Hardness is the nearly universal measurement of heat treating performance. This is because specific materials processed to the required hardness perform well under certain loads. For example, spring steel at a hardness of Rockwell C45 performs well as a spring. Experience has shown that a hardness of Rc45 in spring steel correlates with toughness, resilience, and high fatigue strength. The correlation is consistent and Rc45 is accepted as a quality hardness in heat-treated springs, even though the hardness itself is not an important characteristic of a spring. For many parts where tensile strength, toughness, or fatigue strength may be the desired characteristics, these features correlate with hardness values. Most frequently, hardness is specified as the heat treat requirement only because hardness is easy to measure.
The Brinell Metal Hardness Method
The standard Brinell penetrator is a hardened steel ball (or carbide for use on hard materials), 10 mm in diameter. On softer materials, the ball is pressed into a previously-flattened area of the test piece under a load of 500 kg. With the aid of a precision microscope with a built-in scale, the diameter of the hole is measured in millimeters. After finding this measurement, a table is consulted to determine the corresponding Brinell number, commonly referred to as BHN, or Brinell Hardness Number. For harder materials, the procedure is the same, but with a larger load. When using the Brinell number as a measure of hardness, it is most important that the load being used in the test is specified along with the number itself; for example, 38 BHN (500 kg). The most commonly-used load for soft materials is 500 kg and for hard materials, 3000 kg. Brinell machines come in a variety of models including hand-operated, power-operated, portable, manual, digital and direct reading machines for production work. Some of these use dead weight to apply the pressure to the ball; others employ hydraulic pressure.
The Brinell test is easily and quickly made, and with a little practice, the impression diameter can be read accurately. It is good practice to request that your heat treater’s lab use the average of two readings of the impression diameter made at right angles to each other. Calibration of the machine should be done periodically with the use of metal test blocks of standard hardness.
The Rockwell Hardness Method
The Rockwell method of measuring hardness is a system of several different kinds of penetrators that may be applied to test pieces under a variety of loads. The various combinations of penetrators and loads determine a number of Rockwell hardness scales – each combination of load and penetrator is denoted by a letter. The system is divided into two divisions: superficial and standard. The superficial division employs very light loads and is intended mainly for use on thin work or work with a very thin case. Some Rockwell testing machines are adapted for both the standard and superficial scales and the testing equipment may be manual, which requires an operator, or automated. Many leading heat treating companies will use both methods of testing hardness.
Since the Rockwell hardness measurement is essentially a measurement of the depth of the hole made by the penetrator (read directly on the dial of the machine as the Rockwell number), and the holes made by the penetrator are comparatively small, the test is sensitive to a number of factors:
- Any porosity in the work will cause erratic readings
- The surface to be tested must be smooth and clean
- The surface to be tested must be square with the penetrator
- The test piece must make good contact with the anvil of the machine
- In uncertain situations, a series of readings should be made
- Calibration of testing machines should be done at frequent intervals using metal testing blocks
It is frequently necessary to make Rockwell hardness tests on cylindrical parts that do not have sufficient thickness for the small flat spot required for the test. If the test cannot be made on the ends of the piece, it can be made directly on the curved surface with a correction introduced into the hardness reading; the correction is greater with smaller-diameter test pieces and lower hardness. Readings on round work are always low, and the “round work correction” must be added to the reading obtained from the testing machine.