Evaluation of the Potential for Liquid Metal Embrittlement of 304L Stainless Steel by Galvanized Fasteners in an Industrial Fire Scenario

摘要

Liquid Metal Embrittlement (LME) is a phenomenon that promotes a drastic loss of ductility in normally ductile alloys in the presence of certain liquid metals. Zinc (Zn) at temperatures above its melting point [400°C (752°F)] has been known to promote LME in austenitic stainless steels under tensile loads. Although LME of austenitic stainless steel is more likely when Zn is in its molten state, occurrences of Zn induced LME have been reported at temperatures below 400°C (752°F). Industry has adopted a number of practices to prevent LME of SS pressure-retaining equipment by reducing the contact with Zn. However, the use of mechanically galvanized low-alloy fasteners in SS flange joints, joints under a local compressive load is common practice. Up until now, the potential risk for LME of 300-series stainless steel flanges caused by galvanized fasteners during high temperature exposure (e.g. during an industrial fire scenario) has not been evaluated. This study examines the transfer of Zn from hot-dipped galvanized and mechanically galvanized steel bolting to stainless steel by exposing a 304L stainless steel/Galvanized bolting assembly to temperatures in the range 205°C (400°F) to 537°C (1000°F) for approximately one hour to simulate an industrial fire scenario. Exposed samples were examined using optical and scanning electron microscopy. SEM/EDS analysis shows evidence of Zn transfer to stainless steel surface beneath the fasteners. However, metallographic examination did not reveal any infiltration/diffusion of Zn into the stainless steel matrix.