Rockwell hardness testing for screw

Rockwell hardness testing for screw

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Recently we had a case using SUS 410 screw when penetrate into steel purlin, the tip and threads broke. So we carried out hardness test to prove.

Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation. More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material.

Indentation hardness value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods. Learn more about hardness testing basics here.

The Rockwell hardness test method, as defined in ASTM E-18, is the most commonly used hardness test method. You should obtain a copy of this standard, read and understand the standard completely before attempting a Rockwell test.

The Rockwell test is generally easier to perform, and more accurate than other types of hardness testing methods. The Rockwell test method is used on all metals, except in condition where the test metal structure or surface conditions would introduce too much variations; where the indentations would be too large for the application; or where the sample size or sample shape prohibits its use.

The Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a preliminary test force (commonly referred to as preload or minor load) is applied to a sample using a diamond or ball indenter. This preload breaks through the surface to reduce the effects of surface finish. After holding the preliminary test force for a specified dwell time, the baseline depth of indentation is measured.

After the preload, an additional load, call the major load, is added to reach the total required test load. This force is held for a predetermined amount of time (dwell time) to allow for elastic recovery. This major load is then released, returning to the preliminary load.  After holding the preliminary test force for a specified dwell time, the final depth of indentation is measured. The Rockwell hardness value is derived from the difference in the baseline and final depth measurements. This distance is converted to a hardness number. The preliminary test force is removed and the indenter is removed from the test specimen.

Preliminary test loads (preloads) range from 3 kgf (used in the “Superficial” Rockwell scale) to 10 kgf (used in the “Regular” Rockwell scale). Total test forces range from 15kgf to 150 kgf (superficial and regular) to 500 to 3000 kgf (macrohardness).

 

Test Method Illustration

A = Depth reached by indenter after application of preload (minor load)

B = Position of indenter during Total load, Minor plus Major loads

C = Final position reached by indenter after elastic recovery of sample material

D = Distance measurement taken representing difference between preload and major load position. This distance is used to calculate the Rockwell Hardness Number.

 

The determination of the Rockwell hardness of a material involves the application of a minor load followed by a major load. The minor load establishes the zero position. The major load is applied, then removed while still maintaining the minor load. The depth of penetration from the zero datum is measured from a dial, on which a harder material gives a higher number. That is, the penetration depth and hardness are inversely proportional. The chief advantage of Rockwell hardness is its ability to display hardness values directly, thus obviating tedious calculations involved in other hardness measurement techniques.

The equation for Rockwell Hardness is , where d is the depth (from the zero load point), and N and s are scale factors that depend on the scale of the test being used (see following section).

It is typically used in engineering and metallurgy. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.

Legacy Rockwell hardness testers operation steps:

  1. Load an initial force: Rockwell hardness test initial test force is 10 kgf (98 N; 22 lbf); superficial Rockwell hardness test initial test force is 3 kgf (29 N; 6.6 lbf).
  2. Load main load: reference below form / table ‘Scales and values’.
  3. Leave the main load for a “dwell time” sufficient for indentation to come to a halt.
  4. Release load; the Rockwell value will typically display on a dial or screen automatically.

In order to get a reliable reading the thickness of the test-piece should be at least 10 times the depth of the indentation. Also, readings should be taken from a flat perpendicular surface, because convex surfaces give lower readings. A correction factor can be used if the hardness of a convex surface is to be measured.

Cort part HRC 35, about 320HV.

Surface we use Vickers tester, about 590HV, 53HRC.
Ground screw pile


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