How to choose the gauge for a Rockwell hardness tester?

Taiwan Nakazawa Co., Ltd. answers for you:
1. HRA scale (60 kg test force): Suitable for determining the hardness of hard or thin-hard materials, such as cemented carbides, case-hardened steel after carburizing, thin steel after hardening treatment, thin steel plates, etc. Under the HRC scale (150 kg test force), the diamond indenter with a 120-degree cone angle and a 0.2mm tip radius is easily damaged when pressed into the material surface; therefore, high-hardness materials like cemented carbide steel usually opt for the HRA scale.
2. HRB scale (100 kg test force): Suitable for medium-hardness materials, such as quenched low-carbon steel, medium-carbon steel, malleable cast iron, various copper materials, and various duralumin alloys after solution treatment. Under the applied force, creep behavior may last for a long time, causing the dial indicator to move quite slowly and affecting the measurement reading. Conversely, when the material hardness is greater than HRB100, the steel ball indenter may penetrate too shallowly, affecting measurement precision.
3. HRC scale (150 kg test force): More suitable for determining carbon steel, alloy steel, tool steel, die steel, chilled cast iron, pearlitic malleable cast iron, titanium alloys, etc., after quenching and low-temperature tempering. This is because when the material hardness is less than HRC20, if the HRC scale continues to be used, the diamond conical indenter will penetrate too deeply under the 150 kg test force, and the influence of the diamond indenter cone will increase, leading to a sliding phenomenon that affects measurement accuracy; therefore, it is necessary to switch to the HRB scale for measurement.