NDT - Hardness Testing
Trends in Hardness Testing
November 2, 2012
Develop methods to improve productivity, accuracy and efficiency.
Consistent with the unprecedented advancing technology we all benefit from in just about anything related to computers, communication, digital vision, and hardware engineering, hardness testing has rapidly evolved in technique—more so in the past 20 years than any previous developments since the inception of this important materials test method. Limitations in regards to material geometry, surface finish, productivity, efficiency, data manipulation, and results reporting have been mitigated while continually undergoing enhancement. The result is increased ability and dependence on “letting the instrument do the work,” contributing to substantial increases in throughput and consistency, while freeing up the advanced operator for other responsibilities or allowing less experienced operators to handle hardness data acquisition. With the myriad of fully integrated systems now available, the labor intensive, subjective and error-prone processes of the past are virtually eliminated. More sophisticated, accurate and productive processes can quickly, reliably, and with extreme precision provide useful, material critical information.
Materials testing, including hardness testing, are useful processes for analyzing component properties and can be accomplished through a multitude of methods and techniques. Determining material hardness can provide valuable insight into the performance, durability, strength, flexibility and capabilities of a variety of component types— raw materials to carefully prepared specimens to finished goods. In today’s extremely competitive global market, with high expectations on accuracy and productivity, quality and productivity errors have serious consequences. Manufacturing, research, and quality control now more than ever must depend heavily on new and evolving techniques to revolutionize more traditional processes if they want to maintain a competitive pace.
Some hardness tests types, such the Rockwell method, will yield relatively quick single test results based on indentation depth, providing a standard result in 10 to 12 seconds. If utilizing specialized, custom settings and equipment, a standard result can be produced in as little as two seconds. As effective as Rockwell testing can be, its technique renders it unsuitable as a test method for many material types and geometries; other, more applicable techniques must be employed to achieve valid results. These other commonly used test types, including Vickers, Knoop and Brinell, require a secondary process to determine the size of the indentation surface area, usually in microns or millimeters. These ancillary processes can be very time-consuming, inefficient, and are commonly prone to subjective errors.
Surveys of a wide-ranging user profile for hardness tester operations show that the leading new requirement is realizing increased throughput and efficiency without compromising accuracy and flexibility. Indications are almost unanimous in that hardness tester operators would pay significantly more for a system exhibiting those features and capabilities. One means of improving productivity while providing consistency is through various ways of designing and actuating automatic indentation and indentation traverses and the subsequent impression reading utilizing image analysis. While earlier forms of automated indent reading had value, they were usually restricted in their process contribution due to the fact that the processes were subject to strict surface finish and preparation requirements, lighting factors, material variations, and imaging and stage movement accuracy. This typically meant that the advantages of automation were limited to consistency in material and testing process; flexible operations were rare. Coupled with the often complex operating software and skill level required, the benefits of automation were isolated to a small percentage of test sites.
To put in perspective the challenge of developing a reliable and user-friendly automated method, particularly for reading micron-sized indents, consider the complexities of the hardness test. Two of the more common hardness tests that stand to benefit from improved techniques are Knoop and Vickers. These methods are used in micro and macro testing to determine material hardness based on measuring the size of a diamond-shaped impression left from an application of a specified force. The Knoop diamond produces an elongated rhombic-based, diamond-shaped indent with a ratio between long and short diagonals of about 7 to 1. Knoop tests are typically performed at test forces from 1 to 1,000 grams, are often referred to as microhardness or micro indentation tests, and are best used in small test areas (or on brittle materials) since minimal material deformation occurs on the short diagonal area. The elongated tips of a Knoop and the typically small size of the indent pose a challenge in discerning the extremes of the tip. In comparison, the Vickers diamond produces a square-based pyramidal shape with a depth of indentation of about 1/7 the diagonal length.
The Vickers test has two distinct force ranges, micro (10 to 1000 grams) and macro (1 to 100 kilograms), covering a variety of applications. Vickers tests are typically referred to as macro indentation tests and are used on a wider variety of materials including case-hardened and steel components. The nature of these test types typically dictates a force consistent with the material being tested and resulting in extremely small impressions that must be measured in microns. Traditional techniques involve variations of standard type microscopes with objectives of varying resolution that are integral to the hardness tester (usually on a rotating turret used to locate and manually measure the impression through a filar type eyepiece based on human interpretation). Predictably, this is time-consuming, inefficient, and in today’s environment, increasingly unacceptable. It is not uncommon for a technician to produce and measure by eye hundreds upon hundreds of indentations during a shift, with fatigue likely compromising the measurement process as the indents increase in quantity. Add to this the need to produce a full analysis of a hardness traverse often consisting of more than 15 indents each (many times on a single sample) as well as the importance of accurate, well-reported results, and the need for more advanced, automated techniques becomes evident.
Continual technological improvements in all aspects of the hardware, software and algorithms that contribute to automatic hardness testing have opened the door for testing in applications previously not possible. An automatic hardness system may be comprised of a partially or fully computer-controlled system complementing a modified hardness frame. Fully automated Knoop/Vickers systems include auto-rotating turrets for systemic exchange between indenters and objectives, automating the intent/measure sequence. In addition, precision controlled actuation in the Z-axis direction provides automatic focusing on specimens, a crucial parameter in ensuring accurate measurement as well as enhancing the ability to accommodate multiple samples on a stage system. The autofocus feature eliminates compromise of indent clarity parallelism or position variation. The integration of a powerful computer with enhanced graphic capabilities, newer dedicated hardness software, and a high resolution USB video camera completes the imaging system. Integration of a high accuracy, extremely repeatable automatic XY traversing motorized stage results in a powerful, fully automatic hardness testing system. When commissioning these systems for maximum advantage, the required test parameters are incorporated into a program designed for the particular part and application requisites. Once initial setup is complete, including indentation traverse locations as well as distances between indents and directions, the system can be set up to automatically create, measure and report on an almost unlimited number of indentations and pattern traverses.
