Stainless steel grades guide: families and acid-resistant steel explained

Stainless steels divide into four families: austenitic (such as 304 and 316), ferritic (such as 430), martensitic (such as 420) and duplex (such as 2205). The family sets the balance of corrosion resistance, strength, formability and cost, and within each family the grade is defined by an EN 10088 number such as 1.4301 for 304. On the continent a further distinction is drawn between plain stainless and acid-resistant steel, which is the molybdenum-bearing 316. This guide orients you across the families and grade names so you can choose with confidence.

The four families of stainless steel

All stainless steels share at least 10.5 percent chromium, which gives the self-healing passive film, but they behave very differently depending on their structure. That structure, set by the alloy mix, defines four families. Knowing which family a grade belongs to tells you most of what you need: whether it is magnetic, how strong it is, how well it resists corrosion and how easily it is formed and welded. The grade number then pins down the exact composition.

Austenitic: the 304 and 316 workhorses

Austenitic stainless steels are by far the most widely used, covering the great majority of stainless in industry and construction. They are non-magnetic, tough across a wide temperature range, highly formable and readily welded, and they offer the best general corrosion resistance. 304, or EN 10088 number 1.4301, is the standard grade for general use. 316, number 1.4401, adds 2 to 3 percent molybdenum for chloride resistance. Their low-carbon versions, 304L and 316L, resist weld sensitisation. See our detailed guide to 304 and 316 stainless steel.

Ferritic: magnetic and economical

Ferritic stainless steels, such as 430 (number 1.4016), contain chromium but little or no nickel, which makes them cheaper and less subject to nickel price swings. They are magnetic, moderately corrosion resistant and well suited to indoor and mildly corrosive use: domestic appliances, trim, and interior panels. They are less formable and less weldable than austenitic grades and do not match them for corrosion resistance, so they are chosen where cost and a decorative finish matter more than heavy-duty durability.

Martensitic: hardenable and strong

Martensitic stainless steels, such as 420 (number 1.4021) and 410, can be hardened by heat treatment to high strength and hardness, which is why they are used for knife blades, cutlery, shafts, fasteners and wear parts. That hardness comes at the cost of corrosion resistance, which is the lowest of the stainless families, and of weldability. They are the specialist choice where a stainless steel needs a cutting edge or wear resistance rather than maximum corrosion protection.

Duplex: strength and corrosion resistance together

Duplex stainless steels have a mixed structure that is part austenitic and part ferritic, which gives them roughly twice the strength of standard austenitic grades along with excellent chloride and stress-corrosion resistance. 2205, number 1.4462, is the standard duplex grade, used in demanding marine, chemical and structural work where both strength and corrosion resistance count and where it can allow thinner, lighter sections. See our guide to duplex 2205 for the detail.

Stainless steel versus acid-resistant steel

Across continental Europe a practical distinction is drawn between stainless steel and acid-resistant steel, and it matters when reading a specification. Plain stainless usually means 304 (1.4301), which resists atmospheric corrosion and general use. Acid-resistant steel means 316 (1.4401), whose molybdenum lets it withstand chlorides and many acids that would attack 304, so it is specified for marine, chemical, food-processing and coastal work. When a continental buyer asks for acid-resistant steel, they almost always mean 316 or its low-carbon 316L, and supplying plain 304 in its place would be a costly mistake.

How to read a grade number

European stainless grades are identified by an EN 10088 material number of the form 1.4xxx, which is the authoritative designation on a mill certificate.

The short names such as 304 and 316 are convenient shorthand, but the EN 10088 number is what appears on documentation and removes any ambiguity. Quoting the number on an order and asking for the matching mill certificate is the surest way to get the exact grade intended.

Choosing a grade by environment

The environment, more than anything else, drives grade choice.

Getting the family right first, then the exact grade, avoids both under-specifying, which risks corrosion, and over-specifying, which wastes money. Our guide to why stainless steel rusts explains what happens when the grade is too low for the exposure.

Stainless steel at high temperature

Grade choice also depends on service temperature. The austenitic grades keep their strength and toughness across a very wide range, from cryogenic up to sustained heat, which is another reason they dominate. For genuinely hot service, such as furnace parts and flues, heat-resisting austenitic grades with higher chromium and nickel resist scaling far better than 304 or 316. Ferritic and martensitic grades are more limited at temperature. Where a grade will run hot or cycle through heat, that should be stated on the specification so a suitable austenitic or heat-resisting grade is chosen rather than a standard one.

What the alloying elements do

The behaviour of a stainless grade comes down to a few key elements. Chromium, at least 10.5 percent, forms the passive film and is the basis of all stainless steel, and more chromium means more corrosion resistance. Nickel stabilises the tough, formable austenitic structure and improves resistance to acids, which is why 304 and 316 are so workable. Molybdenum, at 2 to 3 percent, is what lifts 316 above 304 against chlorides and acids. Carbon adds strength but, above about 0.03 percent, risks the carbide precipitation that sensitises welds, which is why low-carbon L grades exist. Titanium in a stabilised grade such as 316Ti ties up carbon to the same end. Reading a grade is largely reading its chromium, nickel and molybdenum.

Grade and cost: the nickel factor

Cost tracks the alloy content, and nickel is the biggest single driver. The austenitic grades carry 8 percent nickel or more, so their price moves with the nickel market and they sit above the ferritic grades. 316 costs more than 304 for its added molybdenum as well as nickel, and duplex sits higher again for its performance. Ferritic 430, with little or no nickel, is the economical choice where its moderate corrosion resistance is enough. This is why the right grade matters commercially as well as technically: specifying 316 where 304 would do, or duplex where 316 would do, adds cost the application may not need.

Matching grade to product form

Grade choice also interacts with the form the steel comes in. Austenitic 304 and 316 are available across the widest range of forms, sheet, tube, bar and profiles, because their formability and weldability suit every process, which is why they dominate fabrication. Ferritic and martensitic grades appear more in specific forms such as sheet, strip and bar for their particular uses. Duplex is common as sheet, plate and tube for structural and pressure work. When specifying, it is worth confirming that the chosen grade is available in the form and size the job needs, which a quotation will confirm.

Stainless steel grades from ULAMEX

ULAMEX has supplied stainless steel since 1988, in the grades and forms that professional buyers specify.

Not sure which grade you need? Tell ULAMEX the application and the environment and the desk will advise on the family and grade and return a non-binding quotation. See our guides to 304 and 316 stainless steel and stainless steel sheet, and our supply capabilities, or contact the export desk at [email protected] or +48 504 424 761.

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