Microstructures and mechanical properties of milled and continuously cast lead sheet
The relationship between some mechanical properties (tensile, creep and thermal fatigue behaviour) and microstructure has been investigated for lead sheet containing <0.06 wt % Cu (the eutectic composition), made either by a conventional milling (rolling) process, or by continuous casting (Direct Method). Milled lead sheet exhibits a recrystallised microstructure, the grain size of which decreases with increasing copper content; copper is present as particles of ≃1-5 μm long, formed from the initial needles or plates of the cast billet by spheroidisation, or by particle break-up during rolling. A comparison of materials produced by several manufacturers indicates that the copper distribution and final grain size are dependent on the thermo-mechanical history of the sheet. Direct Method (DM) sheet exhibits a cast cellular structure within grains which usually extend through the full thickness of the cast sheet; copper is present as a fine dispersion (particles ≃0.5 μm diameter) at cell boundaries. The copper distribution in both materials is stable to prolonged heating (100 hours at 200oC), but some grain growth occurred in the milled sheet. It was not possible to obtain a fully dispersed eutectic microstructure at the eutectic composition; primary lead dendrites (or cells) were always present. This is thought to be due to the difficulty of nucleating a copper particle at the very low copper concentrations used in this work. The tensile behaviour of specimens was investigated at various strain rates and temperatures. DM sheet exhibits an increase in UTS both with increasing copper content at each strain rate, and with increasing strain rate for each copper content; no systematic variation of strain with copper content was observed. The UTS of milled lead sheet (at ambient temperature and slow strain rates) was a maximum at 0.02 - 0.03 % copper. A steady increase in UTS with increasing copper content was obtained at higher strain rates (2.67 min-1 and above) and low temperatures (≃200 K) and indicated that a time and temperature dependent softening process is active at ambient temperature and slower strain rates (up to 1.33 min^-1) which is thought to be grain boundary sliding, although no evidence for this has been detected in the recrystallised microstructure. Values of the work-hardening coefficient (n) and the strain-rate sensitivity (m) were determined for milled and DM sheet, and were found in all cases to be high. DM sheet exhibits good creep resistance, which increases with increasing copper content, owing to the large grains and stable grain boundaries at ≃90' to the direction of stress. Milled lead sheet was less creep resistant, exhibiting maximum creep resistance at 0.03% copper; this is thought to be due to competing processes of strengthening by copper (which pins grain boundaries to some extent) and grain boundary sliding, which increases with the corresponding decrease in grain size. Thermal fatigue tests have been conducted externally, using lead flashing lengths fixed to an outside wall, and in the laboratory using specially developed apparatus. The development of the apparatus, specimen shape and test cycle length is described. Cracking is usually multiple and intergranular; examination of the fracture surface indicates a combination of intergranular fatigue, creep processes (cavitation) and regions of ductile failure. The use of acetate replicas to trace crack growth has shown the migration of grain boundaries to directions of high stress in milled lead, and in 0.01% copper DM sheet. Grain boundary migration was not observed in the more stable DM microstructures of higher copper content.