Q: Are there any issues regarding conversion of hardness from one method or scale to another?
A: In one simple word, yes. Hardness is not a fundamental property of a material. In other words, it is not a property like density or elastic modulus. In the case of fundamental properties, conversion factors from one scale to another (such as from pounds per cubic inch to grams per cubic centimeter for density, or pounds per square inch to megapascals for tensile strength) involve simple unit conversion that can be as accurate as necessary depending on the number of significant digits used in the conversion factor.
The word “hardness” is usually used in reference to indentation hardness, which is the resistance of metal to plastic deformation by indentation. Indentation hardness may be measured by a number of different hardness test methods, including Brinell, Rockwell, Vickers, comparison and ultrasonic contact impedance (UCI) testers, as discussed in the previous column (Spring 2008, page 60). Indentation hardness is also sometimes determined by using a rebound hardness method (such as a Leeb tester) and converting the value to one of the indentation hardness scales.
Unfortunately, these test methods produce and measure the indentations in a variety of different manners. For example, Brinell testing involves using a very high load (usually 3000 kgf) to load a 1 cm tungsten carbide ball into the part, measuring the indentation and calculating the hardness based on an equation. Vickers testing is similar, except it indents the specimen with a square-based diamond pyramid using loads usually ranging from 1 gf to 30 kgf. Rockwell testing uses a round-based conical diamond indenter (A, C and N scales) or a spherical tungsten carbide indenter (B, F and T scales), and loads the material in two stages (minor and major loads). The differential penetration of the indenter between the minor and major loads is measured and used to determine the Rockwell hardness.
Indentation hardness readings are affected to various degrees by the fundamental properties of the material being tested, such as the elastic modulus, the yield strength and the work-hardening coefficient. Since the indentation methods are different, the various methods are measuring different combinations of these factors. This makes correlation of hardness readings taken with various methods difficult, even when only one material is involved.
This fact does not seem to be well-recognized in industry, but is known among hardness testing experts. For example, the following paragraph, extracted from ASTM E140-07 (emphasis added), provides strong indications that hardness conversion is not as straightforward as one would like to believe. Paragraphs 6.1 through 6.3 also include a number of cautionary statements regarding conversions.
1.12 Conversion of hardness values should be used only when it is impossible to test the material under the conditions specified, and when conversion is made it should be done with discretion and under controlled conditions. Each type of hardness test is subject to certain errors, but if precautions are carefully observed, the reliability of hardness readings made on instruments of the indentation type will be found comparable. Differences in sensitivity within the range of a given hardness scale (for example, Rockwell B) may be greater than between two different scales or types of instruments. The conversion values, whether from the tables or calculated from the equations, are only approximate and may be inaccurate for specific application.1
The following examples using the tables in ASTM E140 show that hardness conversion is a very risky business:
- In Table 1 (Approximate Hardness Conversion Numbers for Non-Austenitic Steels [Rockwell C Hardness Range]), 248 Vickers is “equivalent” to 61.5 Rockwell “A”. In Table 2 (Approximate Hardness Conversion Numbers for Non-Austenitic Steels [Rockwell B Hardness Range]), Rockwell A 61.5 is “equivalent” to 240 Vickers. Which is correct?
- In Table 2, 240 Brinell is equal to 240 Vickers, but in Table 1, 240 Brinell is equal to 251 Vickers (by interpolation). Which is correct?
The conversion issue becomes even more problematic for materials that are not covered by the standard conversion tables. Many people use ASTM E140 Tables 1 and 2 for hardness conversions for materials that are not covered in any of the tables in E140. For example, assume a specification (such as one of the NACE sour service standards) calls for a particular maximum Rockwell C hardness for a duplex stainless steel (such as 28 Rockwell C), and the hardness for the part is reported in Brinell (e.g., 286 Brinell). The existing ASTM E140 Table 1 for non-austenitic steels would indicate a conversion of 286 Brinell = 30 Rockwell C, which would cause rejection of the material. However, some private testing indicates that 286 Brinell actually converts to less than 28 HRC in at least one duplex stainless-steel material. Unfortunately, verified and standardized tables of conversion values for duplex stainless steels do not exist. This results in false rejection of materials, leading to increased costs and equipment delivery delays.
In summary, hardness conversion is a very complex subject. Conversion of readings from one scale to another or one method to another should be performed only when absolutely necessary, and with great care and consideration. Furthermore, hardness requirements for materials should be specified using methods and scales that are most appropriate for the material (e.g., Brinell for large castings instead of Rockwell B or C). This approach eliminates the need for conversion and the issues that can result.
1. ASTM E140-07 Standard Hardness Conversion Tables for Metals Relationship Among Brinell Hardness, Vickers Hardness, Rockwell Hardness, Superficial Hardness, Knoop Hardness, and Scleroscope Hardness, ASTM International, West Conshohocken, PA.