The High temperature properties of Fe-Ni-Cr alloys.

A literature review has been presented concerning the influence of the common alloying additions in Austenitic Stainless Steels and Ni-Fe Base Superalloys on their grain coarsening characteristics, precipitation hardening and recrystallization behaviour. The effects of various microstructural features on the high temperature properties of Austenitic Stainless Steels and Ni-Fe Base Superalloys have also been reviewed.The work discribed has shown the grain coarsening characteristics to depend on the type and amount of alloying addition, which influences undissolved particle size and volume fractions. An attempt has been made to compare various mathematical models of the effect of second phase particles on the grain size, using observed data.The age-hardening characteristics of Ni-Fe Base Superalloys have been shown to vary with the morphology of the different types of precipitate such as M[23]C[6], and r , and their nucleation at different sites. The precipitation kinetics have been explained in terms of the relative diffusion rates of the various alloying additions present.The effect of cold working on the relative kinetics of precipitation and recrystallization have been shown to vary with the type of alloying addition . In steels precipitating M[23]C[6], precipitation precedes recrystallization, whereas in r forming alloys, cold working prior to ageing significantly increases the rate of zone formation and growth, thus retarding the recrystallization processes. Both high temperature tensile and creep properties of Ni-Fe Base Superalloys, at varying strain rates, have been shown to depend on the microstructure. Tensile strength increases with increasing volume fraction ofprecipitating phase, whereas the overall creep strength has been shown to be dependent upon the relative strength of the grain boundaries and the matrix. The variation of high temperature ductility of the different initial microstructures at varying strain rates have been explained in terms of the qualitative influence of predominant microstructural features on the deformation and fracture mechanism maps.Serrated yielding has been shown to be a common phenomenon in all steels tested at 700°C at strain rates of 10[-2]/s and 10[-3]/s, and substitutional solutes have been shown to be responsible for this effect. Also, the stress dependence of the creep rate varies with different microstructural features in Ni-Fe Base Superalloys, and the prominence of deformation mechanism controlled by a marked interface reaction has been indicated.