SUS316 Stainless Steel TIG Welding: Process Parameters and Weld Quality Control

SUS316 material characteristics that drive the welding procedure

SUS316 is the JIS designation for the molybdenum-bearing austenitic grade, corresponding to Chinese 06Cr17Ni12Mo2 (GB/T 20878, formerly 0Cr17Ni12Mo2), ASTM 316 / UNS S31600 and EN 1.4401 / X5CrNiMo17-12-2. Its chemistry is C ≤ 0.08%, Si ≤ 0.75%, Mn ≤ 2.00%, P ≤ 0.045%, S ≤ 0.030%, Cr 16.00–18.00%, Ni 10.00–14.00% and Mo 2.00–3.00%.

Corrosion resistance – the 2–3% molybdenum raises resistance to chlorides, sulfuric acid and phosphoric acid well above 304, which is why 316 covers chemical plant, marine hardware, food and pharmaceutical equipment.

Temperature range – usable from −196 °C to about 800 °C, but continuous service in the 425–815 °C band promotes chromium carbide precipitation, so 316L or a stabilised grade is preferred there.

Physical behaviour – thermal expansion is about 16 × 10⁻⁶ /K and thermal conductivity only 16 W/(m·K), roughly one third of carbon steel, so heat concentrates at the arc and distortion appears quickly.

Formability – good plasticity and ductility, but the low thermal conductivity and high expansion make welding heat input and interpass control the critical variables.

Weldability problems and how to control them

Problem	Root cause	Control measure
Intergranular corrosion	Cr₂₃C₆ precipitation at grain boundaries after the weld thermal cycle, leaving chromium-depleted zones	Use ultra-low carbon ER316L filler (C ≤ 0.03%) per AWS A5.9 or ISO 14343, and keep heat input low
Hot cracking	Segregation of sulfur and phosphorus forms low-melting eutectics in the weld pool	Hold base metal S ≤ 0.030% and P ≤ 0.045%; target a weld-metal ferrite number of 3–8 to resist solidification cracking
Porosity	Hydrogen from moisture, oil or condensation entering a slow-freezing pool	Degrease with acetone, dry the joint, use filler from a sealed pack and shielding gas of at least 99.99% purity
Back oxidation	No purge on the root side, so a chromium oxide scale forms at temperature and destroys corrosion resistance	Purge with argon until the outlet oxygen falls below about 0.5%, and hold the purge until the root has cooled below 200 °C
Distortion	High thermal expansion combined with localised heat	Balanced welding sequence, tack spacing of 8–12 times the thickness, minimal restraint and copper backing where possible

Joint preparation and fit-up

Remove oil, rust, moisture and oxide scale with a stainless-only grinding wheel or acetone; never use a wheel that has touched carbon steel, because embedded iron becomes a rust site.

Thin plate up to 3 mm can be welded as a square butt with no gap and no filler on the root pass.

Plate over 3 mm normally takes a V-groove with a 60°–70° included angle, a 1–2 mm root face and a root gap sized to give full penetration without burn-through.

Thick sections benefit from a U-groove, which reduces the number of passes and therefore the total heat input.

Keep a dedicated set of stainless brushes, files and clamps; cross-contamination between carbon steel tooling and stainless work is one of the most common causes of rust marks on delivered parts.

TIG welding process parameters

TIG is the first choice for SUS316 because the arc is stable, the heat input is controllable and the weld bead is clean. The table gives butt-weld starting parameters for DCEN (electrode negative) operation with a ceriated or lanthanated tungsten electrode ground to a 30°–60° included angle with a small flat land.

