Why do we need calibration? Avoid these common errors

The measurement principle of Vickers hardness tester is to press the diamond indenter into the sample with a specific loading force, measure the diagonal length of the indentation, and calculate the hardness (HV=0.1891 × F/d ², where F is the loading force and d is the average diagonal value). If the loading force is inaccurate, the pressure head is worn, or the measurement system deviates, it will directly lead to distortion of the results. For example:

The actual value of loading force is 10% higher than the set value, which may cause the hardness value to be 8% -12% higher;

The indentation measurement system has an error of 0.5 μ m, and for an indentation of 10 μ m, the hardness calculation error can reach over 10%.

Preparation before calibration: Tool and environmental requirements

Environmental requirements: Temperature of 20 ± 2 ℃, humidity of 45% -75%, avoid vibration (such as staying away from punching machines and air compressors) and airflow interference (turn off fans, doors and windows), ensure that the instrument is preheated for more than 30 minutes after startup.

Step by step calibration: from core components to overall accuracy

1. Calibration of loading force: Ensure the accuracy of "force"

Verification steps:

Select 3 commonly used loading forces (such as 1kgf, 5kgf, 10kgf, corresponding to 9.807N, 49.03N, 98.07N);

Connect the force standard device (such as a force sensor) below the indenter, start the hardness tester to apply the corresponding force value, and record the difference between the instrument display value and the standard device;

If the error exceeds ± 1% (as required by GB/T4340.1 standard), the loading system needs to be adjusted:

Mechanical lever type: by adjusting the position of the counterweight or lever fulcrum;

Electronic loading type: Enter calibration mode and recalibrate with standard force values (such as inputting the actual measured 5.05kgf, the instrument automatically corrects to 5.00kgf).

Attention: The loading process should be without impact (the force curve should rise smoothly). If there is a fluctuation of loading force exceeding ± 0.5%, it is necessary to check for hydraulic system oil leakage (hydraulic type) or motor drive failure (electronic type).

2. Calibration of indentation measurement system: solving the deviation of "seeing"

Magnification calibration:

Place the microscope micrometer (with a known scale value, such as 10 μ m per grid) on the workbench and observe it through the eyepiece or camera of the hardness tester;

Measure the display length of 10 grids on the micrometer. If the standard 100 μ m is actually displayed as 102 μ m, it indicates that the magnification is 2% higher and needs to be adjusted to an error of ≤ 0.5% through software or eyepiece scale ring.

Cross line alignment calibration:

Use a standard hardness block to create a clear indentation, rotate the worktable 90 °, and then re measure the same diagonal. If the difference between the two readings exceeds 0.5 μ m, adjust the concentricity between the crosshair and the worktable rotation center (loosen the eyepiece fixing screw for fine adjustment).

3. Inspection and calibration of the pressure head: avoid distortion of the "pressure"

Appearance inspection: Use a 200x microscope to observe the edges of the diamond indenter. If there are cracks, wear (rounded edges), or surface scratches, the indenter needs to be replaced (the angle between the two edges of the new indenter should be 136 °± 30 ′).

Indentation shape verification: Use a standard hardness block (such as HV300) to press the indentation under a loading force of 5kgf, measure the lengths d1 and d2 of the two diagonal lines. If the deviation of d1/d2 exceeds 2%, it indicates that the indenter is installed tilted or damaged, and needs to be reinstalled or replaced.

4. Calibration of the verticality between the workbench and the spindle: ensuring smooth placement

Place the level on the workbench and measure in the X and Y directions. If the horizontal deviation exceeds 0.05mm/m, adjust the bottom leveling screw of the instrument to correct it;

Use a perpendicularity gauge (such as a right angle ruler with a dial gauge) to check the perpendicularity between the spindle (indenter axis) and the workbench. If the deviation exceeds 0.03mm/100mm, loosen the spindle fixing nut and fine tune it to be qualified.

5. Overall accuracy verification: final confirmation using standard blocks

Select three standard hardness blocks with different hardness ranges (such as HV100, HV500, HV1000) and measure them three times each under common loading forces (such as 1kgf, 10kgf);

Calculate the error between each measurement value and the nominal value of the standard block. If all errors are ≤ ± 3% (required for secondary standard blocks), the calibration is qualified;

If the error in a certain range exceeds the standard, it is necessary to retrospectively check the corresponding loading force or indentation measurement system (for example, if the HV100 error is large, it may be due to inaccurate small loading force; HV1000 has a large error, which may be due to wear of the indenter.