6.2: Unit 2: Hardness Testing

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OBJECTIVE

After completing this unit, you should be able to:

Perform a Rockwell Test
Perform a Brinell Test

Beyond verifying our in shop heat treatment, testing hardness is sometime necessary for production work as well. Even though it’s bad planning, occasionally a job arrives at our machine shop with an unknown alloy or maybe its composition is known but the hardness isn’t. It is possible to use a file to roughly test the machinability of that metal, but the best way to select cutter types, speed, and feeds is a true hardness measurement.

Brinell: Testing hardness by reading the diameter of a ball penetrator mark.

Rockwell: Testing hardness by reading a penetrator depth.

The Rockwell Hardness Test

The Rockwell is a widely accepted method for both soft and hard metals. This system gauges malleability by measuring the depth a pointed probe known shape and size will penetrate into the material given an exact amount of force upon it. Due to Rockwell’s range it is the most popular test in tooling and small production shops and training labs.

Rockwell Numbers:

There are several different scales within the Rockwell system. We’ll use the Rockwell C scale, correctly used on the hardened steel. The C scale can be said to start at 0(annealed Steel) and run up to 68, harder than a HSS tool bit, near that of a carbide tool. It is symbolized by a larger R with the scale subscript.

R_C

The two step Rockwell test:

Step 1. Calibrate Load

The test object is set upon the lower anvil such that it’s stable and won’t move when pressed down from above. Next, a cone-shaped diamond penetrator is brought into contact then driven into metal to a predetermined of 20lbs. That cause the conical point to sinks into the metal from 0.003 to 0.006 inch. This is the initial calibration load. At that time, a large dial indicator is rotated to read zero.

Step 2.

Test Load Then with the calibration pressure upon the penetrator and indicator set to zero, a second addition 20lbs test load is added. As the diamond sink farther, its added depth is translated to dial, but in an inverse relationship. The deeper the diamond penetrates, the softer the metal tests, therefore the lower number that must appear on the dial face. Inversely, when the point can’t go very deep, the metal is hard and registers higher on the dial face.

The Rockwell Method

The Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter.

1. Prepare the sample.

2. Place the test sample on the anvil.

3. A preliminary test force (commonly referred to as preload or minor load) is applied to a sample using a diamond indenter.

4. 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.

5. This force is held for a predetermined amount of time (dwell time: 10-15 seconds) to allow for elastic recovery.

6. This major load is then released and the final position is measured against the position derived from the preload, the indentation depth variance between the preload value, and the major load value. This distance is converted to a hardness number.

The Brinell Hardness Test

The Brinell Hardness test is very similar to the Rockwell system in that a penetrator is forced into the sample, however, here the measured gauge is the diameter of the dent made by penetration of a hard steel ball of known size, into the work-piece surface. Hardened tool steel balls are used for testing softer material, while a carbide penetrator ball is used to test harder metals.

Due to the upper hardness, limiting factor of Brinell ball, this test it correctly used as a test of soft to medium hard metals.

The Brinell scale numbers:

The scale runs from 160 for annealed steel up to approximately 700 for very hard steel.

The Brinell hardness test is an alternative way to test the hardness of metals and alloys.

Prepare the sample.
Place the test sample on the anvil.
Move the indenter down into position on the part surface.
A minor load is applied and a zero reference position is established.
The major load is applied for a specified time period (10 to 15 seconds) beyond zero.
The major load is released, leaving the minor load applied.
Follow the process to determine the Brinell hardness of an aluminum sample.
1. Press the indenter into the sample using an accurately controlled test force.
2. Maintain the force for a specific dwell time (usually 10 to 15 seconds).
3. After the dwell time is complete, remove the indenter, leaving a round indent in the sample.
4. The size of the indent is determined optically by measuring two diagonals of the round indent using either a portable microscope or one that is integrated with the load application device.
5. The Brinell hardness number is a function of the test force divided by the curved surface area of the indent. The indentation is considered to be spherical, with a radius equal to half the diameter of the ball. The average of the two diagonals is used in the following formula to calculate the Brinell hardness.

BHN=F2D(D-D2- d2)

Unit Test:

1. Please lists five heat treating safety.

2. What is first important thing to know when heat treating a steel?

3. What is the soak time for 1 to 2” Thickness Of Metal?

4. Please explain Soak time.

5. After the soak time is complete, what is the next step?

6. To temper the sample part it must be placed into the furnace at what temperature?

7. Please explain Austenitize and Quench.

8. What is a Air-cool?

9. Please explain The Rockwell Method.

10. Please explain The Brinell Hardness Test.

Chapter Attribution Information

This chapter was derived from the following sources.

Heat Treating derived from Heat Treatment of Plain Carbon and LowAlloy Steels by the Massachusetts Institute of Technology, CC:BY-NC-SA 4.0.
Brinell Hardness Test Equation derived from Brinell Hardness by CORE-Materials Resource Finder, CC:BY.