Tag: 12L14 Steel

C-1200 Series Steels- Not Recommended for Applications Requiring Mechanical Properties

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Question: Why don’t the Certs for C12L14 and C1215 cold drawn steel bars have mechanical properties routinely reported?
Answer: Because these steels ARE NOT recommended for applications requiring mechanical properties, alternating stress applications, cold metal movement and can also be notch sensitive.

Don't do it!
Don’t use these steels for critical applications!

Here are some details from the application notes provided to me when I started as Plant Metallurgist at Bliss & Laughlin Steel in 1985:

  • The 1200 series steels (1215 and 12L14) are not generally sold for applications requiring high standards of strength, hardness, or other related properties
  • These steels are particularly adapted to automatic screw machine production of small repetitive parts. The ideal application is one where bulk and shape, as dictated by the design, are the chief requirements.
  • They may be used for parts which require only nominal strength values providing the factor of safety as in accordance with established practice. This is especially true where the stresses involved are static tension, compression, or shear. Vibratory, torsional, or alternating stress applications approaching the static limits are not recommended; thus these steels should not be used for line shafting.
  • When cold drawn, the C-1200 steels are notch sensitive and while polished fatigue specimens will show expected endurance limit values, poor finish and processing of parts, or faulty design may cause low or erratic results for finished parts under dynamic or alternating stresses of relatively small intensity.
  • These steels are not recommended for applications involving cold metal movement, such as crimping, forming or bending. Operations such as knurling and character rolling can be done satisfactorily.
  • Because of the smaller amount of hot work in rolling, the large size cold drawn C-1200 bars increase in relative brittleness.  The large sizes may also be expected to reveal stringer sulfide inclusions on polished surfaces. Further removal will likely reveal more inclusions as they are distributed throughout the bar cross-section.
  • A cold drawn surface will not plate to the highest quality finish; a turned or ground surface may prove necessary for critical applications

Your customer wants inexpensive parts, and machining them from 12L14 and 1215 minimizes the cost of machining to the desired geometry. But if the parts are not likely to fulfill the properties expected, that is false economy at best.
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Little Charlie and the Night Cats If you Dig It, Don’t Do it.

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

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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 subsequent 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…