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Welding of Stainless Steel

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Welding of Stainless Steels

Stainless steels are normally divided into different groups related to their microstructure. The welding of stainless steel is related to the microstructure. When welding stainless steels it is adviaisable to follow the general welding guidelines valid for the type of steel, e.g. austenitic, ferritic.
Stainless steels have, due to their chemical compositions, a higher thermal elongation compared to mild steels. This may increase welddeformation. Dependent of weld metal microstructure they might also be more sensitive to hotcracking and sensitive to intermetallic precepitations compared to mild steels.
A general rule might be that stainless steels should be welded with somewhat lower heat-input compared to welding of mild steels.
Stainless steels can be welded with the most common used welding methodes. Welding with oxy-acetylen shall however be avoided due to carbon pick up. Further information can be seen in  the European standard document, EN 1011-3, or in our welding handbook.
A fundamental difference to weldments in stainless steels compared to welds in mild steels is the need for
cleaning after welding. Stainless steel weldments exposed to a wet corrosion exposure normally require removals of weldoxides, slag and other surface contamination.



Weld metal constitution diagrams for estimating weldability

Weld metal constitution diagrams

An aid in determining which structural constituents can occur in a weld metal are the Schaeffler-de-Long or the WRC diagram. With knowledge of the properties of different phases, it is possible to judge the extent to which they affect the service life of the weld. The diagrams can be used for rough estimates of the weldability of different steel grades as well as when welding dissimilar steels to each other. The WRC diagram is today accepted as an improved version of the shaeffler-de-long diagram.




Weldability of Austenitic Steels

  The steels of type ASTM 304, 316, 304L, and 316L have very good weldability. The old problem of intergranular corrosion after welding is very seldom encountered today. The steels suitable for wet corrosion either have carbon contents below 0.05% or are niobium or titanium stabilised.

They are also very unsusceptible to hot cracking, mainly because they solidify primarely ferritic. The higher-alloy steels such as 310S and N08904 solidify with a fully austenitic structure when welded. They should therefore be welded using a controlled heat input, and a recommended welding procedure. Steel and weld metal with high chromium and molybdenum contents may undergo precipitation of brittle sigma phase in their microstructure if they are exposed to high temperatures for a certain length of time. The transformation from ferrite to sigma or directly from austenite to sigma proceeds most rapidly within the temperature range 750-850 C. Welding with a high heat input leads to slow cooling, especially in light-gauge welds. The weld's holding time between 750-850 C then increases, and along with it the risk of sigma phase formation.



Weldability of Ferritic Steels

  These steels are generally more difficult to weld than austenitic steels. This is the main reason they are not used to the same extent as austenitic steels.

The older types, such as AISI 430, had greatly reduced ductility in the weld. This was mainly due to strong grain growth in the heat-affected zone (HAZ), but also due to precipitation of martensite in the HAZ. They were also susceptible to intergranular corrosion after welding. These steels are therefore often welded with preheating and post-weld annealing. Today's ferritic steels of type S44400 and S44635 have considerably better weldability due to low carbon and nitrogen contents and stabilisation with titanium/niobium. However, there is always a risk of unfavourable grain enlargement if they are not welded under controlled conditions using a low heat input. They do not normally have to be annealed after welding.



Weldability of Austenitic-Ferritic Steels

  Today's austenitic-ferritic (duplex) steels have considerably better weldability than earlier grades. They can be welded more or less as common austenitic steels.

Besides being susceptible to intergranular corrosion, the old steels were also susceptible to ferrite grain growth in the heat affected zone (HAZ) and poor ferrite to austenite transformation, resulting in reduced ductility.

Today's steels, which have higher nickel content and are alloyed with nitrogen, exhibit austenite transformation in the HAZ that is sufficient in most cases. However, extremely rapid cooling after welding, for example in a tack or in a strike mark, can lead to an unfavourably high ferrite content.

When welding duplex 2205 in a conventional way (0.5-2.5 kJ/mm) and using filler metals at the same time, a satisfactory ferrite-austenite balance can be obtained. For the super duplex stainless steel SAF 2507 a somewhat different heat input is recommended (0.2-1.5 kJ/mm). The reason for lowering the minimum value is that this steel has much higher nitrogen content than 2205. The nitrogen favours a fast reformation of austenite, which is important when welding with a low heat input. The maximum level is lowered in order to minimize the risk of secondary phases.

The steels are welded with austenitic-ferritic filler metals. Welding without filler metal is not recommended without subsequent quench annealing. Nitrogen affects not only the microstructure, but also the weld pool penetration. Increased nitrogen content reduces the penetration into the parent metal. To avoid porosity in TIG welding it is recommended to produce thin beads. To achieve the highest possible pitting corrosion resistance at the root side in ordinary 2205 weld metals, the purging gas should be 90%N + 10% H2 or mixtures of Ar + N2/H2. The use of H2 in the shielding gas is not recommended when welding super duplex steels.



Weldability of Martensitic Steels

  The quantity of martensite and its hardness are the main causes of the weldability problems encountered with these steels. The fully martensitic steels are air hardening. The steels are therefore very susceptible to hydrogen embrittlement.

By welding at an elevated temperature (= the steel's Ms temperature), the HAZ can be kept austenitic and tough throughout the welding process. After cooling, the formed martensite must always be tempered at about 650-850 C, preferably as a concluding heat treatment. However, the weld must first have been allowed to cool to below about 150 C.

Martensitic-austenitic steels, such as 13Cr/6Ni and 16Cr/5Ni/2Mo, can often be welded without preheating and without post-weld annealing. Steels of the 13Cr/4Ni type with low austenite content must, however, be preheated to a working temperature of about 100 C.

If optimal strength properties are desired, they can be heat treated at 600 C after welding. The steels are welded with matching or austenitic filler metals