Alloy Families

There are five different “metallurgical” structures (or classes) of stainless steel that can be produced by using different alloying elements. Each includes many alloys and is commonly called a “stainless steel family”. These families have different characteristics and were created for different applications. Most stainless steels have relatively high strength, good corrosion resistance, and ductility.

When specifying stainless steels, common names like “304” can be used but it is important to also list the specific Unified Numbering System (UNS) designation for the specific alloy. This identifies the specific chemistry that is desired. Within the ASTM standards and industry association and company literature, alloys are identified by UNS number and, where one exists, the common name. It has been many years since new AISI numbers have been issued so newer alloys will not have them.

1) Austenitic stainless steels (Fe-Cr-Mn and Fe-Cr-Ni)

Austenitic stainless steels grades or alloys contain either chromium and manganese or chromium and nickel. The carbon content is generally held to a maximum of 0.08% (302 (UNS S30200), 309 (S30900) and 310 (S31000) have slightly higher levels). The chromium content can range from 16.0 to 28.0% with nickel between 3.5 and 32.0%.

These alloys cannot be hardened by heat treatment, but they can develop high strength by cold working. They are non-magnetic in the annealed (heat-treated) condition, but cold worked and cast material maybe somewhat magnetic.

They exhibit excellent corrosion resistance and molybdenum is added to some grades for additional resistance to chlorides. In some alloys, nitrogen maybe added to improve strength and corrosion resistance. Austenitic stainless steels offer good formability, high ductility, and good impact toughness. The most common alloys (e.g. 304 and 304L (S30400 and S30403)) are sometimes called 18-8 or 18% chromium and 8% nickel because that is their basic composition. Types 304 and 316 are the most commonly used stainless steels.

Grade	Description	Applications
304	General Purpose	Air sensor bags Automotive wiper arms Baggage handling equipment Baking equipment Bar and fountain equipment Beer barrels Bottling machinery Burial caskets Chemical processing equipment Chimney liners Clothes washer/dryer interiors Coffee makers Column covers Cookware Counter tops Cryogenic vessels and components Dairy handling equipment Dishwasher drums Elevator interiors Entry Doors Escalators Evaporators Fire doors Fireplace fronts Flatware Food processing equipment Gutters and downspouts Hardware, hinges Hose clamps Hospital carts HVAC components Jewelry Kitchen cabinets Kitchen exhaust hoods Meat processing equipment Metal roofing Microwave oven interiors Milking machines Nuclear storage containment Nuclear vessels Oven parts Rapid transit cars Refrigerator panels Retractor springs Roofing tiles Seat belt anchors Signs Sinks Splash shield Spring clips Stove tops Street furniture Tanker trailer bodies and chassis Truck exhaust systems Truck gas tanks Truck mirrors Truck trailer cargo doors and front corner trim Window frames Window louvers (sun screens) Wine tanks
305	Ni increased to lower work hardening	Coffee urn tops Deep drawn parts Expanded metal parts Formed parts (requiring extra low work hardening characteristics) Free spinning operations Mixing bowls Reflectors
309S	Cr & Ni increased for high temperature	Annealing boxes, Chemical processing equipment (elevated temperature), Conveyor parts, Dryers, Electric range elements, Furnace parts
310S	Same as 309, only more so	Annealing boxes, Furnace belts, Chemical processing equipment (elevated temperature), Conveyor parts Dryers, Furnace parts
316	Mo added to increase corrosion resistance	Bus/Traffic shelters Chemical processing equipment Chemical transportation tanks Entry doors Fertilizer handling equipment Fire doors Food processing equipment Hot water tanks Metal roofing Oil refining equipment Pharmaceutical processing equipment Photographic equipment Sea water components Soap handling equipment Storage tanks Street (urban) furniture Swimming pool equipment Ticket dispensers Water treatment equipment Window frames Wine storage tanks
317	More Mo & Cr added for better corrosion performance	Chemical processing equipment, Ink manufacturing equipment, Pharmaceutical processing equipment, Fertilizer handling equipment
321	Ti added to prevent carbicle precipitation	Aircraft exhaust manifold and flanges, Chemical equipment, Fire walls, Pressure tanks
347	Cb added to prevent carbicle precipitation	Aircraft exhaust manifold and flanges, Expansion joints, Furnace stacks and dampers, Welded storage tanks for organic chemicals
330	Si increased for highest heat resistance	Annealing boxes, Combustion chambers, Burners and exhaust parts
409	Lower Cr Primarily used for automotive exhaust	Automotive exhaust systems Bus frames Cargo containers Fins for heater tubes Perforated sheet for silencer housings Smoke control ductwork Transformer and capacitor cases
410/410S	General Purpose	Bushings, Trays for oil fractionating towers, Hardware, Cutlery, Press plates
420	Increased C to improve mechanical properties	Cutlery/Scissors, Dental equipment, Surgical equipment
430	P & S added for improved machining	Architectural applications (interior), Automotive trim, Cooking utensils, Food processing equipment
434	Mo added for improved corrosion resistance in auto trim	Automotive trim
Grade	Description	Applications

