What’s urea grade stainless steel?
Urea grade stainless steel is a kind of austenitic stainless steel specially for use of urea industry. Derivatives such as CO2, NH3, NH2COONH4, (NH2)2CO, etc. in the process of urea production are not highly corrosive when they exist alone, but the reactant formed when they are mixed together will cause strong corrosion to the equipment. Urea-methylammonium solution contains ammonium carbamate and ammonium cyanate, which has strong corrosion under high temperature and high pressure on the stainless steel, can destroy the stainless steel passivation film, so that contains 2% to 3%Mo of Cr-Ni Austenitic stainless steel is difficult to resist corrosion. In addition, the lining of the urea synthesis column, stripper and methylammonium condenser are easily corroded by methylammonium.
In the 1950s, Dutch company Stamicarbon proposed the use of 18-8 austenitic stainless steel under the condition of carbon dioxide and oxygen protection can be used for the urea industry, but there is still serious corrosion. With the development of ultra-low carbon stainless steel smelting technology, the purity of steel is greatly improved and finally formed into stainless steel for urea production. The chemical composition, microstructure and corrosion resistance of urea grade stainless steel are clearly defined, with the purpose of fully inhibiting the ferritic content in steel and obtaining the full austenitic structure, free of intermetallic phase precipitations as far as possible, so as to improve the corrosion resistance and selective corrosion ability of steel. All urea grade stainless steels are required to pass Huey test and selective corrosion test. In addition, metallographic inspection of the material is also required. No sigma phase and metal inclusions are allowed. These harsh test conditions make it difficult to meet the general stainless steel such as 316L.
310MoLN（S31050/725LN /1.4466/2Re69）and 316L mod（724L/316LN）are the most widely used urea grade stainless steels. 310MoLN is urea grade ultra-low carbon Austenitic stainless steel with a density of 7.9g/cm. Its corrosion resistance is similar to that of 904L steel, offers excellent corrosion resistance especially in ammonium carbamate and nitric acid. The 316L Modified stainless steel has low carbon content, extra-low silicon content and substantial higher molybdenum contents. 316L modified is designed for improved corrosion resistance properties in urea-carbonate environments.
The urea grade stainless steel offers excellent welding hot crack resistance and intergranular corrosion resistance, especially has good corrosion resistance for sulfurous acid, sulfuric acid, phosphoric acid, acetic acid, formic acid and chlorine salt, halogen, sulfite, has been successfully applied in urea production device such as a full cycle method of urea equipment, such as synthetic tower, such as high-pressure separator, also has been widely used in the pulp and paper, chemical fertilizers, chemical industry, pharmaceutical, synthetic fiber industry and other industries.