Stainless steel owes its resistance to an extremely thin, chromium-rich passive layer. When welding, what matters is that the weld and the heat-affected zone retain this resistance and that the weld remains free of cracks. This series explains materials, filler metal, processes and gas and shows how to prepare, protect, post-treat and test the weld.
It is based mainly on the DVS technical bulletins 0931, 0937 and 0946, ISO 14343 for the filler metals and research reports.
A good place to start is What makes stainless steel stainless. The other articles follow the path from the material to the tested weld and can also be read individually.
Basics: material, microstructure, filler metal
- What makes stainless steel stainless: Passive layer, material groups from austenitic to duplex, material numbers and PREN.
- Microstructure, delta ferrite and hot cracking: Why a little ferrite in the weld metal protects against hot cracking, ferrite number, Schaeffler and WRC diagram.
- Choosing the filler metal: The right wire for the base material: 308LSi, 316LSi, 318Si, 2209, 309LSi and nickel-based.
Process: method, gas, heat
- Processes at a glance: MAG, TIG, plasma, MMA, flux-cored wire and laser compared.
- Shielding gases: Gas groups to ISO 14175 for MAG, TIG, duplex and flux-cored wire, and what CO₂, oxygen, helium, hydrogen and nitrogen do.
- Heat input and interpass temperature: Arc energy, preheating and interpass temperature for each material group.
Preparation and root protection
- Joint preparation and tack welding: Cleanliness, separate tools, joint shapes and tack spacing.
- Root protection and purging: Backing gases, purging technique, purge time and residual oxygen.
- Temper colours, pickling, passivation: Where temper colours come from, why they have to go and which method removes them.
Special materials and joints
- Duplex and super duplex: The balance of ferrite and austenite, filler metal, gas and heat control.
- Joining stainless to carbon steel: Dissimilar joints with carbon steel: dilution, a Schaeffler example and the right filler metal.
Result: defects, testing, safety
- Weld defects and corrosion damage: Hot cracks, pores, lack of fusion, pitting, extraneous rust: causes and remedies.
- Testing, health and safety: Which test methods work on stainless steel, measuring ferrite, protection against fumes, radiation, forming gas and pickling acid.
Calculators
- Schaeffler, DeLong and WRC diagram: estimate the ferrite content of the weld metal, also for dissimilar base materials.
- Purge time calculator: how long a pipe has to be purged before welding.
- Heat input, cooling time t8/5 and preheat temperature: planning heat control.
Welding wire
- Stainless steel welding wire: 308LSi, 309LSi, 312, 307, 316LSi, 2209, 318Si and 310 with data sheets.
- Nickel welding wire: nickel-based alloys for special cases and dissimilar joints.
Frequently asked questions
Which wire suits 1.4301?
The standard choice is 308LSi. For molybdenum-alloyed steels such as 1.4404, and also for the titanium-stabilised 1.4571, 316LSi is usual. More in Choosing the filler metal.
Does stainless steel have to be purged?
If the root side is to remain corrosion-resistant, yes. Without a backing gas the root oxidises and the chromium-depleted surface rusts. More in Root protection and purging.
Why does a stainless steel weld rust?
Usually because of temper colours that were not removed, iron particles from the wrong tools or slag, more rarely because of the material itself. More in Weld defects and corrosion damage.
About the author
Robert Lahnsteiner, Ing., is an International Welding Engineer (IWE) and managing director of MIGAL.CO GmbH. After training in telecommunications and electronics at HTL Braunau, he spent eighteen years in welding technology at Fronius from 1983. Since 2001 he has run MIGAL.CO, the aluminium welding wire manufacturer in Landau an der Isar, together with Martin Erl. His focus is aluminium filler metals, wire manufacturing and wire feeding, and the monitoring of welding processes. Within DVS, the German Welding Society, he has served for around 30 years on technical committee FA03 “Arc welding” and chairs working group 2.4.4 “MIG welding of aluminium”.