This standard specifies the definition, principle, equipment, method steps, determination, inspection and written procedures to be followed during identification. This standard is applicable to the investigation of electrical fire causes, from the characteristics of the different metallographic structures of burnt molten beads and short-circuit molten beads on copper and aluminum conductors, to identify the relationship between the melting cause and the cause of the fire. That is: is it a burnt molten bead or a short-circuit molten bead? Is it a primary short-circuit molten bead or a secondary short-circuit molten bead. GB 16840.4-1997 Technical identification method for electrical fire causes Part 4: Metallographic method GB16840.4-1997 Standard download decompression password: www.bzxz.net

GB 16840.4-1997
The series of standards of "Technical identification methods for electrical fire causes" consists of 4 parts: Part 1 Macroscopic method; Part 2 Residual magnetism method; Part 3 Component analysis method; Part 4 Metallographic method. This standard is Part 4 of the series of standards of "Technical identification methods for electrical fire causes": Metallographic method. The metallographic method of this standard is a method to identify the cause of fire at the fire scene based on the different changes in the metallographic microstructure of copper and aluminum wires in different environmental atmospheres.
This standard consults and refers to the American papers "Metallographic Inspection and Analysis of Copper Conductors in Building Fires" and "Meltdown Characteristics of Wires in Building Fires".
This standard is proposed by the National Fire Protection Standardization Technical Committee. This standard is under the jurisdiction of the Sixth Subcommittee of the National Fire Protection Standardization Technical Committee. The drafting unit of this standard: Shenyang Fire Science Research Institute of the Ministry of Public Security. The main drafters of this standard: Wang Xiqing, Han Baoyu, Di Man, Gao Wei. 77
1 Scope
National Standard of the People's Republic of China
Technical determination methods for electrical fire cause
Part 4: Metallographic method
Technical determination methods for electrical fire cause Part 4 : Metallographic method GB 16840.4—1997
This standard specifies the definition, principle, equipment, method steps, determination and written procedures to be followed in inspection and identification. This standard is applicable to the investigation of the cause of electrical fire, from the change characteristics of different metallographic structures of fire molten beads and short-circuit molten beads on copper and aluminum wires, to identify the relationship between the melting cause and the cause of the fire. That is: is it a fire molten bead or a short-circuit molten bead? Is it a primary short-circuit molten bead or a secondary short-circuit molten bead. 2 Definitions
This standard adopts the following definitions:
2.1 Melted mark
Circular, pit-shaped, nodular, pointed and other irregular micro-melting and full-melting marks formed on copper-aluminum conductors under the action of external flames or short-circuit arc high temperature.
2.2 Melted bead
Circular bead-shaped melting marks formed on the ends, middle or after the copper-aluminum conductors fall to the ground under the action of external flames or short-circuit arc high temperature.
Primary short circuit melted mark2.3
Short circuit melted marks formed on copper-aluminum conductors due to their own faults before the fire occurs. 2.4 Secondary short circuit melted markSecondary short circuit melted markThe marks left after the insulation layer of copper-aluminum conductors fails due to external flames or high temperature when they are charged and short-circuit occurs. 2.5 Crystal particle
The individual crystals that make up a polycrystal are called crystal particles. They are composed of many unit cells and are often granular and irregular in shape. 2.6 Crystal boundary
The area where two crystal grains with different orientations come into contact, that is, the interface between crystal grains. 2.7 Eutectic cocrystallization When a liquid alloy with eutectic composition solidifies, two solid solutions with different components are generated. The two-phase mixed structure obtained by this eutectic reaction is called eutectic.
2.8 Recrystallization
recrystal
The process of replacing the original deformed grains with new equiaxed grains when a cold-deformed metal is heated is called recrystallization. 2.9 Equiaxed crystal
Under normal solidification conditions, the solid solution of a metal or alloy crystallizes into particles, and a dendrite structure with similar lengths and orientations is formed inside. The branches of a dendrite, with grains of different sizes growing evenly in all directions, are called equiaxed crystals. Approved by the State Administration of Technical Supervision on June 3, 1997, 78
Implementation on May 1, 1998
2.10 Dendrite branch crystal
GB16840.4—1997
The crystal axes that grow one after another, interlaced like branches, are called dendrites. 2.11 Casting-state structure The liquid metal is poured into a mold and solidified. The structure obtained after solidification is called casting structure. 2.12 Afterbirth-like crystal When a solid solution is crystallized, the crystal grows freely on the interface in the form of raised strips in the supercooled zone, forming irregular shapes, strips, and regular hexagons.
