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Ordinary every day experience shows us that steel corrodes. Give it moisture and oxygen and it forms rust. Rust that is porous and will continue to grow and flake until ultimately consuming all the steel. But add enough Chromium to steel and it has the effect of allowing only a thin layer of an oxide to form which does not allow the corrosion to continue. Depending on the amount of Chromium in the alloy and effects of some other elements, various compositions have been established with a variety of combined characteristics as standard alloys.

Specific environment, temperature, strength required, fabricability, and ultimately… cost… are all involved in selecting which stainless steel is used in any given application.

Stainless steels are classified as Austenitic, Ferritic, Martensitic, Duplex or Precipitation Hardening based on their metallurgical structure.

Austenitic Stainless……. the AISI 200 and 300 series…..are not hardenable by heat treatment. HT depends on a change in structure with change a in temperature. These grades remain austenitic from high temperatures up to 1900° to the very low minus 300. There is little or no response to a magnet under normal conditions. Cold working this material can make it slightly magnetic. Common types known generically as “18-8” ( which means there’s a nominal amount of CR at 18 % and Ni at 8% content) are 303, 304 and 316. 316 also has Molybdenum added which boosts the corrosion performance as compared to 304 in many environments. In addition, 316 has more oxidation resistance at higher temperatures. Type 303 has Sulfur added which improves the machining characteristics but sacrifices some corrosion performance. Annealed condition is most corrosion resistant and most commonly used. All exhibit good strength and toughness in cryogenic applications.

Ferritc Stainless – some of the AISI 400 series alloys are not hardenable by heat treatment as these grades remain ferritic over the critical temperature range. Some of these grades respond to a magnet much like ordinary steel. “Straight Cr ” grades with types 405 and 409 relatively low in Cr and are used in auto exhaust applications where appearance is not important. Higher Cr alloys like type 430 resist corrosion at higher temperatures and maintain better cosmetic appearance for applications like automotive or appliance trim at lower cost than the austenitic grades.
Annealed condition is most corrosion resistant.

Martensitic Stainless which consists of  the balance of the AISI 400 series are hardenable by conventional heat treatment. Heat treatment is similar to heat treating alloy steels. They have an austenitic structure at high temperature and will transform with rapid cooling to a martensitic structure.They are typically used in a fully hardened condition for best corrosion performance alongside having high strength and hardness. Depending on type, that can be from mid 30’s Rc (rockwell c)  hardness for types 410 and 416 to 60 Rc for type 440c as heat treated maximum hardness is dependent on Carbon content. Achievable hardness increases with carbon content.These types also respond to a magnet like the ferritic.

PH Stainless the AISI 600 series. PH stands for “Precipitation Hardening”. That means they are hardenable by heat treatment. They also typically respond to a magnet. Most common alloy is type 630 commonly known as “17-4”. Heat treatment consists of a high temperature “solution treatment” (aka annealed or solution annealed) followed by an “aging” at temperatures between 900 F and 1150 F. Strength and general corrosion performance is higher with aging at 900. Strength is reduced with higher aging temperature but toughness increases. Corrosion performance also improves for some specific environments. Strength and corrosion performance are both considerations in applications for these grades.

Lastly there’s Duplex Stainless, which are stainless steels with a mixed structure of austenite and ferrite used in more specific applications than in this introduction to stainless steels. Many are proprietary names and a few like 2205 are considered a standard or common grade designations.

 

I really hope you enjoyed learning about stainless steel. Over the coming weeks I will be discussing the types of stainless, 200, 300, 400, PH and Duplex grades in more detail. Make sure to subscribe and stay tuned to Metal Man Mike for more!