Hardness Testing

Hardness testing is a method of determining the results of heat treatment as well as the state of a metal prior to heat treatment. Since hardness values can be tied in with tensile strength values and, in part, with wear resistance, hardness tests are a valuable check of heat-treat control and of material properties.

Practically all hardness-testing equipment uses the resistance to penetration as a measure of hardness. Among them the better known hardness testers are the Brinell and Rockwell as well as portable-type hardness tester.

Brinell Tester

The Brinell tester (Fig. 2-3) uses a hardened spherical ball, which is forced into the surface of the metal. This ball is 10 millimeters (0.3937 inch) in diameter. A pressure of 3 000 kilograms is used for ferrous metals and 500 kilograms for nonferrous metals.

The pressure must be maintained at least 10 seconds for ferrous metals and at least 30 seconds for nonferrous metals. The load is applied by hydraulic pressure. The hydraulic pressure is built up by a hand pump or an electric motor, depending on the model of tester. A pressure gage indicates the amount of pressure. There is a release mechanism for relieving the pressure after the test has been made, and a calibrated microscope is provided for measuring the diameter of the impression in millimeters. The machine has various shaped anvils for supporting the specimen and an elevating screw for bringing the specimen in contact with the ball penetrator. These are attachments for special tests.

In order to determine the Brinell hardness number for a metal, the diameter of the impression is first measured, using the calibrated microscope furnished with the tester. After measuring the diameter of the impression, the measurement is converted into the Brinell hardness number on the conversion table furnished with the tester.

Rockwell Tester

The Rockwell hardness tester (Fig. 2-4) measures the resistance to penetration, as does the Brinell tester. Instead of measuring the diameter of the impression, the Rockwell tester measures the depth, and the hardness is indicated directly on a dial attached to the machine. The dial numbers in the outer circle are black, and the inner numbers are red. Rockwell hardness numbers are based on the difference between the depth of penetration at major and minor loads. The greater this difference, the less the hardness number and the softer the material.

Two types of penetrators are used with the Rockwell tester, a diamond cone and a hardened steel ball. The load which forces the penetrator into the metal is called the major load and is measured in The principal difference between two tests is the manner in which the specimen is supported. In the Charpy test the specimen is supported as a simple beam with a notch in the center. The specimen is supported so that the notch is on the vertical face away from the point of impact. Fig. 2-9 shows the dimensions of the Charpy test specimen and the positions of the striking edge of the pendulum and the specimen in the anvil.

In the Izod test, the specimen is held on one end and is free on the other end. This way it forms a cantilever beam. Fig.  shows the dimensions of the Izod test specimen and the positions of the striking edge of the pendulum and the specimen in the anvil. In this case the notch is just at the edge of the supporting vise and facing into the direction of impact. As with the Charpy, this position places the notch at the location of the maximum tension.