Deterioration in Fracture Toughness of 304LN Austenitic Stainless Steel Due to Sensitization

The aim of this report is to examine the influence of sensitization on the mechanical properties of AISI grade 304LN stainless steel with special emphasis on its fracture toughness. A series of stainless steel samples has been sensitized by holding at 1023 K for different time periods ranging from 1 to 100 hours followed by water quenching. The degree of sensitization (DOS) for each type of the varyingly heat-treated samples has been measured by an electrochemical potentiodynamic reactivation (EPR) test. The microstructures of these samples have been characterized by optical metallography, scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses, together with measurements of their hardness and tensile properties. The fracture toughness of the samples has been measured by the ball indentation (BI) technique and the results are validated by conducting conventional J-integral tests. It is revealed for the first time that the fracture toughness and ductility of AISI 304LN stainless steel deteriorate significantly with increased DOS, while the tensile strength (TS) values remain almost unaltered. The results have been critically discussed in terms of the depletion of solid solution strengtheners, the nature of the grain boundary precipitations, and the strain-induced martensite formation with the increasing DOS of the 304LN stainless stee

Modification of Sensitization Resistance of AISI 304L Stainless Steel through Changes in Grain Size and Grain Boundary Character Distributions

Sensitization behavior of thermomechanically processed AISI 304L stainless steel has been investigated. The mechanical processing was carried out at deformations of 30 to 90 pct (reduction in thickness), and annealed subsequently at temperatures ranging from 800 °C to 950 °C for 15 to 60 minutes. Stainless steel was then sensitized at 675 °C both for short (2 hours) and long (53 hours) durations. Thus treated specimens were characterized for grain boundary character distribution, grain size, and degree of sensitization (DOS). The increase in annealing temperature and time following mechanical processing showed an increase in grain size (up to 37 μm) and in the DOS. The fraction of coincident site lattice (CSL) boundaries (Σ3 to Σ29) was also noticed to increase with the annealing temperature, which implied that an increasing fraction of low energy boundaries did not cause a decrease in the DOS. The grain size through its effect on grain boundary surface area and the effective grain boundary energy correlated well with the extent of sensitization. Grain growth reduces the grain boundary surface area and the effective grain boundary energy as well, which, in turn, enhanced the DOS. A critical grain size (∼0.05), above which, sensitization reduced to insignificant levels was observed