Producing high quality cold-drawn steel tubes using an optimal thickness of tin as a tribo-layer

Abstract

In this paper, the tin coating layer is utilized as a solid lubricant for the cold steel tube drawing process. The effect of different thicknesses of tin coatings as a tribo-layer has been investigated. The electroplating process has been used for depositing a tin coating layer by deploying an environmentally benign citric acid–based electrolyte bath and the layer thickness is controlled by varying the time of the electroplating process. Characterization of the coating layer is carried out using SEM, EDS, and 3D digital microscopy techniques. Experimental trials of the tube drawing process have been conducted for tin-coated tubes and the effects on drawing force and surface roughness have been analyzed. The draw force gradually decreases and reaches a minimum value as the tin layer thickness increases from 4.5 to 16.5 μm, while for further increase in layer thickness, the draw force increases again, yielding an optimal tin layer thickness of around 16.5 μm. The drawn tube samples are found to have a superior surface finish with roughness (Ra) as low as 0.165 μm even for the smallest tin coating thickness of around 5 μm while it improves marginally up to 0.134 μm when the tin layer thickness is increased gradually to around 25.7 μm. A sizeable thickness of tin coating is retained on the drawn tube, which enhances the surface appearance as well as corrosion resistance. The tape adhesion test was carried out as per the ASTM D3359 standard and it confirmed the adequacy of adhesion of the coating on the drawn tubes. Also, comparison of draw force variation with the well-known Stribeck curve and the insights into the working mechanism of soft metal as a lubricant are presented.