Excellent corrosion resistance
Typical strength (in the annealed condition) 85 ksi – tensile, 40 ksi – yield and 50% elongation
Cannot be heat treated but can be hardened by “cold working” (up to 185 ksi – tensile, 140 ksi – yield
Non-magnetic
Good high and low-temperature mechanical properties
Excellent formability and weldability
All common finishes can be applied

200 Series Austenitic (Fe-Cr-Mn): UNS S20100
300 Series Austenitic (Fe-Cr-Ni): UNS S30100
S30403
S30500
S30908
S31008
S31603
S31703
S32100

2) Ferritic stainless steels (Fe-Cr)

The ferritic stainless steels are sometimes called “non-hardenable” 400 series grades or alloys, and 409 and 430 are the most common. Alloys of chromium and iron with limited carbon contents that are generally below 0.12% (442, 446 are at 0.20). The chromium content can vary from 10.5 to 30.0%. These grades cannot be hardened by heat treatment but they are magnetic. They have good corrosion resistance (particularly to chloride stress corrosion cracking) but are generally not chosen for toughness.

Grade	Description	Applications
304	General Purpose	Air sensor bags Automotive wiper arms Baggage handling equipment Baking equipment Bar and fountain equipment Beer barrels Bottling machinery Burial caskets Chemical processing equipment Chimney liners Clothes washer/dryer interiors Coffee makers Column covers Cookware Counter tops Cryogenic vessels and components Dairy handling equipment Dishwasher drums Elevator interiors Entry Doors Escalators Evaporators Fire doors Fireplace fronts Flatware Food processing equipment Gutters and downspouts Hardware, hinges Hose clamps Hospital carts HVAC components Jewelry Kitchen cabinets Kitchen exhaust hoods Meat processing equipment Metal roofing Microwave oven interiors Milking machines Nuclear storage containment Nuclear vessels Oven parts Rapid transit cars Refrigerator panels Retractor springs Roofing tiles Seat belt anchors Signs Sinks Splash shield Spring clips Stove tops Street furniture Tanker trailer bodies and chassis Truck exhaust systems Truck gas tanks Truck mirrors Truck trailer cargo doors and front corner trim Window frames Window louvers (sun screens) Wine tanks
305	Ni increased to lower work hardening	Coffee urn tops Deep drawn parts Expanded metal parts Formed parts (requiring extra low work hardening characteristics) Free spinning operations Mixing bowls Reflectors
309S	Cr & Ni increased for high temperature	Annealing boxes, Chemical processing equipment (elevated temperature), Conveyor parts, Dryers, Electric range elements, Furnace parts
310S	Same as 309, only more so	Annealing boxes, Furnace belts, Chemical processing equipment (elevated temperature), Conveyor parts Dryers, Furnace parts
316	Mo added to increase corrosion resistance	Bus/Traffic shelters Chemical processing equipment Chemical transportation tanks Entry doors Fertilizer handling equipment Fire doors Food processing equipment Hot water tanks Metal roofing Oil refining equipment Pharmaceutical processing equipment Photographic equipment Sea water components Soap handling equipment Storage tanks Street (urban) furniture Swimming pool equipment Ticket dispensers Water treatment equipment Window frames Wine storage tanks
317	More Mo & Cr added for better corrosion performance	Chemical processing equipment, Ink manufacturing equipment, Pharmaceutical processing equipment, Fertilizer handling equipment
321	Ti added to prevent carbicle precipitation	Aircraft exhaust manifold and flanges, Chemical equipment, Fire walls, Pressure tanks
347	Cb added to prevent carbicle precipitation	Aircraft exhaust manifold and flanges, Expansion joints, Furnace stacks and dampers, Welded storage tanks for organic chemicals
330	Si increased for highest heat resistance	Annealing boxes, Combustion chambers, Burners and exhaust parts
409	Lower Cr Primarily used for automotive exhaust	Automotive exhaust systems Bus frames Cargo containers Fins for heater tubes Perforated sheet for silencer housings Smoke control ductwork Transformer and capacitor cases
410/410S	General Purpose	Bushings, Trays for oil fractionating towers, Hardware, Cutlery, Press plates
420	Increased C to improve mechanical properties	Cutlery/Scissors, Dental equipment, Surgical equipment
430	P & S added for improved machining	Architectural applications (interior), Automotive trim, Cooking utensils, Food processing equipment
434	Mo added for improved corrosion resistance in auto trim	Automotive trim
Grade	Description	Applications

