Microstructural hardness correlation
A metal's hardness depends on its microstructure, including grain size and dislocation density. Fine grains and high dislocation density boost hardness.
Physical-Chemical Mechanism of Action
The hardness of a metal is a measure of its mechanical strength, determined by the arrangement of atoms in its microstructure. Small grains and a high dislocation density (number of dislocations per volume) increase hardness, as they restrict the freedom of movement of atoms.
Metallurgical Relevance & Welding Risks
With insufficient cooling time or too high a carbon content, the structure can become coarsened to glass-hard martensite, which increases hardness but at the same time reduces toughness and dimensional stability. This leads to increased susceptibility to cracks and hot cracks.
Shop Practice & Action Items
To ensure an optimal microstructure, pay attention to the correct cooling time (t8/5 between 5 and 25 seconds) and the carbon content. Use suitable welding methods and parameter settings to control the structure and achieve a balanced hardness.