Urea grade stainless steel is an austenitic stainless steel specifically used in the urea industry. The CO2, NH3, NH2COONH4, (NH2)2CO and other derivatives in the urea production process are not very corrosive when they are present alone, but the reactants formed by mixing them together can cause strong corrosion to the equipment. Urea methyl ammonium solution contains ammonium carbamate and ammonium cyanate, which has a strong corrosive effect on stainless steel at high temperature and pressure, and can destroy the stainless steel passivation film, so the Cr-Ni austenitic stainless steel containing 2% to 3% Mo is difficult to resist corrosion. In addition, urea synthesis tower, steam tower and the lining of the ammonium condenser is susceptible to corrosion by ammonium methane.
In the 1950s, the Dutch Stamicarbon company proposed in carbon dioxide and oxygen protection conditions using 18-8 austenitic stainless steel can be used in the urea industry, but there is still serious corrosion. With the development of ultra-low carbon stainless steel smelting technology, the purity of the steel is greatly improved, the final formation of stainless steel for urea production. Clarify the chemical composition, microstructure and corrosion resistance of urea grade stainless steel, the purpose is to fully suppress the ferrite content of the steel, to obtain a complete austenitic organization, as far as possible without intermetallic phase precipitates, thus improving the ferrite content of the steel. Corrosion resistance and selective corrosion ability of the steel. All urea-grade stainless steels need to pass the Huey test and selective corrosion test. In addition, a metallographic examination of the material is required. No σ-phase or metal inclusions are allowed. These demanding test conditions make it difficult to meet the requirements of general stainless steels 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 a urea grade ultra-low carbon austenitic stainless steel with a density of 7.9 g/cm. Its corrosion resistance is similar to that of 904L steel, with excellent corrosion resistance, especially in ammonium aminocarbon and nitric acid. 316L modified stainless steel has low carbon content, ultra-low silicon content and much higher molybdenum content. 316L modified is designed to improve corrosion resistance in urea carbonate environments.
Urea grade stainless steel has excellent resistance to welding thermal cracking and intergranular corrosion, especially good corrosion resistance to sulfite, sulfuric acid, phosphoric acid, acetic acid, formic acid and chloride, halogen, sulfite, has been successfully applied to urea production equipment such as full cycle method urea equipment, such as synthesis tower, high pressure separator, etc. It has also been widely used in the pulp and paper, fertilizer, chemical, pharmaceutical, synthetic fiber industry and other industries.
