Currently, engineers are actively working on creating new and improving traditional methods for the restoration of parts. And there are objective reasons for this: firstly, in some cases the manufacture of new products from expensive steel is more costly in terms of resources, and secondly, the enterprise simply does not have the technological ability to produce new parts that are complex in form and technical requirements.
Organizations that operate complex and expensive equipment (for example, especially heavy-duty mining dump trucks) are interested in improving various methods of restoring worn parts.
General Provisions
All methods for the restoration of parts are aimed at the regeneration of the operational properties and initial characteristics of the product. In the process, the friction 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 with frequent alternating loads), receive mechanical damage, and change their physical and mechanical properties. A separate type of damage during operation is a violation (damage) of the protective anti-corrosion and wear-resistant coating.
Ways and methods of restoring parts are very diverse. However, wear of machine parts can have different consequences and different formation mechanisms and causes. When choosing a specific technology for restoring worn surfaces, an engineer must first consider what properties (mechanical and physical) a product should have.
So, in some cases, it is necessary to achieve maximum fatigue strength and elasticity. Sometimes the chemical composition of the surface layer is critical, which allows to increase heat resistance, red crack resistance (cold brittleness), resistance to aggressive environments, therefore, in each specific case, one should give preference to the method of restoration of parts that can meet all the requirements. Special technological and design requirements also include integrity (the absence of pores, microcracks, non-metallic inclusions), the mass of individual structural elements and the product as a whole, roughness indicators, mechanical properties (hardness and microhardness), the possibility of cutting and pressure treatment (additional hardening due to deformation surface layer and the occurrence of hardening), the accuracy of geometric deviations of surfaces and shapes.
Classification of methods for restoration of parts by type of defects to be repaired
The whole variety of recovery methods, depending on the nature of the defects, is usually divided into the following groups:
- cutting and metalwork;
- welding and soldering;
- plastic deformation;
- deposition;
- diffusion metallization, as well as spraying;
- galvanic technologies;
- chemical heat treatment (XTO), as well as traditional heat treatment;
- use of composite materials.
Classification of recovery methods depending on the nature of the impact on the part
According to the indicated principle, all recovery operations are divided into three groups:
- processing without removing allowances;
- processing of parts with material removal;
- technological operations associated with coating and materials in one way or another.
It makes sense to give 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, duplication in the name of the method of restoring parts is possible, since one method can simultaneously belong to several groups.
Recovery without removing allowances:
- hardening and shaping by means of cold and hot plastic deformation, calibration;
- chemical-thermal treatment (carried out in order to increase hardness, improve operational characteristics);
- heat treatment (increase hardness, relieve dangerous stresses and so on).
Ways to restore worn parts associated with the removal of 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 reconditioning coated parts include the following:
- application of metallic and nonmetallic coatings in the furnace (metallization, spraying, surfacing and others);
- electrophysical methods of coating (galvanic baths, electrospark methods and so on).
Characteristics of metalwork and mechanical recovery operations
This method of restoration and hardening of parts is used in cases where there is a need to obtain a new or old repair size of the product, as well as when it is necessary to install a new element of the restored mechanical engineering product. So, mechanical and metalworking can serve as a kind of intermediate operation aimed at preparing surfaces for applying and spraying additional hardening coatings. However, most often, cutting 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 in size should provide all technological and design requirements: cleanliness and surface roughness, values and the size of the gap or interference (if the fit is tightened), geometric deviations, and so on.
An engineer makes a choice in favor of one or another mechanical method of repairing a part, taking into account a whole variety of factors. So, if the degree of wear of the part is very large, then it makes sense to install an additional repair part. In this case, surfacing with subsequent processing will cost much more and requires a very high qualification from the contractor. As such parts, basically, are all kinds of bushings and adapters.
Characterization of restoration of parts by plastic deformation
Deformation is used both to change the shape and geometric dimensions of the part, and to improve the operational characteristics of the surface of the product (an indicator of hardness and wear resistance).
