This standard specifies the method for non-destructive measurement of the thickness of non-magnetic (including glaze and enamel) coatings on magnetic metal substrates using magnetic thickness gauges. This method is applicable to measurements on flat specimens. It is not applicable to the measurement of coatings on non-magnetic substrates. GB/T 4956-1985 Magnetic method for measuring the thickness of non-magnetic coatings on magnetic metal substrates GB/T4956-1985 Standard download decompression password: www.bzxz.net
This standard specifies the method for non-destructive measurement of the thickness of non-magnetic (including glaze and enamel) coatings on magnetic metal substrates using magnetic thickness gauges. This method is applicable to measurements on flat specimens. It is not applicable to the measurement of coatings on non-magnetic substrates.

National Standard of the People's Republic of China
Non -magnetic coatings on magetie substrates-Measurement of coating thickness-Magnetie method
UDC 621.793 : 531
717 .11 866
.293/.295
GB 4956-85
This standard specifies the method for non-destructive measurement of the thickness of non-magnetic (including glaze and porcelain) coatings on magnetic metal substrates using magnetic thickness gauges. This method is applicable to the measurement on the specimen. It is not applicable to the measurement of coatings on non-magnetic substrates. This standard is equivalent to the international standard 1S02178·--1982 "Magnetic measurement method for the thickness of non-magnetic coatings on magnetic metal substrates". 1 Principle
Magnetic measuring instrument measures the change of magnetic bow force caused by the existence of coating between permanent magnet (probe) and base metal, or measures the change of magnetic resistance of magnetic circuit between coating and base metal. 2 Factors affecting measurement accuracy
2.1 Coating thickness
The measurement accuracy changes with the change of coating thickness and is related to the setting of the selected thickness measuring instrument. For thin coating, its measurement accuracy is a constant and has nothing to do with the coating thickness. For thick coating, its accuracy is approximately proportional to the coating thickness. 2. 2 Magnetism of base metal
Compensation method thickness measurement is affected by the change of magnetism of base metal (in practical application, the change of magnetism of low carbon steel can be considered as slight). In order to avoid the influence of heat treatment and cold working factors, the instrument should be calibrated with calibration standard sheet with the same properties as the base metal of the sample. It can also be calibrated with the sample to be plated. 2.3 Base Metal Thickness
For each instrument there is a critical thickness of base metal, which varies from instrument to instrument. If the base metal thickness exceeds this critical thickness value, the measured value will not be affected by the increase in base metal thickness. The critical value depends on the instrument probe and the quality of the base metal. If the instrument manufacturer provides a critical thickness value, it should be determined by experiment. 2.4 Edge Effect
This method is sensitive to changes in the shape of the specimen surface, so measurements too close to the edge or inner corner are unreliable unless the instrument is specially calibrated for this measurement. This edge effect can extend to about 15 mm from the reverse edge, but it depends on the instrument. 2.5 Curvature
The curvature of the specimen has an effect on the diameter. The effect of curvature is related to the manufacturer and type of instrument, but this effect always increases significantly with the decrease of the curvature diameter.
When using a bipolar probe instrument, different readings may be obtained if the probe is measured along or along the axis of the cylinder. Measurements made on this sample are unreliable unless the instrument is specially calibrated for this purpose. 2.6 Surface roughness
If the series of values ​​measured on the rough surface within the reference area clearly exceed the inherent reproducibility of the instrument, the number of measurements required should be increased to 5. wwW.bzxz.Net
2.7 Direction of machining of base metal
National Bureau of Standards 198502-14 Issued
1985-12-01 Implementation
GB4956-85
When using a bipolar probe or a worn and uneven monopolar probe, the instrument reading will be affected by the direction of mechanical processing (such as rolling) of the magnetic base metal, and will change with the different positions of the probe on the surface of the test sample. 2.8 Residual magnetism
The residual magnetism of the base metal may affect the values ​​measured by fixed magnetic field instruments, but this effect is much smaller when using alternating magnetic field reluctance instruments.
2.9 Magnetic field
The strong magnetic field generated by various electrical equipment in the surrounding area will seriously interfere with the operation of the magnetic thickness gauge. 2.10 If the material
Since magnetic thickness gauges are very sensitive to adhering materials that prevent the probe from making close contact with the surface of the coating, the adhering materials must be removed to ensure that the instrument probe is in direct contact with the surface of the test sample. 2.11 Conductivity of the coating
Some magnetic thickness gauges operate in the frequency range of 200 to 2000 Hz. Within this frequency range, point conductivity thick coatings will produce diseased currents that affect the readings.
2.12 Probe pressure
The pressure applied to the probe on the sample will affect the measurement readings. Therefore, the pressure applied to the probe must be constant and appropriate, and will not deform the soft coating. Soft coatings can also be measured with a box cover, but the thickness of the foil must be subtracted from the measurement result. This must also be done when measuring the thickness of the phosphate film.
2.13 Probe Orientation
When using a magnetic gravity instrument, the measurement reading may be affected by the direction of the probe relative to the earth's gravity field. Therefore, when the probe is measured at a horizontal or vertical position, the instrument should be calibrated accordingly. 3 Calibration of Instrument
8.1 Overview
Before measurement, each instrument should be calibrated according to the manufacturer's instructions, select appropriate calibration standards for calibration, or select a sample with a cover layer to be tested, and compare the thickness value measured by the magnetic method with the thickness value measured by the national standard method applicable to the relevant special cover layer. During the use of the instrument, calibration should be carried out at regular intervals. And pay attention to the influencing factors listed in Chapter 2 and the operating procedures specified in Chapter 4.
