Failure Analysis of Cracking in T-Type Welded Joints between Tube Sheet and Shell of Low-Pressure Heaters in a Power Plant

Abstract

After 5 a of service, severe cracking occurred at the T-type weld seam between the tube sheet and shell of a low-pressure heater in a certain power plant. The causes of the weld cracking were analyzed by macroscopic inspection, fracture morphology analysis, metallographic structure analysis, energy-dispersive spectroscopy(EDS), chemical composition analysis and hardness testing. Results showed that due to the existence of a hardened martensite structure in the heat-affected zone(HAZ) of the welding, which was sensitive to alkali stress corrosion cracking, and the lack of stress reduction heat treatment of the welded joint, alkali embrittlement occurred in the welding heat affected zone of tube plate, and crack branches propagated and penetrated through the weld, causing equipment failure. The analysis of the failure causes of T-type welded joints is of great significance for the design, manufacture, operation of heat exchangers in similar situations under the same working conditions.

Keywords

• t-type welded joint; martensite; heat affected zone; alkaline embrittlement; heat treatment