Erling Østby

Cleavage Fracture Initiation at M–A Constituents in Intercritically Coarse-Grained Heat-Affected Zone of a HSLA Steel

Local brittle zones, i.e., martensite–austenite (M–A) islands, are formed within the coarse-grained heat-affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ) during welding of many HSLA steels. In the current study, the M–A constituents in the microstructure of simulated ICCGHAZ of an API X80 pipeline steel were investigated using transmission electron microscopy and scanning electron microscopy. The focused ion beam technique was applied to make TEM specimens of M–A constituents that were located in the initiation sites of cleavage cracks. The main purpose of the study was to identify crack-initiation sites of cleavage fracture in ICCGHAZ and to prove the presence of M–A constituents in such initiation sites. Twinned martensite was detected in all local brittle zones that were investigated in the current study, demonstrating that they are M–A constituents. It was also demonstrated that the fracture initiation occurred preferentially at M–A constituents by a debonding mechanism rather than cracking of the M–A constituents.

Investigation of mechanism of cleavage fracture initiation in intercritically coarse grained heat affected zone of HSLA steel

Abstract The degradation of the fracture toughness of high strength low alloy steels is attributed to the formation of ‘local brittle zones’ in the welded joint. These local brittle zones are mainly located within the coarse grained heat affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ). Cracking and debonding of M-A constituents from the surrounding matrix are generally accepted as initiation events of fracture in the ICCGHAZ. In the present work, the low temperature fracture toughness of X80 pipeline steel was examined. The main purposes were (i) to evaluate possible crack initiation sites of cleavage fracture and (ii) to identify the mechanism by which M-A constituents deteriorate the ICCGHAZ toughness. The results revealed that the microstructure of ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries. Finally, it was shown that fracture initiation occurred preferentially at M-A constituents by a debonding mechanism rather than cracking of the M-A constituents.

Multiscale simulations of mixed-mode fracture in bcc-Fe

Mixed-mode fracture of bcc-Fe has been investigated using the quasicontinuum method with an embedded atom method potential. A modified boundary layer approach has been employed to simulate a semi-infinite crack and to investigate the influence of the degree of mode II and mode III loading on the crack tip mechanisms. The analyses have been carried out with four crystallographic orientations and different degrees of mixed-mode loading. The results show that the critical stress intensity factor and the mechanisms at the crack tip are sensitive to both the crystallographic orientation and the mode of loading. Phenomena such as crack propagation, twinning and emission of edge and screw dislocations are observed.

Application of combined EBSD and 3D-SEM technique on crystallographic facet analysis of steel at low temperature

Electron backscatter diffraction has been increasingly used to identify the crystallographic planes and orientation of cleavage facets with respect to the rolling direction in fracture surfaces. The crystallographic indices of cleavage planes can be determined either directly from the fracture surface or indirectly from metallographic sections perpendicular to the plane of the fracture surface. A combination of electron backscatter diffraction and 3D scanning electron microscopy imaging technique has been modified to determine crystallographic facet orientations. The main purpose of this work has been to identify the macroscopic crystallographic orientations of cleavage facets in the fracture surfaces of weld heat affected zones in a well‐known steel fractured at low temperatures. The material used for the work was an American Petroleum Institute (API) X80 grade steel developed for applications at low temperatures, and typical heat affected zone microstructures were obtained by carrying out weld thermal simulation. The fracture toughness was measured at different temperatures (0°C, −30°C, −60°C and −90°C) by using Crack Tip Opening Displacement testing. Fracture surfaces and changes in microstructure were analyzed by scanning electron microscopy and light microscopy. Crystallographic orientations were identified by electron backscatter diffraction, indirectly from a polished section perpendicular to the major fracture surface of the samples. Computer assisted 3D imaging was used to measure the angles between the cleavage facets and the adjacent polished surface, and then these angles were combined with electron backscatter diffraction measurements to determine the macroscopic crystallographic planes of the facets. The crystallographic indices of the macroscopic cleavage facet planes were identified to be {100}, {110}, {211} and {310} at all temperatures.

Quantitative Relation Between Acoustic Emission Signal Amplitude and Arrested Cleavage Microcrack Size

The possibility of obtaining a quantitative relation between acoustic emission (AE) signal amplitudes and arrested cleavage microcrack sizes in the partially transformed coarse grained heat affected zone of a structural steels is explored. Interrupted fracture mechanics tests are performed, and the size of measured arrested cleavage microcracks are compared with recoded AE signal amplitudes. It is shown that the experimentally measured relationship between the arrested microcrack size and AE amplitude closely follows a theoretical relation derived by Lysak (1996). The results may provide quantitative data as input to further development of micromechanically based cleavage fracture models for steels.

Effect of Hyperbaric Chamber Gas on Transformation Texture of the API-X70 Pipeline Weld Metal

The development of the texture components in the X70 weld metal under several shielding environments was investigated using the electron-backscattered diffraction (EBSD) and orientation imaging microscopy (OIM) techniques. A new method for assigning the reference direction (RD), transverse direction (TD), and normal direction (ND) was introduced based on the morphological orientation of the grains. The analyses showed that different shielding gases affect the weld metal texture and microstructure. The shielding environment with pure argon shows the highest orientational pole density values and dominant acicular ferrite microstructure. It was observed that the distribution of misorientation angle and special coincidence site lattice (CSL) grain boundaries play significant roles in determining the tensile characteristics of the weld samples. Moreover, the bainite lattice orientation was found dependent on the directional heat flow unlike the other detected constituents.

3D Fracture Simulations of SENT Specimens Including Welding Residual Stresses

In pipe-laying and reeling operations, high levels of plastic deformation may occur in the pipe wall. The effect of plastic deformation should be investigated and quantified in order to qualify a steel material for a given pipe-laying process. This paper presents a numerical modeling approach for realistic simulation of ductile fracture, including results from FE welding simulations. A 3D model of a SENT specimen is adopted, where crack propagation is modelled using a Gurson model. An elastic-plastic hardening model is used to capture the global deformation of the specimen. Results compare well with experiments. This work is part of an effort to develop an experimental and numerical framework where the influence of welding (HAZ properties, residual stresses and hardening) in fracture assessment. Preliminary results show that there is no influence of residual stresses on fracture strength of an X65 ductile steel.Copyright

3D Ductile Tearing Analyses of Bi-Axially Loaded Pipes with Surface Cracks

This paper concerns 3D ductile tearing analyses of outer surface cracked pipes subjected to tension loading and internal pressure.