1. Why fabrication is the weak link in a stainless line

The base pipe may have left the mill with a perfect ASTM A312 / A213 / A269 certificate, the correct solution-annealed condition, the right ASTM A262 intergranular corrosion test report, and a chemistry that passes PMI. Once that pipe is cut, beveled, fitted and welded, the responsibility shifts from the steelmaker to the fabrication shop. The heat-affected zone (HAZ) can lose corrosion resistance if heat input is too high, the back purge can be ruined by a small leak in the trailing shield, and the wrong filler can produce a galvanic mismatch at the weld.

A good fabrication package treats welding, cleaning, passivation and inspection as one workflow rather than four separate operations. That is also the approach taken in bundled spool deliveries where the same producer supplies the pipe fittings and pipe flanges that will be welded onto the line.

2. Pick the process before you pick the filler

2.1 GTAW (TIG) for the root, GMAW (MIG) for the cap

For thin-wall stainless process pipe (typically 2 mm to 6 mm wall), the industry standard is a GTAW root pass with argon backing, then GMAW or FCAW fill and cap passes. GTAW gives a clean, fully-penetrated root without spatter, while GMAW speeds up the fill. Heavy-wall austenitic spools (8 mm and above) can be welded with SMAW for field joints where portability matters more than cosmetics.

2.2 Filler metal selection

Filler is matched to the base pipe to avoid galvanic and dilution issues:

2.3 Shielding gas and back purge

Argon with 2 to 3 % nitrogen or 2 to 3 % CO₂ is typical for the torch; for the root pass an oxygen-free argon back purge is mandatory on austenitic stainless to prevent sugaring (oxidation of the inside weld bead). On sanitary or pharmaceutical service, a trailing shield carries argon over the cooling weld to keep the HAZ bright and ready for passivation.

3. Heat input and interpass temperature — the two numbers that decide weld quality

Heat input is calculated as HI = (V × A × 60) / (S × 1000) in kJ/mm, where V is voltage, A is current and S is travel speed in mm/min. For austenitic 304/316 stainless, the typical limit is 1.5 kJ/mm to keep the HAZ sensitisation window short; for lean duplex 2205 the limit is tighter at 0.5 to 1.0 kJ/mm to preserve the ferrite-austenite balance.

Interpass temperature is the temperature of the previously deposited bead when the next pass starts. Hold 304/316 below 150 °C and 2205 duplex below 100 °C. The shop floor discipline is simple — measure with a contact pyrometer before each pass, log the number on the weld traveller, and reject the bead if the reading is out of band. Heat input and interpass control are also what separate a long service life from early HAZ cracking in heat efficiency tubes exposed to thermal cycling.

4. Surface finish, cleaning and passivation after welding

The chrome-oxide passive film that protects stainless steel is destroyed by welding heat and by iron contamination from tooling. The standard restore sequence is:

• Mechanical clean: stainless-only wire brushes, flap discs and grinding discs. Carbon-steel brushes are forbidden on the stainless bench.
• Degrease: acetone or isopropyl alcohol to remove oil, marking ink and shop dirt from the weld zone.
• Pickle: a nitric-hydrofluoric or citric acid pickling paste applied to the HAZ to restore the chrome-rich surface layer.
• Passivate: nitric acid or citric acid passivation per ASTM A967 to confirm the passive film is in place.
• Rinse: demineralised water rinse to remove residual chemistry before hydrostatic test.

For pharmaceutical, food and beverage and semiconductor service, a final electropolish of the internal weld seam brings the surface roughness down below 0.4 µm Ra, which is usually the finish spec for hygienic piping.

5. Inspection and non-destructive testing (NDT) on the welded joint

Inspection starts before welding and ends only after the system is hydrostatically tested. The standard sequence on a stainless spool is:

On critical lines — nuclear, offshore, hydrogen, pharmaceutical — the RT or UT coverage jumps to 100 %, and a ferrite count is required on every duplex weld using a calibrated ferroscope (typically 30 to 70 FN for 2205).

6. Common fabrication defects and how to prevent them

Most weld-quality issues on stainless spools come from a small set of root causes. Knowing them in advance saves a lot of rework cost:

7. The fabrication side of the system: fittings, flanges and valves

A welded spool is only as good as the components welded onto it. The first place the line list breaks is at the interface between pipe and fitting — a slightly out-of-square bevel on a butt-weld elbow, a flange face that is not parallel to the pipe bore, or a stud bolt that does not match the flange class will all show up at the fit-up stage. Buying the pipe fittings, pipe flanges and industrial valves from the same mill as the pipe is the simplest way to keep dimensional consistency under control. The supplier can pre-match bevels, factory-weld mock-ups and pre-issue MTRs for the whole pressure boundary in one document set.

8. Sourcing checklist for a fabrication-ready pipe package

• Mill test reports to EN 10204 3.1 (or 3.2 for high-risk service) covering every heat number, with PMI results attached.
• Documented WPS / PQR for every weld procedure the shop will use, qualified per ASME IX or ISO 15614.
• Welders qualified on the actual joint geometry, with current certificates on file.
• A pickling and passivation procedure referenced to ASTM A380 and ASTM A967, with chemistry logs.
• NDT coverage matched to the line class: visual always, plus RT or UT on high-pressure lines and dye-pen or magnetic particle on the ferritic side of any dissimilar-metal joint.
• Bundled delivery: pipe, fittings, flanges, gaskets, stud bolts and valves issued on one PO with one MTR set, to keep the line list traceable from heat number to installed spool.

9. Why EZ Steel Industrial covers the whole pressure boundary

EZ Steel Industrial has been manufacturing and exporting industrial pipe, fittings, flanges and valves since 1994, with an annual capacity above 480,000 tons and a workforce of more than 500 specialists at the Changsha facility. The plant runs API, EN and ASME-certified production lines, an ISO 9001-accredited laboratory, and ASME / AWS-qualified welding procedures, which means the MTRs, WPS and NDT reports the fabrication shop needs can all come from a single supplier.

The catalogue is structured around how projects are actually built — stainless steel pipe for corrosive and hygienic service, carbon steel pipe for utility and structural service, copper-nickel alloy for marine and offshore, plus a full fittings, flanges and valve range to close the system. For a fabrication team working to a tight programme, the bundle is the shortest path from MTR to welded spool.