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Pipe Mills Stainless & Non Ferrous Alloys

Pipe Mills Stainless & Non Ferrous Alloys

Timothy Holding Co.,Ltd.

Most of what has been covered applies to all induction welded tube & pipe, but there are a few special factors to consider when welding Stainless Steels & certain other non ferrous metals.

The “stainless” properties of corrosion resistant steels are due to the fact that the surface is already oxidized! Chromium is an extremely reactive metal that readily oxidizes in the presence of atmospheric oxygen. Chromium oxide is a hard, transparent refractory material which is impervious to most corrosive elements. Unlike iron oxides, which melt at a lower temperature than the base metal, chromium oxide remains a solid, and must be forced out of the weld interface if defects are to be prevented.

Although most chromium oxide is wiped off the metal surface by the fins, it immediately reforms at the elevated temperatures of the weld area. Although atmospheric oxygen is a factor, any water in the weld area is a far greater problem. At weld temperatures, water doesn’t just vaporize it dissociates into its components, oxygen & hydrogen. The monatomic oxygen (O, not O2) is far more reactive than ordinary oxygen, so chromium oxide reforms immediately on the faying edges of the tube.

It is essential to exclude water & preferably oxygen as well from the weld area when induction welding 300 series (austenitic) stainless. 400 series has little or no chromium, so it is less of a problem. Use of water cooled return flow impeders is a requirement for welding stainless, and a low pressure inert gas atmosphere is sometimes used as well.

Because stainless steels are poor thermal conductors, there is a tendency for the corners of the strip to overheat, and for a layer of molten metal to form on the surface of the faying edges. Use of a lower frequency can help to reduce this, but the same effect can be achieved by moving the impeder back toward the entry end of the mill, or eliminating it altogether. More power is required, but the increase is less than with carbon steel because austenic stainless and other non ferrous materials have a much lower magnetic permeability than ferritic steels. This reduces the need for an impeder.

Aluminum, copper & their alloys also have less need for impeders, however their thermal conductivity is high, so short, closely coupled coils should be used to minimise the loss of weld heat by conduction. Aluminum also readily forms insoluble oxides which must be eliminated from the weld area. Remember that grinding wheels & sandpaper are made of aluminum oxide. This is definitely not a material that you want incuded in the weldment! Although water does not dissociate at the temperatures used to weld aluminum, it causes corrosion if left in the tube so return flow impeders are normally required.