Odd M. Akselsen

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.

A comparative study of the heat affected zone (HAZ) properties of boron containing low carbon steels

Quantitative information on the HAZ hardenability of two low carbon microalloyed steels containing different boron levels (11 and 26 ppm B) has been obtained on the basis of the weld thermal simulation technique. The extent to which boron affects the HAZ hardenability was found to depend both on the peak temperature and the base plate boron content, in agreement with predictions based on a theoretical model for quench-induced segregation of boron in steel. At the highest boron level (26 ppm B), the hardenability of the grain refined region was approaching that of the grain coarsened one, which resulted in an approximately uniform microstructure within the transformed parts of the HAZ. However, indications are that additions of boron to conventional low carbon microalloyed steels should be restricted to about 10 to 15 ppm B due to the risk of embrittlement in the grain coarsened region at higher boron concentrations.

Geometrical aspects of hot cracks in laser-arc hybrid welding

Hot cracks were frequently found in double sided laser-arc hybrid welding thick section (20 mm) low carbon steel. Other research has usually investigated the metallurgical resolidification mechanisms of the welded metal alloy, but here possible relationships between the hot cracks and geometrical aspects of their surrounding weld and heat-affected zone cross sections were studied. The motivation behind this research was to identify guidelines for laser-arc hybrid welding to avoid hot cracks. Weld cross sections were used to analyze hot cracking because they are rather easy to prepare and straightforward to alter through the process parameters. In this study, hot cracks were found in a preferred geometrical window, namely, in the middle of the narrow deep region of the weld which was generated by the laser beam. From the cross section analysis, a first indicator was that a more inclined, converging shape of the lower part of the weld cross section can avoid hot cracks, associated with different energy input and resolidification front geometry. Significant reduction of the welding speed has avoided hot cracks, being a second indicator.Hot cracks were frequently found in double sided laser-arc hybrid welding thick section (20 mm) low carbon steel. Other research has usually investigated the metallurgical resolidification mechanisms of the welded metal alloy, but here possible relationships between the hot cracks and geometrical aspects of their surrounding weld and heat-affected zone cross sections were studied. The motivation behind this research was to identify guidelines for laser-arc hybrid welding to avoid hot cracks. Weld cross sections were used to analyze hot cracking because they are rather easy to prepare and straightforward to alter through the process parameters. In this study, hot cracks were found in a preferred geometrical window, namely, in the middle of the narrow deep region of the weld which was generated by the laser beam. From the cross section analysis, a first indicator was that a more inclined, converging shape of the lower part of the weld cross section can avoid hot cracks, associated with different energy inpu...

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.

A model for coupled growth of reaction layers in reactive brazing of ZrO2-toughened Al2O3

In the present investigation, process modeling techniques have been applied to describe coupled reaction layer growth in reactive brazing of ZrO2-toughened Al2O3 with Ag-Ti filler metals. The model takes into account both the successive evolution of the titanium oxide layer at the ceramic/braze metal interface at elevated temperatures and the subsequent decomposition of the reaction products during cooling. The results are presented in the form of novel process diagrams which illustrate in a quantitative manner the microstructural connections throughout the various stages of the process. The diagrams can, in turn, be used to calculate the individual reaction layer thicknesses at room temperature and relate these directly to the content of reacting element in the braze alloy.

A review of cohesive zone modelling as an approach for numerically assessing hydrogen embrittlement of steel structures.

Simulation of hydrogen embrittlement (HE) requires a coupled approach; on one side, the models describing hydrogen transport must account for local mechanical fields, while, on the other side, the effect of hydrogen on the accelerated material damage must be implemented into the model describing crack initiation and growth. This study presents a review of coupled diffusion and cohesive zone modelling as a method for numerically assessing HE of a steel structure. While the model is able to reproduce single experimental results by appropriate fitting of the cohesive parameters, there appears to be limitations in transferring these results to other hydrogen systems. Agreement may be improved by appropriately identifying the required input parameters for the particular system under study. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

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.

The Effects on Process Performance of Reducing the Pressure From 36 to 1Bar in Hyperbaric MIG Welding

Technology for remotely controlled (diverless) repair welding of subsea pipelines from 170 to 1000m water depth is being developed by StatoilHydro. The repair technology is based on a sleeve concept combined with MIG welding and the development is currently nearing completion. Technology for diver-assisted remotely controlled welding down to about 200m has been used in the North Sea for about twenty years. In order to reduce the use of divers, the deep water diverless technology is also being considered for use in shallow waters. The present work has been performed to investigate whether the deepwater welding procedure may also be used in shallow waters, and which modifications for the lower pressure conditions need to be made. Test welding has been performed in the pressure range from 36 to 1bar corresponding to 350 to 0m sea water depth to study the effect of ambient pressure upon the welding process behaviour and weld bead appearance and geometry. For the 12 o’clock welding position tested, welding parameters developed for deep water conditions also worked well for shallow water conditions down to about 2bar. It was also evident that the electrode polarity, which is negative for the deep water procedure, had to be changed to electrode positive for the lowest pressures, which coincides with conventional 1-atm MIG welding. Mechanical property testing and microstructure examinations revealed satisfactory results using the modified welding procedure.© 2009 ASME

Hydrogen Assisted Cracking in Welding of 13% Cr Supermartensitic Stainless Steels

Supermartensitic stainless steels are known to be prone to hydrogen induced cold cracking. Therefore, the objective of this work was to assess the susceptibility to hydrogen cracking in Gas Metal Arc Welding (GMAW) with use of matching base and filler materials (supermartensitic stainless steel), using the Instrumented Restraint Cracking (IRC) test. Root welding in the IRC test did not result in hydrogen induced cracking, neither for low nor high weld metal hydrogen content. Because of the martensitic transformation, the global residual stresses are very low after welding (below 100 MPa). Since the yield strength (Rp0.2% ) of the material is about 600–720 MPa, it implies that the IRC test method is not very suitable for supermartensitic stainless steels. However, by performing IRC test multi-layer welding, micro-cracks were found in the last pass. An increase in the weld metal hydrogen content resulted in reduced fracture stress and ductility, as observed in tensile testing of IRC test specimens directly after welding. Investigation of the fracture surfaces of the specimens with high hydrogen contents showed fish eyes, which are strong indications of hydrogen embrittlement. By performing heat treatment (225°C for 24 hours) of specimens with high hydrogen contents and subsequent tensile testing, the fracture stress and ductility were restored to the initial base metal level. Slow Strain Rate Testing (SSRT) with and without Cathodic Protection (CP) was performed on test specimens sampled transverse to the welding direction. CP has detrimental effect on the fracture stress and ductility due to the high weld metal hydrogen pick up.© 2003 ASME