Leaded steel bars historically have been a mainstay raw material in the screw machining industry. As more applications and newer technology move away to non leaded steel applications, we thought that a brief refresher about Lead and its role in our shops might be timely.
Leaded steel bars are standard steels and widely available. In the U.S. 12L14 is the predominant grade. 11SMnPb30, 11SMnPb28, 9SMnpb28, and 9SMnPb36are German designations nominally equivalent to 12L14.The Chinese version of 12L14 is Y15Pb; Japanese nominal equivalents include SUM22L, SUM23L, andSUM24L.
Leaded steels are selected for use for the savings achievedin producing parts by machining.
Leaded steels are not appropriate for all parts– and parts with low amounts of stock removal may not create any noticeable savings.
Today’s Leaded steels are more consistent, more uniform, than they were when produced by the ingot process.
The decision to use Leaded Steels for a specific part must be based on the economics for that part– volume, stock removal, part complexity, tolerances required, surface finish needed are all factors that contribute to that economic calculation.
There is no sacrifice in mechanical properties when adding lead to steel. neither longitudinal nor transvers mechanical properties are affected by the addition of lead to steel.
Leaded steels are currently permitted under European Union Regulationscovering End of Life Vehicles, RoHS.
The reduction in energy required and time needed (about 25%!) to machine a part make leaded steels environmentally friendlyby reducing Carbon Dioxide emissions to create parts compared to using unleaded materials.
In order to be dangerous to humans, lead must be in a soluble form. The lead in steel bars is a separate solid phase. IARC lists lead under its Group 2B category – “possibly carcinogenic to humans”.
Lead, as well as Chromium, Copper, Manganese, Nickel, and Phosphorous is required to be reported under Sara 313 (40 CFR 372.65) when above thresholds.
Keep these 6 Keys to Using Free Machining (12XX) Steels in mind:
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.
These steels are frequently case hardened or carburized in order to achieve desired surface hardness.
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.
These grades have relatively low impact strength at reduced temperatures and should not be used for sub-zero impact applications.
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.
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 .
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:
Sulfur reduces weldability. The higher levels of sulfur make a slaggy joint.
The high volume fraction of manganese sulfides also hold hydrogen. This hydrogen can then create post weld cracking.
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.
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.