It’s Michael back for the 138th time to talk about the metals…
What do we see everyday in cities walking under bridges or over railroad tracks? Sculptures and buildings where flakes just appear to be falling off? Fences, bridges, and buildings show us rust. We see that steel corrodes. Steel, with moisture and oxygen forms rust. Rust that is porous and will continue to grow and flake until ultimately consuming all the steel.
But if we add enough Chromium to steel, it has the effect of allowing only a thin layer of an oxide to form that does not allow the corrosion to continue. A “stain less” steel.
The degree of corrosion resistance depends on the amount of Chromium in the alloy and the effects of some other elements. Various compositions have been established as standard alloys.
So which alloy do we use?
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, Precipitation Hardening or Duplex based on their metallurgical structure. Let’s discuss each of those breifly.
Austenitic stainless steels……. the AISI 200 and 300 series of alloys…..are not hardenable by heat treatment. . These grades are austenitic from over 1900 deg F to minus 300 deg F. 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. The annealed condition is the most corrosion resistant and most commonly used. All exhibit good strength and toughness in cryogenic applications.
Ferritic stainless steels ….. some of the AISI 400 series alloys……… are not hardenable by heat treatment as these grades remain ferritic over the critical temperature range. These grades respond to a magnet much like ordinary steel. AISI types 405 and 409 are 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. They 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 steels…… 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 in addition to having high strength and hardness. Depending on type, hardness can be from mid 30’s Rc (rockwell c) hardness for types 410 and 416 to 60 Rc for type 440c.
Heat treated hardness is dependent on Carbon content. Maximum hardness increases with carbon content. These types also respond to a magnet like the ferritic.
PH stainless steels ……. 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 deg F and 1150 deg 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.
Duplex stainless steels…. stainless steels with a mixed structure of austenite and ferrite have characteristics of each type. Most common alloy of this type is 2205.
Don’t forget to CHECK THE SPECS !!!!! Twice – This is Michael with Michael Talks Metal. Thanks for watching. For more on stainless steels visit our website www.michlinmetals.com. Missed last weeks video, click here. Still here and haven’t subscribed? Click here. Thanks for watching. See you a week from Thursday, we’re off next week for the holiday. See you 7/14, I’m OUT!