Methods and means of calibration hardness testing machine

GENERAL CALIBRATION PROCEDURE

HARDNESS TESTING MACHINE

1. Scope of application

This technical written provides methods and means of calibration of hardness of metal materials testing machines according to the Rockwell (scale A, B, C), Brinell and Vickers static testing methods.

2. The calibrations

To in turn carry out calibrations listed in Table 1

Table 1

3. Means of calibration

To use the means of calibration listed in Table 2. Used means of calibration must have suitable measuring scope.

Table 2

4. Conditions of calibration

When conducting calibrations, we must satisfy the following conditions:

• Air temperature of the location placing machines must be ensured at 27⁰C ± 5⁰
• Machine placement needs to avoid the effects of chemical corrosion and vibration.

• Machine must be installed certainly as instruction for installation and usage. The standardization is done at the placement of installation.

1. Performing calibrations

• External inspection

Inspect as the following requirements:

• The machine must have brand noting engine number and production place.
• The machine must have all parts and accessories attached using instruction.
• The number of parts directives the hardness value or the number of force pointing board must be clear.
• Technical inspection

Inspect as the following requirements:

• Inspect equilibrium of the machine

Use Nivo to inspect the equilibrium of the machine. Horizontal and vertical deviations shall not exceed 1 mm / m.

• Inspect the working status of the machine
• Inspect the creating force part

Control the transmission parts to create testing force at force levels. Creating force part (including the controlling speed of force testing part, if any) must ensure that the force is created regularly, continuously, without sudden changes.

• Inspect samples setting table and table lifting part

Inspect the uneven of the samples testing table with board ruler and leaf base. The uneven range does not exceed 0.1 mm/100 mm. Control so as for samples setting table to move, the table must be up and down gently, not be jerky, the screw axis of table is not relayed

• 2.2.3. Inspect the hardness measuring part
• Inspect the hardness measuring part of Rockwell hardness testing machine

The meter of compressed trace depth gauges must move gently over the entire measuring scope. In the process of movement, the needle must not jump. After applying a small force to the probe, the needle must back to its original position. Gauges should match with TCVN 257-2: 2000 (Verification and calibration of Rockwell hardness testing machine).

• Inspect measure unit of Brinell and Vickers hardness testing machines
• With the machine having optical measuring part, we must satisfy the following requirements:
  • The observation area must be illuminated;
  • Compressed trace center must be between the observations;
  • Compression trace and number lines of meter bar must be clear.

• Inspect the accuracy of the meter bar in the standardized meter bar.
  • For Brinell hardness testing machine, the error does not exceed 1%;
  • For Vickers hardness testing machine, the error does not exceed 0.1%.
• Inspect the samples clamping part

Samples clamping parts must hold the samples tightly on the table during the process.

• Inspect the measuring tip

Use the magnifying lens to observe the measuring tip. Measuring tip surface is not cracked or defected. Measuring tip should match with (Vietnam Standards) VNS 256-2: 2000 (Verification and calibration of Brinell hardness testing machine) or VNS 257-2: 2000 or VNS 258-2: 2000 (Verification and calibration of Vickers hardness testing machine).

• Inspect the measurement
• General provisions
• Provisions for inspection of testing force
• For Rockwell hardness testing machine, we must check the original force and the total force;
• For Brinell and Vickers hardness testing machines, we must inspect all the force levels;
• For the force level tested according to the upward direction, each level is checked at least 3 times.
• Provisions for errors and scattered of hardness value
• With Rockwell hardness testing machine, we must inspect the absolute error and the scattered of hardness value for all scales. In the case that one measuring scale is carried out to inspect for used errors scale.
• With Rockwell Brinell hardness testing machine having 2 methods, or Vickers – Brinell, we must inspect the hardness error and the relative scattered with 2 methods. In the case that using one method, we must inspect error with used method.
• The relative error permits for the biggest testing power in table 3.

Table 3

- The absolute error and the scattered level largest allow the largest of the hardness value for Rockwell hardness testing machine in Table 4

Table 4

• The relative error of the hardness value of the relative hardness and scattered allow the largest of diameter or diagonal indentations of Brinell or Vickers hardness testing machine in Table 5.

Table 5

• Conduct the inspection
• Inspect the relative error of the testing force

The relative error of testing force at levels is inspected as increasing direction, each level is inspected 3 times.

The relative error of the testing power  expressed in %  is determined by the formula:

Including:

δ_F: Relative error of each testing force level;

F: Nominal value of the testing force;

: The average number of three measurements at each measure point, reading on force machine.

• Inspect the absolute error and the scattered of hardness value with Rockwell hardness testing machine.

For each hardness measuring board, we must use at least 3 standardized hardness rigids for inspection.

Hardness value of the standard rigid must be within the following limitation:

• Scale A:
  • Standardized rigid 1: (20 ÷ 40) HRA
  • Standardized rigid 2: (45 ÷ 75) HRA
  • Standardized rigid 3: (80 ÷ 88) HRA
• Scale B:
  • Standardized rigid 1: (20 ÷ 50) HRB
  • Standardized rigid 2: (60 ÷ 80) HRB
  • Standardized rigid 3: (85 ÷ 100) HRB
• Scale C:
  • Standardized rigid 1: (20 ÷ 30) HRC
  • Standardized rigid 2: (35 ÷ 55) HRC
  • Standardized rigid 3: (60 ÷ 70) HRC

To conduct five measurements on each standardized rigid after removing the first two measurements. Position of traces must to be distributed relatively evenly over the surface of the standardized rigid.

