Duplex stainless steel and use thereof

A method for producing urea comprising reacting ammonia and carbon dioxide under urea forming conditions in a urea production plant, wherein the method comprises forming ammonium carbamate, and wherein the ammonium carbamate has an oxygen level that is less than 0.1 ppm (by weight), wherein at least one part of the equipment of the urea production plant is made from a duplex stainless steel comprising in weight % (wt %):

• C max 0.030;
• Si max 0.8;
• Mn max 2.0;
• Cr 29.0 to 31.0;
• Ni 5.0 to 9.0;
• Mo less than 4.0;
• W less than 4.0;
• N 0.25-0.45;
• Cu max 2.0;
• S max 0.02;
• P max 0.03;
• balance Fe and unavoidable occurring impurities;
• and
• wherein the content of Mo+W is greater than 3.0 but less than 5.0.

A plant for the production of urea comprising reacting ammonia and carbon dioxide under urea forming conditions in a urea production plant, wherein said plant comprises one or more parts comprising a duplex stainless steel comprising in weight % (wt %):

• C max 0.030;
• Si max 0.8;
• Mn max 2.0;
• Cr 29.0 to 31.0;
• Ni 5.0 to 9.0;
• Mo less than 4.0;
• W less than 4.0;
• N 0.25-0.45;
• Cu max 2.0;
• S max 0.02;
• P max 0.03;
• balance Fe and unavoidable occurring impurities;
• wherein the content of Mo+W is greater than 3.0 but less than 4.0,
• wherein the duplex stainless steel has a corrosion rate of 0.186 to 0.191 mm/year, wherein the corrosion rate is evaluated on a sample of the duplex stainless in an oxygen-free carbamate solution at a temperature of 210° C. for 14 days, and wherein the sample is in the form of a coupon having dimensions of 20×10×3 mm obtained from a 5 mm strip produced by hot warming to 1200° C. and cold rolling at room temperature with intermediate annealing at 1100° C. and final annealing at 1070° C., and surfaces of the sample were machined and finished by wet grinding.

A method of modifying an existing plant for the production of urea comprising reacting ammonia and carbon dioxide under urea forming conditions in a urea production plant, said plant comprising one or more components selected from the group consisting of liquid distributor, radar cones, (control) valves and ejectors, wherein said method the step of replacing one or more stripper tubes by a stripper tube comprising a duplex stainless steel comprising in weight % (wt %):

• C max 0.030;
• Si max 0.8;
• Mn max 2.0;
• Cr 29.0 to 31.0;
• Ni 5.0 to 9.0;
• Mo less than 4.0;
• W less than 4.0;
• N 0.25-0.45;
• Cu max 2.0;
• S max 0.02;
• P max 0.03;
• balance Fe and unavoidable occurring impurities;
• and
• wherein the content of Mo+W is greater than 3.0 but less than 4.0,
• wherein the duplex stainless steel has a corrosion rate of 0.186 to 0.191 mm/year, wherein the corrosion rate is evaluated on a sample of the duplex stainless in an oxygen-free carbamate solution at a temperature of 210° C. for 14 days, and wherein the sample is in the form of a coupon having dimensions of 20×10×3 mm obtained from a 5 mm strip produced by hot warming to 1200° C. and cold rolling at room temperature with intermediate annealing at 1100° C. and final annealing at 1070° C., and surfaces of the sample were machined and finished by wet grinding.