2 Surface engineering technology in the engine
Analysis of the form and characteristics of engine component damage can be concluded that the main forms of engine component failure are: wear, corrosion and fatigue, etc., which occur mostly on the surface, or start from the surface, so improve the surface properties of the material to extend the parts. Life and potential play an important role.
Strengthening methods to improve the surface wear resistance of materials often start from two aspects: increasing surface hardness and reducing friction coefficient. So far, the traditional surface quenching and carburizing quenching are the main means to improve the wear resistance of parts. A large number of components such as crankshafts, piston pins, cams and camshafts are used in this process. Gas nitriding, carbonitriding, ion nitriding, etc. mainly use diffused nitride to improve the surface hardness of the material; boronizing, vanadium, chromizing, depositing carbide, nitride, high hardness and The ultra-high hardness compound layer has superior anti-abrasive wear ability and high anti-adhesive wear ability; surface plating, such as chrome plating layer, is also a widely used wear-resistant coating.
Surface treatments that improve friction conditions and reduce the coefficient of friction can, on the other hand, increase the wear resistance of the material. Sulfurizing, sulfur-nitrogen co-infiltration, sulfur-nitrogen-carbonitriding, phosphating, and graphitization are all compounding and non-metallic layers on the surface of the metal to reduce the friction coefficient and avoid direct contact between the friction pairs. Therefore, it is possible to prevent sticking and avoid scratching.
The main methods to improve the surface fatigue resistance of materials are: high-frequency surface quenching, carburizing and quenching hardened layer, high martensite hardness, and residual compressive stress, so that the fatigue strength is improved. Surface deformation strengthening treatment such as shot peening and surface rolling improves the surface hardness and causes surface compressive stress, and also eliminates surface defects, thereby improving the fatigue strength of the material and reducing the sensitivity of the material to the notch. After surface quenching, carburizing and quenching, and then nitriding, the effect of shot peening and surface rolling treatment is better, especially the shaft diameter of the shaft, the root of the gear and other stress concentration points, and the surface rolling processing effect is better.
The main methods to improve the corrosion resistance of the material surface are: (1) chrome plating, because it is very stable in alkali, nitric acid, sulfide, carbonate and organic acid;
(2) Nitriding, because it forms a stable phase layer on the surface of the parts, and has high corrosion resistance to water, humid air, combustion products, especially to nitric acid; (3) aluminizing, siliconizing and acidizing Good corrosion resistance; (4) Chromium-containing nickel-based, cobalt-based self-fluxing alloy sprayed layer has good thermal corrosion and gas corrosion resistance, because of the formation of dense Cr2O3 oxide film; (5) for steel parts For example, piston rings are phosphatized and bluish, which prevent corrosion and reduce mechanical wear. The following highlights several common surface engineering techniques.
2.1 Thermal spraying
The thermal spraying method not only can make the parts have different functions such as wear resistance, corrosion resistance, oxidation resistance, high temperature resistance, heat insulation, etc., but also can have composite properties, that is, it can protect parts and repair parts. At present, in the engine, it is mainly used for surface treatment and repair of crankshafts, piston tops, piston rings, cylinder liners, exhaust valves, valve seats, cams, gears, etc. [6], Table 1 shows the use of thermal spray repair Comparing the economic benefits of the engine block and replacing new parts, it can be seen from the table that the thermal spray 3Cr13 material repair only costs 4.2% of the cost of the new part.
In the engine, the spray materials used to improve the wear resistance of the parts are mainly the following:
1) Molybdenum. As a spraying material, molybdenum needs to have a purity of 99.95% or more. Molybdenum is mainly used as a spray layer of a piston ring, which can obtain a layered molybdenum layer with an oxide layer and a porosity of about 10% to 15%, and the sprayed layer is generally 0.25 mm thick, and has good oil storage performance. The molybdenum layer has a high melting point (about 2620 ° C), a low friction coefficient, and a high hardness (about HV 1070). Therefore, the wear resistance is good, and it is generally used in the first piston ring.
2) Molybdenum based alloy. The plasma sprayed layer of the molybdenum-based alloy is characterized by good tensile strength because it consists of a low hardness pure molybdenum (HV350) and a relatively high hardness (HV700) nickel based alloy. Its thermal stability is superior to that of molybdenum wire spray coating, and its wear resistance is equivalent to chrome plating, especially when the lubrication conditions are not sufficient.
3) Chromium-based alloys. The chromium-based alloy plasma sprayed layer not only has better wear resistance and bite resistance under boundary conditions, but also improves the wear resistance of the grinding material, and simultaneously improves the wear resistance of the cylinder liner and the piston ring. Under boundary lubrication conditions, this coated piston ring is more wear resistant than chrome plating, so it can be used as a coating material for high speed, medium speed and low speed engine piston rings.
4) Ceramics. Ceramic coatings such as chrome oxide and alumina/titanium dioxide have higher hardness (HV1500). Under certain conditions, the piston ring has good wear resistance, but the cylinder liner wears a lot, so it can only be used under special requirements. use.
