Residual alloying elements, effect

Physical Metallurgy. All commerical metal com-ponents are polycrystalline and all metal lattices contain both point and line faults. Heat treatable alloys frequently contain precipitates which may be very small (< 5 pm dia). All alloys contain major alloying elements but they contain also a range of nondeliberate elements, both metallic and nonmetallic in nature. It is possible to show that all these factors, as well as surface finish, residual cold work and metallurgical history (what was done to the alloy in bringing the component to its present shape) may have an effect on... 

Type 304 stainless steel is basically an alloy of 18 to 19 wt% Cr and 8 to 10 wt% Ni. Its corrosion behavior in sulfuric acid is sensitive to both alloy composition and the sulfuric acid environment. Variables with respect to alloy composition include whether the Cr and Ni concentrations are high or low within the allowed range and the concentrations of residual elements such as sulfur, phosphorus, copper, and molybdenum. Thermal and mechanical treatments are also variables but are not considered in the following. Important variables with respect to the sulfuric acid environment include degree of aeration and agitation (velocity effect) and small concentrations of species such as nitric acid, cupric ions, and ferric ions. The net influence of these variables is to find corrosion rates varying from <25 pm/year (1 mpy) to >2500 pm/year (100 mpy) (Ref 3 9). 

The problem facing all smelters is that there is a continuous occurrence of antimony in their lead streams, at a time when there is a rapidly declining demand for the element. Declining sales of the alloy, which coincide with increasing use of soft-lead-based alloys, will result in antimony effectively becoming a major contaminant that requires removal. Whilst it is relatively easy for secondary reflners to remove antimony from the bullion, the antimony-rich residue from the process may have to be stockpiled. The residue, which contains around 80 to 90wt.% lead, effectively ties up lead units and can have a major cost impact on the reflnery. 

Effect of Copper. Residual elements, Cu especially, seriously affect the sensitivity of steel to irradiation. Experiments on iron, simple and complex iron alloys, and pressure vessel steels have shown that irradiation embrittlement is markedly increased in the presence of Cu [58,59]. It was concluded that a Cu content of more than about 0.1 wt% has a determinable effect, although there are some indications that such an effect may be found with Cu contents as low as 0.03 wt% [60]. Increasing the Cu content of irradiated RPV steel can cause an irradiation-induced shift in the ductile-brittle transition temperature ATT (see Figure 4.20). 

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