Why Free Machining and Impact Strength Don’t Mix

November 29, 2011

 It is critical to understand that the selection of free machining steels goes against the ability of those components to withstand impact loads.

Charpy impact values are reduced by free machining additives.Impact values increase with increased hardenability.


Impact strength is often an important design consideration in mechanical components. Cost to manufacture is also an important consideration in mechanical components.

Free machining grades can reduce the cost to manufacture precision machined components. But free machining additives reduce the impact strength of the steel. Materials should be selected on the basis of complying with design requirements, not just low cost to manufacture.

The low carbon free machining steel grade 1215 exhibits a particularly low level of toughness over a wide range of temperatures. Even light impact loadings are a bad fit for this grade of steel. The principal effects of the free machining elements  (Sulfur and Manganese) added to this steel are to lower the upper shelf or ductile portion of the absorbed energy curve.

The effects of hardenability can be seen between the 4140/41L40 and 1141 steels. While the presence of lead in the 41L40 does drop the upper shelf energy somewhat, the biggest difference can be seen  to lie between the 1141 and the two 4140 grades. The lower hardenability of the 1141 on mill cooling in addition to the effect of the manganese sulfide additives explains this difference.

The greatest difference however that can be seen from this figure is the vast difference between the two low carbon steels, Grade 1215 and 1018. Even at 212 degrees F the upper shelf energy of the 1215 is roughly  only a third of that of grade 1018.

Rule of thumb: If a steel grade machines well- it probably has miserable charpy impact properties. 

Figure 1 taken  from The Assessment of The Mechanical behavior Of Free-Machining Steels, J.T. Berry and  R. Kumar, School of Mechanical Engineering, Georgia Institute of Technology; R.G.Kumble, Vermont American Corporation. 1975 ASM  Mechanical Working and Forming Division, International Symposium  on Machinability.

6 Keys To Using Free Machining (12XX) Steels

February 9, 2010

These keys will keep you out of trouble!

 Keep these 6 Keys to Using Free Machining (12XX) Steels in mind:

  1. These steels are not generally sold for applications requiring high standards of strength, hardness or other related properties.  Applications where vibratory, torsional or alternating stresses approach the grades’ static limits  are NOT recommended.
  2. These steels are frequently case hardened or carburized in order to achieve desired surface hardness.
  3. When cold drawn, these steels can be notch sensitive. Highly polished fatigue specimens may achieve expected endurance values, but poor surface finish, tool marks, or sharp corners in the design may cause lower than expected performance.
  4. These grades have relatively low impact strength at reduced temperatures and should not be used for sub-zero impact applications.
  5. These steels are not recommended for applications where severe cold work  follows machining. Crimping, staking and swaging may be performed, especially in non-renitrogenized grades. But severe crimping, cold metal movement, and bending may not be satisfactory in these grades.
  6. The addition of Lead or Bismuth does not alter the mechanical properties in tension. 12L14 and 1215 of same nominal size and process will be indistinguishable by hardness or tensile testing.

Free Machining Steels in the 12XX series- 12L14, 1215, etc., are selected in order to reduce the time needed to make large volumes of complex parts. This  reduces the cost per part. The usual application is one where bulk and shape (mass and geometry) are the chief requirements. The factors that make these steels highly machinable also influence behavior of the products in service. Designers and engineers should keep the above 6 Keys in mind when considering the material for an application.

6Keys: Photo credit .


3 Reasons To Not Weld Free Machining Steels

August 25, 2009

Any type of arc welding of resulfurized steels is generally avoided. This post will give you some  reasons why. Resulfurized steels are free machining steels. This includes steel grades in the 11XX and 12XX series,  such as 1215, 12L14,  1117, 1137, and 1144. These steels contain sulfur and may contain lead. These two elements will create low melting temperature constituents that will cause cracks.

Here are 3 reasons not to weld resulfurized free machining steels:

  1. Sulfur reduces weldability.  The higher levels of sulfur make a slaggy joint.
  2. The high volume fraction of manganese sulfides also hold hydrogen. This hydrogen can then create post weld cracking.
  3. Both sulfur and lead  can become a fume inhalation hazard at welding temperature.

Finally, with the exception of grade 1144, resulfurized steels are generally not sold to mechanical property requirements. Welding implies mechanical property performance.

We have seen 1215 welded using an inertial or friction welding process. But these welds are  usually not subject to mechanical loads, merely attachment. Here’s a video of a friction weld process for truck axles from Thompson Friction Welding in the UK.

Want a second opinion? Dave Barton at Lincoln Electric hosts a column   Barton’s Q&A in Welding Magazine published by Penton.  The second question in this column deals with welding 12L14.

Think of weldability and machinability as two sides of the material coin.

Heads it machines well, welds lousy, Tails...

Heads it machines well, welds lousy, Tails...

You can usually win on one, but at the expense of the other. If you need to weld, a low carbon plain carbon steel is your best bet.