When you ask about types of stainless steel and grades, the short answer is that stainless steel is an iron alloy with a minimum of 10.5% chromium, and it is sorted into five microstructure families and a few hundred named grades.
The chromium reacts with oxygen on the surface to build a passive Cr₂O₃ film. That film heals itself when scratched, which is why the metal does not rust the way carbon steel does. This guide walks through the five families, the common grades (304, 316, 430, 410, 17-4 PH, and duplex 2205), and how to pick the right stainless steel for Indian food, pharma, marine, and infrastructure work.
TL;DR
- Stainless steel is iron mixed with at least 10.5% chromium, which forms a thin chromium-oxide (Cr₂O₃) film that protects the metal from rust.
- The metal splits into five families: austenitic, ferritic, martensitic, duplex, and precipitation hardening.
- Common grades you will meet on the shop floor are 304, 316, 430, 410, 17-4 PH, and duplex 2205.
- Pick the grade by matching the operating environment, the corrosion mode you expect, the strength you need, and the budget you have, not by chasing the highest alloy on the shelf.
The 5 families of stainless steel
Stainless steel is grouped into five families: austenitic, ferritic, martensitic, duplex, and precipitation hardening. Each family has a different crystal structure, a different mix of alloying elements, and a different balance of strength, corrosion resistance, and cost.
Austenitic grades dominate output and handle most general and food work. Ferritic grades are cheaper and magnetic. Martensitic grades are hard. Duplex blends the first two for marine work. Precipitation hardening grades are used when you need very high strength.
The 5 families of stainless steel
Stainless steel is grouped into five families: austenitic, ferritic, martensitic, duplex, and precipitation hardening. Each family has a different crystal structure, a different mix of alloying elements, and a different balance of strength, corrosion resistance, and cost.
Austenitic grades dominate output and handle most general and food work. Ferritic grades are cheaper and magnetic. Martensitic grades are hard. Duplex blends the first two for marine work. Precipitation hardening grades are used when you need very high strength.
| Family |
Microstructure |
Typical Cr % |
Magnetic? |
Best for |
| Austenitic |
FCC |
16–26% |
No (annealed) |
Food, pharma, general |
| Ferritic |
BCC |
11–27% |
Yes |
Trim, decor, appliances |
| Martensitic |
BCC |
11–18% |
Yes |
Cutlery, blades, surgical tools |
| Duplex |
Austenite + ferrite |
19–32% |
Partial |
Marine, oil and gas |
| Precipitation hardening |
Varies |
Varies |
Varies |
Aerospace, high-strength shafts |
Stainless steel grade designation systems
The same piece of stainless steel can carry three different names. The SAE/AISI 3-digit system, used widely in the United States, names a grade as “Type” plus three digits and sometimes a letter, like Type 304 or Type 316L.
The Unified Numbering System for Metals and Alloys (UNS), set up jointly by ASTM and SAE in 1974, uses an alpha-numeric code: an “S” plus five digits, so Type 304 becomes S30400. Europe runs on EN 10088, which classifies stainless steels by chemical composition and mechanical properties and writes the same alloy as 1.4301.
| System |
Example |
Notes |
| AISI/SAE |
Type 304 |
“Type” + 3-digit number |
| UNS |
S30400 |
“S” + 5 digits, ASTM/SAE 1974 |
| EN 10088 |
1.4301 |
European composition-based standard |
In practice, 304, 18/8, and S30400 all point at the same chemical composition. Vendors quote whichever the buyer reads.
Austenitic stainless steel: 300 and 200 series
Austenitic stainless steel is the largest of the five families and accounts for around two-thirds of all stainless steel production. The 300 series carries 16–26% chromium and 6–22% nickel; the 200 series swaps some of that nickel for manganese and nitrogen. Both have a face-centred cubic (FCC) crystal structure, are non-magnetic in the annealed state, and offer excellent corrosion resistance plus good weldability.
Austenitic grades are best for high-temperature environments because of their oxidation resistance, with some grades holding up between roughly 1700°F and 2000°F. The trade-off is cost: austenitic steels are more expensive because of the nickel content.
| Grade |
Composition |
Key property |
Uses |
| 304 |
18% Cr, 8% Ni (18/8) |
General corrosion resistance |
Food processing equipment, kitchen utensils, pharma |
| 316 |
Adds 2–3% Mo |
Chloride and pitting resistance |
Marine, chemical industry, medical tools |
| 310 / 309 |
Higher Cr and Ni |
Heat resistance |
Furnace parts, heat exchangers |
For coastal and seawater work, 316/316L or duplex 2205 is preferred over 304 because chloride ions attack 304 and cause pitting corrosion.
