TASK 70-34-00-200-003
1 . General.
A.The hardness of a metal is the resistance which it exerts against the penetration of another harder body, or to a permanent deformation of its surface.
B.The hardness measurement is an indentation measurement using a calibrated machine to force a penetrator into the surface of the material being measured. The dimensions of the remaining imprint provide a measurement of the hardness.
C.Hardness measurements are often used to verify the heat treatment condition of metals or to determine if overtemperature conditions may have occurred during engine operation on parts which change in hardness and strength when exposed to excessively high temperatures.
2 . Types of Hardness Measurements.
Subtask 70-34-00-200-031
A.Brinell Hardness Test.
(1)The Brinell hardness test uses a calibrated machine to force a ball into the surface of a metal and measures the diameter of the resulting impression following removal of the load.
(2)The Brinell hardness number is the load divided by the spherical area of the imprint.
B.Vickers Hardness Test.
(1)The Vickers hardness test consists of impressing into the metal, without shock, a penetrator in the form of a right pyramid with a square base, under a given load. The penetrator has an angle of 136 degrees at the tip. For the majority of repair procedures, the application of Vickers hardness testing refers to MICROHARDNESS TESTING which is performed on a mounted metallurgical cross-section.
(2)The Vickers hardness number is the load divided by the surface area of the imprint.
C.Rockwell Hardness Test.
(1)Regular tester. The regular Rockwell hardness tester measures the depth of penetration of a penetrator (diamond cone or hardened steel ball) under fixed conditions.
(2)The Rockwell hardness number is related to the depth of penetration caused by two superimposed impressions.
(3)Superficial tester. The tester for making the superficial hardness test is a specialized form of the regular Rockwell hardness tester. It measures hardness by the same principle as the regular tester, but employs a smaller minor load, smaller major loads, and a more sensitive depth measuring system for use on thin strip or sheet material.
D.Portable Rockwell Test.
(1)Portable hardness testing equipment is used when the weight, shape, or condition of the part prevents measurement by other methods.
(2)The tester is attached by clamp or magnet to the surface to be measured or to an adjacent surface. The load is generally applied by means of a screw through a calibrated spring with a dial gage to measure deflection of the spring and another dial gage to indicate depth of penetration.
NOTE:
The portable tester is not considered to be a precise measuring device. It is used only when other testing equipment cannot be practically used. If it is not practical to use a stationary Rockwell tester due to size, geometry, or part environment, a portable Rockwell hardness tester can be used. Refer to TASK 70-34-04-220-010, Portable Rockwell Hardness Testing.
E.Shore Hardness Test.
(1)The hardness testing of plastics is usually measured by the Shore (Durometer) test or Rockwell hardness test. The two methods measure the resistance to indentation and give a hardness value that does not align with other properties. Shore Hardness, measured with the Shore A or Shore D scale, is recommended for rubbers/elastomers and is also used for softer plastics such as polyolefins, fluoropolymers, and vinyls. The Shore A scale is used for softer rubbers while the Shore D scale is used for rubber that is harder.
(2)The Shore hardness is measured with a device known as a Durometer and is thus known as Durometer hardness. The hardness value is measured by the penetration of the Durometer indenter foot into the sample. Because of the resilience of rubbers and plastics, the indentation value can change over time. So the indentation time is sometimes reported along with the hardness number.
(3)ASTM D 2240, the Standard Test Method for Rubber Property - Durometer Hardness, gives the approved procedure to measure Shore Hardness.
3 . Conversion Tables.
Subtask 70-34-00-200-032
A.There is no exact method for converting, accurately, from one hardness scale to another. Such conversion should be made only when necessary. The following tables list some approximate conversion values.
Figure 1   (Sheet 1 ) Hardness Conversion for Nickel and High Nickel Alloys
Figure 1   (Sheet 2 ) Hardness Conversion for Nickel and High Nickel Alloys
Figure 1   (Sheet 3 ) Hardness Conversion for Nickel and High Nickel Alloys
Figure 2   (Sheet 1 ) Hardness Conversion for Steel
Figure 2   (Sheet 2 ) Hardness Conversion for Steel