Abstract
One of the main concerns during manufacturing of timber screws of ultra high-strength steels (UHSS) is hydrogen absorption. Steel may absorb hydrogen either from the gas atmosphere during heat treatment or from aqueous solutions during surface treatments. Once a critical hydrogen content is reached, delayed fracture due to hydrogen embrittlement may cause failure during service of the screws. In this work, hydrogen contents were determined after certain manufacturing step of the screws. The screws were taken directly from the production line and they were stored in liquid nitrogen. A special procedure for cleaning of the screw surfaces was developed, which enabled performing thermal desorption analyses (TDA) using a Bruker Galileo G8 equipped with a thermal conductivity sensor. Significant hydrogen contents were measured, especially after the manufacturing steps of austenitization and pickling. Comparing different zones of the screw showed that the head contained much higher hydrogen contents than the thread. Two different strategies for mitigation of hydrogen absorption were evaluated: outgassing at dry atmospheric conditions after heat treatment and baking/soaking after galvanization. Both strategies reduced the hydrogen content down to an uncritical value, as experimentally validated by slow strain rate testing (SSRT) of the finally galvanized screws, which failed at high force and plastic deformation. Scanning electron microscopy (SEM) of the fracture surfaces revealed fully ductile fracture with micro-void coalescence (MVC).