3.2 Calibration Standard
A foil of known thickness or a sample of known coating thickness can be used as a calibration standard. 3.2.1 Calibration Foil
"Fan" refers to a non-magnetic metal or a metal box or gasket. …Foil is generally used as a calibration standard. In order to ensure good contact, it is not recommended to use foil to calibrate instruments based on the magnetic attraction principle. However, foil can be used as a calibration standard in some cases if necessary measures are taken. “Foil” is more suitable for calibration on the surface and is more suitable than a standard with a coating. In order to avoid measurement errors, it is necessary to ensure close contact between the foil and the base metal and avoid using elastic foil. Calibration foil is prone to formation and should be replaced frequently.
3.2.2 Standard with coating
On a certain metal sheet, a uniform, known thickness of coating that is firmly bonded to the metal sheet should be used as the standard sheet. 3.3 Inspection
3.3.1 The magnetic properties and surface roughness of the base metal of the calibration plate should be consistent with the magnetic properties and surface roughness of the base metal of the test sample. To verify the suitability of the standard, the base metal of the calibration standard can be used to compare the readings measured on the base metal of the test sample.
In order to effectively eliminate the influence of the residual magnetism of the substrate mechanical stress, the probe must be rotated when calibrating the instrument. 3.3.2
3.3.3 The thickness of the substrate metal on the test specimen must be consistent with the thickness of the standard.
GB 4$56—85
Generally, the calibration standard or test specimen is made of the same material of sufficient thickness so that its reading is independent of the thickness of the substrate metal. 3.3.4 If the curvature of the cover to be measured has reached a level that cannot be calibrated on a plane, the curvature of the standard sheet with the cover or the curvature of the substrate metal placed under the calibration foil should be consistent with the curvature of the test specimen. 4 Operating procedures
4.1 Overview
Each instrument should be operated in accordance with the manufacturer's instructions, and attention should be paid to the influencing factors listed in Chapter 2. Before each measurement, the instrument shall be calibrated at the test site and frequently during use to ensure that the instrument is in normal working condition.
The following provisions shall be observed during operation. 4.2 Base Metal Thickness
Check whether the base metal thickness exceeds the critical thickness. If not, it shall be calibrated as described in 3.3.3. 4.3 Edge Effect
Measurements shall not be made near sudden changes in the specimen, such as edges, holes, and internal corners, unless it has been verified that the calibration for this measurement is reliable.
4.4 Curvature
Measurements shall not be made on curved surfaces of the specimen unless it has been verified that the calibration for this measurement is reliable. 4.5 Number of Readings
It is usually necessary to take several readings on each measurement area because the instrument readings are not exactly the same each time. Local differences in coating thickness also require that the disc be retraced several times over any given area, especially when the surface is rough. 4.6 Machining Direction
If the machining direction significantly affects the reading, when measuring on the specimen, the direction of the probe should be consistent with the orientation during calibration, or the probe should be placed in directions 9 degrees apart to perform four measurements. 4.Residual magnetism
If the body metal has residual magnetism, when using a constant magnetic field bipolar probe instrument, the measurement must be carried out in two directions 180 degrees apart.
In order to obtain reliable test results, the magnetism of the sample must be removed. 4.8 Surface cleanliness
Before testing, the attached materials on the surface, such as ash, grease and corrosion products, should be removed, but no covering layer should be removed. When measuring, avoid measuring on the parts with obvious defects that are difficult to remove, such as welding slag, acid stains, dead thorns and oxides.
4.9 Lead coating
When using a magnetic instrument to measure the thickness of the lead coating, the probe may adhere to the coating. In this case, a very thin lubricant film can be applied on the surface of the coating to improve the reproducibility of the measured readings, but the excess oil should be wiped off. When using a tension type instrument to measure verticality, the surface of the coating should be kept dry. Oiling should not be used on other than the wrong cover. 4.10 Technique
The tracing or reading depends on the skill of the operator, for example, each person applies pressure or the rate of balance force to the probe differently. This effect can be reduced or minimized if the operator calibrates and measures, or uses a constant force probe. This method should use a constant force probe as much as possible, or use a measuring stand for measurement. 4.11 Probe Positioning
The probe of the instrument should generally be vertically positioned above the sample surface for measurement, which is especially necessary for magnetic attraction type instruments. For some instruments, it is recommended to tilt the probe slightly and choose the tilt angle that gives the minimum reading. When measuring on a smooth surface, if the instrument reading changes significantly with the tilt angle
GB 496B-85
If the probe of the magnetic thickness gauge is horizontal or inverted from bottom to top for measurement, if the gravity acting on the measuring system is not overcome, it should be calibrated in those positions respectively. 5 Accuracy requirements
The measurement accuracy depends on the performance, operation and calibration of the instrument. This method can make the error between the measured coating thickness value and the true thickness value within ±10% or 1.5 microns, the two errors are the largest. Additional remarks:
This standard is proposed by the Ministry of Machinery Industry of the People's Republic of China and is under the jurisdiction of the Wuhan Material Protection Research Institute of the Ministry of Machinery Industry. This standard is drafted by the Wuhan Material Protection Research Institute of the Ministry of Machinery Industry. The main drafters of this standard are Hu Tieqi and Zhong Wenchang. From the month this standard is implemented, the original Ministry standard JB2119-77 "Method for testing the thickness of metal coatings by magnetic method" will be invalid.
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