Specifying Metal Hardness – Is it Enough?
Rockwell and Brinell hardness tests are common metal characterization methods used to determine whether metal stock or a metal component has the required properties. The reason for this is that these tests are simple and quick to perform, in addition to being inexpensive. However, while these tests do provide useful information, there is a danger to the common practice of specifying only hardness and alloy composition on component design drawings.
There are circumstances where a metal can meet the composition and hardness requirements, and still be unsuitable for use in the intended application. This can occur when the metal microstructure is deficient in a way that is undetectable by Rockwell or Brinell hardness testing.
Consider the microstructure of a steel component. The microstructure consisted of martensite, with ferrite on what were austenite grain boundaries (arrows) before the austenite transformed to martensite. This was the result of improper quenching during heat treatment. The sample had a hardness of Rockwell C 38. However, the ferrite on the prior austenite grains compromised the mechanical properties of the metal by allowing a fracture path through the relatively soft ferrite. Without specifying that the microstructure should be completely martensitic, the component designer risked using components that met the spec, but were potentially unreliable.
Other examples where it is possible to have a Rockwell or Brinell hardness value that meets specification, but still have a component that is unreliable or unacceptable are:
- Decreased corrosion resistance of stainless steels due to the formation of chromium carbide particles on the grain boundaries (sensitization).
- Increased wear of bearing steels with hard carbide particles on the surface.
- Increased susceptibility to stress corrosion cracking of precipitation hardened 2xxx aluminum alloys due to the formation of precipitates on the grain boundaries.
- Orange peel in deep drawn components made from sheet metal with excessively large surface grains and normal size grains at the interior
The main thing to take away from this discussion is that specifying composition and hardness is often not enough. In many cases, it is important to specify the microstructure of stock metal or a component to improve the likelihood of consistently getting the desired metal properties. The importance of this increases as the expected component performance and reliability increases. Other analyses, such as tensile testing may also be required.
More information about the effects of microstructure on properties and the effects of processing on microstructure is in our Principles of Metallurgy, Metallurgy of Steel Heating, and Corrosion of Metals courses. There are also many books that discuss the relationship between processing, microstructure, and properties.
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