Good corrosion resistance
Typical strength 65 – 75 ksi – tensile, 35 – 50 ksi – yield and elongation 20 – 35%
Magnetic
Limited temperature use
Can be polished

UNS S40500
S40910
S40920
S40930
S43000
S43400
S43600
S43035
S44400
S44660
S44735

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The Ferritic Solution

3) Martensitic stainless steels (Fe-Cr-C)

The martensitic structure results when chromium and higher levels of carbon are added to iron. This structure can be heat treated to higher hardness levels and is sometimes called “hardenable stainless” 400 series. 410, 420 and 440 are the most common grades. Carbon can range from 0.08 to 1.20% with chromium levels between 11.5 and 18.0%. The martensitic structure is also magnetic.

Fasteners
Pump shafts
Turbine blades
Surgical instruments
Cutlery

Adequate corrosion resistance
Hardenable by heat treatment
Magnetic
Somewhat limited temperature use
Limited weldability

UNS S41000
S42000
S44002
S44004

4) Precipitation hardening steels[Fe-Cr-Ni (Mo-Cu-Al-Nb)- N]

Precipitation hardening stainless steels develop strength by precipitation hardening reactions as the result of heat treatment. With lower carbon levels (0.09 max.), they have good corrosion resistance and are characterized by ease of fabrication. High strengths can be developed at relatively low temperatures (500-800 °C) so distortion is minimized. The chromium is between 12.25 and 18.0%, with nickel levels of 3.0 to 8.5%. Molybdenum, in some grades, is between 2 and 3%, with additions of aluminum, copper, rare earth elements and nitrogen.

Valves
Gears
Petrochemical equipment

UNS S13800
S15500
S15700
S17400
S17700

5) Duplex stainless steels [Fe-Cr-Ni-(Mo)-N]

These stainless steels combine both the austenitic and ferritic microstructures thus earning the name “duplex”. The carbon levels are very low (below 0.03%). Generally, the chromium composition is between 19.5 and 30.0% with nickel from 1.0 to 8.0%. They may contain molybdenum contents of up to 5% and nitrogen of up to 0.4%. These alloys are magnetic, and offer increased tensile and yield strength over the other categories. They are more resistant to stress corrosion cracking than austenitic, yet tougher than fully ferritic alloys.

Pipelines
Pressure shafting
Structural components
Industrial tanks

UNS S32001
S32003
S32101
S32205
S32304
S32507