Very high cycle fatigue of cold rolled stainless steels, crack initiation and formation of the fine granular area. (July 2017)

• Record Type: Journal Article
• Authors: Tofique, Muhammad Waqas; Bergström, Jens; Svensson, Krister
• Is Part Of: International journal of fatigue. Volume 100:Part 1(2017)
• Page Start: 238
• Page End: 250
• Publication Date: 2017-07
• Subjects: Duplex stainless steel -- Martensitic stainless steel -- Fatigue -- High frequency testing -- Crack initiation -- Fine granular area; Materials -- Fatigue -- Periodicals; Materials -- Fatigue; Periodicals; 620.1122
• DOI: 10.1016/j.ijfatigue.2017.03.037
• Languages: English
• ISSNs: 0142-1123

Abstract

Highlights: Fatigue test data of two cold rolled stainless steels in the VHCF range is generated. Fatigue crack initiation is controlled by formation of the Fine Granular Area (FGA). Presence of the FGA in the failed specimens was observed by electron microscopy. Abstract: Fatigue test data of cold rolled strip steels, a duplex stainless steel and a martensitic stainless steel, was generated in the Very High Cycle Fatigue (VHCF) regime. Testing was conducted using ultrasonic fatigue test instrument operating at 20 kHz in the fully reversed tension-compression load condition. In both tested grades, fatigue failures initiated from surface defects left by cold rolling or from surface located inclusions. The Fine Granular Area (FGA) feature was observed on the fracture surface and analysed using Scanning Electron Microscope (SEM). Transverse cross-sections from fracture surface were extracted using Focussed Ion Beam (FIB) and analysed using Transmission Electron Microscope (TEM). The thickness of the fine grained layer in the extracted cross-section of the duplex stainless steel was 100–350 nm while for the martensitic stainless steel it was only 10–20 nm. The fine grained layers seemed to be composed of nano-sized grains with different crystallographic orientations. The secondary crack was found to grow through the middle of the layer instead of at the FGA-bulk material interface. The grain refinement mechanisms in the FGA were discussed in perspective of the localized plastic deformation ahead of the defect/crack tip or the crack surface pressing due to closure in the wake of the crack tip.