Cr–Mo-Enriched Boundary Networks Govern Repassivation and Intergranular Cracking of 316L Stainless Steel in Oxygenated High-Temperature Water
Abstract
The cracking of 316L stainless steel in oxygenated high-temperature water is controlled by a local competition between film failure, intergranular oxidation, and fast chemical healing at the crack tip. The comparison of three types of 316L stainless steel — wrought conventional 316L stainless steel, cold-worked wrought 316L stainless steel, and laser powder-bed-fused 316L stainless steel — reveals that the presence of defects cannot explain the penetration of cracks. Cold working introduces the dislocation density of $1.6\times10^{14}$ m$^{-2}$ and the low-angle boundary fraction of 44.2\%. Nevertheless, it provides the deepest average intergranular crack length of 515 nm and the largest isolated event of 2000 nm. Laser powder-bed fusion leads to the largest dislocation density of $1.1\times10^{14}$ m$^{-2}$ and the largest low-angle boundary fraction of 65.4\%. However, laser powder-bed fused 316L stainless steel restricts the average crack length to 278.8 nm when applying the highest target stress of 520.6 MPa during the longest exposure of 744.4 h. The discriminating factor is the chemical state of the boundaries: deformation cells of the cold worked sample are poor in chromium and molybdenum; while solidification cell walls of the printed sample have Cr/Mo-enriched and Fe-depleted regions. A repassivation index related to the supply of boundary connectivity is defined by the following parameters: Cr--Mo inventory, low-angle boundary fraction, dislocation density, Cr/Mo cell-wall chemistry, mechanical severity, and grain-boundary segregation energies. For conventional wrought 316L stainless steel, cold-worked 316L stainless steel, and laser powder-bed-fused 316L stainless steel, the values of this index are 32.4, 22.4, and 71.4, respectively. The manipulations by substitutions of alloying elements reveal that eliminating the Cr/Mo cell-wall chemistry reduces the value for printed 316L stainless steel from 71.4 to 21.0, while introducing such chemistry into the cold-worked 316L sample results in an increase in the value from 22.4 to 70.9. Consequently, the structure of cracking is controlled by chemically supplied boundary connectivity rather than by overall defect concentration or total molybdenum concentration.