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Others Joining Techniques for Duplex Stainless Steels

The advantages of joining techniques other then welding (where the base material is melted to produce a joint) include minimum warpage and low residual stresses. The joints can be leak-tight and quite strong. However, the bond never comes close in its properties to a welded bond where the weld metal corrosion resistance and the strength are as high, or nearly as high, as in the base material. This is an important consideration for the duplex stainless steels, which are superior to the 300-series austenitic stainless steels in strength as well as corrosion resistance.

Joint Preparation

For all joining operations, it is very important to thoroughly clean the stainless steel before joining the parts. The surfaces should be free of oil, grease, dirt, dust or fingerprints. A solvent should be used to remove those surface contaminants. Oil or grease can prevent the flux from removing the oxide layer in soldering and brazing. Loose surface contaminants reduce the effective joint surface area. Often a slightly rough surface produces better joints than smooth surfaces. Sometimes roughening with a fine abrasive can help to increase the weldability of a surface, which is critical for a good bond.

Adhesives

A wide variety of commercial adhesives for the joining of metal surfaces is available. Duplex stainless steels are treated in the same way as any other metal for the purpose of joining with adhesives. The adhesives manufacturers can assist in the selection of the proper adhesive for a specific joint strength, service temperature, and service environment.

Soldering

Soldering is distinguished from brazing by the melting temperature of the filler material. The soldering temperature is usually below 450°C (840°F). In general, soldered joints are not as strong and their service temperature is lower than for brazed joints. Typical soldering filler materials include tin-lead, tin-antimony, tin-silver, and tin-lead-bismuth alloys. These low-melting filler materials produce joints of different strength and corrosion resistance and with varying color match.

To produce a good solder joint, the surface oxide layer of stainless steel must be removed with a flux before the soldering takes place. The high stability of the protective oxide layer in stainless steels and, especially molybdenum-alloyed duplex stainless steels, can make proper fluxing very difficult. Typical acid-type fluxes can contain chlorides. If chloride-containing fluxes are used, they must be cleaned with water washing and/or a neutralizer, immediately after the soldering. Failure to completely remove the flux is likely to produce pitting corrosion, possibly even before the equipment is placed in service.

Brazing

Brazing filler material has a melting point above 450°C (840°F). The two most important types of brazing filler metals are silver brazing alloys and nickel brazing alloys. The silver brazing alloys are lower-melting, between 618 to 705°C (1145 and 1300°F), and the nickel brazing alloys are higher-melting, up to 1175°C (2150°F). The nickel brazed joints can withstand a higher service temperature than the silver brazed joints.

The temperature range between 705 and 980°C (1300 and 1800°F) should be avoided with duplex stainless steels. It is, therefore, important to braze at a temperature above 1040°C (1900°F) or below 705°C (1300°F). Nickel brazed joints can be water quenched from the brazing temperature. The proper brazing material should be chosen according to required corrosion resistance, service temperature and joint strength. Nickel brazing materials contain up to 25% chromium which makes them somewhat corrosion resistant, although not quite as resistant as the duplex stainless steel, 2205.

It has been reported that nitrogen-containing stainless steels are difficult to braze. This could affect the second-generation duplex stainless steels that contain increased levels of nitrogen. Few data are available on the brazing of duplex stainless steels, so the fabricator should experiment to find the ideal brazing parameters.

As with soldering, the oxide layer must be removed prior to and during the brazing operation to create a sound brazed joint. Again, this is accomplished with a flux that must be removed after the brazing. The procedure is similar to the clean-up after soldering and includes scrubbing with hot water or a neutralizing chemical.

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