Residuals in engineering steels

Abstract

The very gradual rise in the residuals content of scrap is reflected in the levels of residuals found in engineering steels. A superficial survey of national standards and a few ‘in-house’ specifications shows considerable variation in the limits imposed on residual contents. The normal levels in UK steels are so high that for some specifications purer and more expensive raw materials must be used; this is reflected in the increased price of 'low-residual’ steels. One way of counteracting this trend is to restrict the demand for low-residual steels to applications where they are essential. The most serious detrimental effects of high residuals contents are hot cracking and the incidence of surface defects. The critical contents of Cu and Sn depend on the process route and final application. Steel users should specify residual levels in line with the ‘enrichment factor’ applicable to their processing route. Residuals may raise the flow stress at room temperature and thus increase tool loads and tool wear in cold working. Tool-load measurements have shown a close correlation with hardness. Estimates of the effect of residual elements on hardness increment indicate that current melt-out levels correspond to a hardness increase of about 20 HV or an increase of 80Nmm−2 in UTS. This justifies the demand for low residuals in material subjected to severe forming operations. However, steel users should give serious thought to the actual forming operation before specifying low residuals. The effect of residuals on hardenability is beneficial. The engineering industry in the UK derives considerable benefit from the ability to obtain good mechanical properties by oil quenching what are officially classed as carbon steels. A number of steps which can be taken over the coming years to cope with the rise in residuals are noted.