INTRODUCTION TO VICKERS PYRAMID HARDNESS TESTING
The Vickers Pyramid Hardness Testing Machine was developed as a result of research for a thoroughly reliable and accurate means of testing the hardness of metals, and to be applicable to all classes of work.
The importance of metallurgist science to engineering practice is ever increasing, and modern requirements of high speeds and stress call for rigid metallurgical control of hardness, capable of interpretation in terms of an accepted quantative formula.
In the vickers test, hardness is calculated in terms of the internationally accepted ratio:-
The basic formula is only of value when you have an accurate means of interpreting the result of test.
By the introduction of the 136 degree diamond pyramid, which with its geometric form provided a clear indentation with no burrs, previous problems associated with reading indents was eliminated. Just as important as having a clear indent is maintaining accuracy whilst applying a load, it is for this reason a Vickers Hardness Testing Machine utilises only calibrated weights which are applied through a simple lever mechanism.
It is important that any indent produced, is produced in a controlled manner and that it is not subjected to shock loads, to this end an automatic damping mechanism is employed to control the rate at which the load is applied.
As it is possible to apply loads varying from 1 to 100 kgs utilising the calibrated weights provided, the machine can measure both relatively soft and hard materials. Unlike other types of hardness testing, a Vickers Hardness Test Machine is capable of providing hardness values on one continuous numeric scale.
VICKERS PYRAMID NUMBER HARDNESS TESTING
BRITISH STANDARD REFERENCE: BS 427 : 1990
This standard covers the testing of material hardness in the Vickers range scales of HV1 to HV100 (1 Kg to 100 Kg force loads) and the hardness values between 5 and 3000 HV. The standard also covers the methods for determining and verifying the accuracy of hardness testing machine. The Vickers Hardness Machine is capable of measuring the softest and hardest of metals and depending on the machine model, varying metal thicknesses from say 0.010" to 18" can be measured.
It is highly recommended that an appropriate authority carries out a full calibration on Vickers Hardness Test Machines at least once a year. In between such calibrations at a frequency based upon usage, the accuracy of vickers hardness test machines should be monitored in-house.
Principal of Vickers Hardness Measurement.
An indenter comprising of a diamond in the form of a 'symmetrical pyramid' mounted in a holder, the diamond having an angle of 136 degrees between opposing faces. The diamond is forced into the test piece at a controlled rate with an appropriate force for a standard 'Dwell' period of 10 to 15 seconds. The produced indent is then measured using a microscope that can measure the diagonal length of the indent with an accuracy within 0,001mm. From this figure, which should be the mean dimension calculated from measuring each diagonal, it is possible to calculate the load area of the sloping faces of the pyramid. Given this value it is easy to calculate the Vickers pyramid number for any force employed by dividing the test force by the sloping surface area of the indent viz:-
Vickers Hardness Pyramid No. (HV) = Constant (*) x Test Force
Surface Area of Indentation
(* Constant = 1 = 1 = 1 = 0.102)
9n gravitational force in newtons 9.807n
force = mass (load) x gravity, ie. constant compensates for gravitational effects on load.
2F Sin 136 deg. 2f Sin 136 deg.
Therefore: HV = 0.102 x 2 = 2
Where: g = Gravity in Newtons
F = Test Force in Newtons
f = Load Mass Symbol
d = Mean of the Diagonal Indents
The acceleration effects of the dwell period of the indentation is ignored, as its contributing error is negated by being constant within a negligible limit between all measurements made, and the damping action of the automatic loading mechanism.
Representation of Hardness Values
When defining a hardness value, it is important that it is presented in an unambiguous form, to this end the scale that has been used for the test must be stated, and where necessary the dwell period if different from the standard dwell period of 10 to 15 seconds. The accepted method of illustrating such a measurement where a Vickers Hardness of 526 obtained using a test force of 30 Kgf and a non-standard dwell period of 25 seconds is: "526HV30/25".
Monitoring the Accuracy of Your Machine
It is recommended that the accuracy of your machine should be monitored on a regular basis, in addition to the more stringent test carried out by your calibrating agent. The regularity with which the accuracy monitoring tests are carried out depends upon the usage of the machine and the scales used. Should a machine be partially employed on every working day then daily tests should be carried out, the more frequent the use of the machine, ie/ on a shift system it is logical to confirm the machines accuracy at the start of every shift. Likewise if the machine is used only once or twice per week, then tests should be carried out on each day before the machine is used.
The results obtained from these tests should be recorded conveniently by the machine for examination, an example of a typical accuracy monitoring data sheet is attached.
The tests should be carried out with the machine in its normal operating condition and environment, and with its usual diamond indenter fitted. A test block calibrated in accordance with BS 427 : 1990 should be used, and its value be that of approximately the most measured value of test carried out and of the same scale. The test block should be tested for its value and the result recorded, where a second reading is taken, due to vibration being suspected during indenting for instance, this should be recorded. The mean value obtained should be within the assigned value for the test block within the following tolerances.
+/- 5% for HV1
+/- 4% for HV2.5
+/- 3% for HV5, 10 & 20
+/- 2% for HV30, 50 & 100
It is recommended that where the test machine is regularly used and utilises various load forces, the procedure should be repeated with test blocks of appropriate value to the measurement being conducted.
Typical Accuracy Monitoring Data Sheet
(for location convenient to machine)
ACCURACY MONITORING DATA SHEET
Machine Number: ..............................................
Test No. 1: Test Block Serial No./Value/Load Scale ..................../....................HV.....
Test No. 2: Test Block Serial No./Value/Load Scale ..................../....................HV.....
Test No. 3: Test Block Serial No./Value/Load Scale ..................../....................HV.....
Test No. 4: Test Block Serial No./Value/Load Scale ..................../....................HV.....
Date of Test 1st 2nd Error from O.K.
Test No. Reading Nominal % /X
If error from nominal exceeds tolerance below suspend use of machine and arrange corrective action via
+/- 5% HV1
+/- 4% HV25
+/- 3% HV6, 10 & 20
+/- 2% HV30, 50 & 100