SS 347 Pipe Supplier in India — Niobium Stabilized Stainless Steel

Both SS 347 and SS 321 are stabilized austenitic stainless steels designed to prevent sensitization in the 425-870°C range, but they use different stabilizing elements. SS 347 uses niobium (Nb ≥ 10×C%) while SS 321 uses titanium (Ti ≥ 5×C%). Niobium forms more stable carbides than titanium and is preferred in many refinery and petrochemical applications because niobium transfers well across the welding arc (titanium does not). This means SS 347 weld metal retains its stabilization, whereas SS 321 welds typically require ER347 filler wire. For critical welded refinery piping, SS 347 is often the preferred specification.

In refinery piping operating at elevated temperatures, niobium (columbium) stabilization offers several advantages over titanium: (1) Nb carbides are more thermodynamically stable than Ti carbides at very high temperatures; (2) niobium transfers effectively through the welding arc so weld metal retains stabilization properties; (3) SS 347 shows better resistance to polythionic acid stress corrosion cracking (a common refinery failure mechanism during shutdowns); and (4) many refinery design codes specifically mandate SS 347 for service above 400°C. Creative Metal Industries stocks SS 347 in all common refinery pipe sizes.

Creative Metal Industries stocks SS 347/347H seamless pipes from 1/2 inch (15NB) to 12 inches (300NB) in schedules SCH 10S, 20S, 40S, and 80S per ASTM A312 TP347. Standard lengths are 5-7 metres (random length). Fixed-length cutting is available on request. For larger sizes up to 16 inches, we can arrange supply within 2-3 weeks from mill. Welded SS 347 pipes are available in SCH 5S to 40S. All material supplied with complete MTC documentation and solution annealing certificate.

Use ER347 (AWS A5.9) filler wire for GTAW/TIG welding of SS 347 pipe. For SMAW, use E347-16 or E347-15 electrodes. The advantage of SS 347 over SS 321 is that the niobium stabilizer transfers effectively through the welding arc, so the weld deposit retains its stabilization properties. Pre-heat is not required. Maintain interpass temperature below 175°C to prevent hot cracking. Post-weld heat treatment (PWHT) is generally not required for corrosion service, but may be specified for creep service applications. Always purge with argon during root pass welding.

SS 347 pipe can operate continuously up to 870°C in oxidizing environments. The niobium stabilization prevents intergranular corrosion (sensitization) throughout the 425-870°C range. For intermittent service, SS 347 can handle brief excursions to 900°C. The creep rupture strength of SS 347 is comparable to SS 321 at temperatures up to 800°C. Above 870°C, consider SS 310 or nickel alloys (Inconel 625/600) for superior oxidation resistance. CMI provides grade selection guidance for your specific temperature and environment.

Yes, SS 347 offers excellent resistance to polythionic acid stress corrosion cracking (PTA-SCC), which is a major concern in refinery piping during shutdowns. Polythionic acids form when sulfide scales on sensitized stainless steel react with moisture and oxygen during turnarounds. Because SS 347 is stabilized with niobium and resists sensitization, it is far less susceptible to PTA-SCC than unstabilized grades like 304 or 316. NACE MR0103 and API RP 945 recommend stabilized grades (321 or 347) or low-carbon grades (304L/316L) for refinery service where PTA-SCC is a concern.