Restoration of side cover - DBE, Drawings
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Восстановление боковой крышки.docx
2.2 Development of the part repair process for the specified defect.
2.2.1 Analysis and elaboration of possible methods of part defects repair.
Defect N 1.
Welding and surfacing.
In the manufacture of many parts of cars, gray and forged iron are used. From gray cast iron, such body parts as a cylinder block, clutch crankcase, gearbox crankcase, etc. are made. Ductile cast iron is used in the manufacture of hubs of rear wheels, crankcase gears of rear axles and other parts.
Characteristic defects of these parts are cracks, holes, splits of flanges, damage to threads in holes, etc. The most common way to eliminate these defects is welding.
The main difficulty of welding cast iron is the possibility of bleaching the seam, which occurs as a result of rapid cooling of the built-up metal and burning out of silicon. With rapid cooling, carbon does not have time to release in the form of graphite and remains in a chemically bound state in the form of cementite. Welding seam is very hard, brittle and can not be processed.
During welding as a result of local heating of parts and large shrinkage of cast iron When cooled from the molten state, significant internal stresses arise in the parts. Refractory oxides with a melting point of about 1400 ° C formed during welding of cast iron create a hard film on the surface of the welding bath, which prevents the free exit of gases from the molten metal and thus contributes to the formation of pores and shells.
When restoring cast iron parts, two main welding methods are used: "hot" (heated parts) and "cold" (without heating).
With a hot welding method, the part is first mechanically prepared for welding (drilling the ends of cracks, edge preparation, etc.), and then in special furnaces it is heated to a temperature of 550.. 600 ° С. Welding is carried out with acetylene oxygen flame.
The additive material used is rods with diameter 6... 8 mm cast from gray cast iron with an increased silicon content (up to 3.. .3.5%). Flux consisting of 50% mixture of borax and sodium bicarbonate is used to protect built-up metal from oxidation and removal of oxides.
The welding mode is recommended as follows: the power of the welding torch is selected based on the flow rate of 0.10... 0.12 m3/4 acetylene per 1 mm of the thickness of the welded metal; welding flame shall be neutral or with a small excess of acetylene. After welding, the parts are slowly cooled in thermostats (piggies).
The hot method provides high quality welding, but in technological terms it is very complex and therefore is used relatively rarely, mainly to restore complex and body parts.
The cold method of welding cast iron is technologically simpler and therefore has been widely used in automotive repair production. Most often, manual and semi-automatic electric arc welding with steel electrodes and electrodes made of non-ferrous metals and alloys is used.
Welding of cast iron with steel electrodes is the simplest and most economical method of welding, however, carburization and hardening of the seam are possible, which impairs its workability. In this welding method, it is recommended to use PM4 electrodes made of welding wire St. 08 with a thick coating containing titanium.
Welding of cast iron with non-ferrous metal electrodes is less economical, but gives good indicators in terms of strength, ductility and density of the seam. Most copper electrodes of OZCh1 brand with the covering containing iron powder and copper-nickel electrodes of MNCh1 brand with an UONI55 covering were widely used. The best results are obtained by welding with MPH1 electrodes. Welding seam consists of iron-nickel alloy and has high strength and ductility.
Cold welding, cast iron is recommended to be made with electrodes with a diameter of 3... 4 mm at a direct current of reverse polarity at a voltage of 20. .25 V and a current strength of 120... 150 And.
Semi-automatic welding of gray and ductile cast iron can be carried out with self-protective electrode wire PANCH11 and PANCH12 on the basis of nickel. When welding cast iron parts with this wire, the A547U welding semi-machine is used.
The most prone to bleaching is ductile cast iron. To prevent bleaching, welding of ductile iron should be carried out at a temperature lower than the decay temperature of carbon annealing (950 ° C). The most good results are the use of soldering-welding brass electrodes of LOMNA541040, LOK-59-1-03 and L62 grades. Soldering of cast iron parts is carried out with acetylene oxygen flame using flux of PFSN2 grade containing 50% boric acid, 25% lithium carbon dioxide and 25% sodium carbon dioxide. During soldering-welding, the edges of the parts are heated to 700.. .750 ° С.
Defect No. 4 .
Calibrating the hole.
Restoration of the hole is carried out by the thrower, for more accurate restoration, calibration by the thrower is carried out in 3 stages: the draft thrower cuts the thread in ink, the middle thrower gives a more accurate thread, the finish thrower is used for final accurate thread cutting and its calibration.
