The hardness measurement. The Rockwell hardness test method and microhardness (Part 2).

Irrespective the Rockwell’s scale which is used, the main process operations of the tests in essence don’t change and are provided below:

  1. The preparation of the studied surface;
  2. The placement of the tests subject in the Rockwell test machine;
  3. The preliminary loading application;
  4. The main loading application;
  5. The loading removal;
  6. The instrument reading.

The preliminary loading serves for the compensation of possible errors or gaps in system and allows you to increase the accuracy of tests.

The results received when determining Rockwell hardness can be transformed to hardness according to Brinell and, therefore, to value of the metal strength. As well as in a case with tests according to Brinell, the hardness measurement of metal according to Rockwell can be carried out by means of portable devices. Though work with it can differ slightly from work with bench models, results turn out equivalent.

Now we will consider the following type of tests for the hardness measurement of microhardness. The origin of the name is connected with the fact that the prints created at these tests are so small that carrying out measurements requires repeated increase. The measurement of microhardness is very convenient at the research of microstructures of metal as it is possible to work with separate grain of metal, determining its hardness in microscopic area of grain.

The measurement of microhardness can be carried out by two main methods: on Vikkersa and to Knup. Both methods provide application of the diamond indetnor, but with a little different form.

The Vikkers’s indentorahs with the basis of the square shape leaves a print which both diagonals have approximately identical length. The Knup’s indentorahs creates a print with the different-sized parties. As well as in other methods, it is possible to choose from a wide range of test loadings and types of indentor here. The application of the loadings which aren’t exceeding 1-1000 hs is meant by a concept of microhardness. However in the vast majority of the microhardness measurements loadings in the range from 100 to 500 hs are most often used.

During the microhardness measurement on Vikkersa or to Knup the extreme importance is gained by preparation of the surface. Any roughness of the surface can result in incorrect results of test. Usually, the surface of a sample is prepared for the microhardness measurement precisely as it becomes by preparation for other researches in a metallography. The value of the processing quality of the surface increases in process of decrease in the enclosed loading.

After completion of the sample surface preparation isn’t movably fixed by means of tightening adaptation or the holder so that to achieve the exact provision of prints. Many machines for the microhardness measurement are supplied with a mobile little table by means of which it is possible to move precisely a sample without its removal and repeated adjustment. It is convenient to use such device when it is necessary to receive a number of indications in a certain area of structure of metal. The definition of changes of hardness in a zone of thermal influence can be an example of such use. The result of such measurement is called epure or the microhardness profile.

During the sample microhardness measurement the following steps are carried out:

  1. The preparation of the sample surface;
  2. The sample fixing in tightening adaptation;
  3. The definition of the necessary zone by means of a microscope;
  4. The print formation;
  5. The print measurement by means of a microscope;
  6. The hardness measurement by means of tables or by calculations.
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