With a change in shape, everything is clear: when a considerable load is applied to a solid and then removed, a residual deformation remains. This method of restoring machine parts is used in practice, if necessary, to align products that have received damage as a result of a collision. This type of work includes both bodywork on a car that had an accident, and the straightening of a thick steel sheet. Often the need for pressure treatment arises after welding treatment: when applying a seam, certain local areas are very hot, which leads to a linear expansion of certain elements of the welded structure. When cooling, the reverse process occurs - a decrease in size, which leads to warpage and violation of the geometry of the entire product. Therefore, in the presence of strict requirements in form and design deviations, it is subjected to pressure treatment in order to correct the defect.
Also, pressure treatment can be used to harden the surfaces of the restored product, for example, after surfacing or after mechanical removal by cutting a certain allowance from the part. Strain hardening is a fairly rare way to repair parts. The choice in favor of this technique is extremely rare. This is due to the fact that rather expensive equipment is required to harden surface plastic deformation. It is not economically feasible to acquire such machines in order to occasionally use them in the event of a need for restoration.
The essence of strain hardening. Process physics
What improves the strength properties during deformation of the surface layer? Good question. The answer lies in the radiation theory of the atomic structure of crystalline substances.
Scientists were able to prove that the strength depends on the number of defects in the crystal structure. According to their calculations, a thin metal thread of perfectly pure iron without point and linear structural defects can withstand enormous loads. However, real bodies always have defects, therefore, the bearing strength of such a wire in real conditions is quite small. But when the number of defects increases, a paradoxical phenomenon arises - strength characteristics improve. This is due to the fact that a large number of defects creates obstacles for their movement and exit to the grain surface, that is, it prevents the occurrence of stress concentrators.
It is on this that the strengthening effect of pressure treatment is based: during deformation, a huge number of defects inside the grains arise. 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 the strength and wear resistance, but also to reduce the surface roughness.
Method for restoring parts by surfacing
This method is the most common when restoring the original dimensions of the part. The reason for this is the 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 much broken, then the so-called combined surfacing is used. Its essence is as follows: first, by means of gas or electric arc heating, conventional steel or cast iron is applied. And only then is the electric arc surfacing of a durable alloy having a good set of mechanical and physical properties. The surface quality after surfacing can be characterized as unsatisfactory, therefore, an allowance is necessary. This operation can be carried out on a turning, milling or boring machine. It is also allowed to use chiselling and abrasive tools (if the deposited material is very hard).
Galvanic methods in the restoration of parts
When considering the classification of methods for the restoration of parts, one cannot but mention electroplating. This method is very common. Galvanic baths have long been firmly established in the industry and are actively used both in manufacturing enterprises and in research laboratories. The scope of their application is incredibly vast: from the application of decorative coatings, then the etching of materials.
As a rule, this method is applicable only with a slight degree of wear of rubbing surfaces, since the thickness of the coatings applied by the galvanic method is very small. In addition to restoring the specified size, 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 variety of materials: nickel, chromium, aluminum, iron, copper, silver, gold and so on. Therefore, galvanic coating is used in so many sectors of the economy.
Characterization of thermal and chemical-thermal treatment methods in product recovery
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 (machinability, thermal conductivity) qualities.
Chemical-thermal treatment is a separate issue. In contrast to traditional heat treatment, during the implementation of XTO, 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 inward, thereby changing the chemical composition of the surface layer. The properties of the diffusion layer are significantly different (for the better) from the starting material. So, boronation (saturation with boron atoms) and cementation (saturation with carbon atoms) significantly increase hardness and reduce the coefficient of friction. In practice, silicon, nitrogen, aluminum and other elements are also used as saturating elements.
Conclusion
The above description of the methods for restoring parts is not exhaustive. The idea is given only about the basic and most common methods. In total, there are many more. Moreover, scientists around the world are constantly working on creating new and improving the already known methods of coating and restoration of the geometric dimensions of parts.