Defect No. 3
Methods of electrolytic recovery of parts. Chromium plating
Electrolytic methods of coating parts include deposition of alloys, chromium plating, iron plating, nickel plating, copper plating, galvanizing, etc. More often, when restoring parts in repair practice, chrome plating and ironing are used. The maximum thickness of the coating during chromium plating can reach 0.20.3 mm, and during iron plating - 2-3 mm. This is because iron precipitates 1020 times faster than chromium.
Chroming increases the wear resistance of the part due to the high hardness and wear resistance of chromium. Chrome surfaces also have high anticorrosion properties. Iron coated surfaces have less hardness. The coating layer applied in the course of ironing corresponds to average carbon steels in terms of physical and mechanical properties.
The chroming process is used in restoring the surfaces of machine parts and mechanisms due to the valuable physical and mechanical properties of electrolytically deposited chromium: high hardness, wear resistance, low friction coefficient, good adhesion to the base metal.
Disadvantages of the chroming method: low deposition rate (2450 μm/h) and poor wettability of chromium with oils. Therefore, chrome plating is used only with a small degree of wear. In repair production, the electrolyte containing 150 g/l of chromic anhydride, 1.5 g/l of sulfuric acid, as well as an electrolyte consisting of 250 g/l of chromic anhydride and 2.5 g/l of sulfuric acid, finds the greatest use.
Coatings of higher hardness than those of high concentration can be obtained from a low concentration electrolyte. The disadvantage of such an electrolyte is that chromium anhydride must often be added to it. Therefore, in repair plants, an electrolyte containing 250 g/l of chromium anhydride is more often used.
In addition to chromic and sulfuric acids, some trivalent chromium ions (1.22.5 g/l) are present in the electrolyte at the beginning of the process, which is then formed when hexavalent chromium compounds are reduced at the cathode during electrolysis. In case of excessive accumulation of trivalent chromium, it is oxidized by working out the electrolyte at reduced anode current density. Instead of Cr2O3, a methylene blue dye (2-5 g/l) is used, which allows increasing the current yield of chromium and improving the coating quality.
For chromium plating of parts, an electrolyte is also used, in 1 liter of which 250350 g of chromium anhydride, 50-75 g of calcium carbonate, 520 g of gypsum are contained. In this electrolyte, the optimal ratio of chromium trioxide to sulfate ions is automatically maintained.
The hardness of chromium coatings, as well as the hardness of iron, depends on the deposition conditions. Chrome machining allowance is recommended in the range from 0.08 to 0.1 mm. Before chroming, parts are ground until wear traces are removed, and then washed in organic solvents and wiped with rags. As solvents, gasoline, kerosene, trichloroethylene, etc.
Washed parts are mounted on the suspension and degreased. If it is necessary to degrease only individual places, and the entire part cannot be immersed in the solution, manually degreasing is used by wiping these places with Viennese lime.
Decapitation of steel parts should be carried out for 3090 s at a current density of 2540 A/dm2.
After anode decapitation, parts are loaded into the chromium bath with the current turned off and heated for 5-6 minutes. Then, a full current is given in accordance with the chromium mode of this part. Fluctuations of electrolyte temperature shall be within + 5 ° С. Interruptions in current supply during electrolysis are not allowed, since this causes peeling of the chromium coating. Chroming after a current break is possible if the chromized surface is subjected to anode etching at a current density of 2530 A/dm2 for 30-40 s, and then, changing the current direction, continue the process. In this case, chromium deposition should begin at a cathode current density of 2025 A/dm2 and gradually increase to a normal value.
Chromium anodes are made of pure lead or an alloy consisting of 9293% lead and 7-8% antimony.
In the bath the distance between anodes and parts shall not exceed 3035 mm, the distance of parts from the bath bottom shall not be less than 100150 mm, and the distance of parts from the electrolyte solution mirror shall not be less than 5080 mm. The electrolyte level shall be 100150 mm lower than the upper edges of the bath. In this case, the chromium layer is deposited evenly over the entire surface of the part.
The depth of immersion of anodes and parts in the bath should be the same, since otherwise thickenings are formed on the edges of the parts. The flat parts in the bath shall be arranged vertically. In this case, the hydrogen bubbles released on the surface of the part are freely removed.
Thus, of the above, the most preferred method is chromation in an electrolyte containing 150 g/l chromic anhydride, 1.5 g/l sulfuric acid.
2.2.2 Selection of process bases for part processing.
In our case, we choose a base with a diameter of 62mm because this is most convenient in this case. It was not worn out, so it does not need to be restored .
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