Hardness testing is a simple and relatively inexpensive test. Quicker and less destructive than tensile testing, it is considered one of the most universally valuable mechanical tests available to engineers.
Hardness testing values are empirical, and yet results provide a very good correlation with material strength over a wide range of materials. When a known force is applied in a hardness test, the amount of plastic deformation the material undergoes determines the hardness value.
In Rockwell-type testing, loaded indenters of various sizes and shapes measure hardness. For this experiment, we will measure and compare the hardness of untreated and heat-treated steel using the Rockwell hardness test.
Several common methods to measure the hardness of metals include Brinell, Vickers, Knoop, and Rockwell hardness BNC. Each of these methods utilizes a penetrator, either in the shape of a sphere, a cone, or a diamond pyramid.
An indentation is made into the surface of a metal and a hardness reading is displayed. Of this list, the Rockwell hardness test is the most popular one for structural steels. A standard manual Rockwell tester consists of a lever system to apply the load and an analog readout showing the hardness number.
In the typical Rockwell test, a zero point is established to account for surface variations by applying a preliminary load and measuring an initial penetration depth. Next, the major load is applied to the indenter. Finally, the load is removed and the final penetration depth is measured. The dial gauge on the top of the machine uses the difference between these two values to display a Rockwell hardness value. The harder the material is, the less the indenter will penetrate, resulting in a higher Rockwell hardness value. Thus, the values for Rockwell B and Rockwell C hardness are based on the depth of the penetration, and therefore the test machines are calibrated often using calibration test blocks for specific hardness ranges.
Rockwell hardness testing can be used to evaluate how the strength of a material is changed through processes such as heat treatment or cold rolling. Cold rolling tends to result in stronger, harder materials. Heat treating can result in softer materials through heating but harder structures through quenching.
For example, in the Jominy End Quench Test, a cylindrical specimen is heated uniformly. One end is then quenched with a stream of water. Changes along the length of the specimen from the quenched end to the unquenched end can be seen in hardness values, which are representative of changes in the microstructure.
In the next section, we will measure the hardness along the length of a steel Jominy End Quench Test specimen to observe the transition from untreated to heat-treated steel using the Rockwell hardness test.
Before you begin, familiarize yourself with the testing machine. The anvil, which can be raised or lowered by the capstan handwheel, supports a sample underneath the interchangeable indenter.
A test load is chosen using the selector on the side of the machine and is applied by turning the load lever from the unloading position to the loading position. Correct pre-loading and final measurements are determined using the dial gauge on the top of the machine.
Before inserting your test specimen, confirm that the Rockwell C diamond cone indenter is installed and the test load is set to 150 kilograms. Secure the specimen in the machine with the flat surface against the anvil. For this demonstration, we'll use a Jominy End Quenched specimen that has undergone water cooling treatment.
Move the load lever to the unloading position, and then raise the anvil to bring the specimen close to the indenter. Adjust the specimen position so that the indenter is one-sixteenth of an inch from the end. When the position is correct, re-secure the specimen so that it will not move during testing. Now, raise the sample once again until the needle on the front dial begins to move slightly, indicating that contact with the indenter is established.
Apply the pre-load by continuing to slowly raise the specimen until the needle on the dial has completed three full turns. Stop when the needle has completed the third turn. Adjust the outside ring of the dial gauge so that the initial reading is zero. Then move the load lever to the loading position to apply the test load. The needle will settle to a new value as the load is applied. Wait until it stops moving and then move the load lever back to the unloading position.
Record the Rockwell C hardness from the dial gauge and then lower the anvil to move the sample away from the indenter. Repeat this test along the length of the specimen. The ASTM-A255 guidelines specify that readings should be taken at one-sixteenth inch intervals for the first half inch and at one-eighth inch intervals for the next half inch.
Plot the Rockwell hardness of the specimen as a function of position along the specimen. The hardness clearly decreases as distance from the quenched end increases.
The Rockwell hardness test was used to show that due to the rate of cooling, the change in internal structure of the material affected the hardness of the material, which in turn indicates the strength of the material.
Now that you appreciate the hardness test for its ease of use, let's take a look at how it is applied to assure the quality of everyday products.
Hardness values can easily be converted to strength values using charts derived from empirical equations. As expected, softer materials have lower strength values and harder materials have higher strength values. Because of this, hardness values can be used in place of more costly tensile testing to predict the strength of things we use every day.
Just looking around at a backyard barbecue, you will see many products that are deemed safe due to hardness testing. Metal lawn chairs, grates on the grill over a wide range of temperatures, hardened steel knives, and the metal pool ladder in aquatic conditions. All were likely hardness tested to assure consumer safety.
You've just watched JoVE's introduction to the Rockwell hardness test. You should now understand why hardness testing is commonly used, how to perform hardness testing, and how to analyze the results obtained.
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