MECHANICAL PROPERTIES: Generally, density, thermal and electrical conductivity are considered to be PHYSICAL PROPERTIES. The following represent MECHANICAL PROPERTIES.
If you expect a piece of steel to make a certain part, or provide certain benefits, you should know something about the nature of the steel you are purchasing. Understanding the mechanical properties of the steel will give you a better understanding as to how hard it will be to fabricate (cut, form, drill, tap), as well as an idea about how the steel might perform in your intended application (wear resistance, twist or bow, gouging, etc.).
HARDNESS How hard is it? This relates directly to strength, and/or brittleness.
Note: Hardness could well be an entire study i.e., variables affecting results, and interpretation.
YIELD At what point will it bend? (Plastic Deformation)
TENSILE At what point will it break? (Ultimate Tensile)
% REDUCTION IN AREA Pull it from both ends to the point of fracture. This measures the ratio of the reduced diameter at the break, to the original diameter. FUN WITH MECHANICAL PROPERTIES; roll a piece of Playdough to a pencil shape. Then pull it from each end until it breaks. The longer you can pull it and the smaller the diameter at the break, the more desirable.
% ELONGATION When pulled from both ends to the point of fracture, this measures the ratio of the length, at fracture, to the original length once it has been pulled apart from both ends. FOR MORE FUN SEE ABOVE. Once again, the longer you can pull it, the more desirable it is. There are always exceptions to your particular needs, but greater Reduction and Elongation are most often desired.
LCVN (Longitudinal Charpy “V” Notch) How much impact will it withstand?
Stick a short test specimen vertically protruding from a vice.
Notch it, then strike it above the notch with a weighted pendulum. Measure how far the pendulum travels after the specimen breaks
This is the closest indicator of “Toughness.” (Generally accepted as the standard for measuring impact strength.). “Toughness” is the main deterrent to Fatigue Failure, one of the greatest causes of shaft failure in heavy industrial applications.
RULES OF THUMB – NOT FOR ENGINEERING PURPOSES!
Two of the most common hardness testing methods are The Rockwell Test and The Brinell Test.
30 RC Hardness, in Rockwell “C” scale equals roughly 300BHN in The Brinell Test, 40RC = 400bhn, and so on…
Tensile (Breaking Point) equals about one half of The Brinell (BHN) reading.
EXAMPLE: Where BHN is 300, the Tensile will be approximately 150,000psi.
Yield (Bend Point, or point of plastic deformation) = is approximately 70% of Tensile
Generally, you want the yield strength to be somewhat lower (by 20 to 30%) than
the tensile. “Gives you a chance to get out of the room before the shaft breaks.”
THOUGHTS TO REFLECT ON: The test reports (TRs) for steel mill heat lots (batches of steel), are simply random checks. For 5000pcs of ½” Diameter steel bars, only a sampling will be tested. Even within those tests, results will vary. Recorded readings will usually show some variation which may be due to interpretation of results, test preparation, instrumentation, location in furnace, bar ends, etc. The surface of a wear plate may display several different hardness readings when tested in different places on the plate, by different facilities or by different people. This is not to suggest that readings recorded on Test Reports (TRs) are invalid. It is to encourage perspective within the realm of any mechanical testing. In fact, in situations where criteria are highly critical, the best method is to statistically check the actual finished part. Endurance limit testing is an excellent example of this. Many times, customers will request endurance limits on raw steel. Unless the actual finished part is tested, endurance limits posted on any raw steel product are useful only for the broadest suggestion of potential performance.
There is an excellent movie that illustrates this perspective; I believe it is “No Highway in the Sky”, starring Jimmy Stewart. It is an old movie (1950). Very popular among metallurgists and engineers. Enjoy.
-Howard Thomas, January 7th, 2022