Weld overlay of highly corrosion resistant nickel chromium molybdenum alloys, UNS N06059, on low alloy equipment operating at high temperature

Abstract

Corrosion conditions in the Chemical and Oil and Gas Industries are such that equipment must withstand pressure and corrosion at high temperatures. Due to the fact that in many cases only the internal surface is exposed to corrosive attack, and so only that surface needs to be protected, savings on material costs can be achieved by taking advantage of the weld overlay technique. Weld overlay, through the choice of alloy, is selected to resist to the corrosive attack. Parent material can be selected based on the required strength at temperature. In recent years, highly corrosion resistant Nickel Chromium-Molybdenum Alloys have been developed to operate in the most severe corrosive environments. One of the recent developments in this family of materials, is Alloy 59 UNS N06059 that is replacing Hastelloy C-2000 UNS N06200 and Hastelloy C-22 UNS N06022. Nickel-Chromium-Molybdenum alloys are the most versatile nickel alloys, because they contain molybdenum, which protects against corrosion under reducing conditions, and chromium which protects against corrosion under oxidizing conditions. Wrought products produced from these alloys are processed to obtain a homogeneous austenitic grain structure. Weld overlay or weld deposit, due to the fundamental differences in processing compared to base material, result in a more heterogeneous metallurgical grain structure that in general shows however a good level of corrosion resistance. Weld or Weld overlay features are highly dependent on electric parameters, filler metal, technique, base material temperature, etc., and for this reason the general settings of the welding process need to be finalized and tested, with the intent of optimizing all these parameters in order to allow the best corrosion results. A research activity has been carried out by ALFA LAVAL OLMI to define a processes and parameters' selection in order to obtain a weld deposit with a behavior as close as possible to wrought Alloy 59 base material.