Currently, engineers are actively working to create new and improve traditional methods for restoring parts. And there are objective reasons for this: firstly, in some cases, the production of new products from expensive steel is more expensive in terms of resources, and secondly, the enterprise simply does not have the technological ability to produce new parts with complex shapes and technical requirements.
Organizations that operate complex and expensive equipment (for example, heavy-duty mining dump trucks) are interested in improving various methods for restoring worn parts.
General provisions
All methods of restoring parts are aimed at regenerating the operational properties and original characteristics of the product. During operation, the rubbing surfaces of friction pairs can wear out (as a result of which their sizes change), crumble (as a result of the accumulation of fatigue stresses under frequent alternating loads), receive mechanical damage, and change their physical and mechanical properties. A separate type of damage during operation is violation (damage) of the protective anti-corrosion and wear-resistant coating.
Methods and methods for restoring parts are widely varied. However, wear of machine parts can have different consequences and different formation mechanisms and causes. When choosing a specific technology for restoring worn surfaces, the engineer must first take into account what properties (mechanical and physical) the product should have.
So, in some cases, it is necessary to achieve maximum structural fatigue strength and elasticity. Sometimes the chemical composition of the surface layer is critical, which makes it possible to increase heat resistance, red brittleness (cold brittleness), and resistance to aggressive environments, therefore, in each specific case, preference should be given to the method of restoring parts that meets all the requirements. Special technological and design requirements also include integrity (absence of pores, microcracks, non-metallic inclusions), weight of individual structural elements and the product as a whole, roughness indicators, mechanical properties (hardness and microhardness), the ability to be processed by cutting and pressure (additional hardening due to deformation surface layer and the occurrence of hardening), accuracy of geometric deviations of surfaces and shapes.
Restoration of worn parts by pressure
Damaged and worn parts can be restored by pressure. This method is based on the use of the plasticity of metals, i.e. their ability, under the influence of external forces, to change their geometric shape without collapsing. Parts are restored to nominal sizes using special devices, by moving part of the metal from non-working areas of the part to its worn surfaces. When restoring parts by pressure, not only their external shape changes, but also the structure and mechanical properties of the metal. Using pressure treatment, it is possible to restore parts whose material is plastic in a cold or heated state. Changing the shape of a part and some of its dimensions as a result of metal redistribution should not impair their performance or reduce their service life. The mechanical strength of the restored part must be no lower than that of the new part.
The main types of restoration of various parts by pressure include:
- settlement when restoring bushings, pins, gears;
- distribution when restoring piston pins, machine rollers, etc.;
- compression during restoration of bearing shells and bushings;
- indentation when restoring gears and spline rollers;
- straightening for straightening smooth and crankshafts and levers;
- knurling to increase the diameter of the journals and shaft journals by raising the metal ridges when forming grooves.
The method of plastic deformation when repairing parts is used not only to restore the dimensions of worn parts, but also to increase their strength and durability. Surface hardening of parts increases wear resistance and strength of parts. Plastic deformation of parts is also carried out by processing with steel or cast iron shot, chasing, rolling with rollers or balls.
Classification of methods for restoring parts according to the type of defects being eliminated
The variety of restoration methods, depending on the nature of the defects, is usually divided into the following groups:
- cutting and metalworking;
- welding and soldering;
- plastic deformation;
- fusing;
- diffusion metallization, as well as sputtering;
- galvanic technologies;
- chemical-thermal treatment (CHT), as well as traditional heat treatment;
- use of composite materials.
Methods for restoring and strengthening parts
soot. A more effective method of cleaning pistons is with stone chips and glass beads. Glass beads with a diameter of 1.5–2 mm are supplied to the surface to be cleaned with compressed air at a pressure of 392–490 kPa. After treatment for 1-2 minutes, the piston is washed in a washing chamber with water with the addition of soda ash at a temperature of 80-85 ° C under pressure up to 294.2 kPa. Various diesel filters with filler are cleaned by blowing steam at a temperature of over 100 °C under a pressure of 686 kPa. Steam blowing is alternated with blowing with atomized diesel fuel.
High-quality cleaning from other carbon deposits is ensured in molten salts. The part is immersed in a mixture of salts melted at a temperature of 250 °C: sodium chloride - 5%; caustic soda - 65%; sodium nitrate - 30%. After processing, the parts are placed in a bath with a 12% hydrochloric acid solution, then washed in hot and cold water. This method is effective for cleaning diesel pistons, compressor filter fillers, oil pump parts, etc.
Resinous deposits in the tubes of the oil sections of refrigerators are removed using a synthetic detergent AM-15. First, the sections are purged with compressed air, then immersed in a washing unit and the solution is pumped through the sections for 1-2 hours and purged again.
Old paint is removed by applying paste to the walls of the body: quicklime - 43%; seeded chalk - 30%; caustic soda - 18%; fuel oil - 9%. After 3 - 5 hours, the applied paste is removed with a spatula along with softened old paint and putty. To dissolve and neutralize the remaining alkalis, the surface of the body is generously moistened with a 2% solution of acetic acid and washed with warm water.
Flaw detection. It is produced in order to identify hidden defects in parts that are a potential cause of sudden failures in operation. The following flaw detection methods are used.
Acoustic flaw detection method
(tapping) is used for ongoing monitoring of bolted connections and the tightness of the part. A low tone or rattling indicates the presence of loose connections, seating, the presence of cracks in a prestressed part (wheel set tire), etc.
