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
Optimizing the chemistry to provide for a minimum shear strength
Adding internally contained lubricants
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%.
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…