Stainless and Nickel Alloy Machining

In general terms, machining stainless and nickel alloys is more difficult than machining low alloy steels. However, there are a wide range of stainless alloy compositions and a few of them (such as 416) can actually have better machinability than mild steel.

The austenitic 300 series stainless steels like 304 and 316 comprise the bulk of stainless usage.  Their combination of good corrosion resistance, excellent formability and weldability make them suitable for a wide range of applications. These characteristics are obtained by alloying nickel with the chromium and iron.  The addition of nickel also results in alloys that work harden rapidly (strengthen through cold work). Therefore, while the 300 series stainless steels are the most widely used, as a class, they are also the most difficult to machine.

Recognizing the tendency for stainless and nickel alloys to cold work is probably the most important concept to remember when machining these grades.  When a tool touches a work piece to remove stock, it is work hardening the surface of the work that is left behind.  Even though this is a very thin layer, if on the next pass the tool is in this work hardened layer, the ability of the tool to continue cutting is jeopardized.  In the worst case, it will make the work hardened layer harder and deeper and the tool will stop cutting altogether.

The five guiding principles below are the foundation for good stainless machining practices.

  1. Use a heavier feed than with mild steels to maintain the cut below the work hardened surface. Dwelling on the surface or multiple thin cuts should be avoided.
  2. Taking a deeper cut will require more power.  The equipment capability for stainless and nickel alloys is about 66-75% of that for mild steel. Use a slower speed than with mild steels. This will reduce heat and improve tool life.
  3. Since you are applying greater loads through the tool, the tools and fixtures must be as rigid as possible to avoid chatter and distortion.  Minimize overhang or protrusion of both the tool and the work piece.
  4. Keep tools sharp with recommended rake angles and clearances. Chip breakers may be beneficial for higher nickel containing alloys.
  5. Proper cutting fluids are critical to maintain adequate lubrication and heat removal.  The tool should be flooded with lubricant which will also help remove chips from the work area.

There are many variables in machining operations so guidelines for speeds and feeds are just that, guidelines that can be used as starting points.  Our website provides tables with starting points for many alloys, similar to the table shown here.  Using PRODEC grades, specifically designed for modern CNC machining centers, along with specialized tooling, fixturing and lubrication can result in greatly reduced machining times and increased tool life.

Turning Cut-Off Forming Drilling Reaming End Milling Tapping Threading
Depth/Width, (dia/in) 0.005-
0.200
0.002-
0.004


1/16
1/4


1
2

1/4
1/2
1-2


1/4
1-2
0.050-
1/2
0.050-
1/2

7
threads
per inch


25 threads
per inch
Speed, (sfm)
375-700
600-900

275-400
325-450

375-425
350-400
-
700
800


-
-


300-450
360-500



-



-
Feed, (in/rev) 0.025-
0.030
0.007-
0.010
0.002-
0.004
0.004-
0.008
0.003-
0.006
0.002-
0.004

-
0.005
0.007


-
-


0.004
0.008



-



-

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