Welding with austenitic stainless steel, what are the benefits?

Stainless steels are iron alloys with added chromium and nickel.

A minimum of 10-12 wt% chromium content is required to produce rusting resistance. As more Chromium is added beyond this level a higher degree of corrosion resistance is maintained. However, the lattice becomes less ductile and nickel is usually added to restore toughness. Nickel content at 8 wt% and chromium at 18 wt% then becomes the well-known 18-8 stainless steel, as often seen stamped on cutlery, for instance.

18-8 type stainless engineering steels have the designation 300 series and the most common alloys are 301, 302, 303 and 304, and are referred to as austenitic stainless steels, since their crystal lattice structure in metallurgically known as face-centred cubic (fcc).

303 alloy is a free-machining version containing sulphur and can only be welded with special procedures. 301, 302 & 304 alloys are welded using 308 filler metal. No matching chemistry filler metal is produced. 308 filler alloy contains 2wt% extra chromium content to counter the 2 wt% CR that is lost in the welding arc. Thus, the weld deposit then ends up as type 304.

Carbon content may form chromium carbides and denude the grains of chromium. This reaction occurs at grain boundaries and can lead to localised severe corrosion. To counter this liability, stainless austenitic alloys have a controlled low carbon content and can be further protected with an addition of titanium in type 321 or an addition of niobium in type 347. Since 60-70 % of the added titanium can be lost in the welding arc, usually type 321 alloy is welded using 347 filler metal. The suffix L is used to denote a carbon content usually of less than 300ppm (0.03wt%).

304 base alloy can be protected from pitting and general corrosion by an addition of molybdenum at around 2.5wt%. This version is designated 316 or 316L.

Type 347 can be susceptible to liquation-cracking and/or centre-line cracking. To minimise this susceptibility the filler metal alloy chemistry is adjusted to produce 3-7% delta ferrite in the weld deposit. The islands of delta ferrite absorb carbon and tramp elements within the centre of the austenite grains and thus leave clean grain boundaries.

Quite often austenitic stainless alloys have to be dissimilar welded to structural steel and type 309, 309L or 309LMo welding filler metal is used. Another all-purpose welding filler metal for dissimilar joining applications is type 312 filler wire. The duplex weld deposit using these filler alloys contains various levels of delta ferrite which minimises cracking issues.

Welding Tips & Techniques

Welding procedures require clean joint preparation, controlled lower heat input and minimum interpass temperature in order to offset distortion and to minimise chromium loss and to restrict pick up of nitrogen.

All austenitic weld metal should be metallurgically clean, which requires quality filler material so that the deposit contains very low levels of residual, tramp and unwanted interstitial elements. The COA should always be noted so that the welding alloy is well within specification.

Austenitic stainless engineering alloys also possess excellent heat resisting properties and are widely used for elevated temperature components. For long term elevated application type 310 and 330 fully austenitic alloys are used.

These are fabricated with matching filler alloys using specially controlled welding procedures. No delta ferrite content is required for this class of material because it will transform to sigma phase and eventually become embrittled.