JB/T 8468-1996 Magnetic Particle Inspection Method for Forged Steel Parts
Some standard content:
JB/T8468-96
This standard adopts ASTMA275/A275M90 "Standard Inspection Method for Magnetic Particle Inspection of Forged Steel Parts" in a non-equivalent manner. The main difference between this standard and ASTMA275/A275M--90 is that A275/A275M--90 stipulates that direct current or rectified (half-wave or full-wave) alternating current should be used for magnetization. AC magnetization is widely used in my country, so this standard stipulates that either direct current magnetization or alternating current magnetization can be used.
This standard was proposed and managed by the Deyang Large Casting and Forging Research Institute of the Ministry of Machinery Industry. The responsible drafting unit of this standard: Shanghai Heavy Machinery Plant. The main drafters of this standard: Shen Shunfu, Zhang Yongle, Liu Wu. 256
1 Scope
Standards of the Machinery Industry of the People's Republic of China
Magnetic Particle Inspection Method for Forged Steel PartsWww.bzxZ.net
This standard specifies the magnetic particle inspection method for forged steel parts and the evaluation of magnetic trace defects. This standard is applicable to the detection of cracks and other defects on the surface or near the surface of forged steel parts. 2 Referenced standards
JB/T8468-96
The provisions contained in the following standards constitute the provisions of this standard through reference in this standard. When this standard is published, the versions shown are valid. All standards will be revised, and parties using this standard should explore the possibility of using the latest versions of the following standards. GB9445--88 General Rules for Technical Qualification Assessment of Non-destructive Testing Personnel GB/T12604.5---90 Non-destructive Testing Terminology Magnetic Particle Testing 3 Definitions
The following definitions are adopted according to GB/T12604.5: 3.1 Magnetic traces
Magnetic powder accumulation formed by leakage magnetic flux caused by defects or other factors. 3.2 Magnetic particle inspection method
Non-destructive inspection method using leakage magnetic flux and suitable inspection media to find discontinuities on the surface and near the surface of the test piece. 3.3 Non-related indications
Indications that are not caused by discontinuities. 3.4 Linear magnetic traces
A magnetic trace whose length is not less than three times the width and not less than 1.6 mm. Note: This definition is translated from 3.1.2 of ASTMA275/A275M--90. 4 Application basis
4.1 The supplier and the buyer shall reach an agreement on the following items for forgings that are to be subjected to magnetic particle inspection according to the contract or technical conditions: a) The parts of the forgings that require magnetic particle inspection; b) The type, size, number, location and direction of the magnetic traces that are considered to be defects; c) The type of current, the method of applying magnetic powder, the demagnetization requirements and the magnetic field strength, etc. 4.2 Acceptance criteria.
5 Requirements for inspectors
5.1 The magnetic particle inspectors of forged steel parts shall obtain technical qualification certificates in accordance with the provisions of GB9445. 5.2 The magnetic particle inspectors of forged steel parts shall have basic knowledge of forged steel materials, forged steel processes, forged steel defects, etc. 5.3 The inspectors shall be able to correctly understand and use this standard. Approved by the Ministry of Machinery Industry of the People's Republic of China on September 3, 1996 and implemented on July 1, 1997
6 Implementation period
JB/T8468-96
The acceptance inspection of forgings shall be carried out in the state of surface finishing and final heat treatment of forgings, and may also be carried out in the state of finishing to within the margin of 0.8mm.
7 Magnetizing device
7.1 AC, half-wave or full-wave rectifier or DC power supply may be used. If current is to be passed through the forging, the equipment shall have a fastening device with sufficient contact surface and clamping force to pass the required current without burning the forging being inspected. 7.2 If it can be proved that the sensitivity of using a portable electromagnetic yoke (AC or DC) to detect crack-type defects is at least equivalent to the sensitivity of using the direct electromagnetic method, the electromagnetic yoke may be used as the magnetizing device. 8 Magnetic Particles
8.1 The test medium shall be a finely dispersed ferromagnetic powder which may be used suspended in a suitable liquid medium or in the form of a dry powder.
8.2 The size and shape of the magnetic particles, as well as the magnetic properties of the particles, are important parameters (either individually or in combination) (see Chapter 11).
