Failure analysis of low carbon free-cutting steel wire rods

Abstract

The present work is aimed at investigating the failureof free-cutting grade steel wire rod, which exhibited various undesirable surface imperfectionssuch as slivers/chips while hot rolling the continuous cast billets. Microstructuralcharacterization by optical and scanning electronmicroscopy, and differential thermal analysiswere carried out to identify the phases present in the as-cast and heat treated billets and hot rolledwire rods. By analysing the relative stability of different sulphide inclusions, i.e. FeS or Fe-rich(Fe, )S inclusions (which are undesirable in free-cutting steels, as their meltingpoints are below 1000°C), the cause of failure has been identified. It is found that low meltingpoint sulphides are responsible for hot-shortness and grain-boundary cracking during hot rolling.In the heat treated billets, before rolling, the deleterious sulphides Fe-rich (Fe, )Swere found to be present only in the central region whereas the stable Mn-rich (Mn, )S were found near the edge portion of the billets. Therefore, it is obvious from thecomparison with results obtained on free cutting steels, which did not suffer from sliver/chipproblems, that the transformation of the Fe-rich ternary phase to a more stable Mn-rich ternaryphase was not complete in the central region of the billet due to insufficient heat treatment. Asuitable heat treatment is suggested to facilitate the complete transformation and thereby improvethe quality of the rolled products.