Fatigue crack growth characteristics of nitrogen-alloyed type 347 stainless steel under operating conditions of a pressurized water reactor
Abstract The fatigue crack growth behavior of Type 347 (S347) and Type 347N (S347N) stainless steel was evaluated under the operating conditions of a pressurized water reactor (PWR). These two materials showed different fatigue crack growth rates (FCGRs) according to the changes in dissolved oxygen content and frequency. Under the simulated PWR conditions for normal operation, the FCGR of S347N was lower than that of S347 and insensitive to the changes in PWR water conditions. The higher yield strength and better corrosion resistance of the nitrogen-alloyed Type 347 stainless steel might be a main cause of slower FCGR and more stable properties against changes in environmental conditions.
Weld metal composition and aging influence on metallurgical, corrosion and fatigue crack growth behavior of austenitic stainless steel welds
AISI 316 austenitic stainless steel joints were fabricated using gas tungsten arc welding (GTAW) process selecting two filler compositions viz. AISI 316L (conventional) and AISI 347 (Nb-based). Both the welds were subjected to an aging treatment of 750 °C/24 h. AISI 316L aged weld metal microstructure exhibited interdendritically branched δ-ferrite morphology which facilitated a strong network for the formation of carbides, whereas in 347 weld similar ferrite morphology occurred but branching effect was not observed. EPMA (Electron probe microanalyzer) results show that carbide precipitation in the dendritic regions of both the welds in the aged condition accounted for their respective corrosion that occurred interdendritically. Corrosion performance evaluation of the welds carried out by measuring their degree of sensitization using DLEPR (double loop electrochemical potentiokinetic reactivation) technique showed niobium contained 347 weld performed better than a 316L weld. Fatigue crack growth resistance of both the welds in the aged condition increased due to dispersion strengthening effect that occurred due to carbides formation. Further, 316L weld exhibited better fatigue performance than 347 weld, both in the as-welded as well as in the aged condition. This study establishes that if Nb addition to the weld metal can improve the sensitization performance of AISI 316 welds, it can at the same time; decrease their fatigue crack growth resistance also.
Urea removal strategies for dialysate regeneration in a wearable artificial kidney.
The availability of a wearable artificial kidney (WAK) that provides dialysis outside the hospital would be an important advancement for dialysis patients. The concept of a WAK is based on regeneration of a small volume of dialysate in a closed-loop. Removal of urea, the primary waste product of nitrogen metabolism, is the major challenge for the realization of a WAK since it is a molecule with low reactivity that is difficult to adsorb while it is the waste solute with the highest daily molar production. Currently, no efficient urea removal technology is available that allows for miniaturization of the WAK to a size and weight that is acceptable for patients to carry. Several urea removal strategies have been explored, including enzymatic hydrolysis by urease, electro-oxidation and sorbent systems. However, thus far, these methods have toxic side effects, limited removal capacity or slow removal kinetics. This review discusses different urea removal strategies for application in a wearable dialysis device, from both a chemical and a medical perspective.