Study of martensitic transformation in fatigued stainless steel by neutron diffraction stress analysis

Yu. V. Taran¹, M. R. Daymond², J. Schreiber³

¹ FLNP, Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia

² ISIS, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, UK

³ EADQ, Fraunhofer Institute for Nondestructive Testing, D-01326 Dresden, Germany

The elastoplastic properties of an austenitic matrix and martensitic inclusions induced during cyclic tensile-compressive loading of low carbon metastable austenitic stainless steel were studied during in situ neutron diffraction stress rig experiments on the ENGIN instrument at the ISIS pulsed neutron facility, with the aim of studying the effects of fatigue on the phase elastic stiffness.

Two sets of samples were prepared from the austenitic steel AISI 321. The first set of samples (annealed at 1050 °C and quenched in water) was cycled under strain control with strain amplitude of 1% at a frequency of 0.5 Hz (low-cycled fatigued (LCF) samples). The second set (1070 °C / quenched in air) was cycled under stress control with stress amplitude of 330 MPa at 5 Hz (high-cycled fatigued (HCF) samples).

Subsequent applied stress - elastic strain responses of the austenitic and martensitic phases for both axial and transverse directions relatively the applied load axis were obtained by Rietveld and Le Bail refinements of the neutron diffraction spectra, and were used to determine the elastic constants of the phases as a function of fatigue level.

An unusual phenomenon is observed for both sets of samples, viz. nonlinear behaviour of martensite elastic response in the plastic region, while the austenite elastic response remains linear throughout the measured stress range up to 500 MPa. This effect was interpreted as the additional microstresses induced by the applied load in the martensite phase in the plastic region, providing the most likely mechanism for the unusual strain response of the phase.

Results of LCF-samples study may be summarized in the following way:

- a clear trend of increasing Young's modulus with fatigue level was noted in the austenite matrix;

- the ratios of elastic constants for transverse and axial directions in both austenite and martensite are close to expected based purely on the value of the Poisson's ratio;

- the residual strains in the austenitic matrix were determined as a function of fatigue cycling, using a noncycled sample as a reference sample; a weak tensile strain of the austenite matrix is observed in both directions; such determination for martensite was impossible for lack of a reference sample;

- the residual macrostresses and the deviatoric components of the phase residual microstresses were determined assuming that the elastic properties of both phases are similar; the austenite phase shows a compressive deviatoric stress in the axial direction, while the martensite shows a balancing tensile deviatoric stress in this direction; the magnitude of the austenite deviatoric compressive stress increases with fatigue, however the tensile deviatoric stress in the martensite decreases in magnitude, corresponding to the increasing volume fraction of martensite.

Among results of HCF-samples study note only one more unusual phenomenon in the martensite phase: the axial and transverse elastic constants of austenite and martensite are distinctly different, especially clear in the transverse direction; the ratio of axial and transverse elastic constants for martensite is almost twice that observed (of ≈0.28) in austenite phase of the HCF-samples and in both phases of the LCF-samples, and that expected based purely on the value of the Poisson's ratio; the mechanism for this unusual behaviour is unclear, but may be linked to the shape of the martensite.

Investigations described in literature have indicated a different morphology of the martensite phase as a function of the cycle frequency; they have also revealed a remarkable difference in the martensite transformation properties between the stress- and strain-controlled tests. We plan to perform series neutron experiments with the aim of clarification of the observed phenomena.