Color flaw detection method
is based on the active penetration of a wetting liquid into the cracks and pores of the part being tested, and then into the capillaries of the developing coating. Composition of the wetting liquid: kerosene - 80%; transformer oil -15%; turpentine -5%; paint “Sudan-3” - 10-15 g/l or red penetrating liquid K (MRTU 6-10-750-68). Composition of the developing coating: 600-700 g of kaolin per 1 liter of water or white developing liquid M (MRTU 6-10-749-68).
Testing technology: the part is cleaned and degreased, the surface of the part is generously moistened with penetrating liquid, then a developing liquid is applied, and the part is inspected using a magnifying glass. The crack appears as a clear line.
The method provides detection of cracks with a depth of 0.01–0.3 mm and an opening width of 0.001–0.002 mm or more. It is used primarily for testing large parts: elements of bogie and body frames, wheel centers, automatic couplings, etc.
By magnetic flaw detection method
detect surface and hidden defects. To create a magnetic field in the part being tested, the following types of flaw detectors are used (Table 10).
Magnetic powder PZh-40M (very fine iron powder) or PZh-4M is used as an indicator. Indication is carried out using dry powder or suspension method. The suspension is prepared by mixing 150-175 g of powder with 1 liter of liquid base (a mixture of transformer oil and kerosene). Before testing, the quality of the magnetic mixture and the effectiveness of the flaw detector are first tested on a control standard.
Testing technology: the part is thoroughly cleaned of contaminants to a metallic shine, the flaw detector is applied to the part and turned on, the parts of the part located in the effective testing zone are coated with a suspension and inspected. A discontinuity is revealed by the accumulation of dark powder on the white background of the part. Flaw detection is carried out by sequential movement of the flaw detector and rotation of the part. After checking, the part is demagnetized. Parts with a dark surface are checked with colored magnetic powder.
Table 10
| Type of flaw detector | Technical data | ||
| Diameter of the part being tested, mm | Test area, mm | Application area | |
| DKM-1 | 200-250 | Wheelset axle journals, electrical machine shafts, crankshafts | |
| DGS-M-53 | 200-250 | Necks and the middle part of the axle, tires, automatic coupling and spring suspension parts | |
| GPZ-1 | — | Ring perimeter | Rolling bearing parts |
| DGZ-57 | — | 3-4 teeth | Traction drive ring gears and gears |
The following parts are subject to magnetic flaw detection:
| Mechanical equipment | |
| 1. Wheelset axles: journals for motor-axle and axle bearings, middle part of the axle | For all types of inspection of wheel pairs |
| 2. Gear rings | Same |
| 3. Wheel set tires: inner and outer surfaces | Before installing new ones, after surfacing the flange or correcting defects on the skating circle |
| 4. Main leaf springs, clamps | When repairing springs with disassembly |
| 5. Spring bolts and balancers | When carrying out TR-3 and factory repairs, as well as in all cases of repair by surfacing |
| 6. Axlebox strings, parts of the ball joint of the returning device and the inter-bogie joint | Same |
| 7. Parts of the automatic coupler: automatic coupler body, platen (wedge), pendulum and support bolts, traction strips of the clamp | For repairs involving complete disassembly, as well as in all cases of repairs by straightening or surfacing |
| 8. Parts of the brake lever transmission: levers, brake shaft journals | When carrying out repairs TR-3, as well as in all cases of repair by straightening or surfacing |
| 9. Welds of all connections of the mechanical part | If there is a suspicion of a violation of continuity, welding of cracks in the elements of frames of bogies and bodies |
| 10. Rods of the body tipping mechanism of motor dump cars | During repairs with complete disassembly, as well as after straightening or welding |
| 11. Parts of axle bearings | In all cases of repair with complete disassembly |
| Electrical apparatus | |
| Shaft journals of devices with a group drive, piston rods, gear racks, crankshaft of a pneumatic engine | When carrying out TR-3 and factory repairs, as well as in all cases of repairs by straightening or surfacing |
| Pneumatic equipment | |
| 1. Compressor crankshaft, connecting rods, connecting rod bolts, pins | For repairs with complete disassembly |
| 2. Piston rods of tipping cylinders and brake cylinders | Same |
| Electric cars | |
| 1. Anchor shafts | During repairs with complete disassembly, as well as after repair of journals by surfacing |
| 2. Pole and commutator bolts, parts of motor armature bearings | For repairs with complete disassembly |
| Diesel | |
| 1. Crankshaft, pistons, connecting rods and connecting rod bolts, pins | Same |
| 2. Parts of the gas distribution mechanism | For repairs with complete disassembly |
Ultrasonic flaw detection method
used to identify defects located deep in the part. High resolution allows it to be used also for in-place diagnostics. Several types of flaw detectors are used: UZD-64, 2DM-1M, DUK-PIM.
Testing technology: the operation of the flaw detector is first checked against a standard. The surface of the part is cleaned of dirt, ground and lubricated with machine oil to improve acoustic contact. Press the probe against the surface of the part. Defects are indicated by a pulse on the cathode ray tube screen, as well as a light and sound signal. By moving the probe along the part, the boundaries of the crack are determined, and with a depth gauge, the depth of the defect is determined. The following items are subject to ultrasonic testing: hub parts of wheel pair axles, crankshafts and pistons of diesel engines, pole bolts of electrical machines (without disassembly), welds on critical components.
