Joining of metals

Welding is part of a larger category called metals joining. Within that category, there are three types of operations that can be performed: soldering, brazing, and welding.

Soldering, brazing and welding are operations that involve heat and the processes used in each of these depend on the metals being joined. All of them can be performed on cast iron, however there are some considerations that need to be mentioned.

Soldering

By nature, soldering is a low strength bond. Chances are there are no cast iron applications that will require soldering and it is not necessary to further explain. Soldering is used to join copper plumbing pipe and is used with electrical components. Dura- Bar is not commonly used in either industry. Brazing will be the most common method of joining dissimilar metals using low (below melting point) temperatures.

Brazing

Brazing, by definition, is the joining of metals through the fusion of non-ferrous alloys. The brazing rod, usually a bronze alloy, melts and becomes an interface between the two metals being joined. Brazing alloys have melting points above 800F but lower than the metals being joined.

For cast iron, brazing temperatures need to be below 2050F; the melting point. However, phase transformations in the microstructure of cast iron and steel will occur above 1200F so brazing temperatures should be in the 800 - 1000F range to avoid any softening of the substrate metals.

Brazing is a higher temperature and higher strength bond in comparison to soldering. The strength of the bond depends on: strength of the filler metal and strength of the bond between filler metal and cast iron part.

Major advantages of brazing ferrous alloys

1) There is no reduction in hardness or strength of the metals being joined because the process takes place below the critical temperature.

2) Dissimilar metals can be joined together; for example an alloyed gray iron part can be welded to a steel mounting plate on a road grater. The alloyed gray iron is very hard and wear resistant but would probably crack under the stresses during operation. The steel plate is not nearly as wear resistant as the alloyed iron. Ideally, the wear plate assembly needs to have the impact strength and flexibility of plate steel for mounting but the wear resistance of iron for grating. Brazing the steel plate onto the iron wear pad offers a perfect solution.

3) The surface area of bonding contact can be quite large. The 4-inch steel plate in the above example has a 4 in 2 contact area of bond. If the strength of the brazing rod used to bond the metals together was as low as 30,000psi, it would take nearly 480,000 pounds of force to separate the steel plate from the alloyed iron pad.

"Soldering, brazing and welding are operations that involve heat and the processes used in each of these depend on the metals being joined."

The bonding strength between cast iron and the bronze rod can be weakened because of the graphite flakes or nodules on the surface. The surfaces should be degraphitized before brazing, which is a very simple procedure. The most common method is with an oxygen flame causing the graphite to leave in the form of a gas. A salt bath is also effective for cleaning the surfaces.

Next, there should be a surface cleaning operation - all types of fluxes are used and the procedure for surface cleaning is very standard and will be similar to the cleaning for steel brazing.

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