Failure analysis of low carbon free-cutting steel wire rods

The present work is aimed at investigating the failure of free-cutting grad e steel wire rod, which exhibited various undesirable surface imperfections such as slivers/chips while hot rolling the continuous cast billets. Micro structural characterization by optical and scanning electronmicroscopy, and differential thermal analysis were carried out to identify the phases pres ent in the as-cast and heat treated billets and hot rolled wire rods. By an alysing the relative stability of different sulphide inclusions, i.e. FeS o r Fe-rich (Fe,Mn)S inclusions (which are undesirable in free-cutting steels , as their melting points are below 1000 degrees C), the cause of failure h as been identified. It is found that low melting point sulphides are respon sible for hot-shortness and grain-boundary cracking during hot rolling. In the heat treated billets, before rolling, the deleterious sulphides Fe-rich (Fe,Mn)S were found to be present only in the central region whereas the s table Mn-rich (Mn,Fe)S were found near the edge portion of the billets. The refore, it is obvious from the comparison with results obtained on free cut ting steels, which did not staffer from sliver/chip problems, that the tran sformation of the Fe-rich ternary phase to a more stable Mn-rich ternary ph ase was not complete in the central region of the billet due to insufficien t heat treatment. A suitable heat treatment is suggested to facilitate the complete transformation and thereby improve the quality of the rolled produ cts. (C) 1999 Elsevier Science Ltd. All rights reserved.