Development of a Dissimilar Temper Bead Welding Procedure for an Amine Tower Repair

Abstract

An amine tower was inspected and was shown to have wall loss over the majority of the circumference. A repair plan was developed which included welding onto the tower while the tower remained in operation. The repair plan consisted of two steps the first of which was welding two Inconel 625 rings to the SA516-70N steel tower. Secondly, a stainless steel sleeve was welded to the Inconel 625 rings to cover the corroded area to contain the tower contents if the tower wall was breached and to add structural support until the next shut down. The typical requirements for welds that could possibly be exposed to amine service are outlined in the API Recommended Practice 945 document (API RP 945). API RP 945 recommends a post-weld heat treatment (PWHT) to reduce hardness and relieve stress but since the planned repair was to be made in-service, PWHT was not preferred. The objective was to develop a welding procedure to attach the Inconel 625 straps to the SA516-70N steel amine tower without using PWHT. A temper bead welding technique was used to successfully qualify a welding procedure with hardness values below 200 Brinell without PWHT, in accordance with 2006 NBIC and 2004 ASME Boiler and Pressure Vessel Code Section IX. The temper bead welding procedure required strict welding heat input control and weld bead placement. The heat input can be monitored by controlling the welding parameters or by using the run-out ratio diagram which was developed during this project. The temper bead passes need to be deposited in such a manner that the weld toe of the temper bead is within 3/32 in. (2.4 mm) of the weld toe of the initial layer. When manually welding Inconel 625 to SA516-70N with SMAW electrodes it is recommended that Inconel 182 electrodes be used rather than Inconel 112 electrodes. This recommendation is based on the increase ability of the Inconel 182 electrodes to form manganese oxides, which remove the oxygen from the weld metal. This in turn helps to avoid local brittle zones which caused the Inconel 112 weld to fail qualification.