austenitic stainless steel (300-series stainless steel)

Austenitic stainless steel is the family of stainless steels characterised by a face-centred cubic (FCC) crystal structure stabilised by nickel content of 8% or more, alongside chromium of 16–26%. The FCC structure makes the steel non-magnetic, highly ductile, easily welded, and exceptionally corrosion-resistant in most aqueous environments. The most common Indian grades in biogas and recycling equipment are SS 304 (18% Cr, 8% Ni), SS 316 (16% Cr, 10% Ni, 2% Mo), and SS 310 (25% Cr, 20% Ni) for higher-temperature pyrolysis applications.

SS 304 is the workhorse for CBG plant digester internals, biogas piping, low-pressure storage tanks, and food-contact areas of FOM packaging lines. It tolerates neutral pH process liquid at 35–55°C, is easily welded by qualified Indian fabricators, and lands at ₹220–290 per kg as plate or pipe. SS 316 adds molybdenum that resists pitting in chloride and ammonia-rich environments — making it the right choice for ammonia scrubbers in biogas upgrading, ammoniacal nitrogen storage tanks, and any equipment exposed to LFOM. It costs 25–35% more than SS 304 at ₹290–390 per kg, which is the reason it is specified surgically rather than universally. SS 310 with its higher chromium and nickel content withstands 800–1,000°C in oxidising or reducing atmospheres, making it the standard for pyrolysis reactor shells.

Trade-offs versus other steel families are clear. Versus carbon steel (₹65–90/kg), austenitic stainless costs 3–5× as much but lasts 10–20× longer in corrosive service and avoids the cost and downtime of painting and re-lining cycles. Versus duplex stainless (SS 2205 at ₹400–550/kg), austenitic has lower mechanical strength but better weldability and field-fabrication suitability. Versus high-nickel alloys (Inconel, Hastelloy at ₹1,200–2,500/kg), austenitic is far cheaper but unsuitable for severe chemistries above 600°C. The right specification balances initial capex against expected service life and the cost of unplanned shutdowns — Indian plants that under-specify on MOC routinely lose 5–10% of annual availability to corrosion-driven outages.

• FCC crystal structure, non-magnetic, ductile, weldable, excellent corrosion resistance.
• Common grades: SS 304 (general), SS 316 (chloride/ammonia), SS 310 (high temperature).
• Prices: SS 304 ₹220–290/kg, SS 316 ₹290–390/kg, SS 310 higher.
• 3–5× cost of carbon steel but 10–20× longer service life in corrosive environments.