Typically, aerospace parts such as gears, shafts and bearings are heat treated so that their surface is hardened to resist wear. The depth of this hardened layer is known as the “case depth” and must be quantified.
There are several ways to determine case hardness using microindentation equipment. In all cases, a standardized process must be employed to ensure the greatest repeatability. Proper standardized processes take into account not only the hardness testing procedure itself but also sample preparation and microstructural analysis.
Preparing the Sample for TestingBefore microhardness testing can be performed, it is imperative that a high-quality surface be created to ensure both accuracy and repeatability of the tests. Preparing samples for hardness testing generally involves four steps, broken into two distinct stages – preparation (sectioning, mounting and grinding) and polishing.
The sectioning step involves a small, representative specimen being extracted from a much larger sample. Coolant should always be used during sectioning, preferably one with a rust inhibitor to prevent rusting of saw components. Without a coolant, the specimen can be “burned,” resulting in an altered microstructure and erroneous microhardness readings.
The choice of mounting media plays an important role in the quality of specimen preparation. If very hard materials, such as induction-hardened carbon steel, are mounted in soft mounting media, the mount will grind away more rapidly and leave a slightly domed specimen. This sample geometry can result in misshapen hardness indentations near the sample’s edges. A metallographic laboratory should have a variety of mounting media available – both compression and castables – to ensure that the proper mounting media can be selected for the different materials that are processed.
Of the four steps involved in the preparation process, the one most overlooked is the grinding step. Good grinding techniques constitute approximately 90% of the effort involved in preparing a metallographic specimen. Regardless of what sophisticated microhardness testing system is used, the information obtained can be useless if poor grinding procedures are performed. For instance, residual deformation can be present to such a degree in the specimen that false hardness readings are obtained. If a good grind is not achieved, more often than not the specimen will need to be taken back to a grinding step when it is already through the final polishing stages. Good grinding techniques can usually be developed easily, and the metallographer is urged to cultivate them as soon as possible so they become standard.
The final step in sample preparation is polishing. For most microhardness testing, two diamond-polishing steps will usually suffice. Using 3-micron diamond compound with a silk cloth, followed by 1-micron diamond compound on a red felt cloth (and a suitable a lubricant) will effectively remove the scratches from the final grinding step (Fig. 1). There are literally hundreds of cloths that can be used for polishing, but one criterion should be followed when polishing with diamond compound: A napless cloth, such as silk, should be used to ensure the entire surface of the specimen comes in contact with the diamond particles and the specimen remains flat. Polishing should be continued until all traces of the grinding process are removed.
Performing Case-Depth AnalysisNow that the sample has been properly prepared, it is ready to be analyzed for case depth. In aerospace applications, the test piece will typically have a specification (spec) sheet outlining the test process for the particular sample. This spec sheet will include the test load required, test locations, indentation spacing and case depth min/max values. The heat-treating process typically creates a case hardness of HRC 50, approximately 1.27 mm (0.050 inches) thick. It is critical that the heat treater be able to verify that the proper ratio of hard surface to soft core has been achieved. The case-depth requirement for a particular sample is usually known in advance, so the operator is typically looking for a distance value that has been predetermined. A minimum and maximum value is usually stated for operator reference. The test consists of a single row or column of indentations traversing into the core of the test sample. This traverse will then go through the heat-treatment transition zone in order to determine the distance – 50 HRC was found at 0.81 mm (0.032 inches). Case-depth studies involve the largest numbers of tests to be performed on a hardened steel part.
There are two main types of microindentation hardness tests that can be performed – Vickers and Knoop. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Hardness Number (HV), or Diamond Pyramid Hardness (DPH). HV is determined by the size of the indentation using manual, semiautomated or fully automated methods. Vickers tests have great flexibility since the indenter can be used for most materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe a material’s ability to resist plastic deformation from a standard source.