The results of inspections of parts are recorded in a special journal of the established form. Parts with detected cracks and other defects are taken into account in the registration log of critical parts, etc. The records are certified by a flaw detector and a foreman. Flaw detectors are subject to revision at least once every 6 months. Electrical characteristics are checked at least once a year.
Recovery. Parts are restored by metallization, welding, surfacing or electrolytic coating.
Metallization
When repairing parts, they are used to restore worn surfaces, increase their wear resistance, and correct technological defects. Technical characteristics of electrometallizers are given in table. eleven.
| Device type | Arc voltage, V | Operating current, A | Air pressure, kPa | Compressed air consumption, m3/min | Wire diameter, mm | Wire feed speed, mm/min | Device weight, |
| EM-ZA EM-6 | 30-50 25-50 | 100-150 | 441,3-588,4 490,3-588,4 | 1,2 0,8-0,9 | 1-2 1,15-2,5 | 1,4 0,7-5,5 | 2,2 |
The surface to be restored is cleaned and the metallization process begins immediately after cleaning. The part must have an ambient temperature, but not lower than 7 - 8 ° C. The distance from the nozzle to the product should be 80-180 mm (when applying non-ferrous metals - within 70 mm). The thickness of the layer being built up in one pass should be no more than 0.5 mm. The maximum thickness of the build-up layer on a cylindrical surface of a part can reach 4 mm, and on a flat part - 1.2 - 2.2 mm. After metallization, parts operating under dynamic loads are subjected to sintering in a gas furnace at a temperature of 1030–1050 °C for 3–6 hours.
Turning is carried out with cutters with carbide plates using emulsion cooling. Subsequent grinding is carried out using electrocorundum wheels E46 SMGK. Parts operating under friction conditions, after mechanical treatment, are kept in oil heated to a temperature of 100-120 ° C for 2-3 hours. To check the quality of the coating, a visual inspection method is used to identify external defects: cavities, layer lag at the edges, increase fractions of particles of sprayed metal. The quality of adhesion is determined by tapping.
Parts of mechanical equipment are subject to restoration: spring suspension and brake lever transmission rollers, pins of the inter-vehicle joint and return device, liners of motor-axial bearings; diesel; crankshaft journals, connecting rod heads, piston pins; electrical devices: shaft journals with group drive, pneumatic motor crankshaft journals; pneumatic equipment: compressor crankshaft journals, piston rods of tipping cylinders; electrical machines: shaft journals.
Welding and surfacing
- the most common recovery methods. The requirements for the surfacing process are as follows. Apply automatic or semi-automatic surfacing using highly efficient surfacing materials; the layer application technology must ensure high quality of the deposited metal, good machinability with carbide tools, and the possibility of heat treatment to a given hardness; The wear resistance of the deposited layer must provide the required technical life of the part under given operating conditions.
Progressive technology involves the use of wear-resistant materials. The most rational should be considered the primary structure with a manganese content of 1.7 - 2.0% and chromium up to 2.0 - 2.5%. These corbide-forming elements provide the deposited metal with the necessary hardness and hardenability. The carbon content in the deposited layer in the range of 0.25 - 0.3% ensures the appearance of a hardening structure and guarantees against the appearance of hot cracks. Filler metal with these properties can be obtained by using cored wires or conventional wires and ceramic fluxes. Thanks to the use of flux-cored wires, it is possible to mechanize the surfacing process and restore parts operating in dry friction mode, while ensuring the required wear resistance of the deposited layer, which can be increased by subsequent heat treatment; increase the productivity of surfacing operations by 5 times; get savings of up to 5 thousand rubles per 1 ton of surfacing metal.
The types of flux-cored wires and properties of the deposited metal are given in table. 12, and methods of surfacing with appropriate wires and repair objects are in Table. 13.
Table 12
| Wire grade | Chemical composition, % | Hardness of the deposited layer, HRC, after | |||||||||
| WITH | SG | Mn | Si | W | V | Mo | Ti | surfacing | annealing | hardening and tempering | |
| PP-AN120 | 0,18 | 1,8 | 1,8 | 0,6 | _ | _ | 0,7 | _ | 35-40 | 180 HB | 40-42 |
| PP-AN103 | 1,8 | 12,0 | 0,6 | 0,6 | — | — | 0,8 | — | 40 — 44 | 27 — 29 | |
| PP-AN104 | 1,8 | 12,0 | 0,6 | 0,6 | 1,0 | 0,25 | — | — | 40 — 44 | ||
| PP-AN124 | 2,8 | 17,0 | 1,0 | 0,6 | — | — | — | — | 42 — 48 | ||
| PP-AN121 | 0,2 | 1,1 | 1,2 | 0,65 | — | 0,2 | — | ||||
| PP-TNZOO | 0,1/ | 0,43 | 0,8 | 0,8 | — | — | — | ||||
| PP-TN350 | 0,18 | 0,45 | 0,8 | 0,9 | 0,9 | 32 — 38 |
Approximate surfacing mode, providing the hardness indicated in table. 13, with a wire diameter of 2.0-3.0 mm: current strength 200 - 350 A; arc voltage 24-30 V; wire feed speed 120-160 m/h. The specified mode may vary depending on the brand of wire used and the required hardness of the deposited layer. It is recommended to use semi-automatic machines A765, A1197, surfacing devices A580, A384-MK, machines U651, U654.