9 Preparation of the Surface to be Tested
9.1 The sensitivity of magnetic particle testing is closely related to the surface condition of the forging being tested. Defects are best revealed on forgings that have been cleaned by shot peening or other methods. Defects can also be revealed on forgings that have a small amount of heat treatment scale but have not undergone any special surface treatment, but the loose scale must be removed. In order to detect small defects, the surface roughness (R) of the surface to be tested shall not generally be greater than 6.3 μm.
9.2 The surface of the forging being tested shall not be free of oil, grease, or other substances that may adhere to the magnetic particles. 9.3 A rough surface to be tested will hinder the flow of magnetic particles and sometimes cause false indications. Rough surfaces should be ground. If grinding is not allowed, a paper tape (see 13.2) can be applied to the rough surface to solve the problem of magnetic powder flow. 10 Magnetization method
10.1 The forging can be magnetized by passing current directly through the workpiece; or by using a central conductor or coil to generate an induced magnetic field on the workpiece. To detect surface defects, alternating current or direct current can be used as the magnetization power source. In circumferential magnetization, the "skin effect" of alternating current will reduce the maximum depth of defect detection, so direct current should be used when mainly detecting defects below the surface. 10.1.1 Continuous method
While the magnetization current is not interrupted and the external magnetic field is in effect, magnetic powder is applied to the surface of the forging to be inspected for inspection. The power supply generates a high current intensity pulse current with a pulse time not exceeding 1 s. There should be at least three current pulses within 1, or in other words, when the equipment provides continuous current, the shortest power-on duration should be 0.2 to 0.5 s. 10.1.2 Impact method
This method is limited to DC electromagnetization. First, a higher magnetizing force is applied, then the magnetization is reduced to a lower value, and magnetic powder is applied while maintaining this lower magnetizing force value. 10.1.3 Residual magnetism method
After the magnetizing current is cut off and the external magnetic field is removed, magnetic powder is applied and the residual magnetism on the workpiece is used for inspection. The effectiveness of this method depends on the magnetization intensity and the residual magnetism of the forging. This method is generally not used to inspect forgings. If it is to be used, the purchaser's consent must be obtained. 10.2 Each inspection part should be inspected at least twice. The magnetization direction of the second inspection should be roughly perpendicular to that of the first inspection. Different magnetization methods can be used for the second inspection. 258
JB/T 8468-96
10.3 Magnetization can be divided into two categories according to direction: longitudinal magnetization and circumferential magnetization. 10.3.1 Longitudinal magnetization
For longitudinally magnetized forgings, the magnetic lines of force are generally parallel to the axis of the forging. They have defined magnetic poles and are easily measured with a compass or magnetometer. Longitudinal magnetization is usually achieved by placing the forging in a solenoid wound with cable (Fig. 1). A special yoke may also be used as a longitudinal magnetization device if the requirements of 7.2 are met (Fig. 2). 10.3.2 Circumferential magnetization
Circumferential magnetization can be achieved by passing current directly through the forging (Fig. 3), by generating an induced magnetic field through a conductor (Fig. 4), or by passing a wire through a hole in the forging (Fig. 5). Local circumferential magnetization can be achieved by passing current through a local area of the forging using a contact (Fig. 6).
Figure 1 Longitudinal magnetization
Figure 2 Longitudinal magnetization (using yoke)
Electrode contact
Figure 3 Circumferential magnetization
(current directly through the forging)
Electrode head
Figure 4 Circumferential magnetization
(current through the center conductor)
Center conductor
JB/T8468-96
Circumferential magnetization (current through the conductor passing through the hole of the forging)Figure 5
Contact electrode
Contact electrode
Figure 6 Partial circumferential magnetization (using "contact" type electrode)10.4 If the magnetic field resides almost completely within the workpiece, no external manifestation of the magnetization state may be seen. Magnetic traces are most obvious in the direction perpendicular to the magnetic field.
10.5 Magnetic Field Strength
The minimum magnetic field strength used shall be that which will reveal and distinguish the magnetic traces of objectionable defects. The maximum magnetic field strength shall be just below the critical point at which the magnetic particles begin to be excessively attracted to the surface of the workpiece. 10.5.1 Coil Magnetization
When coil magnetization is used, the magnetic field strength is proportional to the current (or the number of ampere-turns if a coil or solenoid is used) and inversely proportional to the thickness of the section being inspected.
