Role of Phosphorous, Nitrogen and Coldwork on Machinability of Carbon And Alloy Steels

April 26, 2011

Machinability of carbon and alloy steels is a shear process. Working the metal (Shearing to create chip) provides heat. The subsequent sliding of the produced chip on the face of the cutting tool provides heat as well.

Three ways to improve machinability include

  1. Optimizing the chemistry to provide for a minimum shear strength
  2. Adding internally contained lubricants
  3. Adjusting cold work

The steels that we are talking about are in large part composed of the ferrite phase. This is advantageous to us as machinists, because it has a relatively low shear strength.

Because ferrite is also ductile, it does not cut cleanly and tends to tear. Grade 1008 or 1010 are prime examples of  how pure ferrite machines. Long stringy, unbroken chips, torn surface finishes and lots of machine down time to clear “birds nests” are typical results.

Adding carbon up to a point improves machinability by adding a second harder phase (pearlite) into the ferrite. The good news is that up to a point, the chip formation is greatly improved, and surface finish improves somewhat. The bad news is that the shear strength of the steel is also increased. This requires more work to be done by the machine tool.

Addition of Nitrogen and Phosphorous can not only increase the shear strength of the ferrite, but also reduce the ductility (embrittle it).This ferrite embrittlement promotes the formation of short chips, very smooth surface finishes, and the ability to hold high dimensional accuracy on the part being produced. The downside is that these additions can make the parts prone to cracking if subsequebnt cold work operations are performed.

The graph below shows how cold work (cold drawing reduction) works in combination to reduce chip toughness, resulting in controlled chip length, improved surface finish, and improved dimensional accuracy of the part. To read the graphs, the Nitrogen content is shown in one of two ranges, and Phosphorous content is varied as is  the amount (%) cold work. You can see how the synergistic effects of these two chemical elements  when appropriately augmented by cold work, can drop the materials toughness  by as much as 80-90%.

 
 

Phosphorous and Nitrogen affect ductility; Cold work further activates their effect.

Add to that internal lubrication by a separate manganese sulfide phase or a lead addition, and now you can see how these factors can make grade 1215 or 12L14 machinable at speeds far, far, faster than their carbon equivalent 1008-1010. With greater uptime and tool life.

Internal Lubricant- Manganese Sulfides

And you thought that cold drawing just made the bar surface prettier and held closer in size…


4 Ways Nitrogen Affects Your Shop

January 5, 2010
  1. Nitrogen strengthens ferrite.
  2. Nitrogen improves surface finish.
  3. Nitrogen improves production rates.
  4. Nitrogen can contribute to cracking during cold working.

Well 3 out of 4 ain’t bad.

"Three out of four ain't bad"

Nitrogen is a chemical element that can contribute to improved surface finish, especially on side working tools. It does so by strengthening  the chip, resulting in a crisp separation from the workpiece. The bulk hardness of the material increases with increased Nitrogen as well.

Nitrogen is an important factor, especially in free machining steels. Like 1215 and 12L14.

As Nitrogen increases, so does hardness.

Nitrogen is higher in electric furnace melted steels than in steels produced in Basic Oxygen Furnaces.

The down side of higher Nitrogen is that it can result in cracking during cold work- operations such as staking, swaging or crimping.

Nitrogen is “implicitly” specified whenever purchasing chooses a  steel supplier. That supplier’s melt process is a major factor on determining the Nitrogen content that you get in the shop.

For a more complete discussion of the role of Nitrogen and how it can affect your precision machining operations, see our article  in Production Machining here.

Photocredit.

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