Factors affecting the constitution and stability of iron-base austenitic alloys.

A study of the constitution of Fe-Cr-Ni based alloys has been carried out. The isothermal sections of the ternary Fe-Cr-Ni system have been determined for temperatures between 1150 C and 650°C by analysing the composition of the individual phases by a micro-probe technique. The transformation of the delta ferrite, during the isothermal heat treatments, to either austenite or (austenite + sigma phase) has been investigated. In the sigma phase regions the delta ferrite transforms first to sigma phase which forms an envelope around the remaining delta ferrite which then transforms to a cellular structure of austenite + sigma phase. The effect of fourth element additions namely:- (Mn,Cu,C,N, Si,Mo,Nb and Ti) which are known austenite or delta ferrlte/sigma phase formers have been investigated. The constitutional effects of the fourth element additions have been quantified by the determination of nickel and chromium equivalents by a method which compares directly the effect of the fourth element additions against that of the nickel and chromium of the base alloy system. The partitioning of selected alloying additions between the phases present has been investigated in an attempt to explain the microstructural effects of the alloy additions. The results suggest that the amount of partitioning is dependent upon the amount of alloy addition and is somewhat less than would be expected. The concept of electron vacancy numbers has been used to predict the stability of the austenite phase and whether a particular composition is prone to sigma formation. The effects of alloying additions on the void swelling characteristics of selected alloys has been studied using both the High Voltage Electron Microscope and Variable Energy Cyclotron with 46MeVNi 6+ ions.It has been shown that whilst most elements depress void swelling, manganese significantly increases void swelling at moderate levels of addition.