For surfacing work when restoring parts made of carbon steels of all types of equipment, etc. It is recommended to use ceramic flux ANK-18 and ANK-40 in combination with standard carbon wire Sv-08A (GOST 2246 - 70) or Sv-08G2S. The hardness of the deposited metal can be changed in the range from 30 to 45 HRC. The flux has good technological properties: stable arc burning, no
Table 13
| Wire grade | Surfacing method | Surfacing objects |
| PP-AN103 | Automatic submerged arc AN-20 or AN-15 | Spring and balancing rollers, knife supports, spring supports and linings |
| PP-AN104 | Same | Automatic coupler body (side faces of the shank), roller |
| PP-AN120 | Automatic submerged arc AN-348 | Parts of a reinforced automatic coupler: liner, thrust plate, inner and end surfaces of the shank, bead seat and clamp traction strips. Details of body supports, central and side, return device |
| PP-AN121 | Automatic open arc | Brake linkage shafts, spring bolts, spring spindles, spring suspension parts for automatic couplings, tire flanges for wheel pairs (for light duty applications) |
| PP-AN124 | Same | Rollers of the side opening mechanism, shock-traction surfaces of the automatic coupler head. Details of intersection and inter-trolley joints |
| Note. For parts operating under particularly difficult conditions (tire flanges of wheel pairs, parts of impact coupling devices), PP-AN105 grade flux-cored wire should be used. The deposited metal contains Mn - 12-14%, C - 0.4-0.45% and has the property of self-strengthening. |
pores and cracks are formed, a smooth transition to the base metal is ensured, and the slag is easily separated when hot. Chemical composition of the deposited metal, % (by weight): carbon - 0.2 -0.4; manganese - 1.0-1.8; silicon -0.5; chromium - 2.5 -5.0; sulfur - 0.04; phosphorus - 0.04. Recommended surfacing modes are given in table. 14.
Table 14
| Wire diameter, mm | 2,0 | 3,0 | 4,0 | 5,0 | ||||||
| Current, A | ||||||||||
| Arc voltage, V | 22-28 | 28-32 | 35-42 | 24-30 | 32-38 | 37-43 | 31-38 | 37-43 | 24-30 | 33-39 |
Surfacing speed 25 - 35 m/h. The polarity is reversed. Using the technological properties of the flux, it is possible to obtain hardness in a wide range, for which the arc voltage is changed (at a constant current) or the composition of the flux is adjusted. The adjustment consists of mixing ANK-18 and AN-348 fluxes in certain ratios, while maintaining the high technological properties of the ceramic flux and achieving a specified hardness limit. Recommendations for adjusting the flux composition are presented in table. 15.
Mechanical processing of deposited parts is carried out with a tool with cutting plates made of hard alloy T15K6 or T14K8. To process a deposited layer of increased hardness, it is recommended to use cutters with plates made of TT7K12 or TK12V hard alloy, and for a deposited layer of manganese steel - cutters with plates made of TT10K8-B alloy. The hardness specified in the table. 15, achieved by heat treatment of parts. ANK-40 flux is usually not adjusted. When restoring parts by surfacing, plate electrodes and flux-cored tape (PL-AN101, PL-AN102), as well as multi-electrode and vibration-arc surfacing are used.
Table 15
| Blend option | Ratio of components in parts (by volume) | Hardness of the deposited layer, HB | Blend option | Ratio of components in parts (by volume) | Hardness of the deposited layer, HB | ||
| ANK-18 | AN-348 | 2nd | 3rd | ANK-18 | AN-348 | 2nd | 3rd |
| 0,5 | 1,0 | 2,0 | 1,0 | ||||
| 0,75 | 1,0 | 3,0 | 1,0 | ||||
| 1,0 | 1,0 | 1,0 | — |
For welding work when repairing the body of motor dump cars and other work related to welding low-carbon and low-alloy steels with seams of various configurations, flux-cored wires are used (Table 16).
| Wire grade | Diameter, mm | Surfacing mode | Mechanical properties | |
| Current, A | Arc voltage, V | Tensile strength, kPa | Impact strength, kJ/m2 | |
| PP-AN1 | 2,8 | 200-350 | 24-28 | 5,8 |
| PP-AN4 | 2,5 | 200-500 | 25-32 | 12 7 |
| PP-AN8 | 2,5-3 | 200-500 | 25 — 31 | 12,7 |
| PP-AN9 | 2,5 | 360-420 | 27 — 30 | 12,7 |
PP-AN1 wire is recommended instead of rutile-type electrodes with a diameter of 4 - 5 mm. Wires PP-AN4 and PP-AN8 are intended for welding in a carbon dioxide environment instead of wire Sv-08G2S. The quality of the seams of welded joints should be monitored systematically at all stages of the production cycle in accordance with GOST 3242 - 79. After welding cracks and strengthening parts and components of the structure, the quality of welds is checked visually, as well as by color and magnetic flaw detection methods. Seams in critical areas of structures are checked using ultrasonic flaw detection in accordance with GOST 14782 - 76. Defects found in the seams of welded cracks are corrected, for which the seam of the defective area is removed and the crack is re-welded. Corrected seams must be re-inspected.
Electrolytic coating
used to restore worn surfaces, create a wear-resistant layer, protect against corrosion and give the part a decorative appearance. Advantages of the method: simultaneously with restoration, a wear-resistant layer is applied to ferrous and non-ferrous metals, increasing the technical life of the part; multiple restorations of the part are possible; heating and the associated occurrence of internal stresses and deformations are eliminated; a group recovery method is possible; labor productivity increases significantly.
For electrolytic coatings, direct current, pulsating (half-wave), asymmetrical alternating current and alternating polarity current with changing amplitudes and periods of time of current action are used. To improve process efficiency, a specific current waveform should be used.
