Failure analysis of cracking in the thin-walled pressure vessel of electric water heater

Abstract

The inner tank is the kernel component of the electric water heater. However, the stress concentration and presence of corrosive media often result in premature failure of the weld area. In this paper, the leakage failure of the circumferential weld (C-Weld) of the DC01EK inner tank was analyzed by optical microscopy, scanning electron microscopy, energy dispersive spectrometer, and microhardness test. Quantitative calculations of stress distribution of the C-Weld were performed by finite element simulations. Results show that the fracture of the C-Weld was induced by stress corrosion cracking (SCC). The corrosion and crack propagation resistance of the C-Weld are worsened by the presence of coarse grains in HAZ. The high axial stress component generated by the superposition of welding residual stress and working stress is the main driving force of crack expansion. The presence of the root gap of the C-Weld leads to stress concentration, which is more likely to cause scaling and accumulation of chloride ions, resulting in crevice corrosion. Reducing the root gap of the C-Weld, decreasing the welding heat input, and using the butt joint instead of the lap joint are suggested to eliminate such failures.