2.13 Columnar crystal Under normal solidification conditions, when the solid solution of a metal or alloy crystallizes, the dendrites grown from the crystal extend and grow along the branches (trunk) at a special interface, and the final grains are in the shape of long strips. 2.14 Polarized light
The light source in the microscope uses orthogonal polarized light for illumination. 2.15 Fusion transition The melting phenomenon that exists within a certain distance from the molten mark to the conductor is a characteristic of the fire molten mark and the secondary short circuit molten mark. 3 Principle
Whether the copper and aluminum conductors are melted by fire or by high temperature of short circuit arc, except for all the burns, the residual molten marks (especially the copper trace) can generally be found, and the appearance of the molten marks still has the characteristics that can truly represent the environmental atmosphere at that time. The primary short-circuit melting mark and the secondary short-circuit melting mark are both instantaneous arc high-temperature melting, with the characteristics of fast cooling speed and small melting range, but the difference is that the former short-circuit occurs in a normal environment atmosphere, while the latter short-circuit occurs in an atmosphere of fireworks and temperature. The time and temperature of the traces melted by the heat of the usual fire are different from those of the short-circuit. It has a long temperature duration, a large burning range, and a melting temperature lower than the short-circuit arc temperature. Although both belong to melting, due to the participation of different environmental atmospheres in the whole process of melting mark formation, the respective characteristics of the melting mark formation are retained, and the metallographic structures presented also have different characteristics. 4 Equipment and Instruments
4.1 Metallographic Microscope
Magnification 50~2000 times, with camera device (manual, automatic, color photography, polarized light, etc.). Specific components, equipment and operations should be carried out according to the provisions of the instrument manual. When observing the sample, choose according to the required magnification. 4.2 Stereo microscope
Magnification 10-160 times, working distance 97-30mm, field of view up to 20mm, with camera, optical table. 4.3 Ancillary equipment
Metallographic sample pre-grinding machine, polishing machine, metallographic mounting machine, darkroom enlarger, exposure timer, light box, developing and fixing lamp, glassware, forging, mold, hair dryer, etc.
5 Methods and steps
The preparation of metallographic samples includes selection, mounting, grinding, polishing, etching, etc. Ignoring any process will affect the accuracy of tissue analysis and test results, and even cause misjudgment. 5.1 Sample preparation
The prepared sample should have: representative structure, no false image, real structure, no wear marks, pits or water marks, etc. 5.2 Sample Selection
When extracting samples, representative parts must be selected. According to the actual situation of the fire scene, ensure that the parts and traces with melting marks, pits, etc. that can be used for identification are extracted.
5.3 Sampling Parts
GB16840.4—1997
Samples can be taken from the places where the conductor has melting marks and pits, and from the normal parts near them for cross-sectional and longitudinal section inspection and comparison; the cross-sectional view is to observe the microstructure grain size of the melting mark, and the longitudinal section is to observe the microstructure changes in the transition zone between the melting mark and the conductor. 5.4 Sample Size
Sample size: different metal materials of a cylinder with a diameter of 12mm and a height of 10mm or a square cylinder with a size of 12×12×10mm. Samples with special shapes or small sizes that are difficult to hold can be inlaid for the remains extracted from the fire scene. 5.5 Sample extractionWww.bzxZ.net
For small samples, pliers can be used to cut; larger samples can be cut with hand saws or cutting machines, etc., and can also be cut by gas cutting when necessary. However, the burning edge must be kept at a considerable distance from the sample. Regardless of the sampling method used, attention should be paid to the temperature conditions of the sample. If necessary, water should be used to cool the sample to avoid changes in its structure due to overheating. 5.6 Dirt removal
If the surface of the extracted sample is stained with oil, it can be dissolved with organic solvents such as benzene. Rusty samples can be cleaned with ammonium persulfate (NH)2SO: or phosphoric acid. As for other simple methods of removing oil and rust, they can also be used. 5.7 Mounting
If the sample is too small or has a special shape, one of the following methods can be used to mount the sample. 5.7.1 Plastic or bakelite powder inlay method
Bakelite powder, transparent bakelite powder or transparent plastic powder can be used for inlaying on the inlaying machine. When bakelite powder is used, pressurize (170~250)×9.8×10*Pa, heat to 130~150℃ and keep for about 5~～7min, and then cool to form an inlaid sample. When transparent bakelite powder is used, pressurize (170~250)×9.8×10*Pa, heat to 149~170℃, keep warm for 5~7min, then slowly cool to about 75℃, and then water cool to form a transparent inlay. When plastic is used for inlaying, the temperature, pressure and insulation time depend on the properties of the plastic powder used, and insulation should be appropriate not to change the original structure of the sample.
