Crack failure analysis of laser 316L stainless steel edge joints in pillow plate heat exchanger used in oil refinery

The new generation pillow plate heat exchanger (PPHX) is a type of PHXs with a complex wavy structure with desirable thermal-hydraulic characteristics. For the main structure of PPHX, edge welding joints are usually a common and economical design. However, the stress concentration and presence of corrosive media often lead to premature failure of welding structure. In this study, the leakage failure of PHX after 80,000 hours of service was evaluated. The crack at the edge joints of corrugated 316L steel plates was detected by dye permeability detection. The failure of welding joints was studied by optical microscope, scanning electron microscope(SEM), EBSD and X-ray diffraction(XRD). The corrosion product was observed only at inside of the edge joints. The mud pattern morphology and EDS analysis of high Cl ion concentration(2.87 wt.%) on fracture surface revealed the stress corrosion cracking mechanism. Chloride and hydrogen ions were enriched as an occluded cell mechanism on the narrow gap between plates. The carbide precipitated on the grain boundary of the HAZ during the welding process, which initiates cracking under the crevice corrosion environment. Based on the studies carried out, a structure modification welding without narrow gap, duplex or ferrite stainless steel replace the 316L stainless steel, periodic inspection and strict deionization of cooling water were suggested to eliminate such failures.