STRESS AND THE ACICULAR FERRITE TRANSFORMATION

Acicular ferrite is considered to be identical in transformation mechanism to bainite, except that it nucleates intragranularly on inclusions, and hence develops into a morphologically different microstructure. The transformation causes displacements which, on a macroscopic scale, are characteristic of an invariant-plane strain with a large shear component. It is therefore expected to be sensitive to an appropriate, externally applied stress. This is indeed found to be the case. Results are presented which demonstrate quantitatively that large changes in the development of the microstructure are induced when acicular ferrite grows while the austenite is in uniaxial compression. This is in spite of the fact that the applied stress used was below the austenite yield strength. Profound changes in the distribution of acicular ferrite plates are also observed. The results are interpreted in terms of the mechanism of transformation.