Abstract
Ethylene cracking furnace tube coils, which are typically made of high carbon Fe-Ni-Cr alloys, are subject to microstructural changes due to creep deformation, carburization, and thermal aging during high temperature services at temperatures up to 2000 °F (1090 °C). It is common practice to perform repair welding by replacing some degraded tube sections with virgin tube sections using gas-tungsten arc welding (GTAW) process. Welding of the service-exposed tube to a virgin tube can be a challenging task due to the aforementioned microstructural changes. This is particularly to be the case when the tubes are made of high carbon, high Cr micro-alloyed material, such as 35-45Nb MA alloy. The commonly used manual process can be more prone to developing cracking during repair welding due to inconsistent heat input among other factors during manual welding. The present paper discusses the results of mock-up trials involving repair welding of 35-45Nb MA alloy tube after service of 3-1/2 years by comparing the automatic orbital GTAW process with the manual GTAW process. The results of metallurgical evaluation of the weld joints produced by two different welding processes are presented.
