Metallic materials - Rockwell hardness test - Part 2: Verification and calibration of testing machines and indenters
1 Scope
This document specifies two separate methods of verification of testing machines (direct and indirect) for determining Rockwell hardness in accordance with GB/T 230.1, together with a method for verifying Rockwell hardness indenters.
The direct verification method is applicable to determine whether the main parameters associated with the machine function, such as applied force, depth measurement, and testing cycle timing, fall within specified tolerances. The indirect verification method uses a number of calibrated reference hardness blocks to determine how well the machine can measure a material of known hardness.
The indirect method may be used on its own for periodic routine checking of the machine in service.
If a testing machine is also to be used for other methods of hardness testing, it shall be verified independently for each method.
This document is applicable to stationary and portable hardness testing machines.
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.
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 (IS0 6508-1 : 2016, MOD)
GB/T 231.3-2022 Metallic materials - Brinell hardness test - Part 3: Calibration of reference blocks (ISO 6508-3: 2015, MOD)
GB/T 4340.1 Metallic materials - Vickers hardness test - Part 1: Test method (GB/T 4340.1-2009, ISO 6507-1:2005, M0D)
GB/T 13634 Metallic materials - Calibration of force-proving instruments used for the verification of uniaxial testing machines (GB/T 13634-2019, IS0 376 : 2011, IDT)
3 Terms and definitions
No terms and definitions are listed in this document.
4 General
Before a Rockwell hardness testing machine is verified, the machine shall be checked to ensure that it is properly set up in accordance with the manufacturer’s instructions. Especially, it should be checked that the test force can be applied and removed without shock, vibration, or overload and in such a manner that the readings are not influenced.
5 Direct verification of the testing machine
5.1 General
5.1.1 Direct verification involves calibration and verification of the following:
a) test forces;
b) indentation depth measuring system;
c) testing cycle;
d) machine hysteresis test.
5.1.2 Direct verification should be carried out at a temperature of (23 ± 5) °C. If the verification is made outside of this temperature range, this shall be reported in the verification report.
5.1.3 The instruments used for verification and calibration shall be traceable to national standards.
5.1.4 An indirect verification according to Clause 6 shall be performed following a successful direct verification.
5.2 Calibration and verification of the test force
5.2.1 Each preliminary test force, F0, (see 5.2.4) and each total test force, F, used (see 5.2.5) shall be measured, and, whenever applicable, this shall be done at not less than three positions of the plunger spaced throughout its range of movement during testing. The preliminary test force shall be held for at least 2 s.
5.2.2 Three readings shall be taken for each force at each position of the plunger. Immediately before each reading is taken, the plunger shall be moved in the same direction as during testing.
5.2.3 The forces shall be measured by one of the following two methods:
— by means of a force-proving device according to GB/T 13634 class 1 or better and calibrated for reversibility;
— by balancing against a force, accurate to ±0.2 %, applied by means of calibrated masses or by another method having the same accuracy.
Evidence should be available to demonstrate that the output of the force-proving device does not vary by more than 0.2 % in the period 1 s to 30 s following a stepped change in force.
5.2.4 The tolerance on each measurement of the preliminary test force, F0, (before application and after removal of the additional test force, F1) shall be ±2.0 %, see Formula (B.2). The range of all force measurements (highest value minus lowest value) of the preliminary test force shall not be larger than 1.5 % of the nominal value of F0.
5.2.5 The tolerance on each measurement of the total test force, F, shall be ±1.0 % of its nominal value. The range of the force measurements (highest value minus lowest value) of the total test force shall be not larger than 0.75 % of the nominal value of F.
5.3 Calibration and verification of the indentation depth measuring system
5.3.1 The depth measuring system shall be calibrated by making known incremental movements of the indenter or the indenter holder.
5.3.2 The instrument or gauge blocks used to verify the depth measuring system shall have a maximum expanded uncertainty of 0.0003 mm when calculated with a 95 % confidence level.
5.3.3 Calibrate the testing machine’s depth measurement system at not less than four evenly spaced increments covering the full range of the normal working depth measured by the testing machine. For this purpose, the working depth is 0.25 mm for regular Rockwell scales (A, C, D, B, E, F, G, H, K), and 0.1 mm for superficial Rockwell scales (N, T).
5.3.4 Some testing machines have a long-stroke depth measuring system where the location of the working range of the depth measuring system varies to suit the sample thickness. This type of testing machine shall be able to electronically verify that the depth measuring device is continuous over the full range. These types of testers shall be verified using the following steps:
a) At the approximate top, midpoint, and bottom of the total stroke of the measuring device, verify the depth measuring system at no less than four evenly spaced increments of approximately 0.05 mm at each of the three locations.
b) Operate the actuator over its full range of travel to monitor whether the displacement measurement is continuous. The displacement indication shall be continuously indicated over the full range.
5.3.5 The depth measuring system shall correctly indicate within ±0.001 mm for the scales A to K and within ±0.0005 mm for scales N and T, i.e. within ±0.5 of a scale unit, over each range.
5.4 Calibration and verification of the testing cycle
5.4.1 The testing cycle is to be calibrated by the testing machine manufacturer at the time of manufacture and when the testing machine undergoes repair which may have affected the testing cycle. Calibration of the complete testing cycle is not required as part of the direct verification at other times, see Table 10.
5.4.2 The testing cycle shall conform to GB/T 230.1-2018.
5.4.3 For testing machines that automatically control the testing cycle, the measurement uncertainty (k = 2) of the timing instrument used to verify the testing cycle shall not exceed 0.2 s. It is recommended that the measured times for the testing cycle, plus or minus the measurement uncertainty (k = 2) of the calibration measurements, not exceed the timing limits specified in GB/T 230.1-2018.
5.4.4 For testing machines that require the user to manually control the testing cycle, the testing machine shall be verified to be capable of achieving the defined testing cycle.
5.5 Calibration and verification of the machine hysteresis
