February 9, 2010
These keys will keep you out of trouble!
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 .
November 17, 2009
Austenitic Grain Size is a material characteristic that is usually reported on test reports and certification documents for the steel materials that we machine in our shops.
Coarse Austenitic Grain Size is a result of NOT ADDING grain refining elements to a heat of steel. Because these Grain refining elements have not been added, the steel has a “Coarse Austenitic Grain Size.”
This is Coarse Grain Austenite. You like it for machining.
Typically this practice is applied to free machining grades such as 11XX and 12XX steels. These steels are sold primarily for their ability to be machined at high production rates.
What does Coarse Austenitic Grain Size imply for the parts that you make?
- Better Machinability– Coarse Grained Steels are more machinable and provide longer tool life than Fine Grained Steels. (The elements added to make the Austenitic Grain size fine create small, finely dispersed hard abrasive particles in the steel)
- Better Plastic Forming– than Fine Grained Steels
- More Distortion in Heat Treat- than Fine Grained Steels
- Lower Ductility at the same hardness- than Fine Grained Steels
- Deeper Hardenability– than Fine Grained Steels
Coarse Austenitic Grain Size will show up on the test report as an ASTM value of 1-5. Values of 5 and higher are called Fine Grained Steels, and are the result of additions of Aluminum, Vanadium, or Niobium in North American commercial practice for most Carbon and Alloy steels.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods For Determining Average Grain Size.
A nice discussion can also be found HERE.
While we think that chemistry may be the controlling factor for machining performance of the steel in our machines, the contribution of austenitic grain size is also important. As long as you are ordering your free machining steels (11XX and 12XX series) to Coarse Grain Practice, Austenitic Grain Size should not be an issue in your shop.