RU2068025C1 - Method of pieces surface preparation - Google Patents

Abstract

FIELD: methods of pieces surface preparation. SUBSTANCE: before application of coating by gas thermal spraying pieces are subjected to electromechanical working by tools, that exercises reciprocating motion in plane, that is perpendicular to plane of worked surface, Working is exercised by tool with shape, that provides during contact working of 1/4-th of piece surface with periodical switching electrical current off. EFFECT: improved process. 3 cl, 2 dwg, 1 tbl

Description

 The invention relates to mechanical engineering, in particular to thermal methods of coating, for example, in the repair and restoration of parts made of hardened steel.

 A known method of surface preparation [1, 2] comprising shot peening of the surface.

 This method allows to provide the necessary roughness when machining parts with low hardness, but when machining hardened parts, the surface roughness necessary for high adhesion is not provided.

Closest to the proposed invention is a method of preparing the surface of the parts [3] mainly of hardened steel before spray coating, comprising creating a relief of alternating depressions and protrusions, followed by rolling the peaks, in which the relief is formed by machining the surface of the part with a tool with a profile corresponding to the profile of the recesses , and passing at the point of contact of the tool and the electric current part to local melting of the latter at a tool pressure of 50-80 kg cm ² and moving it along a helix.

 The disadvantages of this method is the relatively low adhesion due to the coincidence of the direction of the knurling turns with the direction of the tangential stresses that arise during operation of the coating under friction, tending to tear the sprayed coating from the substrate during operation, as well as a decrease in the fatigue strength of the workpiece due to formation on it surfaces of voltage concentrators connected by a helix.

 The invention is directed to improving adhesion without significantly reducing fatigue strength.

 The solution to this problem is achieved by the fact that the processing is carried out with a tool with a profile that ensures contact with 1/4 of the surface of the part, and the movement is performed reciprocating in a plane perpendicular to the surface being treated with periodic shutdown of the electric current. The electric current is turned off before the tool moves up. Electric current is disconnected after the tool contacts the surface.

 Comparative analysis with the prototype allows us to conclude that since the relief in the proposed method is formed by a tool with a profile that provides for short-term contact a lot of local reflows and metal displacements at the points of contact of the tool with the surface of the part that are not connected to each other, and not by extrusion by a tool moving relative to the part along the helix, the craters formed during processing will not be interconnected, which will not lead to a significant reduction in fatigue strength. The increase in the surface area of the base due to its processing with a tool that provides an increase in the lengths of the guide and forming the surface line (Fig. 1) allows to increase the adhesive strength of the applied coating.

 In FIG. 1 is a diagram of surface formation.

The surface of the part is not processed (Fig. 1a). The surface area to be processed is calculated by the formula:
S ₁ = 2πRh ₁ (1)
where S ₁ surface area (m ² );
R radius (m);
h _{1 the} length of the conditional forming surface (m).

Since 2πR = l ₁ , (2)
where l _{1 the} length of the conditional guide surface (m).

Expression (1) will take the form:
S ₁ l ₁ • h ₁ (3)
The surface of the part is processed by the method described in the prototype (Fig. 1B).

The surface area is calculated by the formula:
S ₂ l ₂ • h ₂ (4)
where l> _{2 is the} length of the conditional guide surface processed by the method described in the prototype (m);
h _{2 the} length of the conditional forming surface processed by the method described in the prototype (m).

Since l ₂ l ₁ , h ₂ > h ₁ ,
the surface area of the base processed by the method described in the prototype will be greater than the area of the untreated surface.

S ₂ > S ₁
The surface of the part is processed by the proposed method (Fig. 1B).

The area of the formed surface is calculated by the formula:
S ₃ l ₃ • h ₃ (5),
where S ₃ the surface area of the base processed by the proposed method (m ² );
l _{3 the} length of the conditional guide surface processed by the proposed method (m);
h _{3 the} length of the conditional forming surface processed by the proposed method (m).

Since l ₃ > l ₂ , h ₃ h ₂ ,
the surface area of the base processed by the proposed method is greater than the surface area of the untreated and processed method described in the prototype:
S ₃ > S ₂
Example. Plasma spraying of samples of steel 45 with a diameter of 50 mm was carried out. Samples are hardened on the HDTV installation at a depth of 1.5-3.0 mm to a hardness of HRC 56-62. Before spraying, the samples were divided into five groups. Their surfaces were subjected to various types of processing.

 To form the surface profile of the samples of the first group, a tool was used, which was a punch with a concave working surface. The length of the arc describing the working surface of the tool is 1/4 of the circumference of the workpiece. The radius of the arc describing the working surface of the tool is 1/2 the diameter of the surface of the workpiece. On the working surface of the tool, made of refractory alloy VN-4, tetrahedral teeth with a pitch of 2.5 mm are milled. The radius of curvature of the apex of the teeth is 0.8 mm.

The instrument, making a reciprocating motion in a plane perpendicular to the plane of the workpiece, in one revolution of the part was in contact with it 4 times with a pressure of 50-80 kgf / cm ² . A current of 500-550 A was applied to the part and tool at a voltage of 2-6 V. In order to prevent the formation of an electric arc when the tool was torn off the surface, before starting to move up, the electric current was turned off.

 The surface treatment of samples of the second group was carried out in the same way as from the details of the first group. But in contrast to the above method, the electric current was switched off immediately after the contact of the tool with the part and local melting of the surface. Further, the tool, continuing to move downward, deformed the cooling metal, thereby strengthening the surface.

 The surface treatment of samples of the third group was carried out by the method described in the prototype.

 The surfaces of the samples of the fourth group were shot blasted.

 Samples of the fifth group were not processed.

Gas-plasma spraying of parts of the first four groups was carried out with a PGUS-25 powder (GOST 21448-75), with a particle size of 5-100 microns and a coefficient of its use of 70-80%. The distance of the plasma torch from the part when spraying is within 50-100 mm, the plasma-forming flow rate gas (argon) 2-Z m ³ / h.

 Samples of the fifth group were not sprayed.

 The sprayed samples were ground to a diameter of 50.8 mm.

 Adhesive strength was determined by the tangential shear force of a segment measuring 10 x 5 mm.

For torsional fatigue strength, the samples were tested on a UM-4 apparatus. The test base is 1.2 × 10 ⁷ loading cycles.

 The test results are presented in the table and are shown in FIG. 2, which shows the adhesion strength of the coating and the fatigue strength of the samples.

 The results obtained allow us to conclude that the proposed method provides an increase in the adhesive strength of the coating without significantly reducing the fatigue strength of the part. This allows us to recommend the use of the invention in mechanical engineering, in particular when restoring worn parts. TTT1

Claims (3)

 1. The method of preparing the surface of the part before applying the thermal spray coating, comprising electromechanical surface treatment of the part with a tool at a pressure of 50 to 80 kgf / cm with passing electric current at the contact point before reflow and moving the tool, characterized in that the treatment is carried out with a tool with a profile that ensures at contact processing 1/4 of the surface of the part, and the movement is carried out reciprocating in a plane perpendicular to the surface to be treated periodically m turning off the electric current.  2. The method according to p. 1, characterized in that the electric current is turned off before the upward movement of the tool.  3. The method according to p. 1, characterized in that the electric current is turned off after the tool contacts the surface.

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