Laboratory Experiment # 1

Hardness Testing

Objectives:

1. To understand what hardness is, and how it can be used to indicate some properties of materials.

2. To conduct typical engineering hardness tests and be able to recognize commonly used hardness scales and numbers.

3. To be able to understand the correlation between hardness numbers and the properties of materials.

4. To learn the advantages and limitations of the common hardness test methods.

Introduction:

It is a common practice to test most materials before they are accepted for processing, and before they are put into service to determine whether or not they meet the specifications required. One of these tests is for hardness. The Rockwell and Brinell machines are those most commonly used for this purpose.

Equipment:

1. Rockwell Standard Hardness Testing Machine,

2. Rockwell Superficial Hardness Testing Machine,

3. Rockwell Calibration Test Blocks,

4. Hardness Test Specimens of various metal alloys.

Procedure and Laboratory Report:

1. Understand thoroughly the operation of each machine, and check its operation before proceeding.

2. Check the calibration of the Rockwell Machines with Standard Calibration Test Blocks for the scale selected.

3. Using the appropriate scale

(a) Check the hardness of each test specimen on a Rockwell Test Machine.

(b) Tabulate the results.

(c) Convert all readings to either RB or RC values.

4. Using Brinell Machine

(a) Find the hardness of the cast aluminum by converting the diameter of the impression to Brinell Hardness Number (BHN).

(b) Convert the BHN to Rockwell B (RB) scale.

(c) Repeat step 4(a) for steel sample and find the BHN.

(d) Convert BHN for steel sample to Rockwell C scale (Rc).

5. Using the hardness conversion chart, find the Tensile Strength of the steel samples.

 

Note: For each hardness number, select three locations on the sample. Read the hardness number at each location and take the average of the three readings.

Background:

A commonly accepted engineering definition of hardness is the resistance to indentation. Resistance to indentation is a function of the mechanical properties of the material, primarily its elastic limit and to a lesser extent, its work-hardening tendency, and the modulus of elasticity. For a given composition with a known history it is possible to relate the elastic limit (for practical purposes, the yield strength) to the tensile strength, ductility, and toughness. Hence, the hardness tests can provide information from which many important mechanical properties can be derived. Since the hardness test can be conducted easily and quickly, they are very popular and are used to control processing and for inspection and acceptance of materials and components.

The common hardness tests rely upon the slow application of a fixed load to an indenter which is forced into the smooth surface of the specimen. Upon removal of the load either the area or the depth of penetration is measured as an indication of resistance to the load. Three types of tests are discussed below.

Rockwell Tests:

The Rockwell tests depend upon the measurement of the differential depth of a permanent deformation caused by the application and removal of differential loads. Various penetrator and load combinations are used to adapt different Rockwell tests to materials of varying hardness and thickness. The penetrators include a cone-shaped diamond , known as a Brale, and hard steel balls from 1/16-inch to 1/2 -inch in diameter.

Standard Rockwell Test:

The Standard Rockwell tests use a light load of 10 Kg to seat the penetrator firmly in the surface of the specimen. This load is known as the minor load. After the application of the minor load, the depth gauge is zeroed and a larger load, known as major load, is applied and then removed. While the minor load still acts, the depth of permanent penetration is measured. The depth gauge which measures the penetration is calibrated to read in hardness numbers directly rather than in inches. Major loads for Standard Rockwell tests are 60, 100 or 150 Kg. The diamond penetrator is marked as "C-Brale".

Superficial Rockwell Test:

Superficial Rockwell tests are used for measuring the hardness of thin specimens and specimens which have only a thin hardened surface layer )known as a case) on a soft base (known as a core). The ball penetrators available for superficial testing are the same as for standard testing. The diamond brale is marked as "N-Brale". The loads for superficial testing are much lower than for standard testing, being 3 Kg for the minor load and 15, 30 or 45 Kg for the major load.

The wide range of combinations of penetrators and loads permit application of Rockwell Test to an equally wide range of materials of varying hardness. The diamond penetrator makes it practical to test the hardest steels and the large balls permit testing of soft metals and even plastics. Generally the Rockwell test is considered to be nondestructive because the light loads and small penetrators produce very small impressions. However, because of the small impressions several readings should be taken to obtain a representative result. Furthermore, the smaller the impression, the greater is the care necessary in preparing the surface. Apart from any special effort required for surface preparation, the Rockwell test is easier and more quickly performed than the Brinell test.

