Stainless Steel – Machining & Machinability

Machinability Graphs

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

The chart shows the machinability of the most popular stainless 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; most grades will have additional alloying elements^[1] such as nickel (Ni)^[13] and molybdenum (Mo^[5]).

Stainless steel is often referred to as a single material. However, more than 150 distinct compositions are formulated to serve specific uses and/or manufacturing requirements. In this article, we will dive into it from the machining perspective.

corrosion resistance

Why does stainless steel have good corrosion resistance?

Chromium (Cr)^[14], in combination with oxygen (O) creates a thin film layer of Cr₂O₃ on the surface of the steel, which provides non-corrosive properties 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.

Chromium effect on corrosion resistance of stainless steel

• A higher chromium content increases the alloy’s corrosion resistance.
• The chart on the left shows the corrosion rate in mpy (Millimeters per year) as a function of the chromium content.
• As you can see on the graph, a small amount of chromium already gives significant corrosion resistance, and above 10%, the effect flattens.
• Chromium alone provides protection in atmospheric and aqueous environments. 
• Adding molybdenum^[5] increases the resistance to chloride penetration.
• Adding nickel^[13] improves the resistance in acid environments.

Corrosion Types

Learn about the mechanisms of the main corrosion types and which stainless steel alloys provide the best protection against them.

Pitting

Pitting is a form of extremely localized corrosion that occurs When the protective chromium-oxide film breaks down in small isolated spots, such as when chlorides and fluorides have direct contact with the surface. Pitting can be avoided by using stainless steels^[6] containing molybdenum, such as Type 316 or 317. (Learn More)

Crevice Corrosion

Crevice corrosion occurs from an attack on the surfaces by a stagnant solution in crevices. A typical case is bolt or rivet heads, Where small amounts of liquid can accumulate. Once an attack begins, its progress is very rapid. Stainless steels^[6] containing molybdenum provide better resistance. (Learn More)

Stress Corrosion

Stress-Corrosion occurs by the combined effects of temperature and tensile stress in corrosive environments. Wet-dry or intermitted heat transfer conditions promote the concentration of chlorides and accelerate the development of stress-corrosion cracking. Duplex stainless steels have excellent stress corrosion resistance. Frenetic stainless steel types 405 and 430 are also recommended among the more common alloys. (Learn More)

Intergranular Corrosion

Occurs when austenitic stainless is heated or cooled through the temperature range of about 800-16500°F (450-900°C). The chromium along grain boundaries tends to combine with carbon^[24] creating carbide precipitation. Low-carbon alloys with an “L” suffix, such as 304L & 316L, are less sensitive because there is not enough carbon to react with the chromium. Alloys with the addition of columbium (SAE 347) or titanium^[20] (SAE 316Ti) are helpful due to their close affinity to carbon. (Learn more).

High Temperatur Corrosion (Scaling)

Scaling occurs when metal is heated to very high temperatures and exposed to oxygen. The metal corrodes and forms unwanted deposits (scaling). The maximum temperature to which the protective film still functions depends on the chromium content. Scaling is a common problem in oil and gas production, water transport systems, and Power generation equipment.

• Stainless alloys with less than 18% chromium are limited to temperatures below 1500°F (800C). This includes most frenetic stainless alloys such as 410, 416, or 420 and PH alloys such as 17-4PH.
•  18-20% chromium content protects up to 1800°F (980°C). This range contains the most popular austenitic stainless steels, such as 304 and 316.
• Alloys with more than 25% chromium, such as 309, 314, 452, and most duplex alloys, protect up to 2000°F (1,100°C)

Which Stainless Steel Aloy is effective in each corrosive environment?

types of Stainless Steel

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^[13] 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^[21] compared to other types of Stainless Steel. Most alloys in this series have low carbon content, below 0.1%. This makes them ductile^[22]; therefore, chip control and BUE^[15] are significant concerns for machinists. Alloys 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^[5] 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:

• High cutting forces.
• Heat.
• Build up Edge^[15] – Workpiece material sticking to the cutting edge^[16].
• Notch Wear (Vg)^[7] – High wear developing at the depth of cut line.
• Higher Nickel (Ni)^[13] and Molybdenum (Mo^[5]) content decrease machinability.

