ALLOY CLASSIFICATIONS - STAINLESS STEEL OVERVIEW

Time:2017-11-20 Click:164

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.


Characteristics

  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

Examples: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.


Characteristics

  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

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


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.


Characteristics

  1. Adequate corrosion resistance
  2. Hardenable by heat treatment
  3. Magnetic
  4. Somewhat limited temperature use
  5. Limited weldability

Examples: 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.

Common Applications: valves, gears, and petrochemical equipment
Examples: UNS S13800, S15500, S15700, S17400, and S17700
Fabrication information can be obtained from the stainless steel producers


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.


Common Applications: pipelines, pressure shafting, structural components, and industrial tanks
Examples: UNS S32001, S32003, S32101, S32205, S32304, S32507