Soldering stainless steels - from http://www.bssa.org.uk/topics.php?article=150

Introduction

Soldering is often used for joining stainless steel in applications such as roofing and water tubing (plumbing). Soldering, like brazing, uses filler alloys with lower melting points than the parent metal (i.e. the stainless steel) being joined. Temperatures are typically below 450 C for soldering stainless steels. The joint strength is generally lower than either brazed or welded joints.

While the presence of a tenacious oxide film makes stainless steels more difficult to solder than carbon steels, stainless steels can be successfully joined by soldering, if the correct techniques are employed.

Surface preparation

Thorough cleaning of the steel surface in the joint area is the key to successful soldering. Oil and grease may be removed with solvents, and wire brushing (with stainless steel brushes) or abrading with emery cloth can be used to eliminate other forms of contamination, leaving a bright, metallic surface. A slightly rough finish from the abrasive is acceptable and will improve adherence of the solder. It is preferable to carry out the soldering operation immediately after cleaning. If this is not possible, the parts may be 'tinned' (pre-coated) with solder to protect them.

Fluxes

Aggressive fluxes are required to remove the oxide from the prepared surface.

Phosphoric acid-based fluxes are generally suitable and have the advantage of being innocuous if any residues remain after the joining process has been completed. Hydrochloric acid is sometimes added to the phosphoric acid based flux where molybdenum containing stainless steels such as 1.4401- 316 are being soldered. Hydrochloric acid based fluxes should however be avoided as corrosion can result to poorly cleaned joints, during service, after soldering.

Although rosin-type fluxes are ineffective, they may be employed when the surfaces have been pre-coated using the appropriate flux. This procedure allows the potentially corrosive flux to be removed completely before the joint is made and is particularly helpful if components contain recessed or blind areas.

Heating

All conventional methods of heating can be used for soldering stainless steels. However, the thermal conductivity of austenitic stainless steels is relatively poor and their coefficient of expansion is high, so that care must be taken to ensure that the whole joint area is brought uniformly to soldering temperature without overheating and distortion is a possibility in long joints. The latter may require jigging.

Solders

Conventional tin-lead solders may be used to solder stainless steels. It is recommended that the tin content should be more than 50% to give good bond strength and minimise the risk of galvanic corrosion to the soldered joint during service. High-tin solders also match the colour of stainless steels well and do not darken significantly in service.

A range of tin-silver solders is recommended for stainless steels, since they offer greater strength than the tin-lead solders, coupled with high ductility. Their corrosion resistance is also superior.

The appropriate solder should be selected for the expected service conditions. Factors to be considered include mechanical loading, potential corrosion, colour match, contact with foodstuffs or water etc.

Post-soldering treatment

Removal of all traces of flux residues immediately after soldering is essential, both to avoid discolouration and to prevent corrosion ensuing in service. Hot water is suitable, possibly with addition of a mild detergent. Flowing water is preferable and a non-metallic fibre brush can be used to speed the process.