Best Practice:

• Use TiAlN PVD^[32] grades or thin-layer CVD^[33] 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^[7]).

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

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^[35] and ISO M^[6] materials. It can be machined with carbide grades^[8] for both Alloy steel^[17] and Stainless steel. Typical wear is usually flank and crater (Like in alloy steel^[17]), with an occasional build-up edge^[15]. 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^[11] 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)^[24] content, which increases the strength and hardness after heat treatment.

Cutting Speeds Recommendations for 400 Series

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:

• Alloy A286, with 26% nickel content, is classified as a PH alloy by some sources. We classify it as a nickel-based superalloy^[25].

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^[36] 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^[12].

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^[26], 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^[2]!

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:

• 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

Machinability Table

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

Stainless Steel Machining

Most stainless steel alloys are challenging to machine because of several reasons:

1. They are very ductile, so it is hard to achieve reasonable chip control.
2. The fast development of notch wear^[7] and built-up edge^[15].
3. High cutting forces and low heat conductivity can cause rapid work hardening.

What can you do to make your life easier when machining Stainless Steel?

Use Free Machining Stainless Steel Alloys

Adding to stainless steel Certain alloying elements, such as sulfur, selenium, lead^[38], carbon, copper, aluminum, or phosphorus, can boost machinability. The enhancement is achieved by reducing the friction and ductility^[22] of the material. Several popular alloys have a Free Machining alternative that preserves the fundamental qualities of the original material while making it much easier to machine. The chart below shows the free-cut alternatives and how much they enhance machinability.

• In addition to the increased cutting speed, also chip control is improved, leading to even higher productivity. 
• The alloying elements that improve free-machining^[11] characteristics slightly affect corrosion resistance, ductility, and weldability. However, these differences are only critical in some cases and might be worth the increased productivity.

No matter if you are a job shop or a machining department in a factory. Contact the relevant engineer and check if he can allow using the Fee-Cut alternative.

Use High-Quality Material even if it costs more

Two suppliers selling the same material (for example, Stainless 316) might have different qualities, which could affect your productivity. Most reputable stainless steel suppliers will stock two (or more) alloys for each SAE number. All these alloys comply with the standard and are legit. But each specific alloy will have enhanced qualities and probably a higher price point.

Raw material properties that will save you money:

• Runout & roundness of the bar – Runout and ovality of a bar make the first cut behave like an interrupted cut instead of a continuous cut. This will shorten the tool life in any material. When machining stainless steel, this factor is magnified because of two reasons:

1. The best inserts for machining stainless steel have sharp cutting edges^[16] (see below). This makes them very sensitive to impacts. If you need to use a wider k-land^[39] to overcome the impacts, you will have poor performance after eliminating the outer layer with the runout.
2. We have encountered enough difficulties machining this material already. It is wiser to get out of the way factors that we can avoid.

• Scale thickness – The existence of scale, its thickness, and uniformity will have an effect similar to a significant runout. (See above). You might save a lot of money, although you will pay more for a well-peeled material without scale.
• Chemical uniformity – Each alloy is manufactured according to a standard that dictates its chemical composition. Each element has a permissible range. The exact chemical composition of the alloy affects (sometimes significantly) the chip behavior and the optimal cutting speed. If the composition varies more between material batches (Or between bars within one batch), your tool life and chip control will be inconsistent. A reputable material supplier will usually control the composition more tightly.
• Chip behavior – Chip control is sensitive to minor material properties changes. If you are not achieving decent chip control, consider checking out another vendor or opt for a higher-quality alloy from the same supplier.

A higher quality material usually costs more but will save much more in productivity and cutting tool costs. 

Lubrication and Cooling

High heat is generated when machining stainless steel because:

•  The low thermal conductivity of the austenitic stainless steel restricts the heat flow away from the machined faces. 
• The considerable depth of cuts and high feed rates needed to reduce the risk of work hardening (see below) increases the amount of heat generated.

