Some Contribution to the Selection of Filler Metals in Welding of Stainless Steels for Urea Service

Urea processes impose severe corrosion conditions on the materials used in equipments such as reactors, decomposers, absorbers, etc. Stainless steel of 316L type has been extensively used by various urea processes. Corrosion rates of SS 316L obtained in boiling nitric acid test (Huey Test) closely resembles the rates obtained in urea equipment and hence the test can be considered as fairly representative for practical purposes. Various elements namely C, Si, N, P, S, Cr, Ni, and Mo affect weldability and sensitisation characteristics of weld metal and HAZ. Proper balancing of the elements in the filler metal is a must for successful application. Nitrogen content in weldmetal in the range of 0.1-0.2% has been found to increase the tensile strength of the weld metal without any detriment to the corrosion resistance properties. Extensive tests on 316L (modified) led to this conclusion. Latest research and development work finds 25Cr-22Ni-Mo-N type of steel with higher Mn-content to be superior to 316L (modified). Use of higher nickel steel and Incoloys may have an edge in certain applications.