Stainless steel can be divided into chromium stainless steel and chromium-nickel stainless steel according to alloy elements; according to the normalized metallographic structure, there are martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, and duplex stainless steel.
martensitic stainless steel
Martensitic stainless steel has better processing performance. It can perform deep drawing, bending, curling and welding without preheating. Typical martensitic stainless steel can be made into corrosion-resistant structural parts such as turbine blades, wear-resistant parts and corrosion-resistant bearings.
ferritic stainless steel
The chromium content of ferritic stainless steel is usually 13%-30%, and the carbon content is less than 0.25%. The metallographic structure in steel is mainly ferrite. There is no phase transformation during heating and cooling, and it cannot be strengthened by heat treatment. The material has strong oxidation resistance. And it also has good hot workability and good cold workability. Ferritic stainless steel is mostly used to produce components with high corrosion resistance but low strength requirements, such as equipment for the production of nitric acid and nitrogen fertilizers.
Austenitic stainless steel
Austenitic stainless steel was developed on the basis of solving the problem of insufficient corrosion resistance and excessive brittleness of martensitic stainless steel. The basic component is Cr18Ni8, also called 18-8 stainless steel. It is characterized by a carbon content of less than 0.1% and the use of chromium and nickel to obtain a single-phase austenite structure. Austenitic stainless steel has many applications.
Although austenitic stainless steel has better resistance to uniform corrosion, there are some problems in local corrosion resistance. For example, intergranular corrosion, stress corrosion, etc.
The main method to avoid intergranular corrosion is to reduce the carbon content in the stainless steel structure, so that the carbon content in the steel is less than the saturated solubility in austenite under equilibrium, which can completely solve the problem of chromium carbide precipitation on the grain boundaries. Generally, reducing the carbon content to less than 0.03% can meet the requirements for intergranular corrosion resistance; or adding elements such as titanium and niobium that can form stable carbides can prevent the precipitation of Cr23C6 on the grain boundaries and avoid austenite. Stainless steel is affected by intergranular corrosion; the ratio of austenite-forming elements and ferrite-forming elements in stainless steel can also be adjusted to give it a dual-phase structure, so that intergranular corrosion is less likely to form. Appropriate heat treatment processes can also be used to avoid intergranular corrosion and obtain good corrosion resistance.
The main way to avoid stress corrosion in austenitic stainless steel is to add silicon and control the nitrogen content below 0.04%. In addition, the content of impurities such as phosphorus and sulfur should be reduced as much as possible.
Austenitic ferritic duplex stainless steel
On the basis of austenitic stainless steel, by appropriately increasing the chromium content and reducing the nickel content, and combining it with the remelting treatment, a stainless steel with a dual-phase structure of austenite and ferrite can be obtained. Typical steel grades are 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti, etc. Duplex stainless steels have better weldability and do not require heat treatment after welding, and their tendency to intergranular corrosion and stress corrosion is also low. Just because the chromium content is high, it is easy to generate σ phase, so you need to pay special attention to this when applying.
language
English
Español
بالعربية