Therefore the best practice is as follows:

1. Use high cutting fluid flow rates.
2. Opt for mineral oils for operations with heavy loads at low cutting speeds^[12] and when using HSS tools. If excessive wear is experienced, consider increasing the dilutions. If the cutting edge tends to burn, consider reducing the dilution.
3. Opt for emulsions when cutting with carbide at high cutting speeds.
4. Use a high-pressure coolant in the range of 600-1500 PSI (40 -100 Bar). Using coolant in this pressure range Vs. the default pressure supplied by most machine manufacturers can dramatically improve chip control and significantly increase cutting speeds.

Work Hardening

All 300 series alloys have a certain degree of work hardening. Stainless steel 316 is especially sensitive in this respect. The best practices to reduce it are:

1. A larger depth of cut and a high feed rate reduce work hardening risk. 
2. Opt for sharp cutting edges and replace tools on time before they show significant wear.

Turning Stainless Steel

When turning^[34] stainless steel, the first two questions that pop up are what cutting speed and carbide grade^[8] should I use? Get a quick answer using our two calculators below.

The above mini-calculators provide a quick answer for general conditions. Use our advanced tools to get more accurate results based on dozens of parameters.

Machining strategy

Notch Wear and Build Up Edge^[15] are the two most common problems that decrease productivity and tool-life when machining stainless steel. We will see what they are and how we can avoid them. However, as you will see, the remedy for one is often the enemy of the other. Therefore, finding the sweet spot in between is a game of balance.

Notch Wear

Notch wear^[7] (Nicknamed Vg) is a wear mechanism that forms on the flank and rake of a turning insert cutting edge at the “Depth of Cut Line” (See picture). It is caused by temperature, chemical, and environmental differences between the section that contacts the raw material and the area just above it.

The best way to prevent notch wear from forming is to change the depth of the cut constantly. For example, ramping down at a slight angle turning in one direction, and then turning back. However, in many cases, this is not possible. In such situations, you can:

• Reduce the cutting speed.
•  Opt for a stronger cutting edge geometry with a larger honing^[40] and/or a smaller rake angle.

You need to be careful with the above two solutions since they have a negative impact on the build-up edge (see below).

Build Up Edge

Built-Up Edge^[15] (Nicknamed BUE) is a wear mechanism caused by the welding of chips to the insert. It appears when the temperature in the cutting zone is too low and therefore is associated with slow cutting speeds. 

The failure due to the Built-Up edge^[15] usually happens when the welded chip breaks and, consequently, tears with it a small amount of carbide, creating a pit on the cutting edge.

You can avoid BUE by:

• Increasing the cutting speed (To increase the temperature).
• Opt for a sharper cutting edge geometry with a smaller honing size and a smaller rake angle.

You need to be careful with the above two solutions since they have a negative impact on the notch wear (see above).

Milling Stainless Steel

When turning stainless steel, the first two questions that pop up are what cutting speed and chip load^[29] should I use? Get a quick answer using our two calculators below.

The above mini-calculators provide a quick answer for general conditions. Use our advanced tools to get more accurate results based on dozens of parameters.

Choice of Tools

Choosing the correct tool configuration for your application is vital when machining stainless steel. Roughing, finishing, and high-speed machining can be optimized for stainless steel by selecting the most suitable flute count and helix angles.

Roughing

A 4-5 flute end mill with 30°-40° is the first choice for roughing. Five-flute end mills will allow for higher feed rates, while a four-flute cutter will enable better chip evacuation.

Finishing

Opt for a higher flute count and an increased helix angle when finishing a stainless steel part. Finishing end mills for stainless steel will have a helix angle of over 40° and a flute count of 5-7. When finishing with a small radial depth of cut^[4], the flute count can be as high as 7-14.

High-Speed Machining (HSM)

When milling^[23] an HSM tool path^[30], the goal is to run extremely fast at a shallow depth of cuts, thus taking advantage of the Chip Thinning^[18] effect. Opt for a milling cutter with the maximum number of flutes.