EXAMINATION OF MICROSTRUCTURES AND MICROANALYSIS OF ALL FE-9-PERCENT NI-ALLOY

Phase transformations in a continuously cooled Fe-9.14 mass%Ni alloy h ave been studied. Samples furnace cooled at 0.13 K.s(-1) gave a therma l arrest of 555 +/- 5 degrees C, while dilatometry indicated that this corresponded to 25 +/- 5% with initial transformation occurring at 57 5 +/- 5 degrees C. Optical examination revealed a typical massive ferr ite microstructure, while examination of pre-polished specimens showed this to consist of grain boundary ferrite and Widmanstatten ferrite w ithin the grains. Transmission electron microscopy confirmed the prese nce of both approximately equi-axed grains and Widmanstatten ferrite w ithin the prior austenite grains. Some areas of martensite/austenite ( MA) constituent were observed indicating that partition of carbon had occurred during transformation. Quantitative analysis of the dilatatio n curve showed it to consist of two portions. One between 575 +/- 5 de grees C and 558 +/- 3 degrees C corresponding to the formation of grai n boundary ferrite while the portion between 558 to 500 degrees C was thought to correspond to Widmanstatten ferrite formation. In the latte r temperature interval - ln (1 - y) varied linearly with temperature, where y = fraction transformed. Microanalysis using a FEG-STEM system with a windowless LINK X-ray detector was carried out. This showed tha t the Ni content across a ferrite grain was constant at 8.8 +/- 0.2 ma ss%Ni with statistically significant values at grain boundaries e.g. 1 1.05 +/- 0.18 mass%Ni and 12.98 +/- 0.43 mass%Ni. The results are disc ussed in terms of paraequilibrium. A short description of microstructu res and transformation temperatures obtained on water quenching & oil quenching (bainitic ferrite) and ice brine quenching, (lath/massive ma rtensite) is also given in the paper.