5) Cermet. The cermet is relatively complicated. It is based on a carbide-based coating (such as tungsten carbide). Even under severe operating conditions, not only the wear of the piston ring is low, but also the wear of the cylinder liner is low. But cermets are expensive. In the engine, the ceramic is generally sprayed on the exhaust valve surface, the piston top surface, and the piston ring working surface.
6) Nickel base alloy. Nickel-based alloys are Ni-B-Si and Ni-Cr-B-Si. Nickel-based alloy coatings have good corrosion resistance and high heat-hardness in addition to good wear resistance. It is mainly used for spray welding (also used for repair) of engine piston rings, valve seats, valves and cams to improve the wear resistance of these parts.
7) High carbon steel and stainless steel. High carbon steel, such as carbon tool steel T8, is often used to repair and strengthen engine cast steel, wire air jet and electric spray on the surface of ductile iron journal. Stainless steel has excellent corrosion resistance and heat resistance, and is used for spraying (flame spraying) on ​​the inner surface of the cylinder liner.
2.2 Brush plating repair
The main failure mode of the engine camshaft journal is wear or scratch. In the past, the camshaft journal was worn or scratched, and it was scrapped. Or the thicker bearing bush was used to grind the journal and used to repair the user. A lot of trouble.
The brush plating technology has the characteristics of simple equipment, convenient operation, safety and reliability, and fast plating speed. It is used to repair the camshaft journal and achieve obvious effect [7]. Table 2 shows the benefit analysis of repairing engine axles and connecting rods by brush-plated nano-Al2O3/Ni. Brush plating technology can greatly reduce maintenance costs.
2.3 Extruded silicon carbide
The process of extruding silicon carbide is to directly extrude silicon carbide particles into the surface layer of the metal, and the surface thereof is discontinuously distributed. The main features of this method are simple process, high production efficiency, low cost, no pollution, and no special equipment. Therefore, it is a promising new process. It is mainly used to improve the wear resistance of engine cylinder liners and piston rings, especially for high-load engines.
2.4 chrome plating
The chrome plating process is mainly used to improve the wear resistance and corrosion resistance of engine parts, such as crankshaft journals, crosshead journals, piston pins, piston ring grooves, piston rods, cylinder liners, piston rings, gas valves and stems. Oil pump plunger and sleeve. At present, there are two types of wear-resistant chrome commonly used in engine parts, hard chrome plating and loose hole chrome plating.
Hard chrome plating refers to the bright chrome layer with high hardness and wear resistance. It is mainly used for the lubrication condition and the load is not very large. Generally, the coating thickness is 0.03~0.3mm, and the thickest can reach 1.0mm. .
Pore ​​chrome plating is mainly used for poor lubrication, and parts with large loads such as engine cylinder liners, piston rings, crosshead pins, etc.
2.5 Nitriding
There are three kinds of nitriding: (1) ordinary nitriding, which refers to the infiltration of pure nitrogen atoms; (2) soft nitriding, salt bath liquid nitrocarburizing, gas nitrocarburizing and ions in a gaseous medium containing reactive nitrogen and carbon atoms Soft nitriding (including titanium ion nitriding); (3) glow ion nitriding.
At present, the most commonly used and most promising are ion nitrocarburizing and glow ion nitriding. They are mainly used to improve the wear resistance, fatigue resistance and corrosion resistance of engine parts such as crankshaft journals, cylinder liners, piston rings, camshafts, intake and exhaust valves, valve lifters and the like.
2.6 light heating surface quenching
Laser heating is used to quench the surface of steel and cast iron parts to improve their wear resistance. In order to solve the strain on the cylinder mouth and improve the wear resistance, the US GM645 series diesel engine has been treated with nitriding, induction heating quenching and flame quenching respectively. However, due to the large treatment surface and high temperature, it appears. Severe deformation, even cracks, and finally the surface treatment with laser heating, achieved good results [12]. The laser-quenched cylinder piston ring has been applied in China and has been well reflected. The method is characterized by simple process and little deformation (such as quenching the surface of the cylinder liner, the local temperature can reach 916 ° C, and the temperature of other parts is lower than 200 ° C, so the deformation is small).
2.7 shot peening
Shot peening does not need to change the shape, material, heat treatment of the parts, and it is easy to improve the strength, especially the fatigue strength. Therefore, there are many applications in the engine, which are mainly used in crankshafts, connecting rods, valve springs, and shakers. Arms and transmission gears. The reinforced layer after shot peening can reach 0.5~0.6mm, and the surface hardness can reach about 40~50 HRC.
2.8 cold calendering
As a means of finishing and surface strengthening, cold calendering has been increasingly used in production to improve the quality of the surface of its parts (roughness, chill hardness, chill layer depth, nature and size of residual stress) . Mainly used in the engine for cylinder liner, piston pin hole, connecting rod small copper bushing, valve guide, valve stem surface, cam journal, crankshaft journal, especially the final finishing of the crank arm transition circle. Reduce surface roughness and its reinforcement and exhibit residual compressive stress.
3 conclusions
In summary, due to the complex working conditions of engine components, damage is often a combination of several failure modes, and the surface performance requirements of components are often not single. Therefore, only by properly selecting the surface strengthening method according to the working condition and failure characteristics of the parts can the purpose of extending the service life of the parts and saving materials can be achieved.
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