What is the Rockwell Scale?
- Rockwell hardness testing is the most commonly used hardness testing method to measure the permanent depth of an indentation, which is finally compared to the Rockwell scale to derive the hardness number.
- Hence, the Rockwell scale specifies the indentation hardness of sample materials using the depth of penetration of an indenter. This indenter is loaded on sample material and compared with the penetration in some reference material. It is one of the major definitions under the topic of hardness testing in materials science. The Rockwell scale applies a minor load followed by a major load. The hardness value is then noted directly from a dial.
- The major advantage of the Rockwell scale is its ability to show hardness values directly. Hence this method successfully avoids further tedious calculations that are involved in other measurement techniques.
- Its setup is relatively simple and cost-effective which enables easy installation even in college laboratories. This method is specifically used in the metallurgy and engineering sectors. It is most commonly prevalent in the USA. Its unique saleable popularity arises from its reliability, speed, resolution, robustness, as well as a small area of indentation.
- There are various alternative scales of the Rockwell method. The most commonly used ones among them are the ‘B’, and ‘C’ scales. Both these scales show hardness as a subjective dimensionless number.
- The Rockwell B-scale can be used in the case of softer materials such as brass, aluminum, and softer steel. It utilizes a hardened steel ball as an indenter along with a 100kg weight in order to derive a value, which is expressed as ‘HRB’. Similarly, the Rockwell C-scale is used for harder materials such as a diamond cone, which is otherwise known as a Brale indenter, along with a 150kg weight to derive a value, which is expressed as ‘HRC’. In a nutshell, it is the depth of penetration that gets converted to a scale where a harder material derive a higher number.
Let us go through the range of hardness values for varied materials as listed below:
- Very hard steel such as a good knife blade will get the hardness value of HRC 55 to HRC 62 or so
- Materials such as axes, chisels, etc. will get the hardness value of HRC 40 – 45
- HRC values below 20 and the readings much above 100 are usually considered as non-reliable.
- Other scales, especially the extensive A-scale, are utilized for specific applications. There are several special scales to measure case-hardened specimens.
Good practice includes the following steps listed below:
- The indenter, as well as the test-piece of the Rockwell hardness tester, should be cleaned well in order to get rid of dirt, rust, grease, or paint
- The measurement should always be taken on a flat, perpendicular surface. The round work correction factors should be raised to regulate the test-piece curvature
- The inspector must ensure that the thickness of the test-piece should be around ten times the depth of the indentation
- A sufficient space should be maintained constantly between multiple indentations
- It is mandatory to control the speed of the indentation while measuring in the Rockwell hardness tester
Standards used for Hardness scales
- ISO 6508-1: This includes the Rockwell test method with varied scales such as A, B, C, D, E, F, G, H, K, N, and T.
US Standard (ASTM International)
- ASTM E18: This is the standard method to determine Rockwell hardness as well as Rockwell superficial hardness of metallic materials using Rockwell hardness tester.
Other than the Rockwell method of hardness testing, there are varied methods such as Brinell, Vickers, and Leeb tests. Though it is not possible to provide a precise conversion in most of the cases, it is easier to provide an estimated material-specific comparison table for the steels as mentioned below:
Hardness Conversion for Rockwell C Scale or High Hardness Range
In the case of Rockwell Scale, Hardness range comes in two types as mentioned below:
- High hardness range
- Low hardness range
High hardness range involves hardness conversion for Rockwell C Scale, while low hardness range involves Rockwell B Scale.
The below mentioned chart will highlight the hardness conversion for Rockwell C Scale, otherwise called as high hardness range.