10.5.1.1 Longitudinal Magnetization
For wound coils (Figure 1), direct current or rectified current shall be used, and the turns of the coil shall be close together. The magnetic field strength decreases with increasing distance from the coil, so long forgings should be magnetized in sections. If the area to be inspected is more than 150 mm on either side of the coil, a magnetic field indicator must be used to verify whether the magnetic field strength is sufficient (see 10.5.6). 10.5.1.1.1 The magnetizing current for small forgings shall be obtained by the following formula: 35000
(ampere-turns)
(L/D) + 2
Where: - coil current, A
N — number of turns of coil or cable, turns;
L — length of workpiece, mm;
D — diameter of workpiece, mm.
The above formula is applicable to forgings with L/D ≥ 4. For forgings with L/D < 4, a magnetic field indicator shall be used to verify whether the magnetic field strength is sufficient (see 10.5.6). The curve shown in Figure 7 can be used to determine the ampere-turns corresponding to each L/D. 10.5.1.1.2 For large forgings, the magnetizing current shall be in the range of 1200 to 4500 ampere-turns. A magnetic field indicator shall be used to verify whether the magnetic field strength is sufficient in the inspected area (see 10.5.6). 10.5.1.2 Circumferential magnetization
For circumferential magnetization using a coil passing through the workpiece (Figure 5), the current used should be the quotient of the ampere given in 10.5.2 divided by the number of turns of the wire, 260
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10.5.2 Direct magnetization
JB/T 8468--96
L=length
D→diameter
1011121314
Figure 7 Curve of the ampere number of the magnetizing current used in longitudinal magnetization as a function of L/D
If the current is directly passed through the forging under inspection for magnetization, the AC and DC or rectified current intensity values I used per millimeter of diameter (or the maximum width of the cross section perpendicular to the current) are recommended as follows: Workpiece diameter D (mm)
≤125
>125~～250
AC current value I (A)
12. 5 ~~17. 5
7.5~12. 5
DC or rectified current value I (A)
25～35
For hollow workpieces, if the wire is clamped directly on the workpiece wall, the current intensity value used should be calculated according to the workpiece wall thickness. If the required current is not available for workpieces with a diameter greater than 250 mm, a magnetic field indicator is used to determine the actual value of the appropriate magnetic field intensity. In all the above cases, a magnetic field indicator can be used to determine whether the magnetizing force is appropriate (see 10.5.6). 10.5.3 Contact magnetization
When the contact is used to produce local circumferential magnetization, the magnetic field intensity is proportional to the current intensity used, but it also varies with the spacing between the two contacts and the thickness of the section being tested.
10.5.3.1 When the material thickness is less than 20 mm, a magnetizing force of 75~~100A should be used for every 25 mm of contact spacing. When the material thickness is equal to or greater than 20mm, a magnetizing force of 100~~125A should be used for every 25mm spacing. 10.5.3.2 The contact spacing should not exceed 200mm. The contact spacing should generally not be less than 75mm to avoid the accumulation of magnetic powder around the contacts. Care should be taken to prevent local overheating or burns on the surface being tested. When the circuit-breaking (poor contact) magnetizing voltage exceeds 25V, it is recommended to use steel contact contacts, aluminum contact contacts or copper brush type contacts instead of integral copper contact contacts to avoid copper penetration into the workpiece surface. Permanent magnet chucks can be used in pairs or together with contacts. Magnets should not be used for currents exceeding 1500A because this will cause magnetization loss.
10.5.3.3 A remote control switch placed in the contact handle can be used. This switch should be turned on and energized only after the electrode contacts are properly positioned. When removing the electrode, turn off this switch first to avoid arcing. 10.5.3.4 Inspection coverage area
There should be sufficient overlap during the inspection to ensure 100% coverage of the inspection surface at the specified sensitivity. 10.5.3.5 Magnetization direction
Each inspection area should be inspected at least twice, and the contacts should be placed so that the magnetic lines of force during the latter inspection are generally perpendicular to the magnetic lines of force during the previous inspection.
10.5.4For circumferential magnetization with current flowing through the inner hole of shaft forgings (Figure 4), a current of 100~125A per 25mm of hole diameter (4~5A/mm) should be used.
10.5.5The specified or mutually agreed current should be measured using a suitable measuring tool such as an ammeter. 61
JB/T8468—96
10.5.6When necessary, a magnetic field indicator (Figure 8) should be used to determine whether the magnetic field is appropriate. The magnetizing current used should be able to produce a clear image on the indicator.