5.7.2 Rapid inlay method
Inlay method using rapid self-curing dental tray water (methyl methacrylate) and self-curing dental tray powder: first place a cylindrical copper tube (or other material tubes) with a diameter of 12mm on a glass plate, then place the sample on the bottom of the mold, and then mix the rapid self-curing dental tray water and self-curing dental tray powder in a certain proportion. When it becomes a paste, inject it into the mold; in winter when the room temperature is low, you can use a hair dryer to heat it to promote rapid solidification, and in summer when the room temperature is high, it can solidify naturally; after solidification, remove the mold to form a mounted sample. 5.7.3 Other methods
In addition to the above two methods, the sample can also be cast in a low melting point material. Such as sulfur, sealing wax, welding alloy (50% tin, 50% lead) or Wurtz alloy (50% bismuth, 25% lead, 12.5% ​​tin, 12.5% ​​chromium), etc., organic plastics and other effective inlay methods that do not affect tissue changes can also be used.
5.8 Grinding of the specimen
When grinding the specimen on sandpaper, do not use too much force, and the grinding time should not be too long each time to avoid deformation. When using a pre-grinding machine for fine grinding, water must be used to cool the grinding surface while grinding to avoid deformation caused by overheating of the grinding surface. 5.8.1 Grinding procedure
The prepared specimen is first ground on the pre-grinding machine on sandpapers of varying sizes from coarse to fine. From coarse sandpaper to fine sandpaper, each time the sandpaper is changed, the specimen must be turned 90° to be perpendicular to the old grinding marks, and ground in one direction until the old grinding marks completely disappear and the new grinding marks are evenly aligned. At the same time, wash and dry the specimen with water each time, and wash your hands at the same time to avoid bringing coarse sand grains onto the fine sandpaper. 5.8.2 Rough polishing
The specimen after rough grinding can be moved to a polishing machine equipped with flat cloth, table cloth or fine canvas for rough polishing. The diameter of the grinding disc can be 200-250mm, the rotation speed can be 400-500r/min, the polishing powder can be fine aluminum oxide powder or silicon carbide powder, etc., the polishing time is about 2-5min, and it is washed with water and blown dry after polishing.
5.8.3 Fine polishing
The sample after rough polishing can be moved to a velvet polishing disc with velvet or other fine and uniform fibers for fine polishing. The diameter of the polishing disc can be 200-250mm, the rotation speed can be about 400-1450r/min, and the polishing powder can be water-selected ultra-fine aluminum oxide powder, magnesium oxide powder or artificial diamond 80
GB16840.4-1997
abrasive paste, etc. Generally, polishing is done until the wear marks on the sample are completely removed and the surface is like a mirror. In addition to rinsing with water after polishing, it is recommended to soak it in alcohol and then blow it dry with a hair dryer so that there will be no water marks or dirt residue on the surface of the sample. 5.8.4 Polishing Notes
When polishing the sample on the polishing disc, use light force and polish from the edge to the center of the disc. Occasionally add a small amount of abrasive powder suspension (distilled water suspension should be used when using magnesium oxide powder) or kerosene. The humidity of the flannel cloth should be such that when the sample is removed from the disc for observation, the surface water film can completely evaporate and disappear within two or three seconds. At the completion stage of polishing, the sample can be polished in the opposite direction to the rotation direction of the polishing disc.
