Copper sulphide Cu_1.8S (Digenite I) precipitation in mild steel

Abstract

The microstructure of mild steel pressure vessel plate and weldments has recently been examined by transmission electron microscopy (TEM) to characterize certain numerous small (<1,000 Å) matrix precipitates and relate them to the bulk mechanical properties of the steel. The TEM examination was undertaken using in-line energy dispersive (EDAX) spectrometry. In view of the strong influence of Cu on radiation hardening and embrittlement in steels¹ particular emphasis was placed on the identification of any copper-bearing precipitates within the grains. We report here the first observation of discrete particles of copper sulphide, positively identified as Cu_1.8S (Digenite I), and found only in the matrix of the mild steel weld specimens. In spite of having a similar copper content to the plate, the precipitation of copper sulphide uniquely in the welds is thought to arise from the high temperature oxidizing environment experienced during welding. This favours the formation of Cu_1.8S in the presence of soluble copper from the less stable FeS commonly found in sulphur-bearing steels. The importance of this observation in relation to the mechanical properties of the steel is briefly discussed.