Austenite, ferrite, martensite, PH, and duplex stainless steel
What are the different types of stainless steel? Can austenitic stainless steel be heat treated? Can ferritic stainless steel be heat treated? What stainless steel can you heat treat? What makes stainless steel rust resistant?
Ordinary daily experience tells us that steel corrodes. Give it water and oxygen, and it will rust. Porous rust will continue to grow and peel off until it eventually consumes all the steel. But adding enough chromium to the steel only forms a thin layer of oxide, which will not allow corrosion to continue. Based on the chromium content in the alloy and the effects of some other elements, various components with various combination characteristics have been established as standard alloys.
The specific environment, temperature, required strength, manufacturability, and ultimately... cost all involve choosing which type of stainless steel to use in any given application.
Stainless steel is classified into austenite, ferrite, martensite, duplex or precipitation hardening according to its metallurgical structure.
austenitic stainless steel
01
The AISI 200 and 300 series... cannot be hardened through heat treatment. HT depends on the structure changing with temperature. From high temperatures up to 1900 ° C to extremely low negative 300 ° C, these grades remain in an austenitic state. Under normal conditions, there is almost no reaction to magnets. Cold working this material can make it slightly magnetic. The common types commonly referred to as "18-8" (which means that the nominal amount of CR is 18% and the nominal amount of Ni is 8%) are 303, 304, and 316. 316 also adds molybdenum, which improves corrosion performance in many environments compared to 304. In addition, 316 has stronger antioxidant properties at higher temperatures. The addition of sulfur to 303 stainless steel improves processing characteristics, but sacrifices some corrosion resistance. Annealed state is the most corrosion-resistant and commonly used. It exhibits good strength and toughness in low-temperature applications.
02
Some AISI 400 series alloys cannot be hardened through heat treatment because these stainless steels still maintain a ferrite state within the critical temperature range. Some stainless steels react to magnets much like ordinary steel. The "pure Cr" grade of 405 and 409 stainless steel has a relatively low Cr content, making it suitable for automotive exhaust applications where appearance is not important. 430 stainless steel and other alloys with higher chromium content can resist corrosion at higher temperatures and maintain a better appearance for applications such as automotive or home appliance decoration at a lower cost than austenitic grade.
Annealed state is the most corrosion-resistant.
Martensitic stainless steel composed of allowances from the AISI 400 series can be hardened through conventional heat treatment. Heat treatment is similar to the heat treatment of alloy steel. They have an austenitic structure at high temperatures and undergo rapid cooling to transform into a martensitic structure. They are usually used under fully hardened conditions to achieve optimal corrosion resistance, while also possessing high strength and hardness. According to different types, the intermediate value of HRc (Rockwell hardness) for 410 and 416 stainless steel can be 60HRc for 440c stainless steel, as the maximum hardness after heat treatment depends on the carbon content. The achievable hardness increases with the increase of carbon content. These stainless steels also react to magnets like ferrite.
PH precipitation hardened stainless steel
03
PH stands for "precipitation hardening". This means that they can be hardened through heat treatment. They usually also react to magnets. The most common alloy is 630 stainless steel, commonly referred to as "17-4". Heat treatment includes high-temperature "solution treatment" (also known as annealing or solution annealing), followed by "aging" at temperatures between 900 F and 1150 F. At 900 hours, the strength and general corrosion performance are higher. The strength decreases with the increase of temperature, while the aging temperature increases but the toughness increases. For certain specific environments, corrosion performance has also been improved. In these applications, strength and corrosion performance are factors that need to be considered.
Finally, there is duplex stainless steel, which is a type of stainless steel with a mixed structure of austenite and ferrite, used for more specific applications than in this stainless steel introduction. Many are proprietary names, while a few, such as 2205, are considered standard or universal stainless steel names.