Each Rockwell hardness scale has a specific usable range, which can be easily determined from the graduations on the hardness tester’s dial. The black scales (HRA, HRC, HRD), which utilize a diamond indenter, are graduated from 0 to 100, while the red scales (HRB, HRE, HRF, HRG, HRH, HRK), which utilize a steel ball indenter, are graduated from 0 to 130; scales on Superficial Rockwell hardness testers range from 0 to 100. In practice, however, the actual usable range for each scale is often narrower than the range of graduations marked on the dial.
The usable range for the HRC scale is 20–70 HRC. When the hardness value falls below 20 HRC, the conical section of the indenter penetrates too deeply, resulting in reduced sensitivity; in such cases, the HRB scale should be used instead. Although the specified upper limit for the HRC scale is 70 HRC, testing materials with a hardness exceeding 67 HRC subjects the diamond indenter tip to excessive pressure, making it prone to damage and significantly shortening its service life; therefore, the HRA scale is generally preferred in these instances.
The usable range for the HRA scale is 20–88 HRA; however, the lower end of this range is rarely utilized in practice, as 60 HRA is equivalent to 19 HRC. Consequently, the HRA scale is typically reserved for testing cemented carbides.
The usable range for the HRB scale is 20–100 HRB. When the hardness value is below 20 HRB, the steel ball indenter penetrates too deeply, leading to increased metal creep and prolonged deformation of the specimen under the test load, which reduces measurement accuracy; the HRF scale should be used instead. Conversely, when the hardness value exceeds 100 HRB, the penetration depth of the steel ball is too shallow, resulting in decreased sensitivity and precision; in this case, the HRC scale should be used. Although the standard specifies an operating range of 20–100 HRB for the HRB scale, its practical application in my country is limited to the 70–100 HRB range due to a shortage of standard hardness blocks in the low-to-medium value range. This significantly hinders the use of the HRB scale—a crucial Rockwell hardness scale—within the country, resulting in the lack of a vital testing method for the hardness of mild steel and copper alloys. Consequently, the adoption of Rockwell hardness testing methods in Chinese product standards regarding material hardness lags noticeably behind international standards.
The operating range for the HRF scale is 60–100 HRF. Widely used internationally, the HRF scale is an excellent method for testing pure copper and softer copper alloys. However, in my country, the shortage of standard hardness blocks has severely restricted its application as well.
The HRG scale is suitable for materials with HRB values approaching 100. When a specimen’s hardness nears 100 HRB, switching to the HRG scale can significantly enhance testing sensitivity, thereby improving measurement accuracy.
The issue regarding the shortage of HRB (low-to-medium range) and HRF hardness blocks is expected to be resolved in the future. Our company Mikrosize plans to commence production of these specialized hardness blocks at an appropriate time; the necessary technical preparations have already been completed, and they will be released for industry use once ready.
