New representation of the hardening processes of lead alloys by transformation-time-temperature (TTT) diagrams

In this paper we present, for the first time, transformation-time-temperature (TTT) curves of different PbCa and PbCaSn oversaturated ct solid solutions. In these alloys the hardening proceeds in sequence via four transformations: two discontinuous transformations, one continuous microprecipitation then one discontinuous reprecipitation of lamellar (Pb-(1-x)Sn-x)(3)Ca compound (overaging). The diagrams presented here are reconstitutions from the different calorimetric studies, hardness measurements and microscopic observations. The TTT diagrams represent very well ail the evolutions for various Sn/Ca ratios or the influence of Ag, Bi, Al impurities. Such TTT representations are the most scientific and rational way for understanding the hardness and structure evolution of industrial alloys during solidification, cooling, heat treatment and service life, particularly when the cooling curve cuts the TTT curves.