Method of pre-machining the surface of the substrate

       Substrate surface preprocessing is to use mechanical processing to clean the surface of the workpiece to remove various damages (such as fatigue layer and corrosion layer), original spray layer, quenching layer, nitriding layer, etc. on the surface of the workpiece to be repaired. Evenly wear the surface, make the thickness of the spray coating uniform, reserve the thickness of the spray coating, etc.



    The amount of pre-processing is determined by the thickness of the designed spray layer. For repairing old parts, it is generally cut to 0.1mm~0.25mm below the maximum wear amount of the part. For new parts, the pre-processing amount is related to the working conditions of the workpiece, such as the pre-processing amount of the sliding surface should be 0.15~0.18 or slightly larger.

    Cylindrical parts that require finishing of the spray coating are generally cut down to the depth of the minimum coating thickness by turning methods. The thickness of the spray coating is usually less than 0.5mm, and the thickness of the ceramic coating is usually 0.2mm~0.35mm.

    When pre-machining, special attention should be paid to corner transitions. This is because there is internal stress in the sprayed layer, and there is a tendency to peel off at the edge of the sprayed layer, especially at the sharp edge. There should be larger rounded corners or chamfers at the corners to prevent the peeling of the spray coating. There should be suitable transition chamfers or transition arcs at the joint parts of the workpiece where spraying and non-spraying and at the end of the shaft to reduce the stress concentration caused by the sharp corner effect of the coating. Usually, the transition chamfer between the sprayed and non-sprayed adjacent surfaces is 30 degrees to 45 degrees; the arc chamfer of the shaft end is generally r=1.0mm; the shaft end surface coating should be covered with the end arc as should.

    Usually the base surface pre-machining is done by turning or grinding. Turning can increase the bonding area between the coating and the substrate by about 30%, and can improve the shear resistance of the coating. After the surface has been turned or ground, the surface of the substrate must also be purified and treated by sandblasting or other roughening methods to further improve the bonding strength between the coating and the substrate.

    Undercutting is a substrate preprocessing method often used in thermal spraying technology. It is to properly remove the surface of the part by turning or grinding. On the one hand, it can remove the surface fatigue layer, and it also provides a space for thermal spraying. a method of operation. When a mechanical part needs to be repaired, the undercut method is usually used.

    Undercuts are also often used in order to obtain a uniform thickness of the finishing coating, or to remove work-hardened surface layers, chemical contamination layers, oxides, and thermally sprayed coatings previously left on the workpiece surface. Although the undercut will reduce the cross-sectional area of the workpiece, because the amount of undercut is small, it has little effect on the tensile strength and fatigue strength of the workpiece.

    At each undercut end of the cylindrical workpiece, a square or small obtuse angle (15 degrees) must be cut at the shoulder, and an acute wedge is not recommended. The radius of each undercut should be 0.30mm~0,50mm, and the undercut should not extend to the end of the shaft, but should leave a certain distance. The correct and incorrect under-shaft sections of the shaft end and thermal spray coating are shown in Figure 2-1.

    At the end of the undercut, leave shoulders wherever possible. Weld overlays are recommended for cylindrical surfaces where the coating is subject to pressure from the ends (eg pump plungers). Machine this weld into a 3.0 shoulder, which should be a bit larger in diameter than the final finish size of the shaft. In pressurized work, this surfacing layer is better than a thermal spray coating.

    It is worth noting that welding can affect the properties of the base metal, which must be paid attention to in repair and maintenance.