Based on the best characteristics of austenite and ferrite, duplex stainless steels offers excellent resistant to corrosion and very high mechanical strength.
Ferretic Steel
A category of stainless steel with high amounts of chromium and no Significant nickel content. resulting in lower corrosion resistant than austenitic stainless steels. However, they have slightly higher yield strengths and much lower strain hardening than austenitics. Ferritic steels have body centered cubic crystal, are less ductile than austenitic steel, and are not hardenable by heat treatment like martensitic steels. Older ferritics (such as AISI 409 and 430) are used mainly for household utensils and other applications not demanding in anti-corrosion properties. Ferritics with high chromium content (such as AISI 446) are used mainly for high temperature (but below 475°C) applications, and those with extremely low carbon and nitrogen content (such as S44400) are used where protection against stress corrosion cracking is required. They are the second largest selling type of stainless steels behind austenitics.
These steels are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold worked and softened by annealing. As a group, they are more corrosive resistant than the martensitic grades, but generally inferior to the austenitic grades. Like martensitic grades, these are straight chromium steels with no nickel. They are used for decorative trim, sinks, and automotive applications, particularly exhaust systems.
Type 430
The basic ferritic grade, with a little less corrosion resistance than Type 304. This type combines high resistance to such corrosives as nitric acid, sulfur gases, and many organic and food acids.
Type 405
Has lower chromium and added aluminum to prevent hardening when cooled from high temperatures. Typical applications include heat exchangers.
Type 409
Contains the lowest chromium content of all stainless steels and is also the least expensive. Originally designed for muffler stock and also used for exterior parts in non-critical corrosive environments.
Type 434
Has molybdenum added for improved corrosion resistance. Typical applications include automotive trim and fasteners.
Type 436
Type 436 has columbium added for corrosion and heat resistance. Typical applications include deep-drawn parts.
Type 442
Has increased chromium to improve scaling resistance. Typical applications include furnace and heater parts.
Type 446
Contains even more chromium added to further improve corrosion and scaling resistance at high temperatures. Especially good for oxidation resistance in sulfuric atmospheres.
