Intergranular Cracking in High-Strength, Cold-Rolled, and Precipitation- Hardened Austenitic Stainless Steel UNS S35500

摘要

When quench annealed, stainless steel UNS 35500 (C 0.12, Cr 15.5, Ni 4.5, Mo 3, N 0.1%) is austenitic and soft. In cold-rolled and tempered condition, heavy cold rolling followed by precipitation hardening considerably strengthens the material (UTS 220 ksi 1517 MPa, elongation 10%). Its strength, combined with good corrosion resistance, makes the material attractive for use in critical load-bearing applications. In one application, helicopter rotor blades are attached to the drive shaft with a component strap pack, assembled from 0.014-in (0.36 mm)-thick laminae of the material. Premature fatigue failures of strap packs have occurred starting from intergranular cracks in single laminae. Chloride salts were detected at crack origins. This intergranular stress corrosion cracking was reproduced under crevices in slow- strain-rate tests conducted in 3.5% NaCl solution at 0.1 V (Ag/AgCl 4M KCl). The potential is typical of those attained by the material under thin, chloride- bearing condensate films exposed to air. Cracking did not occur when crevices were absent. Electrolytic polishing in chloride-free acids, combined with a standard overpassivation treatment in nitric acid, improved the resistance to crevice corrosion. This treatment slowed but did not prevent the onset of stress corrosion cracking (SCC) in slow-strain-rate tests conducted with an artificial crevice on the specimen surface.