Ferritic stainless steel: 400 series
Ferritic stainless steels carry between 10.5% and 27% chromium with very little or no nickel. They have a body-centred cubic (BCC) crystal structure, are magnetic, cost less than austenitic types, and offer moderate corrosion resistance. They are common in automotive components, kitchen appliances, and decorative interior parts where the chemistry is mild.
| Grade |
Cr % |
Key traits |
Uses |
| 430 |
~17% |
Basic, cost-effective, good rust resistance in mild environments |
Appliance trim, exhaust, sinks |
| 434 |
16–18% Cr + 1% Mo |
Better corrosion resistance than 430 |
Automotive trim, exterior parts |
Ferritic welding needs care. The grain can grow during welding and make the joint brittle, so heat input is kept low and post-weld checks are routine.
Martensitic and precipitation-hardening stainless steels
Martensitic stainless steels run 11–18% chromium with higher carbon than ferritic or austenitic types. They are magnetic, can be hardened by heat treatment, and reach high hardness and strength, but their corrosion resistance is lower than 304 or 316. Martensitic steel is suitable for cutlery and surgical instruments because it takes and holds a sharp edge.
Precipitation hardening (PH) stainless steels are aged at moderate temperature so that fine particles drop out inside the structure and lock the metal up. The 17-4 PH grade is a martensitic precipitation hardened stainless steel made up of about 15–17.5% chromium, 3–5% nickel, and 3–5% copper, and it is well known for its high strength and hardness.
| Grade |
Family |
Key benefit |
Uses |
| 410 |
Martensitic |
Hardenable, basic corrosion resistance |
Cutlery, fasteners, valves |
| 420 |
Martensitic, higher C |
Sharpens well, holds an edge |
Knives, surgical blades |
| 17-4 PH |
Precipitation hardening |
High strength + reasonable corrosion resistance |
Aerospace shafts, pump parts |
Pick martensitic or PH grades when wear, hardness, or strength matters more than top-end corrosion resistance. Heat treatment is critical: skip the temper step and the part can crack under load.
Duplex stainless steel: strength + corrosion
Duplex stainless steels have a mixed microstructure of roughly 50% austenite and 50% ferrite, with 19–32% chromium and up to 5% molybdenum. The dual structure gives higher strength than 304 or 316 and improves resistance to chloride stress corrosion cracking, which is the main failure mode for austenitic grades in warm chloride water. Duplex is therefore the working choice for offshore platforms, oil and gas pipework, desalination plants, and chemical industry vessels.
| Category |
Example |
Key traits |
Uses |
| Lean duplex |
2304 |
Lower Mo, lower cost, better SCC than 304 |
Bridges, storage tanks |
| Standard duplex |
2205 |
Balanced Cr-Mo-N, high strength |
Oil and gas, pressure vessels |
| Super duplex |
2507 |
Higher Cr/Mo, PREN > 40 |
Seawater systems, desalination |
For Indian projects, duplex grades are now common in port equipment, desalination work along the western coast, and chemical plants in Gujarat and Maharashtra.
How stainless resists corrosion
Chromium reacts with oxygen and water at the surface and forms a thin passive Cr₂O₃ film. The film is only a few atoms thick, but it self-heals when the surface is scratched, which is why stainless steels do not suffer the uniform corrosion that carbon steel shows when it meets air and moisture. The minimum chromium for the effect is 10.5%.
Pitting corrosion is a localized corrosion mode in which chloride ions break through the passive film at small spots. Resistance is captured by the Pitting Resistance Equivalent Number (PREN), commonly written as PREN = %Cr + 3.3×%Mo + 16×%N. Higher PREN means better chloride resistance.
- For mild indoor or decorative use, basic austenitic 304 or ferritic 430 is enough.
- For coastal air, food acids, and most chemical plants, austenitic 316 (Mo-bearing) is the workhorse.
- For seawater, brine, and the oil and gas industry, duplex 2205 or super duplex 2507 carries the load.
How to choose the right stainless grade
Selection is a checklist, not a ranking exercise:
- Environment: fresh water, coastal, chemical, or food contact?
- Corrosion mode expected: general, pitting, crevice, chloride stress corrosion cracking, or high-temperature oxidation?
- Mechanical demand: tensile strength, hardness, fatigue, creep resistance at elevated temperatures?
- Fabrication route: welding, deep drawing, machining, or forging?
- Budget and life-cycle cost, not just sticker price.
| Application |
Recommended grade(s) |
Why |
| Food and beverage |
304, 316 |
Hygienic, easy to clean, no nickel pickup issues at use temperature |
| Marine and coastal |
316L or duplex 2205 |
Mo and duplex structure resist chloride pitting |
| Cutlery and blades |
410, 420 |
Hardenable, holds an edge |
| Automotive trim and exhaust |
430, 434 |
Magnetic, low cost, fine for mild exposure |
| Aerospace shafts and high-load parts |
17-4 PH |
High strength after aging |
| Pressure vessels |
304L, 316L, duplex 2205 |
Pressure vessel quality plates and good weldability |
When asked “best grade for marine work?” the answer is 316L for fittings and 2205 for structures. For “best grade for food processing?” 304 covers most lines and 316 takes over near brines and acidic cleaners. See the complete steel grades list for full datasheets.
FAQs on stainless steel types and grades