Power is supplied to the galvanic baths from single-phase and three-phase thyristor converters. The main quality criteria for electrolytic coatings are: hardness, wear resistance, adhesion to the base metal, porosity, corrosion resistance. For decorative coatings: shine, color, uniformity.
Coating thickness when restoring a cylindrical part
,
where d
H
, d
p—diameters of the part, respectively, nominal and before restoration, mm; 2δ — allowance for machining, mm.
Chrome plating
used to restore the worn layer and create a protective shiny coating. Chrome plating restores crankshaft journals, piston pins of compressors and diesel engines, parts of fuel equipment, cylinders of pneumatic drives and other parts. Reflectors, handrails, control handles, etc. are subjected to a protective coating. Depending on the composition of the electrolyte and the mode of the process, coatings are obtained: shiny, milky and porous. A porous coating (dotted or channeled) is obtained by anodic etching. It has high oil-retaining properties and high wear resistance. The compositions of electrolytes are given in table. 17.
Table 17
| Type of coverage | Electrolyte composition, g/l | |||
| Chromic anhydride | Sulfuric acid | Chromium anhydride | Iron | |
| Hard chrome plating | 150-200 | 1.5-2,0 | Up to 4 | Until 6 |
| Protective and decorative chrome plating | 300-350 | 3,0-3,5 | More than 6 | More than 4 |
For hard coatings, an effective tetrachrome electrolyte is recommended, having the following composition, g/l: chromic anhydride - 380 - 460; caustic soda – 50 – 70; trivalent chromium - 8 - 25; sulfuric acid - 0.8 - 1.6. The process is carried out at a temperature of 15–25 °C and a current density of 2000–10000 A/m2. The deposition rate is about 0.025 mm/h.
Copper plating
used to restore worn surfaces, create anti-friction pairs, and also as a sublayer for chrome and nickel plating. The composition of sulfuric acid electrolytes and recommended modes are given in table. 18.
Table 18
| Electrolyte composition | Component content, g/l | Current density, A/m2 | Electrolysis time, min. | Coating thickness, mm |
| Anode | Cathode | |||
| Number | Components | |||
| I | CuSO4- 5H,O H2SO4 NiSO4 CH4N2S | 188,4 107,2 0,05 | 0,8 | |
| II | CuSO4- 5H2O H2SO4 Dextrin | 6,6 |
Sulfuric acid electrolytes are used to build up worn brush windows of brush holders of traction motors and auxiliary machines, and liners of motor-axial bearings.
Remaining -
the process of precipitation of iron from an aqueous solution of ferrous chloride. Compared to chrome plating, the cooling process has the following advantages: higher productivity, the ability to obtain a deposit up to 5 mm thick, no scarce chemical reagents are required, the ability to obtain a deposit hardness in the range of 170 - 250 HB, good workability, the range of repair objects is significantly expanded. Parts restored by this method can be divided into the following groups. Parts with restorable seating surfaces: conical parts and shaft journals of electrical machines, bearing rings, conical parts of pins. Rubbing parts: spring suspension rollers, brake lever transmission, side opening mechanism, inter-bogie joint and return device.
Steel 10 and steel 20 are used as anodes. The configurations of the part and the anode must be identical, and the ratio of the anode surface area to the cathode surface area is recommended to be maintained at 1:2. The composition of electrolytes and electrolysis modes during hot cooling are given in Table. 19.
Table 19
| Electrolyte composition | Concentration, g/l | Electrolysis modes | Sediment deposition rate, mm/h | |||
| Number | Components | Temperature, °C | Density, A/m2 | Type of current | ||
| I | Ferric chloride Manganese chloride Hydrochloric acid | 200 100 0,8-1,0 | 60-80 | 2000-6000 | Constant | 0,2-0,5 |
| II | Ferric chloride Sodium chloride Hydrochloric acid | 400 50 1,0 | 18-25 | 3000-6000 | Variable asymmetric | 1,0-1,2 |
The electrolyte is prepared in a galvanic shop from defatted steel filings treated with an aqueous solution of hydrochloric acid with the addition of an appropriate amount of sodium chloride or manganese chloride. Then the acidity is adjusted.
Along with hot cooling, the cold cooling method is used. Electrolyte composition: ferric chloride - 200 g/l; potassium iodide - 20 g/l; hydrochloric acid to pH = 1.3 ÷ 1.5. When remaining, an alternating asymmetric current is used. The hardness of the layer is increased by heat treatment.
Nickel plating
used as an anti-corrosion and decorative coating. Nickel is deposited onto the copper underlayer, then ground and polished. For nickel plating with an easily polished deposit, an electrolyte of the following composition is used: nickel sulfate - 70-100 g/l; boric acid - 20 g/l. The process is carried out without stirring the electrolyte at a temperature of 20 - 40 °C and a current density of 100 A/m2. Nickel plating is used for handrails, reflectors, control handles, window grilles, etc.
Galvanizing
used to protect equipment parts and fasteners made of ferrous metals from corrosion. Electrolyte composition, g/l: zinc sulfate -215; aluminum sulfate - 30; aluminum alum - 45 - 50; sodium sulfate - 50-160; dextrin - 10. The process is carried out without stirring the electrolyte at a temperature of 18 - 25 ° C and a current density of 100 - 200 A/m2. Anodes are made of electrolytic zinc. After galvanizing, the parts are passivated and dried.