The Knoop hardness test is used particularly for very brittle materials, thin sheets or coatings/layers where a small indentation is required. A rhombic-based pyramidal diamond indenter is pressed into the polished surface of the test material with a known force for a specified dwell time, and the resulting indentation is measured using a microscope. One advantage of this test is that a very small sample area is required.
Testers Used During Case-Depth AnalysisManual Hardness Testing with Ocular
Using a manual microindentation tester, the operator makes the appropriate indentations using a manual stage to move to each indentation location (being sure to accurately measure the indentations between each movement). Precision microscopes, with a magnification of 400X or greater and measuring to an accuracy of +0.1 micrometers, are used to measure the indentations. These measurements are then converted to a hardness value – either Knoop or Vickers – using a formula or look-up table (Fig. 2).
Performing this conversion manually is time consuming. The need for quicker testing coupled with an increased demand for high-volume microindentation testing led to the development of digital testers. While eliminating the need to manually calculate hardness values, they do not address the repeatability issue caused by the operator sitting in front of the tester for hours at a time peering through the digital ocular measuring indentations. These operators also need extensive training to repeatedly and consistently measure indentations. The tedious and time-consuming nature of this process does not allow the operator(s) to perform additional laboratory duties.
Video Measuring Systems
As most employees have a certain degree of computer experience, PC-based video measuring systems are becoming more and more popular in all types of workplaces, including test labs (Fig. 3). The use of these systems, however, still necessitates a standardized process in order to obtain the best repeatability. The operator loads a preset program based on the spec sheet to make the appropriate indentations. Using the manual stage, the operator then follows the program to move to each indentation location (being sure to accurately measure the indentation between each movement). These measurements are then automatically converted to a hardness value – either Knoop or Vickers. These types of systems make reading a high volume of sample indentations much less tedious and more productive.
Automated microindentation systems also require a standardized process in order to obtain the best repeatability. These systems offer the ability to follow the exact test specifications on a given part, including the ability to accurately follow all angle, distance and spacing requirements that may be involved. The case-depth sample’s spec sheet still must be followed to do the analysis. These types of systems offer minimal operator involvement due to their pre-programming, auto-traversing and auto-measuring capabilities and offer the most repeatable results (Fig. 4). Once the correct analysis application is chosen, the motorized stage moves the part into the desired position, makes and then measures the indentations. Software then calculates the results and plots the case depth, printing a test report in the desired format. These systems usually have the ability to share results with customers or other company facilities, or incorporate data into another analysis. Another advantage of automated systems is the ability for electronic data storage, allowing for future evaluations or data tracking for quality control.
Automated systems use image analysis to measure indentations. This greatly improves the repeatability of the measurement throughout the days, weeks and months of analysis. There are systems that use traditional “threshold-based” measurement techniques (gray-scale threshold). These systems can suffer from sample preparation limitations as well as illumination problems. Once again, good sample preparation is important, as light variations may cause shading errors while thresholding the indentation. Another measurement method uses image recognition, which utilizes advanced algorithms to accurately measure the indentation by finding the shape of either the Vickers or Knoop indentation. This technique is not limited by illumination problems or less-than-perfect sample preparation, and it can even locate and measure indentations in an etched microstructure. Automated systems using image analysis routinely outperform manually operated equipment due to their better day-to-day consistency.
The latest generation of automated systems provides a reliable means for performing high-volume, high-quality hardness testing while eliminating the need for costly manual intervention. Once the operator initiates the analysis, the system will perform the required steps fully unattended, allowing the operator to complete other laboratory duties. Once trained, a proficient operator can run a wide variety of case-depth analyses that may be required throughout the day.
SummaryWith the advances of technology in microindentation testing, productivity, accuracy and repeatability have been dramatically increased due to the implementation of automation. What was once the most difficult or time-consuming hardness test performed has now become easier. The greatest advantage, at least from a management perspective, is the return on investment of any automated system – from sample preparation to image analysis. Automated systems save time and money by increasing laboratory productivity. The ability to perform very precise and delicate material characterization ensures the parts and components manufactured will deliver the right results.IH
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