Hardness Testing: Problems and Solutions
Rockwell hardness testing is arguably the most common tool used by the heat-treating industry to measure the success or failure of a heat-treatment process. It is one of the seemingly easiest tests to perform on the shop floor, in the quality-control department or in the metallurgical laboratory but arguably one of the hardest tests to perform properly. Let’s learn more.
What is hardness?
A common definition of hardness is the measure of the resistance of a material to an applied force and involves the use of an indenter of fixed geometry under static load. However, hardness can also refer to stiffness (temper resistance) or to resistance to scratching, abrasion or cutting. It can be thought of as the ability of the material to resist permanent deformation (i.e., to be bent, broken or have its shape changed) in service. The greater the hardness, the greater its resistance. It is important to recognize that the hardness of a material is highly dependent on its microstructure, however, which in turn is influenced by the heat-treatment process.
Hardness is not a fundamental material property but rather a composite value. As such, it is an indication that all is well, not a guarantee that it is so. It is of great interest because hardness can be directly related to the expected strength of the material, which would otherwise require destructive testing to measure. The location of the hardness indentations, away from working surfaces, is often an important consideration if hardness testing is to be considered a nondestructive test.
Hardness measuring methods fall into three general categories, depending upon the manner in which the tests are conducted: scratch hardness, indentation hardness (Fig. 1) and dynamic rebound hardness (Fig. 2). Rockwell hardness testing machines can be either benchtop or portable devices.
Rockwell Testing Tips
Many different factors can affect the results of a hardness test. In most instances, incorrect or misleading readings are the result of poor testing practices that violate simple yet basic testing rules. This leads to good parts being discarded or scrapped or, worse yet, bad parts being accepted and used. By following the rules, accurate hardness readings can be obtained. To aid in this endeavor, we have gathered some of the most common Rockwell hardness testing problems and solutions faced by the heat treater (in no particular order).
- Follow the testing guidelines outlined in ASTM E18 (latest revision), “Standard Test Methods for Rockwell Hardness of Metallic Materials” or the appropriate ASTM standard for the material being tested.
- Always use a hardness tester that has an up-to-date calibration sticker from an approved outside service provider.
- Use a test block to verify hardness readings. Typically, three readings are performed on a test block and the average compared to the value stamped on the block. This should be done daily. Never use a test block on both sides. It is intended to be used on one side only because diamond marks on the bottom side facing the anvil will cause incorrect readings. Old test blocks can be ground down, marked as scrap and used to “set the indenter” when either the indenter or anvil is changed. Remember, the first reading on a part after an indenter or anvil change should be discarded.
- Inspect the indenter for damage (chipped or cracked diamonds or flattened balls) that will produce erroneous readings. Perform the inspection on a weekly basis by removing the indenter from the machine and inspecting the tip using a low-power magnification (10-50X) such as a stereomicroscope or jeweler’s eye loop to check for damage. Flattened balls are sometimes difficult to detect unless you inspect all surfaces – often at an angle. If you see consistently high or erratic readings, perform this inspection immediately. An old heat-treater’s trick is to place one finger over the tip of the indenter when removing it to prevent accidental damage.
- Cleanliness of the part and tester is paramount. Remove any scale, debris, dirt and oil on the part or the machine before testing. Even a small amount of debris can alter the reading by as much as several Rockwell points. Remove and clean the indenter and anvil prior to operation and at shift change. Lightly sand then clean both the bottom and top surfaces of the part before hardness testing.
- Non-flat surfaces can alter readings. Extremely rough or textured surfaces (e.g., machining marks) may give inconsistent readings. Lightly sand both the bottom and top surfaces before hardness testing.
- Take into account the curvature of the surface. Remember that a correction factor must be added to the hardness reading of small-diameter shapes for Rockwell A, C and D scales and varies with the apparent hardness and part diameter. The correction factor to be added is shown in the appropriate ASTM E18 tables. In addition, do not rotate a previous diamond mark downward toward the anvil because this indent will cause microscopic movement and a resultant low reading.
- If the part moves, the reading is invalid (even if it is within the specified range). Discard these readings and do not include them in your average. In many instances the readings are recorded with an appropriate note as to why they are not being used.
- Remember that a minimum case depth is needed to support a given Rockwell scale and produce a valid reading (see ASTM E18 for details). On the Rockwell-C scale, for example, a minimum case depth of 0.030 inch is needed to hold a 60 HRC reading. A shallower case depth may yield a soft reading, so changing to either the A scale or a superficial scale such as the N scale is appropriate.
- Surfaces not perpendicular to the indenter will give false readings. Remember that surfaces should be flat within 2 degrees. Be careful when taking readings on mounted samples. They must be flat, thick and not flex under load. A microhardness test may be more appropriate.
- Remember that the A scale spans both the Rockwell-C and Rockwell-B ranges and is often a good referee.
- Readings taken too close to the sample edge may damage the indenter and will produce false readings. Per ASTM E18, indentations should be spaced no closer than 2.5 times the indenter diameter from the edge. If the metal buckles outward, the indenter is too close to the edge and the reading is invalid.
- Readings taken too close together will give false (higher) hardness readings. This is also true on the test (calibration) blocks, where often one tries to place too many readings in one area to save the cost of buying a new block. Indentations should be spaced per ASTM E18 – three diameters apart.
- Parts that are not properly supported will give false readings. Large and irregularly shaped parts need to be well supported. Parts that move, even slightly during the test, produce a false reading – even if that reading falls within the desired hardness range. Change the anvil to one that keeps the part stationary. Additional outside support devices (such as a Steady-Rest®) may also be required.
- A sample that is too thin will yield false readings. The material being tested should have a thickness at least 10 times the depth of the indentation. The minimum acceptable thicknesses can be found in ASTM E18 tables. Special (prehardened) anvils can be used when hardness testing thin sheet or foil material.
Finally, put up wall charts and have laminated cards in convenient places that show the various hardness testing scales and their relationships to one another. Almost all companies that manufacture hardness testing equipment offer them for free.
Most people need not be experts in all the intricate details of hardness testing. However, it is important that the user selects the appropriate hardness testing method and scale, considers part geometry and test location, and accounts for equipment and testing limitations. Failure to do so can lead to improper interpretations of the true material condition, properties and hardness.
Should you find yourself in a dispute regarding hardness and hardness testing methods, the first item to confirm is that the specified hardness is appropriate for that material. Then confirm how the hardness was measured and if the method was appropriate for that sample. While there can be varying levels of uncertainty between hardness testing machines or laboratories, expect some level of consensus if the methods are correct.
- ASTM E13 (www.astm.org)
- Herring, Daniel H., Atmosphere Heat Treatment Volume II, BNP Media, 2015
- Specht, Fred, “Testing of Induction Hardened Work Pieces,” Thermal Processing, September 2013
- Midea, Sandra J., “Hardness Testing: The Basics of Rockwell Hardness Testing,” HOT TOPICS in Heat Treatment and Metallurgy, November 2003