Hardness and Hardenability - Part One: Tips and Techniques for Hardness Testing
Hardness measuring methods can be divided into three general categories depending on the manner in which the tests are conducted: scratch hardness, rebound (dynamic) hardness and indentation hardness. For most of us, hardness is simply the measure of the resistance of a material to a force applied by a hardness tester and involves the use of an indenter of fixed geometry under static load (Fig. 1). The ability of the material to resist plastic deformation depends on the carbon content and microstructure of the steel. Therefore, the same steel can exhibit different hardness values depending on its microstructure, which is influenced by the cooling (transformation) rate.
Hardness testing is one of the most common (and often the only) quality control check performed. It is relied upon to determine the success or failure of a particular heat-treatment operation or to understand the material’s current condition. Hardness testing is one of the easiest tests to perform on the shop floor or in the laboratory, but it can be one of the hardest tests to do properly.
Tips for Successful Macrohardness Testing (Rockwell, Rockwell Superficial, Brinell, etc.)A number of simple rules should be followed when hardness testing. These include:
1. Select the proper hardness scale. Understand which scales are best suited for which materials and be careful when hardness testing at the extreme end ranges of a particular scale. For example, performing Rockwell testing in the 90 HRB to 25 HRC scale might best be performed using the “A” scale.
2. Use your calibrated test block (and be sure to select the anticipated hardness range). As a minimum, calibration blocks should be run at the beginning and end of each day, preferably at the beginning and end of every shift. In extreme situations, calibrations might need to be run before and after a particular set of hardness tests.
3. Clean the part and tester. Take the time to remove and clean the indenter and anvil prior to each operation and at shift change. A small amount of debris can alter the reading by several points.
4. Check the curvature of the part surface. A correction factor must be added to the hardness reading of small-diameter shapes and varies with the scale, apparent hardness and part diameter. Wall charts are available from all major hardness-tester manufacturers that show the correction factors. ASTM E18 also lists these corrections.
5. Watch out for non-flat surfaces. Extremely rough or textured surfaces may give inconsistent readings. Lightly sand both the top and bottom side of a sample to ensure flatness and removal of all marks. Remove any scale, debris, dirt and oil before testing.
6. Keep surfaces perpendicular to the indenter. Surfaces should be flat within 2 degrees. Be careful when taking readings on mounted samples. They must be flat, thick and not flex (inside the mount) under load. A microhardness test may be more appropriate.
7. Do not take readings taken too close to the sample edge. Indentations should be no closer than 2½ 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.
8. Do not take readings too close together. Indentations should be a minimum of three diameters apart.
9. Beware of parts or section of parts that are too thin. Unless a special anvil is used, the material should have a thickness at least 10 times the depth of the indentation. ASTM E18 contains thickness guides.
10. Adequately support the parts. Large and irregularly shaped parts need to be well supported. Parts that move, even slightly during the test, produce a false reading. If possible, change the anvil to one that keeps the part stationary. Also, a component may require the application of lower loads because case depths are too shallow or samples are too small for the applied load. The specimen may not be able to physically support the hardness test load without deflection. Samples may need to be externally supported or even fixtured.
11. Check frequently for a damaged diamond or flattened ball. At least once a day, or if readings are suspect, remove the indenter from the hardness tester and inspect the tip using a low power (10-50X) stereomicroscope or loop.
12. Apply common sense. For example, poor lighting or lack of proper magnification on the sample are common sources of accuracy errors. Clamp-on accessories are available to correct these situations but are seldom used.
13. Throw out the first several readings after changing the indenter or anvil, even if they are within range.
Tips for Successful Microhardness Testing (Vickers, Knoop, etc.)Microhardness tests are typically used for very small, intricate shapes, thin parts and case-depth determination. It helps to:
1. Be sensitive to where microhardness tests should be performed on the part. Microhardness tests are conducted on very small areas, so choose where to do the testing carefully. Also, testing machines can only accommodate samples or mounts within a certain size range.
2. Mount samples whenever practical. Where possible, mount and polish samples using standard metallographic techniques. There is a common misconception, however, that all samples must be mounted in order to be microhardness tested. This is not the case if a special jig or fixture is used.
3. Watch sample orientation. The sample or mount has to be flat so that the indenter is in contact with the surface evenly. Furthermore, the sample surface to be tested must be perpendicular to the indenter.
4. Be aware of the effect of surface finish on readings. The condition of the surface affects the ability to read and interpret results.
5. Be aware of sample-preparation-induced effects. Certain materials are more affected by sample-preparation methods than others. For example, aggressive grinding of stainless steels or nonferrous alloys can work-harden material surfaces.
6. Provide the best surface possible for testing. While it is possible to microhardness test a sample after grinding (only), the accuracy of the test may be improved by polishing the specimen as well. Freedom from scratches should be the goal.
7. Be sensitive to vibration. Microhardness testers are sensitive to vibration, which can cause erroneous readings. Do not lean on the bench or table during the test. Location and shock mounting pads will isolate the tester from excessive vibration. Remember, microhardness testers seldom change locations, but the addition or movement of other equipment can change their environment.
8. Watch out for multiphase sample materials. A microhardness test is often used to show hardness gradients within multiphase materials. For general microhardness testing, a homogeneous sample is optimal, but in the case of a multiphase alloy, it is often necessary to take multiple hardness measurements so as to obtain an adequate sampling of different phases (grains) within the alloy. Microhardness testing of cast iron is another example. Often, the sampling scheme is determined on the basis of statistical calculation or understanding which areas to measure for representative results.
9. Try to establish the same testing regime. Ideally, the same operator would perform tests using the same testing apparatus. However, do not assume that your operator or quality control person is performing the tests or documenting results accurately – double check. Repetitive actions lead to problems such as improper rounding to the nearest integer, using the wrong conversion scales and the like.
10. Watch out for the springboard effect. Taking readings too close to the edge (<0.0025 inch, 0.06 mm) of a material with too heavy a load will cause erroneous readings. If lighter loads (<500 grams) are used, comparative results (only) are possible.
11. Remember, not all loads can be converted to Rockwell “C” readings. ASTM E140 states loads of 500 grams or above can be converted to Rockwell “C” readings. The conversion error is skewed higher with increased hardness or decreased loads.
12. Select the heaviest test load weight practical. The lighter the load, the more inaccurate the readings.
13. Know as much of the manufacturing history and heat treatment as practical. The manufacturing processes to which a sample is subjected prior to testing can cause microhardness data to display extreme scatter or be skewed.
Summing UpMost 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 be aware of equipment 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. Next, investigate how the hardness was measured and if it was a suitable method for that sample. While there can be shades of gray and varying levels of uncertainty between hardness-testing machines or laboratories, expect some level of consensus if the methods are correct.
ASTM guidelines state the Rockwell readings should be reported to the nearest integer. Although machines do vary (for example, if measuring on the Rockwell “C” scale), different machines should vary by no more than one HRC point of the accuracy range.
Everyone involved with hardness testing should have and be familiar with the appropriate ASTM specifications, including E3, E10, E18, E103, E140, E384 and others as necessary. These specifications address proper sample preparations, selection of loads and penetrators, sample geometry, minimum sample thickness considerations, roundness corrections, spacing and edge considerations and conversions between scales.
Part two will focus on issues concerning hardenability and hardenability testing. IH