With these newer capabilities, system integration and hardware requirements have been mitigated. Many system components that caused operational challenges in the past (cluttered workspace, intensive user training and specialized calibration) have been alleviated. Current automatic stages are moved through a virtual joystick, and on some systems, stage controllers are fully integrated into the stage assembly. Advances in stage movement algorithms and mechanical design have made XY accuracy and repeatability better than ever—paramount in precision traverse requirements such as case depth analysis and the dramatically improved video technology has provided imaging on a wide range of material types while increasing field-of-view possibilities. It is now possible to accurately and repeatedly read smaller indents while locating and analyzing on surfaces previously not possible such as glass, micro coatings, and extremely thin wire.
Expanding productivity even further is the introduction of more accurate, larger automatic traversing XY stages capable of holding multiple samples at a time in an array of methods and future arrangements. Pre-programmed and saved traverses are opened, samples are aligned in holders, and with a single click, the indentation, reading, and reporting of a multitude of traverses on each sample is initiated. Newer software even allows different scales, forces and microscope objectives within and between traverses, creating new possibilities for multi-sample and case depth analysis. This fully frees the operator from manually moving the sample from test to test for both the indentation as well as the measurement process. The flexibility also provides an immediate ROI and clearly increases the ability to evaluate a variety of materials.
Automated testing is also increasingly beneficial for other test methods such as Rockwell and Brinell. In particular, Rockwell often requires indentation in repetitive patterns. One application where automation can significantly increase productivity is in Jominy testing. A Jominy test is a method for determining the hardenability of steel: important information for determining the ability of the steel to be hardened to a particular depth so that the proper material and heat treatment process can be formulated. In the process, a test piece that typically measures 25 millimeters (mm) in diameter and 100 mm long is heated to a predetermined temperature and quenched by a jet of water sprayed onto one end. When the specimen is cold, a flat is ground lengthwise on opposing sides to provide the test surface. Hardness measurements using the Rockwell HRC scale (10 kg minor and 150 kg major forces) are then made at specific intervals, typically at 1.5 mm on alloy steels and 0.75 mm on carbon steels, with the initial indent closely performed just inside the quenched end. Test results are then plotted on a standard chart. The hardness values proportionally decrease away from the end of the bar. With the manual process taking valuable setup time as well as posing difficulties in ensuring accurate indent positioning and spacing on the bar, it becomes evident that automating the process can increase productivity as well as the accuracy of the testing.
As in Knoop and Vickers testing, Brinell testing by nature is a labor intensive and manual process that, in its conventional state, requires constant human intervention and processing. Since the traditional Brinell test consists of a single, controlled test force made with a specified diameter tungsten carbide ball, the resulting impression must be optically measured (diameter in millimeters) as part of the process to determine Brinell hardness. In conventional form, this is typically achieved by using a low-power, handheld microscope—a process that is both laborious and subjective. As in Knoop and Vickers, fatigue induced errors from performing measurements repeatedly are common; the process itself can be inefficient and time-consuming. With many processes requiring 100% inspection and productivity depending on quick results, it is no surprise that a means to both accelerate the process as well as one that mitigates the possible manually induced errors is in demand. The method that may be most applicable is dependent on a variety of factors including test time requirement, specimen geometry, loading and unloading technique, material properties, ASTM standards adherence, and budgetary alignment.
The Production Brinell test is one unique method of automatically and accurately determining Brinell hardness in a production environment. These systems can be integrated to production automation lines or used as individual stations to perform quick, consistent Brinell testing. The systems are available in a variety of formats and frame configurations. Plus, each can be customized to meet an abundance of application requirements. An advantage in newer optical reading Production Brinell as compared to depth reading predecessors is their adherence to common optical Brinell standards such as ASTM E-10. When using a conventional Brinell floor or bench model tester to perform indentation, an alternative to the handheld manual process involves a handheld digital camera that can accurately and efficiently measure the diameter of the impression, automatically using image analysis techniques similar to Knoop and Vickers. As a result, it has become relatively easy to measure Brinell indents through a camera. If a handheld imaging system, which requires manual intervention is lacking in the desired production level, then a fully automatic optical Brinell system can provide adherence to ASTM E-10 while allowing for fully automated optical testing. A fully integrated automatic optical Brinell testing system can efficiently perform the entire Brinell test process with accurate indentation and image analysis. All of this, coupled with flexible user-friendly software, gives the operator extensive capabilities to generate tests, complete analysis and generate reports. An operator only has to locate the sample in the tester and press the start button. Indentation is automatic as is the rotation of a revolver or turret that turns the measuring objective/microscope into position. The automatic focus and imaging process then is initiated and results are returned quickly in both indent diameter and Brinell hardness.
Materials testing and verification is of significant importance in today’s fast-paced, mechanized world. Ensuring efficient, accurate verification of components provides valuable information about a material’s ability to perform and endure; this knowledge is critical to the safety and operation for the most basic of things in everyday life. For most producers, keeping up with current technology is instrumental in aligning an accurate and productive test process with the bottom line. These newer and developing processes are proving to save significant time and money while contributing to more precise and consistent test methods and results generation. Future use of automatic hardness, representative stress and strain, and new techniques will undoubtedly provide the basis for material testing for years to come.