Parameter	Thin plate 1–3 mm	Medium plate 3–6 mm	Thick plate over 6 mm
Welding current, DCEN	80–150 A	150–250 A	250–350 A, multi-pass
Travel speed	15–30 cm/min	10–20 cm/min	8–15 cm/min
Tungsten electrode diameter	2.0–2.4 mm	2.4–3.2 mm	3.2–4.0 mm
Shielding gas	Pure argon, 99.99% or better	Pure argon or argon–helium mixture	Pure argon, mandatory root purge
Gas flow, front / back	8–15 / 5–10 L/min	10–15 / 8–10 L/min	12–18 / 10–15 L/min
Filler wire	ER316L, 1.6–2.4 mm	ER316L, 2.4–3.2 mm	ER316L, 3.2 mm
Interpass temperature	≤ 100 °C	≤ 150 °C	≤ 150 °C
Heat input	0.5–1.0 kJ/mm	0.8–1.5 kJ/mm	0.8–1.5 kJ/mm, controlled per pass

One correction to the common process sheets: an argon mix containing 2–5% oxygen belongs to MIG welding, not TIG. Oxygen-bearing additions oxidise the tungsten electrode, destroy arc stability and remove the oxide-free finish the process is chosen for. For thicker sections, use an argon–helium blend to raise penetration, or add up to 5% hydrogen to pure argon when welding fully austenitic stainless steel; both keep the tungsten clean.

Two operating rules matter more than the headline amperage. First, current must follow the tungsten diameter, because an electrode that is too small for the current melts back and contaminates the pool. Second, welding speed and current must be balanced: too fast gives porosity and lack of fusion, too slow raises heat input and increases sensitisation risk in the heat-affected zone.

Post-weld treatment, purge practice and inspection

Cleaning – brush the heat tint away with a stainless wire brush, then pickle and passivate the weld and heat-affected zone so the chromium oxide layer is fully restored.

Purge discipline – maintain back purge for the full root pass and until the joint is below 200 °C; a straw-coloured root is acceptable for many duties, but a dark blue or black root indicates oxidation and needs removal.

Corrosion testing – for chemical service, qualify the procedure and the production weld with an intergranular corrosion test and a ferrite check on the weld metal.

Non-destructive testing – dye penetrant for surface defects, radiography or phased array ultrasonic testing for full-penetration butt welds.

Post-weld heat treatment – not required for 316L; where a procedure calls for it, full solution annealing at 1,040–1,120 °C followed by rapid cooling is used, never a slow stress-relief cycle in the sensitisation range.

FAQ

Q: Why is ER316L filler preferred over ER316 for SUS316 welding?
ER316L contains no more than 0.03% carbon, so chromium carbides do not precipitate at the grain boundaries of the weld metal during cooling. The 0.02–0.03% lower carbon has a negligible effect on strength and a large effect on corrosion resistance after welding.

Q: What shielding gas should be used for TIG welding SUS316?
Pure argon of at least 99.99% purity covers thin and medium sections. Argon–helium blends increase penetration on thick plate, and up to 5% hydrogen in argon can be used for austenitic stainless to improve travel speed. Never add oxygen to a TIG shielding gas.

Q: What interpass temperature should be maintained?
Keep it at or below 150 °C and at or below 100 °C for thin, restrained sections. Interpass temperature is measured on the base metal close to the weld, and letting it climb is the fastest way to lose corrosion resistance in the heat-affected zone.

Q: Is post-weld heat treatment needed after TIG welding 316?
Normally no. The austenitic structure does not harden and residual stresses are managed by weld sequence rather than by a stress-relief cycle. Slow cooling through 425–815 °C must be avoided because it sensitises the material.

Q: How do I prevent oxidation on the root side?
Purge the pipe or the underside of the joint with argon before welding and continue the purge until the root has cooled below 200 °C. Confirm purge quality at the outlet, since even a small residual oxygen level will tint a stainless root.

Q: What ferrite number is acceptable in a 316L weld?
A weld-metal ferrite number of about 3–8 is the usual target: enough delta ferrite to resist solidification cracking, but low enough to keep toughness and corrosion resistance. Ferrite is checked with a calibrated ferrite scope on the production weld.