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August 12, 2015
Repair of body parts
The body parts, in addition to ICE cylinder blocks, include the housings of speed gearbox bodies, reducing gearbox bodies, cranes, arms of construction machines, etc.
The body parts, in addition to ICE cylinder blocks, include the housings of speed gearbox bodies, reducing gearbox bodies, cranes, arms of construction machines, etc. They are made of cast iron, steel, aluminum. Main defects of body parts are the wear of the holes for bearings, cracks in the walls, puncture holes. The last two defects are eliminated by welding operations and are not difficult to repair.
The wear of the bearing holes in the body results in violation of the axial distance between the shafts in the mechanism, which provokes the wear of other mated parts, for example, gear teeth. Therefore, when repairing the body parts, special attention is paid to determining the initial location of the axis of the bearing hole. This work is performed on vertical or horizontal boring machines. After hole boring, the repair liner is usually made and installed it into the bore hole, fixed, if necessary, bored again to provide the necessary fit for the bearing.
Diesel engine cylinder blocks are made of gray cast iron. Welding heating and subsequent cooling changes the structure and properties of cast iron in the fusion zone and the heat-affected zone so that it is very difficult to obtain weld joints without defects with the required level of properties.
A well-made weld joint shall have the necessary level of mechanical properties, density (impenetrability), and satisfactory machinability.
The reasons that make it difficult to obtain high-quality welds on cast iron parts are as follows:
- high cooling rates of the weld metal and heat-affected zone, corresponding to the thermal cycle of welding, lead to chilling of cast iron, i.e. the appearance of areas with the release of cementite of one form or another in different quantities. Their high hardness practically makes it impossible to machine a part with a cutting tool;
- due to local uneven heating of the metal, welding stresses occur, which, due to the very low ductility of the cast iron, lead to the formation of cracks in the weld and heat-affected zone. The presence of chilled areas that have a higher density than gray cast iron creates additional structural stresses that contribute to cracking;
- intense gassing from the weld pool, which continues at the stage of crystallization, can lead to the formation of pores in the weld metal;
- increased fluidity of cast iron makes it difficult to stop the molten metal from dripping out and the formation of the weld;
- the presence of silicon and other elements in the metal of the weld pool contributes to the formation of refractory oxides on its surface, leading to lack of penetration.
Cold welding of cast iron with electrodes does not provide positive results, since at high cooling rates a structure of white cast iron in the weld, as well as high-temperature region of the heat-affected zone, are formed, and also the hardening of the metal base of the HAZ sections occurs. Resulting deformations exceed the deformation ability of the weld metal and heat-affected zone, leading to the formation of cracks.
Developed technology of semi-automatic cold welding ensures the tensile strength of the weld of 420-480 MPa with a hardness of 150-170HB. At the same time the absence of cast iron chilling in the heat-affected zone is guaranteed.
It is possible to obtain high-quality weld joints only on perlite and perlite-ferrite cast iron with a ferrite content of not more than 10-15%. Therefore, before starting work, a metallographic study of the material of the welded part in the crack area is carried out.
The cylinder head (cover) is the most complex in design and one of the most thermally loaded engine parts. The heat flow discharged into it exceeds the flow discharged into the piston, and complexity of the design leads to large unevenness of thermal loads on its individual elements and, as a consequence, to high temperatures and, especially, temperature stresses. The latter is the main reason why the failure of cylinder heads (covers) is related to chronic defects inherent in many engine models. At the same time, in most cases, the cause of failure is the appearance of through cracks in the combustion pot cover.
During operation of the cylinder head (cover), the seating surfaces under the intake and exhaust valves wear out. The generally accepted method of restoring such heads consists in boring of holes and mounting of repair seats. Its disadvantages are as follows:
- weakening of the combustion pot cover in the place of boring;
- possible formation of cracks and falling out of the seat in the process of diesel engine operation;
- the appearance of a barrier, which is the gap between the bore in the head and the seat, for heat removal to the cooling system from the head and the valve itself. This leads to an increase in their temperatures and adversely affects the reliability of operation.
Diesel engine cylinder heads are made of low alloy gray cast iron with perlite structure. Poor process weldability of this material can lead to the following defects:
- chilling of cast iron, i.e. the appearance of areas with the release of cementite in one form or another. High hardness of chilled areas practically makes it impossible to process cast iron with a cutting tool;
- formation of cracks in the weld and heat-affected zone due to local uneven heating;
- formation of pores due to gas release from the weld pool, which continues at the stage of crystallization;
- lack of penetration due to the formation of refractory oxides on the surface.
Cylinder heads with full wear of the seal band in the holes for valves can be restored only if they do not have through cracks on the combustion pot cover.
Technological process of restoration of the cylinder head includes:
- preliminary machining of holes;
- deposition of the buffer layer;
- machining for subsequent seat deposition;
- seat deposition with austenitic heat-resistant steel (mechanical properties of the weld metal shall be not less than: ultimate tensile strength - 540 MPa; relative elongation - 30%; impact toughness - 98 J/cm2; hardness after deposition - 240 HV). In the process, the surface of the seat shall be work-hardened to a hardness of 450 HV;
- The final boring of the seat shall be in accordance with the drawing.
Welding of cracks in the coupling
Restoration of the seating surface in the bearing body
Deposition of the buffer layer on the sealing bevel
Головка (крышка) цилиндра после восстановления