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Rockwell Hardness Testing
- Reference Information and Standards -


Rockwell is the most common
type of hardness testing.

The Rockwell test is the most commonly used indention hardness test because the hardness number is always indicated directly on a dial gage or digital readout.  The Rockwell number is based on the additional depth to which a steel ball or diamond penetrator can be forced by a heavy major load beyond the depth of a previously applied minor load (Figure 5).

When testing materials such as steel, cooper alloys, aluminum and plastics, a steel ball penetrator that is 1/16-inch, 1/8-inch, ¼-inch, or ½-inch is used. Materials such as hardened steels and cemented carbides require a diamond Penetrator.

Rockwell testing is divided into two categories, regular and superficial. With regular Rockwell testing, the minor load is 10 kilograms and the major load can be 60, 100 or 150 kilograms. Superficial minor loads are 3 kilograms and major loads are 15, 30 or 45 kilograms. Rockwell hardness numbers are lower the deeper the penetration and they are usually prefixed by a letter signifying the value of the major load and the type of penetrator. A letter has been assigned for every possible combination of load and penetrator (Tables II and III).

To select the best combination of load and penetrator, the type of material and the thickness of the specimen should be considered. Tables such as Table IV have been compiled to suggest which Rockwell method will provide the most sensitivity and repeatability.

A Rockwell test cold works the area immediately surrounding it. The extent of the area cold-worked depends on the type of material and its previous work-hardening. The depth of material affected has been found to be about ten times the depth of the indentation. Therefore, unless the thickness of the material being tested is at least ten times the depth of the indentation, accurate Rockwell test cannot be expected. A minimum thickness chart has been compiled using the ten to one approximation and by making actual tests on varying thicknesses of low carbon steels and hardened and tempered strip steel. See Table V.

 

If a choice between two or more scales exists, the scale applying the heavier load should be chosen. The heavier load will produce a larger indentation covering a greater portion of the material; thus, the resulting Rockwell hardness number will be more representative of the material as a whole. The same principles apply in determining the hardness of a case hardened part which is of a known depth.

Minimum thickness charts and the ten to one ratio can be used as guides, but a trial test should be made before a scale is chosen. If a bulge develops on the underside of the test area, the material was not sufficiently thick for the applied load. This condition, known as the anvil effect, indicates that the Rockwell scale applying the next lighter load should be used.
Too much force on softer materials of insufficient thickness will result in flow of the material. When the anvil effect or flow exists, the Rockwell hardness number obtained may not be a true value. The use of several specimens piled to an adequate thickness is not recommended; the slippage between the contact surfaces will make a true value impossible to obtain.
If an indentation is placed within an area that has been cold worked by a previous indentation test, accuracy will be affected. Usually the readings will be higher than they would be for an unworked area. For this reason, the distance from center to center of indentations should be at least three diameters. Generally, the softer the material, the more critical the spacing; but the three diameter gage is enough space for most materials.
If the indentation is too close to the edge of the specimen, the material will yield and the
Rockwell hardness number will decrease accordingly. To avoid this factor, the distance from the center of the indentation to the edge of the specimen should be at least two and one-half diameters.

Slippage or movement of the tested material can also introduce an error factor. The surface being tested must be near normal to the penetrator tip during the entire test. If there is any movement during indentation, the displacement will be transferred to the dial gage. A movement of 0.001 inch could cause an error of over ten Rockwell numbers because one point of hardness represents a depth of only 0.000080 inch. The support which holds the specimen must b rigid enough to avert such movement and to prevent its own deformation.

 

The main disadvantage of the traditional Rockwell instruments is that the accuracy of testing depends to a large degree on the perfect contact between the work piece and its support, usually called the anvil.

When removing the additional load and returning to the preload, the unique deformation registered by the dial indicator should be the penetrator displacement.

This can happen only if the work piece is in perfect contact with the anvil; if there is a slight oil coat, some grease or other, a little movement occurs during load application, when added to the indentation depth, gives a wrong result, decreasing the hardness value.

Because it is not always possible to work in perfect conditions, for example in heat treat or shop environments, this is an important consideration. To overcome this problem, most of the Ernst Hardness Testers work according to a modification to the Rockwell principle, shown in Figure 6.

A support on the test surface gives the penetration depth. Therefore, any movement of the part, elevating screw or stand, does not influence the test result.

 The assembly (a + b) moves down on to test piece and the penetrator moves back producing a resistance equivalent to the preload value. Zero setting is automatic.

The test load is applied.

The load is removed and the dial indicator shows the penetrator’s displacement.

In case of movement of the test piece, the relation between a and b stay constant, avoiding the typical error of the original Rockwell principle. The same method is used also in Superficial Rockwell measurement.

Ernst Hardness Testers have a third component that should not be confused with the penetrator shroud (b). In bench testers, this component is called the clamping shield. The clamping shield is used for locking of the large work pieces, avoiding the use of any special support; the clamping shield can be easily removed if needed.


Rockwell Hardness Chart (+PDF)
(aka Hardness Scale Conversion Chart)

Round Correction Factor (+PDF)
Hardness Minimum Thickness (+PDF)

ASTM Hardness Test Methods (+PDF)
Rockwell Hardness Scales (+PDF)

Brinell Hardness Chart (+PDF)
Vickers Hardness Chart (+PDF)

Rockwell Hardness Testing
Brinell Hardness Testing
Microhardness Testing


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