Operation of Hardness Testing Equipment:

 

(1) Select the correct combination of weights (at the rear of the machine) and penetrators (diamond brale, 1/16-inch ball, etc.) for the hardness scale you wish to use. The numbers given in black represent the scales that use brale and the numbers given in red represent the scales that use ball penetrators.

(2) Make certain that the crank(4) is in forward position (nearest to you).

(3) Place sample on the anvil.

(4) Slowly turn the wheel spokes(1) clockwise. This raises the anvil and sample toward the penetrator tip. After contact is gently made, continue raising sample until small pointer(5) is about in line with small black dot and large pointer(6) is within colored sector(7). The minor load has now been applied to the sample.

(5) After step 4, large pointer(6) on the dial is nearly vertical. Now, turn the knurled collar(2) until "SET" line on the dial scale is in line with large pointer(6).

(6) Depress trip lever(3). This triggers the mechanism that applies the major load. Crank(4) will automatically move away from you.

(7) After the crank(4) has come to rest (against a "stop" and away from you), gently pull the crank toward you as far as it will go. If this is done abruptly, a false reading will be obtained because of jarring.

(8) Now record the scale reading of large pointer(6). The black scale is read for the diamond penetrator (Example: Rockwell C), and the red scale is for ball penetrators (Example: Rockwell B).

(9) Remove the minor load, which remains on the specimen, by lowering the anvil (Turn the wheel(1) counterclockwise). Move the sample to position for next test and repeat the steps above.

Brinell Test:

The Brinell test relies on mechanical or hydraulic loads as large as 3000 Kg. acting through a 10 mm hard steel or carbide ball. In order to compensate for variations in the response of materials to the application of the load, the time for which the load is applied is specified. For hard materials such as steel, a 30-second loading period is adequate. Softer metals and alloys such as brass or aluminum require about 60 seconds. After the load is removed, the diameter of the impression made by the ball is measured in millimeters. The Brinell hardness number, abbreviated as BHN, is the quotient of the load, P (kg), divided by the area, A, of the impression:

 

P

BHN =

_______________________________

 

( D- (D2 - d2) 1/2 ) p D/2

 

Where D is the diameter of the ball penetrator (mm) and d is the diameter of the impression(mm). In practice, the BHN is read directly from a table listing different values of d for various values of load, P.

The Brinell test makes a large impression on the surface of the piece tested. Unless such a large impression can be tolerated, and often it can not, the test is destructive. However, the large impression is advantageous because it gives a more representative result than would a smaller impression which would be more sensitive to local soft or hard inhomogeneties. The size of the impression also renders the test less sensitive to the presence of rough surface finish and mill scale than is the case when tests are used which rely on small indentations.

Operation of Brinell Testing Machine:

(1) Turn air on

(2) Set the required load on the dial.

Note: For steel and other hard materials the load is 3,000 kg. for 30 seconds. For non-ferrous materials a 500 kg. load is used for 60 seconds. Thin specimens should not be tested by this method.

(3) Place the specimen on the anvil and apply a preload by bringing the specimen surface to contact with the ball penetrator.

(4) Pull the load knob and apply the appropriate timing at that load level.

(5) Release the load by pushing the load knob back into the initial position.

(6) Remove the specimen and measure the diameter of the indentation. The Brinell Microscope reads in millimeters. Take several readings and average them.

(7) Look up BHN from chart or calculate from the formula.

LINKS

 Hardness Conversion Chart (I)

Hardness Conversion Chart (II)

Hardness Conversion Chart (III)

Hardness Conversion Chart (IV)

Hardness Testing Machines (I)

Hardness Testing Machines (II)

 

The following is a sample hardness data as presented in a laboratory report. Use the same format in your report.

Material

Rockwell Hardness Scale, Major Load, Type of Penetrator

Rockwell Hardness Number

Brinell Hardness Number

Brinell Load, Indentation Diameter

Tensile Strength (Ksi)

Example 1 (steel)

Rc

150 Kg

C-Brale

51

or

Rc 51

 

 

253

Example 2 (steel)

15 N

45 Kg

N-Brale

42

or

15N42

 

 

182

Example 3 (Steel)

 

 

352

3000 Kg

3.25 mm

176

 

 

 

 

 

 

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Last Update: May 1, 2001

By: Serdar Z. Elgun