• Insp\ct the absolute error of the Rockwell hardness value

The absolute error of the Rockwell hardness value is determined by the formula:

Including:

• Δ: Absolute error of Rockwell hardness value;
• H: Nominal hardness value of the standardized hardness rigid;
• H: The average value of 5 hardness values measured on a standardized hardness rigid.

• Inspect the scattered of Rockwell hardness value

The scattered of hardness value is determined by the formula:

Including:

• R : The scattered
• Hmax, Hmin: The smallest and largest hardness values in 5 values measured on one standardized hardness rigid.

• Inspect the relative error of the Brinell or Vickers hardness values and the relative scattered of diameter of indentation or diagonal of compression trace.
• For Brinell hardness testing machine, we must to use at least 2 Brinell standardized hardness rigids with the testing force level to inspect the machine. The hardness value of 2 standardized hardness rigids must be within the following limitation:
  • Standardized rigid 1: (100 ÷ 200) HB
  • Standardized rigid 2: (250 ÷ 350) HB
• For Vickers hardness testing machine, we must use at least 3 Vickers standardized hardness rigids with the testing force level to inspect the machine. The hardness value of 3 standardized hardness rigids must be within the following limitation:
  • Standardized rigid 1: ≤ 225 HV
  • standardized rigid 2: (400 ÷ 600) HV
  • Standardized rigid 3: > 700 HV
• To conduct five measurements on each standardized hardness rigid. Position of measured traces must be relatively distributed on the surface of standardized rigid.
  • Inspect the relative error of the Brinell or Vickers hardness values.

Relative error of the hardness value expressed in% is determined by the following formula:

Including:

• δH: Relative error of the Brinell or Vickers hardness values;
• H: Nominal value of hardness of Brinell or Vickers standardized hardness rigid;
• : The average value of 5 hardness values measured on a standardized hardness rigid.
• Inspect the relative scattered of diameter of indentation or diagonal of compression trace.

The relative scattered of diameter of indentation or diagonal of compression trace expressed in % is determined by the following formula:

Including:

• Rd: The relative scattered of diameter of indentation or diagonal of compressed trace.
• dmax, dmin: The average diameter of the largest and smallest of the five measured compressed indentations or traces on a standardized hardness rigid.
• : The average value of 5 average diameters (*) of indentation or compressed trace.

Remark (*):The average diameter of indentation or average diagonal of compressed trace is the average value of the diameter or diagonal of an indentation or a compressed trace measured in two mutually perpendiculars.

1. General handling

• The hardness testing machine after being calibration is certificated accompanying the calibration results.
• Calibration cycle of the machine is 1 year.

APPENDIX 1

GUIDELINES FOR DETERMINING THE UNCERTAINTY

The standardized synthetic measurement uncertainty is determined by the formula:

Including:

• u_f : Components of measurement uncertainty is caused by the testing force. With triangular distribution probability, we have:
• with a_f is a half of the width of the relative error
• u_tm: Components of measurement uncertainty caused by the scattered of hardness value. With rectangular distribution probability, we have:
• With a_tm is a half of the width of the relative scattered
• u_md: Components of measurement uncertainty caused by measurement error tip.
• u_ct: Components of measurement uncertainty caused by the error of the measurement unit (depth measuring gauge or diameter, diagonal of compressed trace measuring part).
• u_tc: Components of the measurement uncertainty of the standardized rigid.

The extended uncertainty of measurement of hardness testing machines is determined by the formula:

U = k.u_c

Including:

• k: covered coefficient; k = 2 with reliable probability 95,6%;
• u_c: standardized synthetic uncertainty.

APPENDIX 2

Measuring device name:………………………………………………………………………………………………………….

Model:…………………………………………………………………….. No:……………………………………………………..

Production facility:…………………………………………………… Production year:…………………………………..

Technical reatures:………………………………………………………………………………………………………………….

Using place:…………………………………………………………………………………………………………………………..

Method of implementation:……………………………………………………………………………………………………..

…………………………………………………………………………………………………………………………………………….

…………………………………………………………………………………………………………………………………………….

Standard of main used equipment:……………………………………………………………………………………………

…………………………………………………………………………………………………………………………………………….

Environment conditions:

_ Temperature:……………………………………………………………………………………………………………………….

_ Humidity:…………………………………………………………………………………………………………………………..

Presenter:………………………………………………………………………………………………………………………………

Perfoming date:……………………………………………………………………………………………………………………..

RESULTS

1. External inspection

            - Brand:

– Completeness:

– Directive number:

2. Technical inspection

            - Horizontal deviation:                           …………………………………. mm/m

– Vertical deviation:                               …………………………………. mm/m

– Uneven of the samples setting table:   …………………………………. mm

– Technical requirements of the measuring section:

– Measurement error:                               …………………………………. mm

– Technical requirements of measuring tip:

3. Measurement inspection

• Results of relative error of testing force.

• Results of Rockwell hardness testing error (Brinell, Vickers) and Rockwell scattered hardness value.

• Results of testing relative scattered of Brinell or Vickers hardness values.

• Measurement uncertainty:

u = …………………….

Conclusion:………………………………………………………………………………………………………………….

…………………………………………………………………………………………………………………………………………….

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