials (usually metal) by rotating a round cutter as it moves along a workpiece. The cutter is either solid carbide or a steel body with indexable inserts. It can have one cutting edge or several (Up to dozens). More cutting edges yield higher material removal rates.
23. Steel ( ISO P ) Steel (ISO P) 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. By “playing” with the mix and amount of the different alloying elements, Iron gains a range of unique mechanical properties. However, some of these additives also harm the machinability rating.
24. Grade ( carbide grades ) In the machining jargon, the term “Carbide Grade”, or simply “Grade” refers to the combination of sintered tungsten carbide, coating, and other treatments that the cutting indexable insert or solid carbide tool are made off. Each grade is suitable for different materials, cutting conditions and applications
25. Carbon (C). Carbon is a chemical element designated by the symbol C; moreover, carbon is a critical factor in several machining topics. Although it is added in small doses of 0.03% up to 3% (Usually between 0.1 -1%) to metals, it is the most influential element in many materials used in machine shops. It is also a primary component in Carbide Grades
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