Hardness testing is one of the most common quality control checks performed. It is often used 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.
Hardness is a property determined by measuring the resistance to deformation from an applied load. Alignment of the indenter to the sample surface is crucial to correct hardness measurements. The penetration is measured by either depth or area, which is then translated into a hardness number. It is important to realize that all conventional hardness-testing methods involve sampling some volume of material. The amount of material actually sampled is a function of the indenter selected, the applied load and the material properties. If the sampled volume is limited by the physical size of the piece to be tested, then you may actually be sam-pling the underlying anvil or pushing out beyond the edge of the sample.
The most commonly used indentation hardness tests are the Brinell, Rockwell, Rockwell Superficial and microhardness methods. The Brinell test is used primarily for inhomogeneous materials such as forgings and castings, particularly cast iron. The Rockwell test (Table 1) is used for both ferrous and nonferrous materials, which have been annealed, hardened, case hardened or tempered. Sheet materials in heavier gages and cemented carbides can also be tested. Rockwell Superficial is used where lighter loads are required for testing thin case hardened surfaces, decarburized layers and sheet material in thin gages. Microhardness tests are typically used for very small, intricate shapes, thin parts and for case depth determination.
Types of Indenters & Test MethodsBrinell testing uses larger indenters (e.g. 10 mm) and heavier loads (e.g. 3000 kg) to minimize localized feature effects such as graphite flakes or voids in castings. Common indenters for Rockwell testing include diamonds and steel or carbide balls (although steel is being phased out). Applied testing loads range from 15kg for the Rockwell superficial scales to 150kg for the "C" scale.
To illustrate the concepts of hardness testing, imagine a pen being pushed into a small cubic chunk of clay and then retracted (Figs. 1, 2, 3). You are left with a hole where depth is dependent on the force applied load and the resistance of the clay to the indentation. A different sized or shaped pen (Fig. 4) produces a different type of indentation. As shown, the heavy loads produce deeper indentations and sample more volume of material. The higher the hardness of the material, the more resistant it is to penetration and the shallower the resultant indentation.
Indenters for microhardness testing (Fig. 5) include Knoop and Vickers with applied loads ranging from 1 gram to 1000 grams. The indentations are typically measured optically at magnifications from 400 to 1000°-. High quality optics is critical especially when using magnifications of 500°- and greater. Accurate measurement of the indentation boundaries, especially when using lighter loads, is mandatory
Microhardness testing is sensitive to a number of variables such as indentation spacing (Fig. 9), material segregation, inclu-sions, edge effects and cold working.
Summing UpMost people need not be experts in all the intricate details of hardness testing.
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.
Everyone involved with hardness testing should have and be familiar with the appropriate ASTM specifications. These address proper sample preparations, selection of loads and penetrators, sample geometry, minimum sample thickness con-siderations, roundness corrections, spacing and edge considerations, and conversions between scales. IH