Investigation of hydrogen assisted cracking in acicular ferrite using site-specific micro-fracture tests

Abstract

Hydrogen assisted cracking (HAC) is a common type of failure mechanism that can affect a wide range of metals and alloys. Experimental studies of HAC are cumbersome due to various intrinsic and extrinsic parameters and factors (associated with stress, hydrogen and the materials microstructure) contributing to the hydrogen crack kinetics. The microstructure of many materials consists of diverse constituents with characteristic features and mechanical properties which only occur in very small material volumes. The only way to differentiate the effect of these individual constituents on the hydrogen crack kinetics is to miniaturise the testing procedures. In this paper we present a new experimental approach to investigate hydrogen assisted crack growth in a microstructural constituent, i.e. acicular ferrite. For this purpose, sharply notched microcantilevers were fabricated with a Focus Ion Beam within this selected microscopic region. Acicular ferrite can be found in many ferrous alloys including ferritic weld metal and has specific features that control its intrinsic susceptibility to HAC. These features were characterised via Electron Backscatter Diffraction and the specimens were subsequently loaded under uncharged and hydrogen charged conditions with a nanoindenter. The outcomes of the testing, demonstrated that the threshold stress intensity factor, Kth, to initiate crack propagation in acicular ferrite ranges between 1.56 MPa m1/2 and 4.36 MPa m1/2. This range is significantly below the values of Kth reported for various ferrous alloys in standard macro-tests. This finding indicates that the mechanisms and resistance to HAC at micro-scale could be very different than at the macroscale as not all fracture toughening mechanisms may be activated at this scale level. Hydrogen assisted cracking (HAC) is a common type of failure mechanism that can affect a wide range of metals and alloys. Experimental studies of HAC are cumbersome due to various intrinsic and extrinsic parameters and factors (associated with stress, hydrogen and the materials microstructure) contributing to the hydrogen crack kinetics. The microstructure of many materials consists of diverse constituents with characteristic features and mechanical properties which only occur in very small material volumes. The only way to differentiate the effect of these individual constituents on the hydrogen crack kinetics is to miniaturise the testing procedures. In this paper we present a new experimental approach to investigate hydrogen assisted crack growth in a microstructural constituent, i.e. acicular ferrite. For this purpose, sharply notched micro-cantilevers were fabricated with a Focus Ion Beam within this selected microscopic region. Acicular ferrite can be found in many ferrous alloys including ferritic weld metal and has specific features that control its intrinsic susceptibility to HAC. These features were characterised via Electron Backscatter Diffraction and the specimens were subsequently loaded under uncharged and hydrogen charged con...

Microstructure and Strengthening/Toughening Mechanisms of Heavy Gauge Pipeline Steel Processed by Ultrafast Cooling

“…During fracturing, AF can deflect crack propagation, which consumes the crack-propagation energy. The crack-arresting property is improved, which results in good DWTT properties [8][9][10]. Hence, it is necessary to establish the effect of UFC on AF in different heavy-gauge pipeline steel positions.…”

Microstructural, Mechanical, Texture and Residual Stress Characterizations of X52 Pipeline Steel

“…TMCP promotes the formation of an acicular ferrite (AF)-based microstructure, which is the preferred microstructure for pipe steels. Steels with AF microstructure normally possess higher strength and toughness, as well as superior stress corrosion and fatigue resistance than steels with ferrite and pearlite (P) microstructure [1,2]. API X52 steel is used widely as pipeline material in Australia and other countries.…”

Effects of Different Parameters on Initiation and Propagation of Stress Corrosion Cracks in Pipeline Steels: A Review

“…As shown with red circle, the SCC crack is deflected about 45 • when it reaches grain number 30. Calculation of misorientation between grains number (30,31) and (31,32) proves that the misoriatation angles between them are 12.3 • and 36.1 • , respectively. This proves why the SCC crack deflects when it reaches grain number 30.…”

“…Besides the SCC phenomenon, the SIF plays a key role in HIC crack propagation in pipeline steels. Costin et al [31] reported that the threshold SIF (Kth) range for crack propagation is between 1.56 MPa√ and 4.36 MPa√ . This range is lower than the Kth value which is calculated for ferrous alloys.…”