Hardening. Increasing the service life of new and restored parts by hardening the working surfaces is the most important stage of the production cycle. The most common methods of hardening are rolling (hardening), thermal and thermochemical treatment, surfacing and spraying with wear-resistant materials.
Hardening
used to increase the wear resistance of rubbing pairs and the fatigue strength of steel parts subject to cyclic loads.
The most common are shot peening and centrifugal peening. Shot peening is performed using pneumatic or mechanical shot blasters. In this case, a jet of metal shot falls with high kinetic energy onto the surface of the part at an angle of 70°.
The fatigue strength of a part after shot peening increases by 20 - 40%. Centrifugal hardening is carried out using rotary hardeners and by applying steel balls to the surface of the part. The depth of the hardened layer reaches 1.5 mm, the hardness increases by 50%. Leaf springs, springs, hub parts, fillets of wheel pair axles, and crankshaft journals are subjected to hardening.
To increase the fatigue strength of welds and deposited metal layers, surface hardening with multi-ram hardeners is used. The strengthener consists of a bundle of strengthening wires (27 - 60 pcs.), mounted in a KM-5 pneumatic hammer with a number of blows of 1600 per minute. The method is effective for seams of bogie frames, center beams, upper and lower dump car frames.
Roll hardening
carried out by exposing the surface to be treated with a ball or roller under a certain pressure. Preferably, hardening-smoothing rolling is used, which, along with increasing fatigue strength and hardness, ensures a reduction in surface roughness by 2 - 3 classes. Parts made of ferrous and non-ferrous metals are subjected to hardening by rolling: fillets and journals of axles, the surface of the rolling circle of tires of wheel pairs, journals of crankshafts, and commutators of electrical machines. However, the positive effect of hardening is observed only up to a certain degree of hardening for a given metal. If a certain degree of hardening is exceeded, microscopic cracks appear, brittleness occurs, and the hardened layer peels off. This applies primarily to bandages, the surfaces of which during operation are subject to continuous hardening by rolling.
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Classification of restoration methods depending on the nature of the impact on the part
According to this principle, all recovery operations are divided into three groups:
- processing without removing allowances;
- processing of parts with material removal;
- technological operations associated with the application of coatings and materials in one way or another.
It makes sense to provide a more detailed classification of these groups, since each of them includes many processing methods using a variety of equipment and principles. In some cases, there may be duplication in the name of the method of restoring parts, since one method can simultaneously belong to several groups.
Restoration without removing allowances:
- hardening and shaping through cold and hot plastic deformation, calibration;
- chemical-thermal treatment (carried out to increase hardness and improve performance characteristics);
- heat treatment (increasing hardness, relieving dangerous stresses, and so on).
Methods for restoring worn parts that involve removing a layer of material:
- machining by cutting;
- electrophysical processing;
- combined methods.
The last subgroup includes methods that allow you to apply an additional protective layer of material to the surface of the part. The main methods for restoring coated parts include the following:
- application of metallic and non-metallic coatings in furnaces (metallization, spraying, surfacing and others);
- electrophysical methods of coating (plating baths, electrospark methods, and so on).
Restoration by surfacing and welding
Restoring parts by welding and surfacing is one of the most common methods.
When surfacing, the following operations are performed sequentially. Treatment of a worn surface, the purpose of which is to remove the boundary layer of deposited metal from the treatment zone. Surface surfacing with an allowance sufficient for further processing. Processing of the deposited surface in accordance with the requirements of the drawing.
Characteristics of plumbing and mechanical restoration operations
This method of restoring and strengthening parts is used in cases where there is a need to obtain a new or previous repair size of a product, as well as when it is necessary to install a new element of a mechanical engineering product being restored. Thus, mechanical and metalworking can serve as a kind of intermediate operation aimed at preparing surfaces for the application and spraying of additional hardening coatings. However, most often cutting processing is final and is aimed at correcting defects in shape and surface that have arisen for one reason or another. Such reasons may be surface and volumetric deformation of parts and workpieces in order to give them greater strength and the most favorable performance characteristics, surfacing of metal powder and electrode, and so on.
Processing to size must ensure all technological and design requirements: cleanliness and roughness of surfaces, values and magnitude of the gap or interference (if the fit is carried out with interference), deviations of the geometric shape, and so on.
The engineer makes a choice in favor of one or another mechanical method of restoring a part, taking into account a number of different factors. So, if the degree of wear of a part is very high, then it makes sense to install an additional repair part. In this case, surfacing with subsequent processing will cost much more and require very high qualifications from the performer. Such parts mainly include all kinds of bushings and adapters.
Non-metal coating
The essence of this method is:
- in applying a layer of a two-component polymer composition to a previously cleaned and degreased surface;
- in fixation with the help of auxiliary devices (if necessary).
Compared to galvanization, the application of non-metallic coatings has a number of advantages:
- simplicity, no need for preliminary mechanical treatment of the surface being repaired;
- possibility of applying a thick (10 – 15 mm) layer of polymer.
At the same time, such coatings are noticeably inferior to metals in wear resistance and durability.
Characteristics of restoration of parts by plastic deformation
Deformation is used both to change the shape and geometric dimensions of a part, and to improve the performance characteristics of the product surface (hardness and wear resistance).
