Increasing Weldability of Service-Aged Reformer Tubes by Partial Solution Annealing

Abstract

A dissimilar joint of 25Cr-35Ni/30Cr-48Ni (HP/HV) heat-resistant steels was evaluated. The investigations indicated that the as-cast HP alloy contained M 7 C 3 , M 23 C 6 , and NbC carbides and HV alloy with 5 wt.% tungsten, contained M 23 C 6 and M 6 C carbides embedded in an austenitic matrix. After 8 years of ex-service aging at 1050°C, the ductility of HP/HV reformer tubes was decreased dramatically, and thus, the repair welding of the aged HP/HV dissimilar joint was at a risk. In order to repair the aged reformer tubes and increase weldability properties, a new partial solution annealing treatment was designed. Mechanical testing results showed that partial solution annealing at 1200°C for 6 h increased the elongation and toughness of the aged HP and HV alloys drastically. Also, a mechanism for constitutional liquation cracking in the heat-affected zones (HAZ) of the HP/HV dissimilar joint was proposed. In the HAZ of the aged HP/ HV welded joint, the cracks around the locally melted carbides were initiated and propagated during carbides solidification at the cooling cycle of welding associated with the decrease in the ductility of the aged HP and HV alloys. In addition, Varestraint weldability test showed that the susceptibility to hot cracking was decreased with partial solution annealing.

Evaluation of creep rupture behaviour for weld joint and base metal of the Cr35Ni45Nb alloy

“…As the weldability of the ethylene pyrolysis furnace tubes is deteriorated due to microstructure deterioration [18,19], carburisation and oxidation [20] in the service process, many researchers have studied welding repair parameters of ethylene cracking furnace tubes after a long-term service [21][22][23]. As the weak part of the ethylene pyrolysis furnace tube, the microstructure and mechanical properties of the Cr35Ni45Nb alloy weld joint after service are rarely reported.…”