Welding stainless steel: joint preparation, cleanliness and tack spacing

By Robert Lahnsteiner, International Welding Engineer (IWE) · Updated: November 2026

Cleanliness determines corrosion resistance

With stainless steel, the corrosion resistance of the weld begins before the arc is struck. Joint faces and their surroundings must be bright metal and free of grease, oil, paint, scale and iron particles.

Separate tools: use grinding discs, brushes and files only for stainless steels, never alternately for carbon steel. Iron particles on the surface become extraneous rust and can trigger pitting corrosion.
Stainless steel brushes that have not previously been used on any other material.
Remove grease and oil: organic residues can carburise the weld metal and form inclusions.
No chlorinated cleaners: solvents based on chlorinated hydrocarbons form the highly toxic phosgene under the UV radiation of the arc. DVS 0931 names acetone as an alternative.
Clamping fixtures in contact with the workpiece made of stainless steel or heat-resistant plastic, never of unalloyed steel.

Preparing the joint faces

The grooves are produced mechanically or thermally. Thermally cut edges must be freed of oxide before welding.

Mechanically: shearing, planing, milling, grinding or water jet cutting.
Thermally: plasma or laser beam cutting. The cut edges are usually lightly ground over, because adhering oxides reduce the corrosion resistance of the weld.
Abrasives: resin-bonded corundum discs, free of iron and sulphur, with fine grit, used only for stainless steels.

Joint shapes and gaps

Joint shapes follow ISO 9692-1. There are deviations for duplex and for the beam processes.

Case	Recommendation
Joint shape in general	according to ISO 9692-1, depending on workpiece thickness and process
Duplex, super duplex	larger groove angle, 70 to 80° instead of 60°, against lack of fusion in the viscous weld pool
Plasma, keyhole technique	square butt joint without gap up to about 10 mm; thicker as Y groove with about 5 mm root face; gap over 0.08 × sheet thickness requires filler metal
Laser without filler metal	gap at most 1/10 of the sheet thickness, with thick sheets at most half the focus diameter; larger gaps only with filler wire

According to DVS 0931, DVS 0946 and ISER information sheet 823 (2019). When heated, duplex steels expand less than austenitic and more than ferritic grades. This must be taken into account for the gap width when tacking.

The processes in detail: Processes at a glance.

Tack spacing by sheet thickness

Austenitic steels must be tacked more closely than carbon steel. When heated, they expand around a third more and conduct heat only about a third as well.

Sheet thickness	Spacing of tack welds
1 to 1.5 mm	20 to 40 mm
2 to 3 mm	50 to 70 mm
4 to 6 mm	70 to 100 mm
over 6 mm	100 to 150 mm

Recommendation according to ISER information sheet 823 (2019), paraphrased. DVS 0931 gives a coarser 10 to 100 mm for sheets up to about 3 mm and 80 to 200 mm above. Tack duplex more closely because of its higher strength.

Further measures against distortion: Heat input and interpass temperature.

Tack welding correctly

A tack weld is part of the later weld. The same requirements apply to it as to the joint weld.

Tack strongly enough. Tack welds that are too weak can crack during subsequent welding.
Melt tack welds completely or remove them before welding over. With fully austenitic steels, grind the tack welds and remove crater cracks.
Strike the arc only in the groove. Arc strikes on the sheet can initiate cracks and reduce corrosion resistance there.
Tack duplex with filler metal. Short tack welds cool quickly, become ferrite-rich and can crack. Without filler metal only if the tack weld is later completely remelted. Preheat if necessary.
Purge during tack welding if the root is exposed to corrosion and cannot be treated later. Otherwise an oxide spot remains under every tack weld.
Check clamping fixtures: on large components, clamping lugs can come loose because of distortion during welding.

How purging works: Root protection and purging.

Weld pool backing

As a rule, stainless steel is welded without weld pool backing, using backing gas instead of a backing bar.

The exception according to DVS 0931 is the root with slag-forming flux-cored wire. Here a ceramic backing is recommended.

Frequently asked questions

May I use the angle grinder from carbon steel for stainless steel?

No. Discs and brushes that have been used on carbon steel transfer iron particles. These turn into extraneous rust on the stainless steel.

Do I have to regrind plasma-cut edges?

Usually yes, at least lightly. The oxide layer on the cut edge must be removed before welding.

Why tack more closely than with carbon steel?

Because austenite expands more and dissipates heat less well. With wide tack spacing, the gap opens or closes during welding.

What do I degrease with?

With a chlorine-free cleaner such as acetone. Chlorinated solvents form toxic phosgene under the arc.