With a change in shape, everything is clear: when a significant load is applied to a solid body and then removed, residual deformation remains. This method of restoring machine parts is used in practice when it is necessary to align products that have been damaged as a result of a collision. This type of work includes both body work on a car that has been involved in an accident and straightening of a thick steel sheet. Often the need for pressure treatment arises after welding treatment: when applying a weld, certain local zones become very hot, which leads to linear expansion of certain elements of the welded structure. When cooling, the reverse process occurs - a decrease in size, which leads to warping and disruption of the geometry of the entire product. Therefore, if there are strict requirements for the shape and deviations of the structure, it is subjected to pressure treatment in order to correct the defect.
Pressure treatment can also be used to strengthen the surfaces of a restored product, for example, after surfacing or after mechanical removal of a certain allowance from a part by cutting. Strain hardening is a rather rare method of restoring parts. The choice in favor of this technique is extremely rare. This is due to the fact that rather expensive equipment is required for hardening by surface plastic deformation. It is not economically feasible to purchase such machines in order to use them occasionally in case of need for restoration.
Electrochemical methods for restoring parts
To restore parts by applying metal coatings, the galvanic method is used, with which the following is applied:
- chromium;
- nickel;
- iron.
Chrome and nickel coatings have a thickness of 0.25 - 0.3 mm, iron 2 - 3 mm or more. Iron plating in its parameters is close to surfacing, however, it provides relatively low hardness. There are smooth or porous coatings used for movable and fixed joints.
The essence of hardening by deformation. Physics of the process
How do the strength properties improve when the surface layer is deformed? Good question. The answer to this lies in the radiation theory of the atomic structure of crystalline substances.
Scientists were able to prove that strength depends on the number of defects in the crystal structure. According to their calculations, a thin metal thread made of ideally pure iron without point or linear structural defects is capable of withstanding colossal loads. However, real bodies always have defects, so the load-bearing strength of such wire in real conditions is quite small. But when the number of defects increases, a paradoxical phenomenon arises - the strength characteristics improve. This is explained by the fact that a large number of defects creates obstacles for their movement and emergence to the grain surface, that is, it prevents the occurrence of stress concentrators.
This is precisely what the strengthening effect of pressure treatment is based on: during deformation, a huge number of defects appear inside the grains. At the same time, the grains themselves acquire a characteristic shape - the so-called texture. It should be noted that this method allows not only to increase strength and wear resistance, but also to reduce the roughness of the treated surface.
“Repair dimensions” method and others
The most common defect, as already mentioned, is surface wear. Therefore, the main direction of restoration technologies is to bring worn surfaces to their original parameters. For this, standard technological methods are used - welding, soldering, surfacing, spraying of metal coatings, metal deposition, application of polymer materials and some others.
When choosing a recovery method, you should pay attention to a number of issues. For example, with the help of surface spraying you can obtain the desired surface hardness, increase the wear resistance of the working surface of a part, reduce the impact of the fatigue factor, and enhance anti-corrosion qualities, therefore the spraying material, as well as the method of its application, is the most important stage of restoration. But, if a decision is made to apply a coating to a defective surface, it is necessary to find out how well the metal of the part is combined with the applied coating, as well as how the surface of the mating part will “treat” this coating. It is also necessary to know whether the chosen method and material can create a coating thickness that would compensate for wear and allowance for subsequent processing.
But, if the mutual wear of parts working in pairs is significant, then usually the landings are not sprayed, but restored, changing the original dimensions to the so-called “repair” ones. The most valuable part of the pair is processed, for example, increasing the diameter of the worn-out hole to the repair one, while the geometry of the hole is restored, traces of wear are removed, and the necessary cleanliness of the processing is restored. And the mating part is either made new, but with an increased diameter corresponding to the repair size of the first part, or machined to a cylindrical surface, eliminating ellipse, and a sleeve is installed, the outer diameter of which corresponds to the repair diameter.
The “repair dimensions” method is undesirable where parts wear out intensively and, accordingly, are often repaired or replaced. During repairs, when one part of a pair is rejected, the paired part also has to be changed. It also often happens that the repairman does not know about the repair dimensions of the part and prepares a standard part for replacement. Then you urgently have to decide what to do and look for a new spare part. Ultimately, simple repairs can take longer.
More universal is the so-called. method of “setting an additional element”. In this case, worn holes and shafts are processed until the correct geometric shape is restored, and then bushings are installed in the hole or on the shaft, restoring the original drawing fit dimensions of the mating parts.
The initial shape of frames, outrigger beams, slewing rings, portal struts and booms that have received residual bending and twisting is returned by straightening. Upsetting restores the nominal dimensions of hollow parts of hydraulic cylinders. To do this, a rod is inserted into the hollow part, the diameter of which corresponds to the one being restored, and the cylinder is compressed on a press. Bushings are restored in a similar way, crimping them on a press, which is equipped with special punches and dies.
Method for restoring parts by surfacing
This method is the most common when restoring the original dimensions of a part. The reason for this is relative cheapness and simplicity. To restore the geometry of the product, you only need a welding machine and the necessary material for surfacing.
In the event that the size is very broken, then the so-called combined surfacing is used. Its essence is as follows: first, ordinary steel or cast iron is applied using gas-flame or electric arc heating. And only then electric arc surfacing of a durable alloy is carried out, which has a good set of mechanical and physical properties. The quality of the surface after surfacing can be characterized as unsatisfactory, so an allowance is necessary. This operation can be carried out on a lathe, milling or boring machine. It is also possible to use chiselling and abrasive tools (if the deposited material is very hard).
