Heat Treatment Drives Evolution of Rockwell Hardness Testing
This article was originally published on January 8, 2015.
Unlike the Wright Brothers, Hugh Rockwell and Stanley Rockwell were two engineers working for the same company who shared nothing more than a common last name and an interest in developing a better type of hardness test, specifically to test the hardness of bearing races.
The year was 1914, and both Rockwells were employed by the New Departure Mfg. Company of Bristol, Conn., a maker of automobiles, as well as ball bearings for automobiles. As is so often the case, the mother of invention was necessity to develop a better method. Brinell was too slow, not good for small-radius, curved surfaces or hardened steel, and it also used a large indent that caused sample destruction. For hardened steels, the Scleroscope test was usable but quite difficult. And a third option, the file test, provided no data beyond go or no-go.
Together, the two Rockwells pursued a testing machine method that could measure indentation hardness via the application of a minor and a major load. They applied for a patent that took nearly five years to win approval. By then, both men had left New Departure and went in different directions. However, Stanley Rockwell continued to refine the design and focus on heat treatment of metals. He presented his test during the 1922 convention of ASM’s predecessor, the American Society for Steel Treating, and the Rockwell method of hardness testing gained acceptance throughout the steel and metals industry.
The efficiency and range of the Rockwell process, and its ongoing refinement since the ‘20s and into the digital age, has made it the most widely preferred and used method for hardness testing. Hardness is somewhat of an elusively defined material property (and not to be confused with hardenability, a measure of potential, or toughness, which in metallurgy means resistance to failure under sudden or impact loading).
Given the existence of so many metals with so much hardness variance and that testing is so critical to quality control of metal material advantages, Rockwell has become the go-to method for commercial hardness testing answers. One big reason is that its test results can offer a reliable sense of the yield strength of the material. Another is that hardness testing can aid in comparing property differences of two materials. Lower hardness usually means higher ductility and lower yield strength plus the potential for premature wear. Higher hardness equates with more brittleness and higher yield strength.
According to Daniel Herring, author of Common Pitfalls in Hardness Testing, “Hardness testing is arguably the most common quality-control check performed throughout industry. It is often used to determine the success or failure of a particular heat-treatment operation or to accept or reject material. Hardness testing is thought to be one of the easiest tests to perform on the shop floor or in the metallurgical laboratory, but it can be one of the hardest tests to do properly.”
Herring identifies Rockwell as “used for testing ferrous and nonferrous materials, which have been annealed, hardened, tempered or case hardened, sheet materials in heavier gauges and cemented carbides. Rockwell Superficial is used where lighter loads are required such as testing thin case-hardened surfaces, decarburized surfaces and sheet material in thin gauges. Microhardness tests are used for very small, intricate shapes, thin parts and for case-depth determination.”
In Rockwell testing, a material’s resistance to being indented is evaluated by a steel ball or a diamond cone (the latter is known as a Brale indenter). If the material is known to be exceptionally hard, it is better to use the diamond cone to ensure the steel ball does not get deformed. The steel ball is preferred for all soft materials (those testing less than HRB-100). Since there are no hardness units, Rockwell assigns values in a series of scales (30 in all). In each scale, the higher the number, the harder the material.
The most commonly used Rockwell scales are “C” and “B.” The B-scale is used for softer materials (such as aluminum, brass and softer steels). It employs a tungsten-carbide ball as the indenter and a 100-kg weight to obtain a value expressed as “HRB.” The C-scale for harder materials uses a diamond cone and a 150-kg weight to obtain a value expressed as “HRC.” There are several alternative scales for other purposes. Refer to ASTM E18 to determine the correct Rockwell hardness scale to use. The scale is typically based on case depth and sample size.
Rockwell Superficial Hardness
A second test, Rockwell Superficial hardness, is for use with thin, smaller, or more delicate or surface-sensitive samples. It employs significantly reduced loads. For instance, in a standard Rockwell test, the minor load is 10 kgf and the major load is 60, 100 or 150 kgf. In a Rockwell Superficial test, the minor load is 3 kgf, and the major load is 15, 30 or 45 kgf.
There are numerous applications involving a broad spectrum of metals where high hardness numbers are desired. For instance, the American Iron and Steel Institute identifies nearly 100 different grades of tool steels. More often than not, various heat-treatment processes are employed to increase the metal’s overall hardness because hardness nearly always is a measure of heat-treat performance. The most typical heat treatments include: stress relief, modification cold-worked material, development of physical properties in solid-solution alloys, change in surface composition and development of special characteristics.
It is important that those involved in part design work closely with those doing heat treatment and hardness testing in order to minimize cracking at various stress locations, such as notches, sharp corners and variances in the thickness of sections. Too much cool down of the material can contribute to cracks. In matching a hardness level to the intended application, it is important to keep temperature ranges in mind. You do not want one in which tempering leads to less toughness of the metal. Distortion is another risk, so always bear in mind machining allowances. Finally, stress relief is worth specifying right on the part drawings.
Measurable results and verifiable documentation at every critical step has led to improved accuracy in measurement, improved data collection, and enhanced overall monitoring and control of the hardness testing process. Rockwell and Rockwell Superficial hardness testers typically come with advanced digital-control interfaces for serious precision and at-a-glance presentation of critical data. When conversion between different types of hardness scales is critical, hardness testing software usually has that capability built right in. For instant or ongoing comparative analysis, a wide range of reports is easily generated.
Stanley Rockwell and Hugh Rockwell would be humbled to see how vital a quality-control technology their original invention has become, especially for testing the hardness of metals that are heat treated. They would also be amazed at how many innovations have taken place in the ongoing refinement of the Rockwell process. As long as materials engineers are seeking new advantages in properties and performance, the evolution of Rockwell testing will continue unabated.