Eight pieces of mild steel brazed together by heating
Nonferrous metal handles of any suitable length
≤.0.8mm
Artificial defects (interfaces of all sectors) Brazed or machine-connected nonferrous metal trunnions
Copper sheet (thickness 0.25mm)
Figure 8 Magnetic Powder Field Indicator
10.5.6.1 When using a magnetic field indicator, first place the indicator on the forging to be inspected, then apply the required current and magnetic powder. When the magnetic powder produces a recognizable image (usually a "cross" line), it indicates that sufficient magnetic field strength has been generated in the forging to be inspected. 10.5.7 Magnetization of Yoke Iron
When an electromagnetic yoke is used to magnetize a local area, a longitudinal magnetic field is formed between the two poles. 10.5.7.1 Equipment
The yoke can be fixed or articulated. 10.5.7.2 Yoke Verification
AC electromagnetic yokes shall have a lifting force of at least 45N when the pole spacing is 75 to 150mm. DC electromagnetic yokes shall have a lifting force of at least 180N when the pole spacing is 75 to 150mm. 10.5.7.3 Magnetization Direction
Each inspection area shall be inspected at least twice, and the magnetic lines of force during the two inspections shall be substantially perpendicular to each other. 10.5.7.4 The pole spacing shall be limited to a range of 50 to 200mm. 10.5.7.5 Inspection Area
The inspection area shall be limited to a maximum of 1/4 of the pole spacing on both sides of the line connecting the two poles. The pole spacing shall overlap by more than 25mm each time.
11 Application of Magnetic Powder
When the forging is properly magnetized, the magnetic powder may be applied by one of the following methods. 11.1 Dry method
11.1.1 When using the dry powder method, you can use a manual sieve, a mechanical sieve, a powder sprayer or a mechanical powder sprayer to apply magnetic powder. The sieve can only be used for surfaces that are flat or nearly flat facing upward, and the powder sprayer can be used for vertical and downward surfaces. The magnetic powder should be applied evenly to the forging surface. The color of the dry powder should have an appropriate contrast. The magnetic powder should not be applied too much because it will cover the magnetic traces. 11.1.2 When blowing off excess magnetic powder, be careful not to damage the magnetic traces. 11.2 Wet method
11.2.1 Oil
It is recommended to use light refined kerosene with a high flash point and low viscosity as the carrier liquid of the magnetic suspension. 262
JB/T8468—96
Generally, a magnetic suspension with a volume of 1% to 2% magnetic powder should be used. The oil magnetic suspension is sprayed on the surface to be inspected, and the color of the magnetic particles should be selected to give it an appropriate contrast.
11.2.2 Aqueous agent
The magnetic particles are suspended in clean water or clean water with an appropriate wetting agent to prepare a magnetic suspension. The volume of magnetic particles in the magnetic suspension should generally account for 2% to 2.5%.
11.3 Fluorescence method
11.3.1 Fluorescent magnetic particle inspection is an improvement on the wet method. The magnetic suspension used is similar to that used in the wet method, but the magnetic particles are covered with a layer of fluorescent material that can fluoresce when excited by black light or ultraviolet light. 11.3.2 The preparation method of the magnetic suspension is the same as that of the wet method. The volume of fluorescent magnetic particles in the magnetic suspension should account for 0.1% to 0.7%. 11.3.3 The carrier liquid must not be fluorescent.
11.3.4 If fluorescent magnetic particles are used, the inspection shall be carried out in a dark place with a "black light" and the illumination shall be at least 9701x during operation. The "black light" shall radiate ultraviolet light with a wavelength of 33 to 39 um, so that the irradiated magnetic particles will emit bright fluorescence. The lamp shall be preheated for at least 5 minutes before inspection.
12 Demagnetization
12.1 If specified, forgings shall be fully demagnetized after inspection, and the residual magnetic field strength after demagnetization shall not exceed 240A/m. 12.2 When direct current is used, demagnetization is usually achieved by repeatedly changing the direction of the current and gradually reducing the magnetizing current strength. The starting magnetic field strength used during demagnetization shall be equal to or greater than the original magnetizing force. When the current is reduced to zero, the workpiece is actually demagnetized. Direct current demagnetization is recommended for large forgings.
12.3 When using alternating current, demagnetization can be achieved by reducing the magnetizing current intermittently in small increments, or continuously reducing the magnetizing current to a very low value.