To restore, you need to know the defects
If we conditionally divide all the parts that are most often subject to restoration, then 53.3% of all restored parts have a cylindrical shape, both external and internal. 12.7% of all restored parts are threaded parts, and approximately 10% each are geared (gears, sprockets, etc.) and splined (shafts, bushings) parts. Flat parts are repaired least often, in only 6.5% of cases out of 100% of repaired parts. This is due to the relatively low cost of such parts in mass production and the relative complexity of their restoration.
If we consider the restoration process itself, then at the first stage the most thorough cleaning of the part is necessary. If you do not pay due attention to cleanliness, then, for example, during surfacing, the remaining dirt can most likely cause the formation of pores and cavities. And when coating by galvanic or chemical methods, grease or other contaminants lead to peeling of these coatings during operation.
At the next stage, parts are defective, first by external inspection, and then using a universal measuring tool. They identify cracks, nicks, dents, areas significantly damaged by corrosion, surfaces and landings that have significant deterioration. To identify hidden defects, check for leaks, and also to determine the correct relative position of mating parts, there are special measuring tools and standard devices. The quality of the restored part largely depends on the thoroughness of defect detection.
Of course, many hidden defects are difficult to detect in the field. Therefore, say, if internal cracks are suspected of occurring in solid parts, it is advisable to detect them magnetically using universal magnetic flaw detectors. Specialized enterprises, as a rule, have similar equipment. To detect internal defects in parts made of non-ferrous metals, fluorescent flaw detection is used.
To detect cracks in body parts, a hydraulic method is used. All standard holes are first closed with plugs, then the part is installed on a special stand and the internal cavity is filled with water, pressure is created and held for some time.
In emergency cases, cracks can be clearly identified by treating the degreased metal surface with kerosene, to which transformer oil and turpentine are added, approximately 150 and 50 g per 1 liter of kerosene, respectively. Having treated the part with such a solution and left it for 5-10 minutes, wipe the kerosene dry from the part and apply a layer of chalk to the surface under study. Residues of the kerosene solution will certainly appear on the cracks and show the size and shape of the defect.
Galvanic methods in the restoration of parts
When considering the classification of methods for restoring parts, one cannot fail to mention galvanization. This method is very common. Galvanic baths have long been firmly established in industry and are actively used both in manufacturing enterprises and in research laboratories. The scope of their application is incredibly wide: from applying decorative coatings to etching materials.
As a rule, this method is applicable only when the degree of wear of the rubbing surfaces is insignificant, since the thickness of the coatings applied by galvanic method is very small. In addition to restoring specified dimensions, such a coating can act as a protective film and prevent corrosion and oxidation of materials.
The advantage of this method is the ability to obtain coatings using a wide variety of materials: nickel, chromium, aluminum, iron, copper, silver, gold, and so on. Therefore, galvanic coating is used in many sectors of the national economy.
Plastic deformation of restored parts
Restoring parts by plastic deformation involves recreating their shape and size due to the redistribution of metal under the influence of a load applied in a certain place and in a certain direction.
Products made from low-carbon steels (less than 0.3% carbon) and non-ferrous alloys are restored without heating. Medium and high carbon steels are heated to a temperature determined by the formula: Theating = (0.70.9) Melting
Main types of plastic deformation:
- upset or settling - changing the diameter of a cylindrical product by applying an axial load to the ends;
- expansion and compression - recreating, respectively, the outer and inner working diameters of a hollow body of rotation by increasing (decreasing) the internal non-working diameter;
- hood – increasing the length of a product due to local narrowing of its cross-section;
- knurling - processing surfaces using a toothed roller;
- straightening - recreating the shape and eliminating bending and twisting (can be done under pressure by creating local surface hardening and using local heating);
- The electromechanical method for restoring parts, used, as a rule, for processing bodies of rotation, includes two operations: creating a microrelief on the surface in the form of a spiral line; smoothing to a given size using a deforming plate.
Characteristics of methods of thermal and chemical-thermal treatment in the restoration of products
It is difficult to exaggerate the role of heat treatment in general in mechanical engineering, and in the field of restoration of parts in particular. It allows you to obtain the necessary operational (wear resistance, hardness) and technological (cutting ability, thermal conductivity) qualities.
Chemical-thermal treatment is a separate topic. Unlike traditional heat treatment, when carrying out chemical treatment, the product is exposed not only to temperature, but also to a chemical reaction with atoms and ions of other substances. Atoms diffuse to a certain depth inside, thereby changing the chemical composition of the surface layer. The properties of the diffusion layer differ significantly (for the better) from the original material. Thus, boriding (saturation with boron atoms) and cementation (saturation with carbon atoms) significantly increases hardness and helps reduce the coefficient of friction. In practice, silicon, nitrogen, aluminum and other elements are also used as saturating elements.
Types of surfacing of cylindrical surfaces
In cases where the wear of the mechanism exceeds the standards established by the manufacturer, another option may be used. Removal of the damaged part mechanically. Manufacturing a new product and welding it in place of the removed one. Heat treatment (if necessary). Final machining.
Welding is widely used in the repair of body parts in which cracks have formed. The technological process includes several operations:
- Determination of crack direction.
- Drilling into metal at a distance of 6–10 mm from the visible end of the crack.
- Removing the crack mechanically with simultaneous cutting for welding.
- Welding of a crack with a slight excess above the surface of the base metal.
- Surface treatment of the weld metal flush with the base metal.
- Checking geometric parameters.
- Treatment of mating surfaces (if necessary).
Preparing a crack for welding:
- cleaning the crack;
- drilling the ends.
