Figure 1. The common austenitic stainless steel grades.

The commercial welding processes are all suitable for the welding of austenitic stainless steel. The austenitic stainless steels are metallurgically simple alloy as their microstructure is either 100% austenite or predominantly austenite with a small amount of ferrite. The austenitic stainless steels do not undergo phase transformation during the heating and cooling cycles during welding as is experienced by carbon and low alloy steels. Preheating is thus not required to slow the cooling rate in the heat affected zones to prevent the formation of hard brittle microstructures, such as martensite, which promote cracking. Austenitic stainless steels are prone to weld metal cracking during solidification of the weld pool – hot or solidification cracking

Hot cracking

All the austenitic stainless steels are susceptible to hot cracking. The fully austenitic types such as 310 are more sensitive to hot cracking. The impurities such as sulfur and phosphorus are low melting point elements which will congregate on the grain boundary areas. The higher the amount of impurities the greater the risk of solidification (hot) cracking when welding.

To prevent hot cracking some ferrite is required. The ferrite can dissolve more of the sulfur and phosphorus than the austenite so these elements are retained in solution and not available for the liquid films along the grain boundaries. The amount of available grain boundary are increases significantly with small amounts of ferrite present in the austenite matrix so that the liquid films have to spread over larger surface area preventing 100 % coverage of the grain boundaries.

So how much ferrite is enough. Experience and the various constitutional diagrams indicate that if the ferrite number (FN) is 4 or greater then there is no risk of hot cracking.

The consumable manufactures all formulate the  chemistry of the welding consumables to have a small amount of ferrite (FN 5 to 10) in the weld metal to prevent hot cracking, except for those cases where almost no ferrite is required to give adequate toughness at cryogenic temperatures.

However care during welding must be taken not to destroy the ferrite in the weld metal. The ferrite content in the weld can be destroyed by additions of carbon  and nitrogen, loss of chromium and severe dilution.

When using ‘stick’ electrodes care must be taken to use a short arc length. Longer arc lengths can pull nitrogen from the atmosphere into the arc, resulting in the nitrogen becoming an alloying element in the weld. Nitrogen is an austenite promoter and hence the amount of ferrite will decrease to almost zero.

Similarly using gas shielded arc welding processes (GMAW,FCAW and GTAW) care must be taken that the gas shield is not disturbed which can result in nitrogen entering the arc, ending up as an alloying element and reducing the ferrite content to almost zero.

Adding CO₂ to the shielding gas can result in some carbon pickup from the shielding gas. Increasing the weld metal carbon content reduces the ferrite content which can result in hot cracking of the weld.

When welding with submerged arc welding, the choice of flux has an influence on the amount of Chromium that will be retained in the weld. High silica fluxes can lose up to 5% of the chromium content of the wire. Some fluxes will add carbon which will also reduce the ferrite content to near zero.

Excessive dilution normally becomes an issue when a dissimilar weld joint is made between a C- Mn steel and the austenitic stainless steel. Once again submerged arc welding is prone to having high dilution which can result in almost no ferrite in the weld metal resulting in solidification cracking.

Contamination

One of the most common mistakes made during fabrication is not to ensure that the stainless steel is fabricated and welded in clean conditions.

Contact with carbon steel must be avoided during handling, transportation, storage and fabrication.

Tools that have previously been used on carbon steel cannot be used on stainless steel. The trace amounts of carbon steel will contaminate the stainless steel and ultimately rust.

All material handling equipment and storage space must provide protection against contamination by iron or C-Mn steel.

Shot blasting if done must be done with shot that has not previously been used for the cleaning of C-Mn steel or cast iron alloys.

Grinding carbon steel in the vicinity of the stainless steel fabrication will lead to the fine carbon steel dust contaminating the surface of the stainless steel and eventually rusting.

Corrosion due to welding

Welding can produce metallurgical changes in the heat affected zone which can affect the corrosion behavior of the alloy. Weld decay or sensitization can occur in the HAZ of the austenitic stainless steels if incorrect welding parameters were followed.

The precipitation of chrome carbides in the HAZ where the temperature was between 600 and 850 °C can lead to selective corrosion adjacent to these carbide precipitates.

The chrome carbides preferentially precipitate on the grain boundaries. Next to the carbides the region is depleted of chromium. This precipitates requires short range diffusion of the large Cr atoms causing a zone depleted in chromium. The lower chromium region has reduced corrosion properties, leading to this area being preferentially attacked when in contact with a corrosive medium.