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Welding Dissimilar Metals Guide

In Uncategorized on December 15, 2009 at 7:36 pm

Many welding methods metals are available to sear joints and ridges together, but not all metals are made equal. As a result, some welding methods are better suited for different kinds of metals. This situation is made slightly more difficult when a job calls for welding two different kinds of metals together. Various options to combine these metals exist, but, again, different methods are better suited for different metals. There are also situations where welds can benefit from transition welding.

Deciding what kind of weld is necessary for a particular job is the first step in any welding process, and can depend on a wide variety of factors. Additional preparation requires purchasing transition metals (if necessary) and weld materials and equipment.

Materials and Metals

Often, a decision to use dissimilar metals in a weld is one of financial reasons. Certain kinds of steels can be priced up or down depending on the current raw materials market as well as the relative purity—and therefore strength—of the steel. In order to achieve financial savings without jeopardizing structural integrity, welders will often employ lower grades of steel in joints in a staggered fashion, welding stronger, higher grade types of steel with lower grade steels as spacers of transition pieces. Additionally, not all parts of a structure are susceptible to the same kinds of corrosive elements or pressure requirements. As a result, many welders use less expensive metals where these considerations are not required, and weld these metals to more durable pieces, like stainless steel, where needed.

However, depending on the weld, this staggered welding can be a difficult process. Different grades of steel and different kinds of metals have varying melting points and molten metals do not always join easily. To join two dissimilar metals, they must have mutual solubility. This means they melt at similar temperatures into molten forms that are relatively equal. Additionally, the welded forms of these metals must have intermetallic chemistries that do not easily corrode, crack or wear.

Nickel for Metallurgical Transition

The problem with welding seemingly similar metals is that the potential for corrosion and wear is often evident only at the microstructural level. For instance, welding stainless steel with a low grade steel might seem like a decent match, but there are chemical considerations that aren’t so apparent. The carbon in the steel might react poorly if the weld is performed at two high a temperature, which could result in a metal known as chromium carbide. While chromium carbide is very hard, it has low ductility, which makes the welding process extremely difficult. Many such dissimilar welds can pose these kinds of unforeseen consequences.

The solution is the introduction of transitional metals. This means the welder interposes a third metal into the weld that will counteract the negative effects of welding dissimilar metals based on its own reaction to the weld. Research and trial and error have resulted in nickel being the metal of choice for transition needs. When welded, nickel dilutes the carbon, chromium, and other chemicals that form tough, hard and inductile metals, making the metal that much easier to bend and shape.

Cladding

Another solution for welding dissimilar metals is by using clads. Clads are pieces of dissimilar metals that have been bonded, usually through extruding, pressing or rolling. The resulting metal form consists of an interior of one type of metal with a coating of another type.

When using clads, the metal on the outside surface of the piece is usually the metal that is intended to make the welding process more appropriate for welding. The interior metal is intended for longer lasting situations, the life of the piece; for instance, a stainless steel piece clad with some other type of alloy, perhaps nickel. The weld is performed on the clad side, which then can be removed through a carbon gouging process, resulting in a weld of stainless steel and another type of metal.

High Temperatures to Prevent Problems

One final method for establishing dissimilar welds is the use of high temperatures. With a transition or filler piece of nickel, using high temperatures can make an austenitic weld. An austenitic weld means the iron of the steel undergoes a phase transition to an austenite form, which results in a face-centered cubic configuration of the iron. This cubic configuration provides extra ductility and flexibility for the weld, which is much easier for a welder to shape and form, but when it sets it will be incredibly strong.

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