Braze welding of cast iron GeneralT

Braze welding of cast iron
General
The name braze welding comes from the relatively slow- flowing nature of the filler material, which gives the joining process much in common with ordinary gas welding. A condition for using braze welding on cast iron is that it must be possible to preheat an area on each side of the weld zone to 300-400 oC.in practice this limits braze welding to smaller parts and thinner dimensions. For larger components “ cold “ electric arc welding is recommended. In braze welding the cast iron is not melted and braze welding is thus a form of mechanical bonding, as opposed to gas wwelding, where the metal is melted and forms a chemical bonding with the filler metal. The use of braze welding on cast iron has the decided advantages of low heat and ductility, both of which reduce and/ or eliminate the two serious problem traditionally associated with welding cast iron.
These are
I. The tendency for high heat to form unwanted white cast iron, a form of martensitic cast iron that is very brittle and hard due to the presence of iron carbide. The low heat used in braze welding greatly reduces the possibility of this formation.
II. The problem of contraction during cooling. These are avoided partly due to the lower heat required, and partly because the brazing filler alloy has a great capacity to yield and accommodate any cooling stresses. By braze welding cast iron one is to a certain degree independent of the quality of the base material.
In braze welding the cast iron is heated to bonding temperature. This is the minimum temperature to which it must be heated to form a bound ( Surface alloy ) between the filler metal and the cast iron. The phenomenon, which then occurs, is called “ tinning” ( also called “ wetting out” or “ bonding”). Actually, it refers to the almost microscopically thin layer in which the alloys of both the cast iron and the molten filler metal intermix. While there is several processes for braze welding, the use of an oxy- acetylene torch to provide the necessary heat will be the method employed on board.
Preparation of the workpiece
Remove oil, paint and rust from the surface. Grind off casting skin to a width of 20mm on each side of the edges to be welded. If the damage is in form of a crack in the material, it is recommended to use the crack detector set to find the complete extent of the crack. Drill 3mm holes at each end of the crack, at a distance of about 3mm from the ends to orevebt the crack from opening further during brazing.
Groove preparation
Prepare a 70- 90o grove. Remember that braxe welding is a surface bonding and that the lager the surface of a joint the better the bonding. If the part has been broken into two or more pieces, bevel each side of the fracture to 45, use a file to remove surface graphite from the groove, and round off edges and sharp corners.