Fatigue Strength, Crack Initiation, and Localized Plastic Fatigue Damage in VHCF of Duplex Stainless Steels

• Published in: Steel research international 2016-07, Vol.87 (7), p.899-910
• Main Authors: Tofique, Muhammad Waqas, Bergström, Jens, Burman, Christer, Hallbäck, Nils, Gåård, Anders
• Format: Article
• Language: English
• Subjects: Alloys; Austenitic stainless steels; Compression tests; Compressive strength; Crack initiation; Crack propagation; crystallographic crack growth; Crystallography; Damage accumulation; Damage localization; duplex stainless steel; Duplex stainless steels; Extrusion; Fatigue cracks; Fatigue failure; Fatigue strength; Fatigue testing; Fatigue tests; Ferrite; Ferritic stainless steels; Fracture mechanics; Grains; Maskinteknik; Materials Engineering; Materialteknik; Mechanical Engineering; Metal fatigue; Microstructure; plastic fatigue damage; Quality; Stainless steel; Steel alloys; Test equipment; ultrasonic fatigue testing; Ultrasonic testing; very high cycle fatigue
• ISSN: 1611-3683; 1869-344X; 1869-344X
• DOI: 10.1002/srin.201500263

The fatigue strength of two‐duplex stainless steel grades, 2304 SRG and LDX 2101, with austenitic–ferritic microstructure is tested using ultrasonic fatigue testing equipment operating at 20 kHz. The testing is conducted in tension‐compression mode with the load ratio R = −1. The fatigue strength is evaluated at 107, 108, and 109 load cycles and the estimates of fatigue strength are higher for the LDX 2101 grade. The fatigue crack initiation mechanisms are analyzed using a scanning electron microscope. The fatigue cracks, in all cases, appear to initiate due to accumulation of plastic fatigue damage at the surface. In the 2304 SRG grade, accumulation of fatigue damage occurs at the external surface of fatigued specimens in the form of extrusions at the grain/phase boundaries and in the form of individual slip lines in the austenite phase. Meanwhile, in the LDX 2101 grade accumulation of plastic fatigue damage in the form of extrusions and intrusions occurs mainly within the ferrite grain. When the crack is microstructurally short, the crack growth appears to be crystallographic in nature and the crack appears to change its direction propagating from one grain into another. The fatigue strength of two‐duplex stainless steel grades, 2304 SRG and LDX 2101, with austenitic–ferritic microstructure is tested using ultrasonic fatigue testing equipment operating at 20 kHz. The fatigue crack initiation mechanism analysis shows that the fatigue cracks, in all cases, appear to initiate due to accumulation of plastic fatigue damage at the surface.