Deformation Fatigue and Fracture vis-a-vis Deformation Induced Martensite in AISI 304LN Stainless Steel

Understanding of deformation, fracture or fatigue behaviour of AISI 304LN grade stainless steel with reference to in-situ evolution of deformation induced martensite (DIM) is important for the structural integrity of numerous critical engineering components made of this steel. The primary objective of this report is to present a concise overview on the state-of-the-art of these aspects based on a series of investigations by the authors and their co-workers through over more than a decade. The major experiments involved are determination of tensile, fatigue and fracture behaviour of the steel using standard testing procedures. The associated structural and sub-structural changes in the deformation volume or at local regions such as fracture surfaces or crack tips are characterized. The nature and amount of DIM have been detected through microstructural analysis, X-ray diffraction, hardness measurement, ferrofluid based technique, ferritoscope assessment and TEM, in addition to extensive fractographic analysis by SEM. The major highlights of the investigations centre on revelations of the role of DIM on tensile deformation of 304LN stainless steel at various strain rates and temperatures, illustrating the association of DIM with constrained and unconstrained deformation ahead of crack tips in monotonic and cyclic fracture tests, and examination of the extent of DIM transformation during stress controlled and strain controlled cyclic loading and fatigue crack growth, with an underlying theme of continuously emphasizing the nature, location and amount of DIM formed.