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August 12, 2015
Restoration of parts by depositing the metal
You can do without buying new spare parts: just restore them by depositing the metal on the worn-out part.
Those who have to operate various kinds of mechanisms sometimes face the need to deposit the metal in one place or another on a worn-out part, be it a broken bearing liner, a broken seat, a surface worn out during operation or chipped edge of the metal-cutting tool. In all these situations you can do without buying a new spare part, just restore the existing one by depositing the metal on the worn-out part. Restoration of parts by depositing allows you not only to go back to nominal parameters (properties of the part), but also to give it completely new valuable properties from time to time (as a result of mixing the main and filler layers during deposition). In particular, wear-resistant cutting tool can be obtained by depositing the layer of hard-alloy metal onto the low carbon steel base.
Deposition is the process of applying one molten (filler) metal to the surface of another (base metal). In this case, the base metal is also melted to a small depth to form a homogeneous compound. The purpose of deposition can be different: restoration of the lost geometry of the part or giving it a new shape, the formation of a surface layer with desired physical and mechanical properties (such as increased hardness, wear resistance, antifriction properties, corrosion resistance, heat resistance, etc.), hardening by deposition.
Almost any surface can be restored by deposition - flat, conical, cylindrical, spherical. Within large limits, its thickness may vary - from a few tenths of a millimeter to 1 centimeter or more.
Basic principles of the restoration of parts by deposition
In its main points, the technology of restoration by deposition is similar to welding technology. It faces the same tasks as welding - protection of the deposited metal from gases contained in the air, obtaining dense weld metal, without pores, cracks and foreign inclusions. When depositing, one should follow the basic principles consisting of a number of requirements:
· It is necessary to strive for minimum penetration of the base metal. This is achieved by tilting the electrode in the direction opposite to the deposition direction.
· There should be as little mixing of the deposited metal with the base metal as possible.
· It is necessary to try and achieve minimum residual stresses and deformations in the part. This requirement is largely ensured by the observance of the previous two requirements.
· It is necessary to reduce the allowances for subsequent processing of the part to acceptable values. In other words, one should deposit the metal to a required level, and not more than that.
Different methods of restoration of various metal parts are used - electric arc, gas, electroslag, induction, plasma, pulsed arc, vibratory arc, powder deposition. The most widespread method is arc deposition.
Deposition materials exist in various forms. These can be filler rods, powder mixtures, coated electrodes, and flux-cored and solid wire. Mainly coated electrodes, filler rods and wires are used in electric arc deposition.
Parts made from low carbon and low alloy steels are usually subjected to deposition without preheating. But preheating and subsequent heat treatment is often required to relieve internal stresses. More detailed requirements for deposition can be found in the documents for deposition electrodes in use. For example, the following process features are listed for the OZI-3 electrode: Deposition is performed in one to four layers with preheating to a temperature of 300-600°C. Slow cooling shall be performed after it. It is possible to deposit using the tub method at elevated modes. Annealing before deposition: 350°C, 1 h.
The surface of the part shall be cleaned of oil, rust and other contaminants before deposition.
Various schemes for the location of deposition welds are applied. In case of flat surfaces, there are two main types of deposition - the use of narrow beads that overlap each other by 0.3-0.4 of their width, and the use of wide beads, obtained by increased transverse movements of the electrode in relation to direction of the pass.
Another way would be the laying of narrow beads at some distance from each other. In this case, the slag is removed after applying several beads. After that, the beads are built up in the gaps too.
In order to avoid distortion of parts, deposition shall be carried out in separate sections, "randomly", and the laying of each subsequent bead shall be started from the opposite side with respect to the previous one.
Restoration by deposition of a cylindrical surface can be carried out in three ways - by applying the beads along the generatrix of the cylinder, around closed circumference and along a helix. The latter option (along a helix) is particularly convenient in case of machine deposition, in which parts during the deposition process are evenly rotated.
To restore and increase the service life of the cutting, stamping and measuring tools, as well as parts of mechanisms operating under intensive wear, hard-alloy deposition on working surfaces is used. Hard alloys can be the compounds of metals such as titanium, tungsten, tantalum, manganese, chromium and others with boron, carbon, cobalt, iron, nickel, etc.
When manufacturing new tools and parts with hard-alloy deposition, parts made of carbon or alloy steels are used as workpieces (bases). In case of repair of parts with high wear, preliminary depositing using mild steel electrodes is usually carried out prior to hard-alloy depositing.
In many cases it is advisable to heat the workpieces to a temperature of 300°C and higher to obtain higher quality deposition, prevent the formation of cracks and reduce stress.
Deposition on parts, working to abrasion without impact loads. If it is necessary to get the deposited metal of very high hardness, one can use T-590 and T-620 electrodes. They are specifically designed to cover parts that work to intense abrasion. Their core is made of mild steel, but the coatings include ferrochrome, ferrotitanium, ferroboron, boron carbide and graphite. Thanks to these materials, the hardness of the deposited metal can reach 62-64 HRC units.
Due to the fact that the deposited metal is brittle and prone to cracking, the products subjected to deposition with T-590 and T-620 electrodes, are not designed for operation under conditions of significant impact loads. Hard-alloy metal is deposited in one or two layers. If it is necessary to deposit a large thickness, the lower layers shall be deposited with mild steel electrodes and only the final ones - with hard-alloy electrodes.
Deposition on parts working to abrasion with impact loads. Parts made of manganese steels (110G13L and similar) working under conditions of intense surface wear and high impact loads (in particular, working bodies of construction and earth-moving equipment) shall be subjected to deposition with OMG-N, TsNIIN-4, OZN-7M, OZN-400M, OZN-300M electrodes and electrodes of other grades. When using them, the hardness of the deposited metal in the second layer is 45-65 HRC with high toughness values.
Deposition on stainless steels. TsN-6L, TsN-12M-67 electrodes and electrodes of other grades are used for deposition on parts made of stainless steels. The core of these electrodes is made of high-alloy stainless wire. In addition to high corrosion resistance, the deposited metal also has resistance to tearing, which makes it possible to use these electrodes for deposition of sealing surfaces in reinforcement products.
When using some electrodes for deposition on stainless steels, it is recommended to carry out preliminary and concurrent heating of the part to a temperature of 300-600°C and carry out heat treatment after deposition.
Deposition of copper and its alloys. Deposition of copper and its alloys (bronze) can be carried out not only on a copper or bronze base, but also on steel and cast iron base. In this case, bimetallic products are created that have the necessary performance qualities (high resistance to corrosion, low friction coefficient and other valuable properties inherent in copper and its alloys) and have a much lower cost in comparison with parts made entirely of copper or its alloys.
Aluminum bronzes, in particular, possessing high anti-friction properties, work very well in friction joints, thus, they are deposited on worm wheels, sockets and other parts operating under friction conditions.
Deposition on parts made of technically pure copper can be performed using Komsomolets-100 electrodes or filler rods made of copper or its alloys. When depositing copper on copper, preheating shall be carried out to a temperature of 300-500°C. The deposited layer shall be preferably forged at a copper temperature above 500°C.
If bronze depositing is required, OZB-2M electrodes can be used, which contain, in addition to the copper (base) component, tin, manganese, nickel and iron. Products subjected to deposition using the OZB-2M electrodes have high surface wear resistance.
Deposition on copper and its alloys is performed using direct current of reverse polarity in the lower position.
Metal depositing with narrow beads
Metal depositing on barrel surface
Screw after depositing
Processed screw after depositing
Deposition on parts (hammers) of the mill
Collar of the shift sleeve