Traditionally, indentation hardness has been reserved for use in the laboratory. More recently, however, it has moved to the production floor and has even evolved to integration of online testing and analysis within the work cell to improve yields, reduce costs, maximize resources and generally improve the overall productivity of an operation. The goal is to produce, test and make a product more readily available to its intended market as quickly as possible without compromising quality.

There are many common applications of indentation hardness used around the globe. Assessing a metal or plastic material’s elastic properties based on a penetration depth will determine the material’s hardness after heat treating or other metal preparation processes, validate a material’s integrity (such as the coalescence quality of welds and overlays), and provide a general quality determination of a material’s tolerance to wear based on physical properties. These are just a few of the many possible applications.

Rockwell Method Explained

The Rockwell hardness test method, as defined in ASTM E-18, is the most commonly used hardness test method. The Rockwell test is widely used because it is generally easy to perform, the equipment is relatively inexpensive, and the testing process is quick and more accurate than other types of hardness testing methods due to reduced operator influence. The Rockwell test method can be used on most metals, except in conditions where the test metal structure or surface conditions have too many variations, where the indentations would be too large for the application or where the sample size or shape prohibits its use.

  The Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a preliminary test force (preload), commonly referred to as the minor load, is applied to a sample using a ball or diamond indenter. This load represents the zero, or reference, position that breaks through the surface to reduce the effects of surface finish. After the preload, an additional load, called the major load, is applied to reach the total required test load. This force is held for a predetermined amount of time – dwell time – to allow for elastic recovery. This major load is then released, and the load is returned to the minor load. This final position is measured relative to the position of the indenter at the original preload. The distance between the primary and secondary minor value is converted to a Rockwell hardness number.

Automatic Microhardness Testing Using Rockwell Method

Traditional microhardness testing, including Vickers and Knoop testing, is a time-consuming process. As operators looked for ways to improve production efficiency by increasing speed and accuracy and reducing operator influence, specialized fixtures and innovative techniques became incorporated into the operator’s overall production strategy. 

There are two ways to shorten the microhardness testing process. The first is to automate the traditional optical microhardness with a camera to view the impressions together with software to facilitate the measurement of the impressions. From there, it is possible to add a motorized table to allow complete automation of the measuring process. This is particularly useful for the measurement of case depth on samples. 

The basic process for the operator is as follows. Place the sample in the microhardness tester and then use the software to locate the edge of the part and the direction of the measurement. The software then directs the system to make impressions at predetermined locations, and the system repeats the movements and automatically measures all the impressions. This type of system not only eliminates the operator error inherent in having the operator manually measure the impressions, but it allows them to perform other functions while the tester is running. However, the time-consuming surface prep of the samples still remains.

  The second way to decrease case-depth measurement time is by using a fully automated system that uses the Rockwell test method. By reducing the major load of the test from the traditional range of 15-150 kg to 1,000 grams, a Rockwell test can be performed in the traditional microhardness testing range. This type of system takes measurements much faster than traditional methods – as quickly as a measurement every seven seconds – and because the tester does not read the impressions optically, part preparation is much less critical and, therefore, much quicker. The expense of consumables used in prep is also greatly reduced.

Automatic Case-Depth Analysis Using Rockwell Method

Case-depth analysis is one of the most frequent uses of microhardness testing. Case-depth analysis involves making a series of measurements, usually starting from the surface of a sample, to chart changes in the hardness. The test is used to find the location where the heat-treatment method has changed the metallurgical structure, creating a hard case while leaving the core soft. Case-depth analysis is a critical measurement on heat-treated parts such as gears, piston pins and crankshafts. If insufficient depth of case is created on a part, premature failure due to wear may occur. If too deep of a case is created, cracking can occur. At the least, excess time has been spent in the heat-treat process, which is a waste of money. 

  Rockwell-type automatic microhardness testers – complete with a camera for setup, a motorized X-Y table and software – can be used for automatic rapid case-depth analysis. A traverse, or a series of programmed indentation locations, can be created using software allowing the case-depth test to be performed in an automatic mode. Complex traverse profiles can be created with up to 24 individual traverses having as many as 50 individual test points each. 

  The operator can then select a traverse from a list that has been created. The sample is placed in a self-leveling vise in the tester. The camera is used to locate the starting point and direction of the traverse. The tester will then make a series of measurements on the sample. When completed, the data can be used to automatically create a case-depth chart. Readings can be displayed in converted Vickers, Knoop or (most commonly) Rockwell C scale. Tolerances can be entered for case depth and even individual test locations so that operators can instantly identify nonconforming parts. Modifications to the heating process can then be made.

Conclusion

Microhardness is a routine test that has long been a source of problems for companies. This is due to its time-consuming nature, both during part prep as well as during testing, and the fact that the test results are easily influenced by the operator. Although automated traditional Vickers or Knoop testers can increase throughput, a microhardness tester that uses the Rockwell principle can easily meet accuracy requirements and vastly improve measurement efficiency.

  Microhardness testing will continue to play an important role in qualifying products, materials and processes. Using a fully automated Rockwell microhardness testing system allows measurements to be taken much faster than traditional methods and provides accuracy unmatched by other microhardness testing methods. IH

For more information: Please contact Rich Wismer, sales manager at Newage Testing Instruments, 820 Pennsylvania Blvd. Feasterville, PA. 19053; tel: 440-308-6834; e-mail: rich.wismer@ametek.com; web: www.hardnesstesters.com. Newage is a part of AMETEK Test & Calibration Instruments.