Ferritic stainless steels have the body-centered cubic structure of ferrite, but contain between 11 and 30 wt% chromium in solid solution . As in other types of stainless steels, it is the content of chromium that imparts the property of corrosion resistance to the alloy. Many grades of ferritic stainless steel also contain smaller amounts of alloying elements such as molybdenum, nickel, titanium, and niobium. In general, ferritic stainless steels are cheaper than the austenitic and duplex grades, and they usually possess better deep drawability and resistance to stress corrosion cracking.
Duplex Stainless Steel
They are called duplex grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) in roughly equal proportions. They are used for their good mechanical properties in the as-cast (and therefore as-welded condition) and/or their excellent corrosion resistance properties (particularly to stress corrosion cracking).
They are usually divided in three groups:
Standard Duplex Grade EN 1.4462 initially developed. and represents the “mid-range” of properties and is perhaps the most used today
Super Duplex Grade EN 1.4410 up to so-called Hyper duplex grades developed later to meet specific demands of the oil& gas as well as those of the chemical industries. They offer a superior corrosion resistance and strength but are more difficult to process because the higher contents of Cr, Ni, Mo, N and even W promote the formation of intermetallic phases, which reduce drastically the impact resistance of the steel.
Precipitation Hardening Stainless Steel
Precipitation Hardening (PH) stainless steels are classified as martensitic or semi-austenitic. They develop their high strength and hardness through a variety of heat treatments resulting in a very high strength-to-weight ratio.
17-4 PH is a martensitic, chromium-nickel-copper precipitation-hardening stainless steel. 17-4 PH stainless steel is capable of attaining a wide range of strength and toughness properties depending on the precipitation or aging temperature used in hardening. Its valuable combination of high strength, good corrosion resistance and toughness in both base metals and welds gives designers opportunities to add reliability to their products while simplifying fabrication and often reducing costs. High strength is maintained to approximately 600°F (316°C). This material has good resistance to stress corrosion cracking in the lower strength conditions.