Low-load hardness testing
The low-load hardness test creates a small, precisely measurable indentation in the material with low test force. This way, the inspector measures hardness precisely in narrow zones—for example, in the heat-affected zone of a weld seam. A high hardness value there indicates glass-hard, crack-prone martensite.
Physical-Chemical Mechanism of Action
In the low-load hardness test, the inspector presses a diamond pyramid with a defined, low force into the surface. The force is typically between 1.961 N and 29.42 N. The indenter leaves a tiny, pyramid-shaped indentation. The inspector measures the diagonals of this indentation under the microscope. From the indentation area and the test force, he calculates the hardness. Due to the small force, the indentation remains very shallow and locally confined. This method barely damages the component and allows measurements directly in narrow microstructural zones.
Metallurgical Relevance & Risks in Welding
The hardness in the weld zone indicates the microstructure and thus the crack risk. In welding, hard microstructures such as martensite often form in the heat-affected zone. A hardness that is too high—over 350 HV for unalloyed steels—means brittle material and a high risk of cold cracking. Hardening in the weld metal or in the dilution zone also indicates critical cooling rates. Only with precise hardness measurement can such danger points be reliably detected. A general measurement on the component overlooks local hardness variations.
Workshop Practice & Instructions
For weld seams, the low-load hardness test with a Vickers indenter and test forces from HV0.5 to HV3 is suitable. The inspector grinds and polishes the measuring point across the weld seam. He makes at least three indentations per zone: weld metal, heat-affected zone, and base material. He keeps the test force constant for 10 to 15 seconds. For unalloyed steels, a hardness over 350 HV indicates martensite. Then the welder must extend the cooling time t8/5—for example, by preheating or increasing the heat input. After heat treatment, he checks again whether the hardness has dropped below the limit value.