12.4 If the workpiece is to be austenitized before use or machining, such workpiece does not need to be demagnetized. 13 Defect Recording
13.1 Use a schematic diagram to mark the size, number and location of all magnetic traces. 13.2 Permanent Record of Magnetic Traces
13.2.1 Carefully cover the surface of the magnetic trace with a transparent cellophane tape, allow the magnetic powder to stick to it, then peel off the tape and stick it on white paper or card, and copy it by photography or other means.
13.2.2 If you need to copy the defective magnetic trace more accurately, you can use the following method: When the magnetic trace is found by conventional inspection, remove the magnetic powder from the collection and cover the part showing the magnetic trace with a white paper tape with a smooth surface and a resin adhesive on the back. Turn on the power again and apply the magnetic powder. At this time, the magnetic powder will show the defective trace on the paper. While continuing to power on, spray a layer of acrylic paint on the surface of the paper tape. Then disconnect the current and peel off the paper tape, and you can get an accurate copy of the magnetic trace on the front.
14 Identification and evaluation of magnetic marks
The following items shall not be used as acceptance criteria for forgings, but they are helpful for the identification and evaluation of the obtained magnetic marks. 14.1 When explaining the cause of a magnetic mark, the following factors must be considered: a) the appearance of the magnetic mark;
b) the direction and shape of the magnetic mark;
c) the type of material used to make the forging;
d) the production process, processing type and heat treatment of the forging; e) the experience gained from destructive tests such as disassembly, corrosion, breaking, cutting, grinding, etc. on the production of similar forgings in the past. 14.2 Magnetic marks can be divided into three categories:
14.2.1 Magnetic marks of surface defects
JB/T8468-96
This type of magnetic mark has a clear outline and the magnetic powder is tightly gathered and well adhered. Various surface defects can be identified from the following characteristics: a) Delamination: Its magnetic traces are thick and parallel to the surface; b) Heavy skin and folding: Its magnetic traces may not be very thick and not straight, but distributed along the metal flow lines; c) White spots: It may appear in machined parts, and its magnetic traces are irregular and scattered; d) Heat treatment cracks: Its magnetic traces are thick and often appear in corners, grooves and parts with cross-sectional changes; e) Shrinkage cracks: Its magnetic traces are very thick and clear, generally continuous and have several branches, appearing in parts with cross-sectional changes; f) Grinding cracks: Its magnetic traces often appear in groups and are perpendicular to the grinding direction; g) Corrosion or electroplating cracks: Its magnetic traces are thick and perpendicular to the direction of residual stress. 14.2.2 Magnetic traces of subsurface defects
This type of magnetic trace is relatively wide, with unclear outlines and poor magnetic powder adhesion. Various subsurface defects generally have the following characteristics: a) Linear non-metallic inclusions: their magnetic traces are similar to surface wrinkles, often relatively thick, but usually intermittent or relatively short, appear in groups, distributed along metal flow lines, and only appear when the defects are located near the surface; b) Large non-metallic inclusions: their magnetic traces can appear in any part of the forging, ranging from clear to scattered distribution; c) Cracks under welds: their magnetic traces are wide and scattered; d) Forging cracks: their magnetic traces are scattered and irregular. 14.2.3 Irrelevant indications or false indications
These indications are often unclear, but can generally be identified as follows: a) Magnetic writing indications: The indications are unclear and can be destroyed by demagnetization rain. They are caused by contact with other steel parts and magnetic bodies during magnetization; b) Cross-section change indications: The indications are wide and unclear. They are caused by the increase of magnetic field strength at gear teeth, part fillet transitions or chamfers, keyways, etc.
c) Weld edge indications: The indications are poor magnetic powder adhesion. They are caused by magnetic changes caused by diffusion; d) Streamline indications: The magnetic traces are distributed in large pieces and are parallel to each other. They are easy to appear when the magnetizing current is too large. 14.3 All doubtful indications can be regarded as irrelevant until they can be eliminated by surface finishing or confirmed as irrelevant indications by rechecking with the same or other non-destructive test methods. 15 Inspection report
The flaw detection report shall include the following contents:
Forging name, production number, material, and dimension diagram; the date, unit, and number of the commissioned inspection; the inspection standard, conditions, and location;
Inspection results and conclusions;
Inspection date, signatures of the inspector and auditor, and qualification certificate number. 264
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