Hardness Testing
Hardness testing presses a test body with defined force into the workpiece surface. The size of the remaining indentation gives the hardness value. This detects dangerous hardening in the heat-affected zone during welding, indicating brittle martensite.
Physical-Chemical Mechanism of Action
A diamond or hard metal body penetrates the surface under a specified test force. The material offers plastic resistance to penetration. The harder the structure, the smaller the indentation remains. In welding, Vickers (HV10) is commonly used. This method uses a diamond pyramid and a test force of 10 kiloponds. The measurable hardness value is directly derived from the diagonal length of the indentation. Besides Vickers, Brinell and Rockwell are common, but for weld seams, Vickers HV10 is best because the indentation remains small and can be precisely placed in the narrow heat-affected zone.
Metallurgical Relevance & Risks in Welding
After welding, the seam cools down. Rapid cooling creates hard, crack-sensitive martensite in the heat-affected zone. Hardness testing reveals such zones. Values above 350 HV10 are considered critical for unalloyed and low-alloy steels. There is a risk of hydrogen-induced cold cracking. A too-soft zone can also indicate softening due to tempering, especially in quenched and tempered steels.
Workshop Practice & Instructions
For weld qualification to standards, at least three indentations each are checked in the weld metal, the heat-affected zone, and the base material. The surface must be ground flat and free of scale. Measurement points are 0.5 to 1 mm from the fusion line. If the hardness value exceeds the permitted limit, the welder must increase heat input, raise preheat temperature, or perform post-weld heat treatment. Then re-test.