Abstract
This study is a detailed failure analysis of galvanized high carbon steel wires, which developed coating cracks during the torsion test performed as a quality control at the end of the manufacturing process. Careful visual inspections showed that the cracks are already present in the coating before the torsion test. In order to explain the origin of these cracks, systematic metallographic investigations were performed by means of optical and scanning electron microscope on both the wires and the rods that have been cold drawn to produce the wire. The chemical composition of the galvanized coatings was evaluated by means of energy dispersive spectroscopy. Micro bidimensional X-ray diffraction experiments were also performed to measure the residual stresses in the galvanized coating. The results showed that the failure is related to two main factors: the relatively high content of silicon in the steel and the unsuitable cooling rate of the rods at the exit from the galvanizing bath. The mechanism proposed to explain the origin of the defects was supported by Finite Elements Methods simulations and verified with in-plant tests. The proper countermeasures were then applied and the problem successfully solved.
Highlights
The manufacturing process of galvanized high carbon steel wires starts from the hot-rolled steel rods, which are austenitized in a furnace and maintained in a lead bath at a temperature in the range of 520–560 ◦ C
The lead particles remaining attached to the rod surface are removed by acid pickling, which is a necessary surface pre-treatment for the following hot-dip galvanizing step
The steel rod is maintained in contact with the molten zinc at around 440–460 ◦ C for a few minutes to form the galvanized coating on its surface [2]
Summary
The manufacturing process of galvanized high carbon steel wires starts from the hot-rolled steel rods, which are austenitized in a furnace and maintained in a lead bath at a temperature in the range of 520–560 ◦ C. The lead particles remaining attached to the rod surface are removed by acid pickling, which is a necessary surface pre-treatment for the following hot-dip galvanizing step. Before the immersion in the molten zinc bath, the steel rod is fluxed in an aqueous solution containing chloride salts to avoid any surface oxidation. The steel rod is maintained in contact with the molten zinc at around 440–460 ◦ C for a few minutes to form the galvanized coating on its surface [2]. At the exit from the molten zinc bath, the inert gas wiping dies accurately control the thickness of coating on the surface by removing the excess of molten zinc
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