Stainless steel is not one material but a family of families, and the cost of choosing the wrong one shows up years later as pitting, stress corrosion cracking or a weld that fails inspection. This guide sets out how the main stainless families differ, which specification axes actually predict field performance, and what to verify when sourcing from China.
The stainless families and where each belongs
Austenitic, the workhorse group
304 and its low carbon variant 304L are the baseline, with roughly 18 percent chromium and 8 percent nickel. They handle general atmospheric and fresh water service and most food contact work. 316 and 316L add 2 to 3 percent molybdenum, which raises resistance to chlorides and to pitting, and is the default for chemical, marine and pharmaceutical service. 321 and 347 are stabilised with titanium and niobium respectively to resist sensitisation during welding or elevated temperature service, and they appear in heat exchangers and aircraft exhaust components where intergranular corrosion is the risk.
Super austenitic, for aggressive chloride service
904L, 254SMO and AL-6XN carry much higher molybdenum plus nitrogen and, in some grades, copper. They bridge the gap between ordinary austenitic grades and nickel alloys and are specified for offshore, desalination, flue gas desulphurisation and phosphoric acid service where 316L pits. They are substantially more expensive and are not a general purpose substitute for 316L.
Duplex and super duplex, strength plus corrosion resistance
2205 is the standard duplex grade and 2507 the super duplex grade. Both combine a ferritic and austenitic microstructure, which gives roughly twice the yield strength of austenitic grades together with good chloride stress corrosion cracking resistance. That higher strength lets a designer use thinner wall, which partly offsets the higher price per kilogram. They are specified for pressure vessels, offshore piping, pulp and paper digesters and seawater systems. Duplex grades are less forgiving in fabrication: they require controlled heat input and interpass temperature, and they must not be used above the temperature at which the microstructure embrittles.
Martensitic and precipitation hardening, for hardness and strength
410, 420 and 440C are martensitic grades hardened by heat treatment and used for cutlery, bearings, valves and wear parts where hardness matters more than corrosion resistance. 17-4PH is a precipitation hardening grade combining high strength with moderate corrosion resistance, widely used in aerospace, pumps and valve stems.
Heat resistant grades
309S, 310S and 253MA are designed for elevated temperature service rather than corrosion resistance, and are specified for furnace parts, heat exchangers and exhaust systems on the basis of oxidation resistance and creep strength rather than PREN.
The specification axes that actually separate grades
Corrosion resistance expressed as PREN
The pitting resistance equivalent number, PREN, is calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. It gives a first order ranking of pitting resistance across grades and is the single most useful number when comparing quotations. Values increase from around 18 for 304 through the mid twenties for 316L, around 34 for 2205 and 904L, and above 40 for 2507, 254SMO and AL-6XN. Treat it as a ranking rather than a guarantee, because the actual service environment, surface condition and welding all matter.
Mechanical properties and the strength question
Specify yield strength, tensile strength and elongation, and be explicit about whether the requirement is annealed or cold worked condition, because cold working raises strength at the cost of ductility and magnetic behaviour.
Welding and fabrication behaviour
Sensitisation, intergranular corrosion and sigma phase formation are fabrication risks that are invisible in a mill certificate. For 321 and 347 the stabilising element content matters. For duplex grades the ferrite to austenite balance is controlled by composition and thermal history, and the welding consumable must match the base metal.
Form, tolerance and surface
Specify whether the material is plate, sheet, coil, bar, pipe or wire, the thickness and its tolerance, the width for coil and strip, and the surface finish such as 2B, BA, No.4 or No.1. Surface finish affects both corrosion performance and downstream processing, and it is often the parameter where an attractive quotation has quietly downgraded the product.
Incoming verification checklist
- Check the mill certificate against the ordered grade and confirm heat number traceability back to the original mill.
- Verify chemistry by independent analysis on a sample per heat, especially chromium, nickel, molybdenum and nitrogen, since these drive both grade identity and PREN.
- Confirm mechanical properties by tensile or hardness test as appropriate to the grade and condition.
- Measure dimensions and tolerance across the batch rather than on a single sample.
- For duplex, check the ferrite to austenite ratio and confirm the material has not been exposed to improper heat treatment.
- Run a corrosion test relevant to your service, such as an intergranular corrosion test for stabilised grades or an ASTM G48 type pitting test for duplex.
Common sourcing mistakes
- Buying on the grade name alone without confirming chemistry, so a 316L quotation turns out to be a leaner chemistry.
- Substituting 304 for 316L on a chloride duty to save cost, then discovering pitting within a season.
- Specifying duplex without agreeing the welding procedure, then finding cracking during fabrication.
- Ignoring surface finish, so a 2B sheet is quoted where the application needs a No.4 or better finish.
- Comparing only price per kilogram without normalising thickness tolerance and yield strength.
Sourcing stainless and duplex steel from Hunan and China
China is a major producer across the full stainless range, including austenitic, super austenitic, duplex and super duplex, supported by integrated mill capacity and a mature downstream processing base. Hunan hosts advanced steel and metal material suppliers within its advanced materials industrial base, serving both domestic equipment makers and export customers. LiiFoo maintains verified supplier profiles in the advanced steel materials cluster covering stainless, duplex and specialty steel products.
For grade matched quotations, mill certificates and sample material, see the cluster above or contact our sourcing team.
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