Carbide ball
A hard metal ball is made of tungsten carbide in a cobalt binder phase. It serves as the indenter in Brinell hardness testing. Its extremely high compressive strength prevents the ball from deforming during testing—so you always measure the true hardness of the material.
Physical-Chemical Mechanism of Action
The ball is made of tungsten carbide grains (WC) in a tough cobalt matrix. Tungsten carbide is extremely hard (hardness about 1500–2000 HV) and has very high compressive strength. In the Brinell test, you press the ball with a defined force into the workpiece surface. The ball itself hardly deforms. The resulting indentation shows only the plastic deformation of the material being tested. From the diameter of the permanent indentation, you calculate the Brinell hardness (HBW).
Metallurgical Relevance & Welding Risks
In welds and heat-affected zones, hard microstructures like martensite often form. These zones can reach hardness values above 450 HBW. A steel ball would plastically deform on these hard spots. The indentation would be larger, and the measured hardness would come out too low. You then overlook embrittled zones and misjudge the weld joint. The hard metal ball stays form-stable even at high hardness and delivers reliable values. Risk: a damaged or worn ball leads to faulty measurements. So check before each measurement series.
Workshop Practice & Instructions
Always use a hard metal ball when the expected hardness is above 450 HBW (typical for martensite in welds). Check the ball for cracks, flattening, or chipping before use. Replace damaged balls immediately. Keep test force, ball diameter, and loading time exactly as per standard (e.g., 10 mm ball, 29.42 kN test force, 10–15 s loading time). Use the correct indentation diameter for the calculation. Document the symbol HBW, not HB. Calibrate the hardness testing machine regularly with certified hardness reference plates.