Transformation and toughness of iron-9 percent nickel alloy.

Phase transformation studies have been carried out on VS2241A, Fe-9.14Ni -0.002C alloy. This alloy was chosen because it was expected that the speed of austenite decomposition would be slow enough to allow continuous cooling transformation and isothermal transformation experiments to be carried out on the same alloy. Upon furnace cooling, massive ferrite was the predominate phase formed. TEM inspection observed low angle sub-boundaries inside ferrite grains. Pre-polished surface examination showed the presence of Widmanstatten ferrite as evident by tent-shaped surface relief. Martensite-austenite (M-A) constituent was also observed under TEM inspection indicating that partition of carbon had occurred during transformation.Quantitative analysis of the dilatation curve showed continuous cooling consisted of two portions. One between 575 +/- 5°C and 558 +/- 3°C corresponding to the formation of grain boundary nucleated massive ferrite while the portion between 558°C to 500°C was thought to correspond to Widmastatten ferrite formation. The observation of an experimental T[0] temperature of 623 +/- 5°C and a theorectical T[0] temperature of 614 +/- 5°C implied that transformation took place below T[0] in the two phase field.Microanalysis using a FEG-STEM system with a windowless LINK X-ray detector was carried out at Liverpool University. This showed that the Ni content across a ferrite grain was constant at 8.8 +/- 0.2wt%Ni and 12.98 +/- 0.43wt%Ni was detected on a grain boundary confirming that the massive transformation was composition invariant, but local partitioning occurred in the interface during transformation.Thermal arrest experiments observed bainitic ferrite and lath martensite transformed at temperatures ~ 486°C and ~ 384°C respectively.Incomplete transformation was observed for all the isothermally transformed structures in the Fe-9Ni alloy below T[0]. It was suggested that this phenomenon would apply to all transformations occurring in the two phase field below the T[0] temperature. Separate but overlapping C-curves in a TTT diagram for Fe-9Ni were proposed to account for the coexistence of massive ferrite and Widmanstatten ferrite at the same temperature. The lath formation of Widmastatten ferrite was interpreted as a product of partial coherent interfaces propagated by means of a ledge mechanism, thermally activated by the trans-interphase diffusion of solute atoms. A thermal arrest at 707 +/- 5°C was observed on air cooling an Fe- 3.5Ni alloy,VS2239A. This corresponded to equi-axed ferrite transformation in the single phase region. In an iced brine quenched Fe-4Cu alloy, massive ferrite and Widmastatten ferrite were observed. A 6° misorientation was calculated between two adjacent ferrite grains seperated by a ragged grain boundary.Charpy impact testing of Fe-9Ni alloy, VS2241A gave a DBTT of -140°C and -100°C for massive ferrite and bainitic ferrite respectively. Massive ferrite showed a higher upper shelf energy on the transition curve.