Welding Inspection & Metallurgy API ICP Self Study Notes

10.4.3 Hardness
The hardness of a material is defined as the resistance to
plastic deformation by indentation. Indentation hardness may be measured by
various hardness tests, such as Brinell, Rockwell, Knoop and Vickers.
Hardness measurements can provide information about the metallurgical
changes caused by welding. In alloy steels, a high hardness measurement
could indicate the presence of un-tempered martensite in the weld or heataffected
zone, while low hardness may indicate an over-tempered condition.
There is an approximate interrelationship among the different hardness test
results and the tensile strength of some metals. Correlation between
hardness values and tensile strength should be used with caution when
applied to welded joints or any metal with a heterogeneous structure.