Abstract
It was shown in previous report that the fatigue strength of welded joint with reinforcement is remarkably improved by hot galvanizing treatment in comparison with that of non-treated joint. However, the reason why the fatigue strength of joint is improved by the treatment has not yet been made clear. Therefore, each step in hot galvanizing process is analysed and the effects of steps on the fatigue strength of welded joint are experimentaly investigated in this report.Generally, hot galvanizing process contains five steps, that is, pickling, rinsing, flux coating, dipping in zinc bath and cooling. At first hydrogen absorption in steel during pickling, rinsing and cooling in water are investigated. Hydrogen in steel during galvanizing process is released with time, but galvanized zinc layer retards the releasing rate of hydrogen. The fatigue strength of hot-galvanized steel decreases owing to the hydrogen, but it recovers if the hydrogen is released. Dipping of the joint for long time in rinse bath containing a little acid has no good effect on the fatigue strength. The higher is the temperature of zinc bath, the more effective is hot galvanizing for improving the fatigue strength of steel welded joint. The iron-zinc alloying phase formed at higher than 495°C is a ductile δ1 phase. When the joint is cooled in water bath immediately after dipping in zinc bath, the fatigue strength of it increases, but decreases by air cooling. The alloying layer formed during air cooling is harmful for the fatigue strength of welded joint. The iron-zinc alloying layer by hot galvanizing has hardly any influence on the fatigue strength of steel welded joint.
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