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Machinability of Stainless Steel

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Machinability is roughly defined as how easy it is to cut a particular kind of metal.  If a metal is easy to machine, it will require low power and will cut quickly without excessive wear of the cutting tool.  There is typically an inverse correlation between a metal’s durability and machinability.  The challenge for manufacturers and designers is to find materials that are easy to machine while maintaining performance.

Stainless steels are more difficult to machine than ordinary carbon steels.  Let’s look at why that is, and which stainless steel grades are most suitable for applications that require machining.

Definitions of machinability

Machinability is a complex subject with inherent variable factors.  What constitutes an easily machined grade to one process may not apply to others.  The below list expands on the factors affecting the concept of machinability:

  • Tooling wear and tool life
  • Post machining surface finish
  • Chip removal from work area
  • Maximum cutting rate
  • Productivity

Carbon Steel vs. Stainless Steel

When it comes to comparing carbon steel and stainless steel, there’s a significant difference in terms of machinability. Stainless steel alloys are generally more difficult to machine than carbon steel.  The various grades are likely to be tougher and gummier, and work harden very rapidly.

How is machinability rated?

It is important for metals to have a reliable machinability rating to help manufacturers distinguish between various alloys and grades.  For this reason, the American Iron and Steel Institute (AISI) formulated a rating system to make it easy to determine the machinability of any grade of stainless steel.

To create the system, the institute ran turning tests on various grades at 180 surface feet per minute.  For the base number, the AISI uses 160 Brinell B1112, arbitrarily assigning it a rating of 100 percent.  The rating considers weighted averages for the normal cutting speed, surface finish, and tool life.  The result is that a metal with a rating lower than 100% is more difficult to machine than B1112, while a score over 100% indicates the metal is easier to machine.

Which stainless steel grades are best for machining?

There are two primary austenitic stainless steel grades perceived as the most suitable for machining applications.  Most manufacturers will be familiar with 304, which is one of the oldest and the most widely used of all the stainless steel grades.  Traditionally made with 18% chromium content and 8% nickel content, it is often referred to as 18/8 stainless steel.  It has a machinability rating of 48%.

However, despite its popularity, 304 is not especially well-suited to machining.  303 was created by adding sulfur, the result being a “free-machining” stainless steel grade with a machinability rating of 75%.

The other stainless steel grade most often used for machining is 416.  Stainless steel 416 is perhaps the most machinable of all stainless steels, thanks to its high sulfur content, and has a machinability rating of 85%.  A martensitic grade, 416 can also be heat treated to enhance mechanical properties.  Generally, the 400 series martensitic and ferritic grades exhibit improved machinability performance over the austenitic 300 series.

What affects the machinability of stainless steel?

There are several alloying elements and other materials that can be added to a metal that affect its machinability.  Many of the elements added to stainless steel to increase a grade’s strength also result in it becoming more difficult to machine. Examples include chromium, nickel and molybdenum.  On the other hand, certain chemicals and elements can be added to stainless steel that boost its machinability.

Sulfur is the most popular option to increase the machining performance of stainless.  When added, sulfur will lower the shear strength of inclusions in the cutting zone.  The inclusions then act as stress risers and weaken the steel, allowing it to be cut more easily. This allows the machined chip to break away from the work area with greater efficiency, avoiding the buildup of machined waste metal around the cutting tool known as “bird-nesting.”

Selenium is also used to improve machinability and used to be very popular for use with 303 stainless.  303Se is good for screw machine production of valves, valve trim and associated products.

Calcium is another additive to stainless steel that improves its machinability.  It has the additional benefit of not affecting the weldability of the grades that employ it.

Your Technical Services Professional

While not known for its ease of machining, stainless steel has certain grades, namely 303 and 416, which are specially designed for increased machinability.  No matter what your requirements are for a stainless steel, it’s critical to work with a metal supplier who has a proven track record of partnering with its clients through every step of their procurement process.

At Clinton Aluminum, our number one priority is helping support the success of our customers. Our dedicated and knowledgeable staff average almost thirteen years of experience at the company, and they are standing by ready to answer your questions. Contact us today to learn more and discuss your stainless steel supply needs.

 

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