GB/T 230.3-2022 Metallic materials - Rockwell hardness test - Part 3: Calibration of reference blocks
1 Scope
This document specifies a method for the calibration of reference hardness blocks (hereafter referred to as reference blocks) to be used for the indirect and daily verification of Rockwell hardness testing machines, as specified in GB/T 230.2-2022.
Attention is drawn to the fact that carbon tungsten alloy ball indenter is considered to be the standard type of Rockwell indenter ball. Steel indenter balls may be used only when complying with GB/T 230.1-2018, Annex A.
This document is applicable to the calibration of Rockwell hardness reference blocks.
2 Normative references
The following referenced documents are indispensable for the application of this standard. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 230.1-2018 Metallic materials - Rockwell hardness test - Part 1 : Test method (ISO 6508-1:2016, MOD)
GB/T 230.2-2022 Metallic materials - Rockwell hardness test - Part 2 : Verification and calibration of testing machines and indenters (ISO 6508-2:2015, MOD)
JJG 144 Verification regulation for standard dynamometers
3 Terms and definitions
No terms and definitions are listed in this document.
4 Manufacture of reference blocks
4.1 The reference block shall be specially manufactured. Attention is drawn to the need to use a manufacturing process, which will give the necessary homogeneity, stability of structure, and uniformity of surface hardness of reference blocks.
4.2 Each hardness reference block shall be of a thickness not less than 6 mm. To minimize the effect of hardness change with increasing number of indents, thicker blocks should be used.
4.3 The reference blocks shall be free of magnetism. The manufacturer should ensure that the blocks, if made of steel, have been demagnetized at the end of the manufacturing process (before calibration).
4.4 The deviation from surface flatness of reference blocks shall not exceed 0.01 mm. The bottom of the blocks shall not be convex. The deviation from parallelism of reference blocks shall be ≤0.02 mm per 50 mm.
4.5 The test surface and bottom surface shall be free from damage, such as notches, scratches, oxide layers, etc., which can interfere with the measurement of the indentations. The surface roughness, Ra, shall not exceed 0.0003 mm for the test surface and 0.0008 mm for the bottom surface. Sampling length is l = 0.80 mm (see GB/T 3505-2009, 3.1.9).
4.6 To verify that no material is subsequently removed from the reference block, the thickness at the time of calibration shall be marked on it, to the nearest 0.1 mm, or an identifying mark shall be made on the test surface.
5 Calibration machine and calibration indenter
5.1 General
5.1.1 Calibrations and verifications of Rockwell calibration machines and calibration indenters shall be carried out at a temperature of (23 ± 5) °C.
5.1.2 The instruments used for calibration shall be traceable to national standards.
5.2 Calibration machine
5.2.1 In addition to fulfilling the general conditions specified in GB/T 230.2-2022, Clause 4, the calibration machine shall also meet the requirements given in 5.2.2, 5.2.3, 5.2.4, 5.2.5, and 5.2.6.
5.2.2 The machine shall be directly verified in intervals not exceeding 12 months. Direct verification involves calibration and verification of the following:
a) test force;
b) indentation depth measuring system;
c) testing cycle; if this is not possible, at least the force versus time behaviour.
5.2.3 The test force shall be measured by means of standard force-proving device (according to JJG 144) class 0.03 or better and calibrated for reversibility, or by another method having the same or better accuracy.
Evidence shall be available to demonstrate that the output of the force-proving device does not vary by more than 0.1 % in a period of 1 s to 30 s, following a stepped change in force.
5.2.4 Each test force shall be measured and shall agree with the nominal preliminary test force, F0, to within ±0.2 % and the nominal total test force, F, to within ±0.1 %.
5.2.5 The measuring system shall have a resolution of 0.1μm ) and a maximum expanded uncertainty of less than 0.2μm, when calculated with a confidence level of 95 % over its working range.
5.2.6 The testing cycle shall be timed with an uncertainty less than ±0.5 s and shall conform to the testing cycle of Clause 6.
5.3 Calibration diamond indenter
5.3.1 The geometric shape and performance of calibration diamond indenters shall be calibrated as defined below. Direct verification of the geometric shape shall be made before first use and at a frequency of no greater than five years. Verification of the indenter performance, as specified in 5.3.3, shall be made before first use and at a frequency of no greater than 12 months.
5.3.2 The diamond indenter shall be measured on at least eight unique axial section planes equidistant from each other (e.g. the eight cross-sections will be spaced approximately 22.5° apart at 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, 157.5°), and shall meet the following requirements:
a) The cone angle shall be measured adjacent to the blend. The diamond cone shall have a mean included angle of (120 ± 0.1)°. In each measured axial section, the included angle shall be (120 ± 0.17)°.
b) The mean deviation from straightness of the generatrix of the diamond cone adjacent to the blend shall not exceed 0.0005 mm over a minimum length of 0.4 mm. In each measured section, the deviation shall not exceed 0.0007 mm.
c) The radius of the spherical tip of the diamond shall be measured adjacent to the blend. The tip shall have a mean radius of (0.200 ± 0.005) mm. In each measured section, the radius shall be within (0.200 ± 0.007) mm and local deviations from a true radius shall not exceed 0.002 mm.
Note: The tip of the diamond indenter is usually not truly spherical, but often varies in radius across its surface. Depending on the crystallographic orientation of the diamond stone with respect to the indenter axis, diamond tends to preferentially polish away more easily or with more difficulty at the tip, producing an increasingly flat or sharp surface in the central indenter axis region. The sphericity of the diamond tip can be better evaluated by measuring multiple measurement windows of varying width. The measurement window would be bounded by widths measured along a line normal to the indenter axis.
For example, the following window sizes can be evaluated:
— between ±80 µm from the indenter axis;
— between ±60 µm from the indenter axis;
— between ±40 µm from the indenter axis;
d) The surfaces of the cone and the spherical tip shall blend in a smooth tangential manner. The location where the spherical tip and the cone of the diamond blend together will vary depending on the values of the tip radius and cone angle. Ideally for a perfect indenter geometry, the blend point is located at 100 µm from the indenter axis measured along a line normal to the indenter axis. To avoid including the blend area in the measurement of the tip radius and cone angle, the portion of the diamond surface between 90 µm and 110 µm shall be ignored.
