The principle of oxygen acetylene flame spray welding with metal powder oxyacetylene flame as the heat source, the self fluxing alloy powder spray after surface pretreatment, then higher than the melting point of coating but below the melting temperature of base metal parts will melt to form a coating, and the surface of base metal is brazed joint with surface cladding characteristics.
Spray welding includes two processes: one is the spraying process; the two is the remelting process. The purpose of the remelting process is to obtain a cladding layer with high strength, no pores, no oxides, and the surface of the workpiece. To achieve this goal, we must have the following two conditions:
(1) the melting of the coating to the surface wetting the workpiece, which must be in a solid liquid alloy on the surface of the work piece is not free flowing, spreading, and cemented the ball;
(2) after wetting liquid alloy workpieces, must be able to appropriate physical and chemical effects and the surface of the workpiece, when the molten alloy is cooled, firmly combined with the workpiece surface to form, rather than the adhesion on the surface of the work piece.
The wettability of the molten coating on the surface of the workpiece is mainly determined by the following factors:
(1) coated alloy and metal components of the workpiece. When the coating and alloy workpiece metal in liquid and solid are not interacting with each other, the wettability between them is very poor; if the alloy coating and metal workpiece to mutual dissolution and diffusion coating alloy melt can better wetting the surface of workpiece.
When the substrate surface is easy to generate passive film, it is difficult to wet the surface of the molten coating. The composition and properties of passive films are determined by the substrate composition. For example, the nickel based alloy spray welding, it is difficult to wet the chrome layer and carburizing layer; in heat resistant steel on wettability, good wettability in the structure of ordinary carbon steel and alloy steel, in the iron on the wettability is very good. If the workpiece wettability is not good, can try spraying a thin layer on the surface of the molten iron to spray, can greatly improve the wettability, improve the quality of spray welding layer.
(2) cleaning degree of workpiece surface. The surface is clean, the better wettability. If there is a workpiece surface oil, dirt and oxide film, will seriously affect the wetting, must be removed before injecting in. As for the process of preheating and spraying, the thin layer oxide formed on the surface can be reduced by the boron and silicon in the alloy powder. If the oxide surface too much, or alloy powder containing oxygen is too high, as the reducing agent of silicon boron, insufficient quantity, deoxidation is not sufficient, has a negative impact on the performance of a good combination between the coating and the workpiece and the spray welding layer. Therefore, it is necessary to control the oxygen content in the alloy powder, the process should try to reduce the surface of the workpiece oxidation.
(3) surface roughness. The surface roughness of the working surface is better than that of the surface. This is due to the rough surface of the groove plays arranged in a crisscross pattern of capillary action special, is conducive to the molten alloy flow along the surface of the workpiece, improve the wettability.
(4) remelting temperature. The remelting temperature, the surface tension of molten alloy coating is reduced, resulting in liquid alloy coating is easy to spread, so as to improve the wettability of the alloy melt coating on the surface of workpiece. But the remelting temperature can not be too high, too high will lead to excessive flow of coating alloy, resulting in flow, making the spray welding layer is not smooth, uneven. The temperature is too high will make the melting of the coating on the surface of the alloy melt, change the performance of the spray layer. Therefore, the remelting temperature should be controlled properly.
During the operation of the oxygen acetylene flame powder spray welding, the control of the heating temperature is usually obtained from the "specular reflection" of the spray welding layer. This feeling in the spray welding of nickel based self fluxing alloy is obviously self fluxing alloy is not obvious, so the operator to control the reheating temperature is not very accurate.
It is important to know the behavior characteristics of the spray welding layer during the remelting process:
1) because of self fluxing alloy melting point is lower than the melting point of steel (parts), so when the remelting of spray welding layers are heated to a molten state and the wetting and spreading in the surface of base metal. Melting alloy powder and silicon excellent reduction of oxide melting alloy and base metal surface and its effect on the formation of boron silicate slag, low melting point, and the surface to form a protective film, a layer of good.
2) between the melting spray welding alloy layer and base metal surface at high temperature, very conducive to the self fluxing alloy layer elements to the base metal diffusion, base metal elements in the spread of remelting layer. The mutual diffusion of elements ensures the bonding between the cladding layer and the base metal matrix.
3) when the temperature exceeds the melting temperature of the remelting base metal surface, the melting layer self fluxing alloy and the base metal surface near the base metal surface cladding layer of alloy composed of molten pool, condensation will become a part of the cladding layer. At this time, the bonding between the cladding layer and the base metal is metallurgical combination or micro metallurgical bonding.
In the actual self fluxing alloy oxygen acetylene flame spray welding, bonding between the cladding layer and the substrate is mainly diffusion brazing with the combination of characteristics, sometimes also micro metallurgical bonding. The choice of the two forms is mainly determined by the control of the remelting temperature.
