| Zinc kettle made of iron is the most common type of zinc kettle which is one of the most important equipment of general hot-dip galvanizing. Modern zinc kettle is usually stitched by multi U-type segment and 2 head, so that weld is almost inevitable. The weld of zinc kettle would bring out some problem, such as welding defects, uneven texture and composition, residual stress, which make the welded joints of zinc kettle becoming one of the most special part of the zinc kettle and the part which is often caused premature failure. However, both at home and abroad, theoretical research on the corrosion behavior in liquid zinc of zinc kettle welded joints is few, not to mention systematic theoretical research, especially the corrosion on the weld metal and the metal near it. Based on this, three kinds of zinc kettle welded joints(joint A, B, C) whose welding respectively using common welding manual J422 and J427 A and new low carbon and silicon submerged arc welding manual provided by the large domestic hot-dip galvanizing equipment manufacturer and made respectively were selected as the research object. The microstructure and corrosion behavior in liquid zinc saturated with iron at 450℃ for long-term immersion of those parent metal, hot affected zone metal(HAZ) and weld zone metal were systematic studied, moreover, its mechanism of corrosion were analyzed. The microstructure of welded joints was analyzed by the ways of scanning electron microscope(SEM) and optical microscope(OM). The corrosion rates for different immersion time of parent metal, HAZ and weld metal of those three kinds of zinc kettle welded joints were measured by weight loss method. And those microstructure and growth situation of intermetallic compounds of coating at Fe/Zn interface were analyzed by the ways of SEM and energy diffractive spectrum(EDS). The main results are listed as follows:The microstructure of welded joints have typical characteristics of the multi pass welding: uneven HAZ grain and weld zone grain which composed of coarse grain and fine grain structure with staggered distribution. The weld metal structure is composed by ferrite and pearlite.For the welded joint A and B, the corrosion rates of those character areas dipped in liquid zinc saturated with iron at 450℃ for same immersion time are in the following rules: υweld metal > υHAZ metal > υparent metal. And with the extension of immersion time, this difference was reduced, however for 720 h immersion time, the corrosion rates of weld metal(4.22 g·m-2·s-1) of welded joint A is still over 10% of that of its parent metal(3.80 g·m-2·s-1), and that of weld metal(5.13 g·m-2·s-1) of joint B is over 35% of that of its parent metal(4.02 g·m-2·s-1). But for welded joint C, the corrosion rate of parent metal, HAZ metal and weld metal are very similar.For the welded joint A, there are abnormal growth phase ζ of HAZ metal and weld metal when dipped in liquid zinc for 1 hour. Specifically, the intermetallic compounds on the HAZ metal are similar with that on typical Sandelin steel which has the burst phase ζ, and the intermetallic compounds on the weld metal are similar with that on typical hyper-Sandelin steel which has loose and thick massive growth phase ζ. Similarly, for the welded joint B, here are abnormal growth phase ζ of HAZ metal and weld metal when dipped in liquid zinc for 1 hour. The phase ζ on the HAZ metal is large and loose bulk and that on weld metal with loose bulk and super thick growth is similar with that on typical Sandelin steel. But for welded joint C, the intermetallic compounds microstructure of parent metal, HAZ metal and weld metal when dipped in liquid zinc for 1 hour or 24 hours are similar and they are also similar with those on typical low carbon and silicon steel. The morphology of intermetallic compounds on each region of the three kinds of welded joints are similar when dipped in liquid zinc for 24 hours, mainly consisting of thick δ layer and thin ζ layer.The chemical composition of weld metals of welded joints A, B and C was determined by the way of chemical analysis. Their equivalent silicon(Si+2.5P) content are 0.333wt%, 0.127wt%, 0.040wt%, respectively in the high silicon area, Sandelin area and low silicon area of Sandelin curve. The high content of silicon causes a high growth of intermetallic compound thickness on weld metal of welded joint A dipped in liquid zinc, and then reflecting a high corrosion rate; The weld metal of welded joint B occurs Sandelin effect dipped in liquid zinc for short time with abnormal growth of intermetallic compounds, and that reflects a greater corrosion rate. However the silicon content of weld metal is similar with parent metal of welded joint C for both in low silicon area, so its intermetallic compounds growths normal and that reflects a similar corrosion rate with parent metal. The effect of silicon is no longer evident when immersion time is over 24 hours.The equivalence silicon content of weld metal and HAZ metal of welded joints A and B is the main cause of greater corrosion rate in liquid zinc than parent metal, and the special structure of the heat affected zone and the weld zone has no obvious influence on the corrosion rate. |
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