This is the knowledge library from Nandini Steel, an ISO 9001:2015 certified stainless, nickel-alloy and pressure-vessel steel supplier in Mumbai since 2003. Covering PREN, sensitization, sour service, duplex metallurgy and the science behind grade selection.
Written and reviewed by our engineering team. Browse the topics below, or send a spec and we will point you to the right grade and standard.
Stainless and nickel metallurgy
Stainless and nickel-alloy performance starts with crystal structure and alloying content, not just a grade number. The austenitic family, which includes 304, 304L, 316, 316L, 321 and 347, is built around a chromium-nickel matrix that stays face-centered-cubic at room temperature, giving good ductility and weldability; austenitic stainless steel metallurgy covers how chromium forms the passive oxide layer these grades depend on. Raising nickel and molybdenum produces the super-austenitic class, where 904L carries roughly 25% nickel, 4.5% molybdenum and 1.5% copper for aggressive acid and chloride duty, explained in super-austenitic stainless steel metallurgy. Duplex stainless steel instead mixes austenite and ferrite grains for higher strength with strong chloride resistance, detailed in duplex stainless steel metallurgy. Nickel-base alloys such as Incoloy 825, Inconel 625 and the Hastelloy C series replace iron as the primary matrix element; nickel alloys explained sets out how that shift changes resistance to reducing and oxidizing acids. Carbon vs stainless vs alloy steel, role of alloying elements in steel and mechanical properties of stainless and alloy steel explained build the vocabulary used across this knowledge base, and stainless steel metallurgy fundamentals ties the composition-to-property relationships together.
Corrosion mechanisms: pitting, crevice, stress-corrosion cracking and intergranular attack
Most stainless steel failures trace to a specific corrosion mechanism, not uniform wastage. Types of corrosion in stainless and alloy steel is the overview, branching into pitting corrosion in stainless steel, where chlorides break down the passive film at a localized point, and crevice corrosion mechanisms, its close relative that initiates under gaskets and deposits where oxygen cannot replenish the passive layer. Galvanic corrosion between dissimilar metals covers the risk when stainless is bolted or welded directly to carbon steel or aluminum in an electrolyte. Stress-corrosion cracking in austenitic stainless steel explains why chloride exposure combined with tensile stress can crack grades that otherwise show no general corrosion, while intergranular corrosion and sensitization covers the chromium-carbide precipitation that follows welding or improper heat treatment in the 425–815°C range. Two medium-specific pages close the set: sulfuric acid resistance by grade, where copper-bearing 904L outperforms standard austenitic grades, and sour service and NACE MR0175 qualification, covering sulfide stress cracking in wet H2S environments.
PREN and grade selection
The Pitting Resistance Equivalent Number condenses chromium, molybdenum and nitrogen content into one comparative figure, calculated as percent chromium plus 3.3 times percent molybdenum plus 16 times percent nitrogen. On that scale 304 sits around 18, 316 around 24, 904L around 34 and duplex 2205 around 35, with super-duplex 2507 reaching roughly 43; the full derivation is in PREN and pitting resistance equivalent explained. PREN is a starting point for grade selection, not the final answer, since it ranks pitting resistance only and says nothing about strength, weldability or cost; the stainless and nickel grade selector tool weighs PREN alongside those other factors.
Heat treatment and testing: MTC, NDT and PMI
Metallurgical condition is set before a product ships, then checked again before it is accepted. Solution annealing of stainless and nickel alloys describes the high-temperature, rapid-quench cycle that redissolves carbides and restores corrosion resistance after hot working or welding, while normalizing of pressure-vessel-quality plate covers the equivalent treatment carbon steel plate needs for low-temperature impact toughness. Passivation of stainless steel explains the acid treatment that removes free iron from the surface so the passive layer can reform cleanly. MTC, NDT and PMI testing explained and material traceability from heat to certificate cover how a mill test certificate, non-destructive examination and positive material identification confirm a delivered heat matches its paperwork. Weld decay and filler metal selection and welding stainless and nickel alloys connect the metallurgy to fabrication, and pressure vessel steel explained summarizes how these combine in a vessel.
How this content is reviewed
This content is reviewed by the Nandini Steel Engineering Team against the ASTM, ASME, EN and NACE standards cited on each page, and is updated when a referenced standard is revised. Nandini Steel has stocked and supplied stainless, nickel-alloy and pressure-vessel-quality steel since 2003, and every order ships with an EN 10204 3.1 or 3.2 mill test certificate.
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