by John Du Plessis of Spesmet Technologies.
In order for the alloy to be considered to be stainless steel it must contain a minimum of 10.5 % chromium. The chromium combines with oxygen to form a passive chrome oxide layer which prevents corrosion and oxidation of the underlying material. This oxide layer is self-healing in the presence of oxygen. If it is damaged by mechanical or other means it will reform if there is oxygen present, to restore the passive chrome oxide layer.
Stainless steel is not a single alloy but rather a large range of different alloys which can be grouped in five main groups. The five groups are Austenitic stainless steels, Ferritic stainless steels, Martensitic stainless steel, Duplex stainless steels and the Precipitation Hardening stainless steels.
Most people are not aware of the wide range of alloys and when they speak about stainless steel, they are addressing the austenitic stainless steels, typically the ‘300’ series of alloys.
Martensitic stainless steel is essentially alloys of chromium, carbon and iron. The chromium content normally ranges between 12 and 18 percent. The microstructure is matensitic. These alloys are ferro-magnetic and can be hardened by heat treatment.
Ferritic stainless steels are mainly alloyed with chromium and carbon. The carbon levels are much lower than compared with the Martensitic stainless steels and the chromium content( 11 to 28%) can be significantly higher.
Austenitic stainless steels in addition to chromium contain austenitizing elements such as nickel, manganese and nitrogen. Typically the steel composition will range from 16 to 16 % Chromium and 8 to 22 % Nickel. These steel are essentially non-magnetic and can only be hardened through work hardening.
Duplex stainless steels have a mixture of ferrite and austenite in the microstructure. The duplex stainless steels have higher strengths as well as better pitting and stress corrosion properties than the austenitic stainless steels
Precipitation hardening stainless steels are predominantly alloyed with chromium, nickel and precipitate forming elements such as aluminium, titanium, niobium and copper.
The weldability of each of the five groups is determined by the metallurgical features of each group. Each of the five alloy groups behave differently during the weld thermal cycle, due to their unique metallurgy. Hence there are a different set of rules for each alloy group to weld them successfully.
Due to the brevity of the article we cannot have an in-depth discussion on each of the alloy groups. The focus would be on the most widely used alloy group – the austenitic stainless steels.
Austenitic Stainless steel
The austenitic stainless steels consist of the 200 and 300 types. The 300 series are by far the most widely used of the austenitic grades.
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
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.
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.
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.
Figure 2. Chrome carbide precipitation leading to sensitization
The use of the low carbon grades or the stabilized grades (alloyed with Ti or Nb) reduces the risk of sensitization.
The use of low heat input during welding, which results in faster cooling rates through the susceptible temperature zone where the carbides form, also minimize the risk of sensitization.
Austenitic stainless steels have much lower coefficient of thermal conductivity and a higher coefficient of thermal expansion than ferritic steel. This result in a narrower heat affected zone with higher expansion when welded. The end result is higher residual stress and more distortion in the austenitic stainless steels compared to ferritic steels.
Higher heat input result in larger distortion. Thinner material is more sensitive to distortion than thicker material.
Some general rules for controlling distortion are:
The correct welding consumable grade must be used for each of the different austenitic stainless steel grades. There are matching consumables for most of the grades, the exceptions being grade 304 which is normally welded with a 308 type consumable and grade 321 which is normally welded with a 347 type consumable.
Consumables are either a straight consumable or carry a suffix. The suffix can be –L, H or N. The L suffix means it is a low carbon alloy. The N indicates it is nitrogen bearing and the H indicates higher carbon.
For shielded metal arc welding there are different types of coating. The three types are “15”, “16” and “17”.
A “15” electrode has a lime based coating and is intended for DC+ polarity only. The slag covering is not as thick as that found on the “16” and “17” type coatings.
The weld bead is normally convex in a horizontal fillet weld with excellent crack resistance. The “15” electrodes gives the best all positional weldability. However, the arc is harsher than the other types.
The “16” electrode has a rutile based coating and can be used with both DC and AC polarity. The weld bead in a horizontal fillet is almost flat. The arc is much softer than the “15” type electrodes with good all positional welding.
The “17” electrode has a silica-rutile type coating. They can be used with both DC and AC polarity. The additional silicon in the coating acts as a wetting agent, having the effect of increasing puddle fluidity. The “17” type electrodes produce a concave weld bead in a horizontal fillet weld, and is often used for flat and hand a smooth horizontal position welding. These electrodes have limited vertical welding capability. The arc is smooth and relatively soft when welding.
Figure 3. Fillet weld bead shape differences using different coating type electrodes.
Welding consumables from different manufacturers, although having the same classification, are by no means equal. There are significant differences in the consistency from batch to batch, chemistry, ferrite number and impurity levels.
The weldability of flux based consumables such as flux cored arc welding and shielded metal arc welding electrodes are greatly influenced by the flux composition and ratio of the various ingredients in the flux.
Spend time to familiarize yourself with the consumable, read the manufacturers literature etc. in order to make the best choice before purchasing the consumable.
Welding austenitic stainless steel successfully is no more difficult than welding C-Mn steel (mild steel) successfully, if one follows the rules which are well established.
Some of the pitfalls have been illustrated in this article and hopefully in future will be avoided