The inspection of manufactured goods has majorly become standard practice to guarantee that products meet increasingly demanding specifications. In certain cases, this can mean that long established methods of quality control will have to be pushed to their limits. One example refers to the heat treatment of steel. Hardness testing has been established to check heat treatments for more than 100 years.
As technical developments have become more refined, and applications more specific, the test methods have developed further alongside technology. In the greatly competitive cutting blades industry, it is essential for the blade to have suitable hardness in order to retain an edge, while excessively hardening can cause brittle failure. When hardened materials are ground, residual heat may alter the microstructure and also soften the blade edge.
By design, the blades have minimal mass, thus control of residual heat during the final finishing process is also important. Accurate testing for hardness is thus importantto ensure a high quality product. Testing the hardness of the cutting blades becomes interesting as indents must be made and then measured accurately, making use of low loads and correspondingly small indents. This needs proper specimen preparation and careful use of hardness testing equipment. This article reviews a guide to metallographic preparation and microhardness test processes for this application; even though the principles are applicable to the use of Vickers testing for any hard material applications.
Before diving in to specimen preparation, it is important to select methods suitable for the specimen material and expected hardness. For the blade example, the expected Vickers Hardness (HV) value may be upward of 700 HV and can extend into 1200 HV range. For such a hardened part, the inspection sample will have to be mounted, ground and then polished while maintaining a flat edge. Flatness is of utmost importance for attaining valid symmetrical hardness indentations, as rounding or tilt in the specimen can cause significant errors in measurement.
The ASTM-E-92 Standard for Knoop and Vickers hardness testing suggests that any indent shall be 2.5X its diagonal size away from any edge or other indent. Placing an indent next to the blade tip thus needs low loads, so that the expected indent diagonals will be extremely small, i.e. 4 to 5 μm diagonal length. With such a small indent size, accurate measurement would be extremely difficult if the hardness tester does not have high magnification objectives. Even with a 100X objective, manually measuring such small indents with an eye-piece can differ significantly between operators. These factors affect reproducibility and repeatability of hardness test results.
With an in-depth focus on specimen preparation, one can greatly reduce the amount of time for getting the specimen ready for hardness testing. Using a high precision sectioning saw can allow sectioning extremely close to the area of interest without the risk of specimen damage or heating. Cubic Boron Nitride blades are more ideal for ferrous materials than diamond, even though it is possible to use abrasive blades on some machines.
The cut should be at a distance away from hardness test plane and that takes into account the anticipated thickness for grind and polish removal and the thickness of the sectioning blade. Usually, with less damage produced during sectioning, less grinding will be needed (with coarser grits); in turn, this reduces the risk of damaging the area of interest.
With cutting blades or other specimen materials having high aspect ratio, it is desirable to mount several blade samples together. This can be more efficient, and also having multiple specimens will keep the mount more stable during preparation and help retain flatness. Using support clips is a good way to hold the samples perpendicular to the mount's bottom plane. The mounting material should be selected with characteristics of maintaining best edge retention – low shrinkage and high hardness. Two types of mounting routes are available to choose: Castable Mounting or Hot Compression. With castable mounting, the best selection would be a hard, very low shrinkage acrylic material, such as VariDur 3003. With a compression mounting process, the best choice would be a fine grain, hard, mineral filled, epoxy material, such as Epomet F.
It is essential to clean and then dry the samples well prior to mounting; and not doing so can lead to shrinkage gaps between the mounting material and specimen. Shrinkage gaps prevent the edges of the specimen being supported during preparation, which results in edge rounding, and are also sites that collect and then disperse contaminants during the grinding and polishing process.
The use of a semi-automatic grinder/polisher permits more reproducible and consistent preparation. Diamond grinding disks (DGD) and no-nap cloths are recommended in order to get the specimens as flat as possible. Central force grinding was also used to maximize planarity and guarantee uniform grinding. With preparation of extremely hard material, it is vital to minimize the amount of time polishing on soft surfaces; otherwise it will lead to edge rounding. Polishing steps should be optimized and not be excessive. If the finish is not good enough after the final stage, instead of polishing it is suggested to go back and repeat earlier stages.
The polishing route employed is outlined in Table 1 below. A series of Apex DGD grinding disks were used to planarize the sample, and consecutively reduce scratches. Diamond disks are far superior to silicon carbide paper for retaining flatness. The TriDent cloth used for 3 μm diamond and the ChemoMet used for the 0.05 μm final polishing step were both selected for attaining best flatness in the sample.
|Grinding/ Polishing, High Hardness ferrous material
>> Comp • Last 15-20 seconds flush platen with water * 1.25” mount, scale load by specimen mount diameter *Table 1: Metallographic preparation method
Selecting the correct set-up for microhardness testing on cutting blade specimens is critical. Proper system configuration is also important. The tester should be isolated from environmental vibration for low load microhardness testing. Inaccurate load application can occur if vibration is an issue. Given that the test area is limited to the blade tip, repeating an indentation may require re-preparing the specimen. Thus, an indent to be placed at the blade tip leaves no room for error.
The hardness tester will have to be accurate and repeatable at low loads. A load cell tester permits highly accurate application of load, whereas drop-weight testers can be susceptible to slight overloads. Regardless of which type of tester is selected, the tester must be repeatable, accurate and in compliance.
A 100X objective is a must when measuring low load indents. ASTM E92 addresses the inherent difficulties with making and measuring indents less than 20 μm due to the various possible measurement error. A 10% error (0.4 μm) in measuring 4 μm size indent at 100X via an eye-piece may not be uncommon, as the limits of optical resolution are being reached. Measurements made on a monitor improve accuracy and are more repeatable than those measurements taken via a filar eye-piece, due to enhanced visibility. When diagonals are down to 4 μm size, a digital image makes measuring such small indent easier. The option of digital magnification would further improve measurement accuracy and repeatability.
Current automatic hardness system uses computers and integrated software to control the hardness tester. It becomes an intelligent tester using sophisticated measurement algorithms in order to capture an indent’s image and measure its diagonal lengths automatically. Auto-measurement allows fast, accurate and repeatable results and will convert the measured diagonals directly to a hardness value without needing an operator to carry out any calculation or use a look-up table. All of these functions. significantly help to reduce the error and variance between operators.
The tester has to be capable of locating and then placing indents at designated points. An automatic tester will permit programming of hardness traverses with multiple indents at designated locations. The more intelligent software system allows specimen tracing and indent placements over developed templates. For high volume quality verification testing, multi-sample testing can majorly reduce testing operation time. Time studies on automatic hardness testing have demonstrated a time saving of more than 80% compared to manual testers. Certainly, the test process will differ for each situation; however, actual time saving is generally significant.
Figure 1. Microhardness indent performed on Knife Blade Tip, HV10gf 747 @ 26 µm from blade tip. Tester: VH3300 automatic hardness tester with DiaMet software.
Image Credit: Buehler
Quality assessment of cutting blades has its challenges, but these can be overcome. Technique and attention to detail with specimen preparation is a key factor to success. Having the specimen flat with no edge rounding will enable an indent to be located near the blade tip. The use of fully or semi-automatic grinder/polishers will provide reproducible, uniform specimens.
The use of automatic hardness testers with integrated software systems and high quality optics is the other key factor. They reduce operator error and variance between varied operators. Auto-measurement of indents is accurate and quick; indent placement is precise and repeatable. And the total hardness test time is significantly reduced.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
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