Phosphorus - Sulfur - Copper

Phosphorus is considered a tramp or residual element in steel and is carefully  restricted to levels generally below 0.02%. However, like carbon, phosphorus is an interstitial element that can substantially strengthen iron. For this reason, phosphorus is added to a special class of steels called rephosphorized steels for strength. Rephosphorized steels also have enhanced machinability.

Sulfur is also considered a tramp element in steel and is usually restricted to below about 0.02%. Although an element with a small atomic diameter, sulfur is not considered an interstitial alloying element because it is insoluble in iron. However, as in the case of phosphorus, sulfur is added to a special class of steels called resulfurized steels that have improved machinability. These steels are called free-machining steels.

In most steels copper is considered a tramp (residual) element and is restricted to levels below 0.04%. Copper, having a much lower melting point than iron, can create a detrimental steel surface condition known as hot shortness. Although not generally added to steel, there is a very special class of steels that contain high levels of copper to take advantage of the precipitation of copper particles during aging (a tempering process). These copper particles increase strength and hardness. Copper is also added to low-alloy steels for  atmospheric corrosion protection (these steels are called weathering steels). One problem with copper in steel is that it cannot be oxidized and removed during steel refining. Thus, over time, the copper level of steel  produced from steel scrap is slowly increasing.

Bruce L. Bramfitt
International Steel Group, Inc.
Research Laboratories
Bethlehem, Pennsylvania

Reprinted from Handbook of Materials Selection, Wiley, New York, 2002, by permission of the publisher.
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.

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