6 Stainless Steels
Stainless steel was invented by Englishman Harry Brearley in 1913. While attempting to find a more corrosion resistant steel for gun barrels, he alloyed a sample with a larger than normal quantity of chromium. (Chromium had been used as an alloy in steels for a few years but in only very small quantities.) The result was highly resistant to the acid etching pen that he used to mark his specimens. As is often the case with invention, serendipity brings forth something new and unexpected. In this instance, a stainless steel was invented while attempting to find a more wear resistant steel.
Over the years many alloys have been mixed into stainless steel; however, the most common alloying elements are chromium and nickel. Therefore, carbon and chromium/nickel percentages give the best profile of this steel.
Number designations for Stainless Steels
The UNS designation system for stainless steel follows the AISI standard . By prefacing it with an S and adding two more digits to the end it becomes a UNS number. For example AISI/SAE 304 stainless steel is converted to S30400 UNS.
Key to the AISI/SAE stainless steel numbering system
S300xx (AISI/SAE 300) series are austenitic steels that provide the best corrosion resistance of all the stainless steels. However, they will not heat treat like carbon steels, and this limits them in strength. 300 series is easy to segregate from other steels because it is non-magnetic.
- The first digit is “3” as in 3xx
- The second and third digits do not correlate to any particular chemical combination or process. However, in general, chromium content increases with the last two-digit number.
Following are a few 300 series stainless steels familiar to most machinists (note how carbon, chromium, and nickel effect the properties of the steel).
S30300 (AISI/SAE 303)
- Carbon 0.15%
- Chromium 17 to 19%
- Nickel 8 to 10%
- Other alloys manganese, sulphur, phosphorus, silicon, zirconium, molybdenum
- Machinability excellent
- Hadenability none
- Weldability not recommended
- Applications nearly everything
- Comments one of the most popular easier machining stainless steels. Available in all sizes and forms.
S30400 (AISI/SAE 304)
- Carbon 0.18% max
-
Chromium 18 to 20%
- Nickel 8 to 12%
- Other alloys manganese, sulphur, phosphorus, silicon
- Machinability slow speeds heavy feeds, use chip breaker, will work harden
- Hardenability none
- Weldability fusion welding and resistance welding
- Applications nearly everything
- Comments This is the oldest and most widely used austenitic stainless steel. It possesses an excellent combination of strength, corrosion resistance and machinability. All sizes and forms are available.
S31600 (AISI/SAE 316)
- Carbon 0.08%
- Chromium 16 to 18%
- Nickel 10 to 14%
- Other alloys manganese, sulphur, phosphorus, silicon, molybdenum
- Machinability slow speeds heavy feeds, use chip breaker, will work harden
- Hardenability none
- Weldability fusion and resistance – no
- Applications marine hardware
- Comments best corrosion resistance of the three, widely available.
Martensitic Stainless Steels
S400xx (AISI/SAE 400) series are martensitic and steels that are not quite as corrosion resistant as the 300 series. However the non-ferritic varieties will heat treat quite well with ordinary methods. This provides a great deal of utility because the engineer can design machinery that will stand up to caustic cleaning solutions – like that present in food processing plants – yet still have strong hardened machine components. Most 400 series stainless steel is magnetic like carbon steel.
Martensitic and Ferritic Stainless Steels
- The first digit is always “4” as in 4xx
- The second and third digits do not correlate to any particular chemical combination or process. However, in general, chromium content increases with the last two-digit number.
Following are three 400 series stainless steels which the machinist will be familiar: Note how carbon, chromium, and nickel change their properties.
S41600 (AISI/SAE 416 )
- Carbon 0.08%
- Chromium 16 to 18%
- Nickel 10 to 14%
- Other alloys manganese, sulphur, phosphorus, silicon, molybdenum
- Machinability easy to machine
- Hadenability good oil hardening
- Weldability not recommended
- Applications shafting, axles, screw machine parts
- Comments the first free machining stainless. Widely available
S43100 (AISI/SAE 431)
- Carbon 0.2%
- Chromium 15 to 17%
- Nickel 1.25 to 2.5%
- Other alloys manganese, sulphur, phosphorus, silicon
- Machinability galling chips and inferior finishing
- Hadenability oil or air quenching
- Weldability arc welding only
- Applications aircraft fasteners and bolts
- Comments excellent strength, high hardness levels, best corrosion resistance of any 400 series stainless steel.
S44000 (AISI/SAE 440)
- Carbon 0.60 to 1.2%
- Chromium 15 to 17%
- Nickel none
- Other alloys manganese, sulphur, phosphorus, silicon, molybdenum
- Machinability tough stringy chips, use carbide chip breaker, will grind nicely
- Hadenability good hardening with air or oil quench
- Weldability do not weld
- Applications molds, dies, valve components
- Comments this steel has three sub-parts, 440A, 440B, and 440C, all of which can harden progressively from Rockwell 56 to Rc 60. Poor corrosion resistance.
Martensitic Stainless Steels that provide good corrosion resistance
S1xxxx (AISI/ASE 15 and 17) series are martensitic steels that provide good corrosion resistance. They are similar to 400 series in most ways. However, they do not use same method of heat treating that is usually used with carbon steels or 400 series stainless steels. They are heat treated using the Precipitation Hardening (PH) method. Also, this steel – like 400 series – is magnetic.
Precipitation Hardening was first used on Aluminum in 1909 by Alfred Wilm. However, the application of these principles was not used on stainless steels with any significance until after WWII when the aerospace industry began to demand steel with the characteristics PH ultimately provided. These unique heat-treating process can be used with both austenitic and martensitic steels which means high corrosion resistance as well as hardenability are available with a single stainless steel.
Precipitation Hardening Stainless Steel
- The first digit is “6” as in 6xx
Following are two of these PH stainless steels familiar to most machinists:
S15500 (AISI/SAE 15-5 or 15-5PH)
- Carbon no data
- Chromium no data
- Nickel no data
- Other alloys no data
- Machinability machinable in most conditions
- Hadenability precipitation method
- Weldability resistance methods or shielded fusion
- Applications aircraft parts, valves, fasteners
- Comments martensitic, precipitation hardening, chromium-nickel-copper stainless steel, corrosion resistance comparable to 304 stainless.
S17400 (AISI/SAE 17-4 or 17-4PH)
- Carbon no data
- Chromium no data
- Nickel no data
- Other alloys no data
- Machinability gummy chips, stringy chips, use chip breaker
- Hadenability precipitation method
- Weldability resistance methods or shielded fusion
- Applications jet turbines, chemical plants
- Comments the most widely used precipitation hardening stainless, good corrosion resistance, high harness, toughness and strength.
Derived from Wikipedia (Stainless steel – Wikipedia) accessed and available online 13 January 2024 and The Virtual Machine Shop:
- http://jjjtrain.com/3engineering/7eng_metalsl_hist/eng_metal_hist_03.html
- http://jjjtrain.com/3engineering/7eng_metalsl_hist/eng_metal_hist_04.html
- http://jjjtrain.com/3engineering/7eng_metalsl_hist/eng_metal_hist_05.html) accessed and available 13 January 2024.
retrieved from Wayback Machine 16 January 2024.