When polishing a sample, if it is found that there are coarse abrasion marks that are difficult to remove or pits are found in the polished sample under a microscope, which affects the test results, the sample should be re-ground. 5.9 Etching of the sample
After fine polishing, suitable samples can be immersed in the etchant contained in the glass dish for etching or rubbing for a certain period of time. During etching, the sample can be slightly moved from time to time, but the polished surface must not contact the bottom of the dish. 5.9.1 Etching time
Etching time depends on the nature of the metal, the concentration of the etching solution, the purpose of the inspection and the magnification of the microscopic inspection. Usually, when observing at high magnification, the etching should be slightly shallower than that of observing at low magnification. - It ranges from a few seconds to thirty minutes, so that the metal structure can be clearly shown under the microscope. 5.9.2 Etching
After etching, take out the specimen immediately and quickly wash it with water, then wash the surface with alcohol and blow it dry. - If the etching degree is insufficient, it can be etched continuously according to the specific situation, or it can be etched again after re-polishing on the polishing disc. If the etching is excessive, it must be re-grinded on the grinding disc or sandpaper before etching. - After etching, if there is metal disturbance on the surface of the specimen and the original structure cannot be shown, it can be etched again after lightly polishing on the polishing disc. Generally, this can be repeated several times to eliminate the disturbance. If the disturbance is too serious and cannot be completely eliminated by this method, the specimen must be re-grinded. 5.10 Etching agent
The following chemical etchants are recommended for copper wire, aluminum wire and steel metal: The ratio of metallographic etching is shown in Table 1.
Steel wire
Aluminum wire
Steel
5.11 Microstructure inspection
Etching agent ratio
H, O or (alcohol)
98~96mL
Etching time
Metallographic inspection can use various types of metallographic microscopes. The microscope should be installed in a dry and clean room and mounted on a stable table or base, preferably with a vibration reduction device.
5.11.1 Sample inspection
Sample inspection includes inspection before etching and inspection after etching. Before etching, the main inspection is the smoothness and wear marks of the sample, and after etching, the main inspection is the microstructure of the sample.
5.11.2 Sample observation
When observing the sample under the microscope, generally use 50-100 times first, and then switch to high magnification when observing fine tissue conditions. 81
5.11.3 Attention when observing the sample
GB 16840.4—1997
When taking the lens, avoid touching the surface of the lens with your fingers. 一 Be particularly careful when taking the lens, and put it back in the box after use. 一 When the objective lens is close to the surface of the sample, it should be adjusted with a fine adjuster. When adjusting, pay attention to the fact that the objective lens does not touch the sample. When there is dirt on the lens surface, first use a soft brush or a grease-free feather to brush it, then wipe it with lens paper or soft deerskin, and use xylene to clean it if necessary.
The lens should be stored in a dry and clean place, and the microscope should be covered with a dust cover when not in use. 5.12 Microphotography
The specimens to be prepared for microphotography should be finely ground and kept clean. The degree of etching of the specimen depends on the magnification of the photograph. 5.12.1 Magnification
The magnification of the photograph is generally 50 to 1500 times. The choice of lens depends on the required magnification (appropriate selection according to the microscope manual). Under low magnification (100 times), a three-prism light reflection is used on the microscope to increase brightness and contrast; under high magnification, a flat glass reflector is used to increase resolution.
5.12.2 Light Source
The light source used for photography needs to be adjusted appropriately. The light emitted must be stable and have sufficient intensity. When taking pictures, the position of the light source and the focusing light should be adjusted so that the light beam can just hit the center of the vertical illuminator inlet, so that the brightness of the image obtained is uniform. 5.12.3 Filter
The filter should be determined according to the type of objective lens. If it is an achromatic lens, use a yellow-green filter; if it is a fully achromatic lens, use yellow, green, or blue filters.
5.12.4 Sample placement
The sample should be placed steadily on the microscope stage so that its plane is perpendicular to the optical axis of the microscope. After the sample is placed, move the stage and select the appropriate tissue part on the sample. And adjust the focus of the microscope to make the image clear. 5.12.5 Aperture adjustment
The aperture diaphragm (i.e., aperture) of the microscope must be adjusted to an appropriate size so that the image seen by the microscope is the clearest. The field diaphragm (i.e., aperture) of the microscope must be adjusted to an appropriate size so that the brightness range of the image can be within the size range of the film, and the best image contrast can be obtained. 5.12.6 Exposure time
The exposure time of the film depends on the sample conditions (metal type and whether it is etched or not), the properties of the film, and the brightness intensity. If necessary, the segmented exposure method can be used for preliminary testing, and automatic exposure can be ignored. 5.13 Development and Fixing
5.13.1 Development
Select the appropriate developer according to the type of film. The development temperature and time should be carried out in accordance with the provisions of the film manual. The general development temperature is about 20°C.
