Stainless Steel Machinability Graphs
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:
- High cutting forces.
- Heat.
- Build up Edge[6] – Workpiece material sticking to the cutting edge[7].
- Notch Wear (Vg)[8] – High wear developing at the depth of cut line.
- Higher Nickel (Ni) and Molybdenum (Mo) content decrease machinability.
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.
