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 .
Stress and stress risers are words that we often hear in our shops. Usually when bad things have happened to our work. Here are 5 ideas to reduce stress in precision machined products and a brief tutorial on what it is when the engineers say “stress.”
Stress (when used by designers, engineers, and metallurgists) refers to the measurement of load on a part or test specimen related to the area under that load. Stress can be considered to be have three modes, axial (in line) , bending (you know what that means), or torsional (twisting or torque). The following graphic illustrates some stress states our parts may encounter.
Residual stress can be considered to be a kind of “internal pressure” in the material which may act in the same direction as the stress applied to the part. Because of this, it can actually reduce the load carrying ability of the part. This is what usually results in failures. Characteristics of the part may also contribute to the concentration of these internal stresses, leading to premature failure of the parts once in service and subjected to load.
Here are 5 ideas to reduce stress in precision machined parts.
Assure a smooth surface.
Use a larger not smaller diameter for threading.
Always maximize the fillet or radius between section or diameter changes.
Provide both pads and relief areas on parts where applicable.
Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction.
Assure a smooth surface. The creation of a smooth surface prevents the concentration of internal stresses at sharp changes in surface. Parts with smooth surface finish are much less likely to fail than parts where deep grooves, tool marks or pits can allow stresses to build up. Use a larger not smaller diameter for threading. This is both related to the strength of the additional material as well as to the geometry and radii between change of dimensions. The more generous radius possible with the larger diameter for threading can improve the endurance limit of the part substantially. In heat treated 4340, the increase in radius from 0.015″ to 0.090″ increase the endurance limit from 34,000 to 65,000 psi. Always maximize the fillet or radius between section or diameter changes. Any design which allows stress to concentrate locally will promote fatigue failure. Generous radii and fillets are inexpensive insurance against premature failure. Make sure that the designer has provided both pad and releif areas on parts joining perpendicularly. Instead of having a single point or locus for the change in forces to be distributed through the part, pads and relief areas diffuse the stresses that would otherwise be concentrated, improving the performance of the part. Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction. Many of the materials that we prefer to machine are resulfurized, and in these steels, the manganese sulfides can in fact lower the steel’s transverse mechanical properties. Also, cold drawing and or forging prior to machining can influence grain flow which can enhance the ability of the material to carry the load. The material the designer selected could be a large reason for the material’s ability to handle stress, or not.
There you have it. Stress = Load. Don’t give it places to concentrate on your precision machined parts.