5.13.2 Fixing
The fixing temperature should be below 23℃. The film stays in the fixing solution for 20 to 30 minutes. After fixing, rinse the film with running water for no less than 30 minutes, and then dry it in a dust-free room. If the room temperature exceeds 23℃, in order to avoid softening of the film, it can be hardened after development and before fixing. Generally, it stays in an aqueous solution of 2% chrome alum: KCr(SO)z·12H,O and 2% acidic sodium sulfite: NaHSO: for 3 to 5 minutes. When developing and fixing, the latex side of the film must face up. The film must be completely immersed in the solution and shaken frequently. 5.14 Exposure
When exposing, choose the appropriate number of photographic paper and exposure time according to the condition of the film and the intensity of the light. The exposure time should not be too short or too long, and the fine image lines of the darker part of the film should be clearly displayed. 5.14.1 Development and fixation after exposure
The developer is selected according to the type of photographic paper. The development time is generally about 1 to 3 minutes. After development, the photographic paper can be slightly immersed in a 1.5% acetic acid aqueous solution to neutralize the alkaline developer to stop the development effect, and then the photographic paper is immersed in the fixer for fixation; when the photographic paper is in the developer and fixer, the latex surface must be facing up and completely immersed in the solution. The time that the photographic paper stays in the fresh fixer is about 15 minutes. If it is an old fixer, the time can be extended as appropriate. After fixing, the photos should be rinsed in running water 12 times, each time for about 5 minutes, and then dried.
6 Determination
6.1 Identification of Wire Melt Marks
6.1.1 Fire Melt Marks
The metallographic structure of the fire melt marks presents coarse equiaxed crystals, no voids, and very few shrinkage holes on the grinding surface of individual melt beads (except for the melt marks of multiple strands of wires). 6.1.2 Differences between primary short-circuit melt marks and secondary short-circuit melt marks The metallographic structure of the primary short-circuit melt marks is composed of fine cellular crystals or columnar crystals; the metallographic structure of the secondary short-circuit melt marks is divided by many pores, with more coarse columnar crystals or coarse grain boundaries. The internal pores of the metallographic grinding surface of the primary short-circuit melt beads are small and few, and are relatively neat, while the internal pores of the metallographic grinding surface of the secondary short-circuit melt beads are many, large, and irregular.
The transition zone boundary at the junction of the primary short-circuit melt beads and the wire is obvious; the transition zone boundary at the junction of the secondary short-circuit melt beads and the wire is not so obvious.
The grain boundary of the primary short-circuit bead is fine, and the copper and cuprous oxide eutectics around the void are less and less obvious; the grain boundary of the secondary short-circuit bead is coarse, and the copper and cuprous oxide eutectics around the void are more and more obvious. -When observed under polarized light, the color around the void and the wall of the primary short-circuit bead is not obvious; the color around the void and the wall of the secondary short-circuit bead is bright red and orange.
When judging the primary short-circuit melting mark and the secondary short-circuit melting mark in more complicated situations, it is necessary to combine the macro method, component analysis method and the actual situation of the fire scene for comprehensive analysis and judgment.
7 Written procedures to be followed when submitting for inspection and appraisal 7.1 When submitting for inspection, the unit submitting for inspection shall first fill in the application form for technical appraisal of the cause of electrical fire, which shall include the name, address and contact person of the unit applying for appraisal; the name of the unit on fire, the name and quantity of the sample, the sampling location, the sampler and the purpose of appraisal. 7.2 After accepting the appraisal task, the appraisal unit shall fill in the sample receiving form, task form, reception record and original record. 7.3 After the appraisal is completed, the appraisal conclusion shall be filled in the appraisal report approval form, signed by the head of the laboratory, and submitted to the leader for approval after the quality review is correct.
7.4 The original of the approved appraisal report shall be submitted to the inspection unit, and a copy shall be kept on file for inspection. 83
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