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.

GradeDescriptionApplications
GradeDescriptionApplications
304General PurposeAir 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
305Ni increased to lower work hardeningCoffee urn tops
Deep drawn parts
Expanded metal parts
Formed parts (requiring extra low work hardening characteristics)
Free spinning operations
Mixing bowls
Reflectors
309SCr & Ni increased for high temperatureAnnealing boxes, Chemical processing equipment (elevated temperature), Conveyor parts, Dryers, Electric range elements, Furnace parts
310SSame as 309, only more soAnnealing boxes, Furnace belts, Chemical processing equipment (elevated temperature), Conveyor parts
Dryers, Furnace parts
316Mo added to increase corrosion resistanceBus/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
317More Mo & Cr added for better corrosion performanceChemical processing equipment, Ink manufacturing equipment, Pharmaceutical processing equipment, Fertilizer handling equipment
321Ti added to prevent carbicle precipitationAircraft exhaust manifold and flanges, Chemical equipment, Fire walls, Pressure tanks
347Cb added to prevent carbicle precipitationAircraft exhaust manifold and flanges, Expansion joints, Furnace stacks and dampers, Welded storage tanks for organic chemicals
330Si increased for highest heat resistanceAnnealing boxes, Combustion chambers, Burners and exhaust parts
409Lower Cr Primarily used for automotive exhaustAutomotive exhaust systems
Bus frames
Cargo containers
Fins for heater tubes
Perforated sheet for silencer housings
Smoke control ductwork
Transformer and capacitor cases
410/410SGeneral PurposeBushings, Trays for oil fractionating towers, Hardware, Cutlery, Press plates
420Increased C to improve mechanical propertiesCutlery/Scissors, Dental equipment, Surgical equipment
430P & S added for improved machiningArchitectural applications (interior), Automotive trim, Cooking utensils, Food processing equipment
434Mo added for improved corrosion resistance in auto trimAutomotive trim
  1. Excellent corrosion resistance
  2. Typical strength (in the annealed condition) 85 ksi – tensile, 40 ksi – yield and 50% elongation
  3. Cannot be heat treated but can be hardened by “cold working” (up to 185 ksi – tensile, 140 ksi – yield
  4. Non-magnetic
  5. Good high and low-temperature mechanical properties
  6. Excellent formability and weldability
  7. 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.

GradeDescriptionApplications
GradeDescriptionApplications
304General PurposeAir 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
305Ni increased to lower work hardeningCoffee urn tops
Deep drawn parts
Expanded metal parts
Formed parts (requiring extra low work hardening characteristics)
Free spinning operations
Mixing bowls
Reflectors
309SCr & Ni increased for high temperatureAnnealing boxes, Chemical processing equipment (elevated temperature), Conveyor parts, Dryers, Electric range elements, Furnace parts
310SSame as 309, only more soAnnealing boxes, Furnace belts, Chemical processing equipment (elevated temperature), Conveyor parts
Dryers, Furnace parts
316Mo added to increase corrosion resistanceBus/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
317More Mo & Cr added for better corrosion performanceChemical processing equipment, Ink manufacturing equipment, Pharmaceutical processing equipment, Fertilizer handling equipment
321Ti added to prevent carbicle precipitationAircraft exhaust manifold and flanges, Chemical equipment, Fire walls, Pressure tanks
347Cb added to prevent carbicle precipitationAircraft exhaust manifold and flanges, Expansion joints, Furnace stacks and dampers, Welded storage tanks for organic chemicals
330Si increased for highest heat resistanceAnnealing boxes, Combustion chambers, Burners and exhaust parts
409Lower Cr Primarily used for automotive exhaustAutomotive exhaust systems
Bus frames
Cargo containers
Fins for heater tubes
Perforated sheet for silencer housings
Smoke control ductwork
Transformer and capacitor cases
410/410SGeneral PurposeBushings, Trays for oil fractionating towers, Hardware, Cutlery, Press plates
420Increased C to improve mechanical propertiesCutlery/Scissors, Dental equipment, Surgical equipment
430P & S added for improved machiningArchitectural applications (interior), Automotive trim, Cooking utensils, Food processing equipment
434Mo added for improved corrosion resistance in auto trimAutomotive trim
  1. Good corrosion resistance
  2. Typical strength 65 – 75 ksi – tensile, 35 – 50 ksi – yield and elongation 20 – 35%
  3. Magnetic
  4. Limited temperature use
  5. 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
  1. Adequate corrosion resistance
  2. Hardenable by heat treatment
  3. Magnetic
  4. Somewhat limited temperature use
  5. 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