Urea grade austenitic stainless steel U3 has all the elements of A4 steel (A4 equivalent to 316), and adds new elements of urine resistance, and has more erosion resistance and corrosion resistance. P4 valves used in fertilizer industry are often used to make valve core, seat and other valve components. The most important corrosion resistance of U3 steel is that it has much higher passivation ability than 00Cr17Ni14Mo2 (316L) steel. Even under severe reductive conditions (when the oxygen content of steel in passive state is very low in ammonium carbamate solution), the passivation film of U3 steel still exists, which ensures that the steel has good corrosion resistance. It is pointed out that 00Cr17Ni14Mo2 is in the active state when the oxygen content in ammonium carbamate solution is only 3 ppm at 200 C. The corrosion rate of 00Cr17Ni14Mo2 is as high as 40 mm/a. For 00Cr25Ni22Mo3N steel, the corrosion resistance is still the same, and the change is not significant. The higher nitrogen content in U3 steel also increases the strength of the material.