Filtern
Dokumenttyp
Sprache
- Englisch (6)
Schlagworte
- Microstructure (6) (entfernen)
Organisationseinheit der BAM
Spectacular failure cases of fossil power stations in the recent years exhibited severe cracking in T24 welds. The results show that hydrogen-assisted cracking up to 200 °C cannot be excluded. Hence, it is important to gain a basic understanding on how hydrogen might affect the basic material properties in the respective weld microstructures. The present study focuses on hydrogen degradation of the respective weld microstructures, i.e., the weld metal and the coarse grained heat affected zone, where actually cracking appeared in practice. Tensile tests were carried out for coarse grain heataffected zone (CGHAZ) and the weld metal in uncharged and electrochemically hydrogen-charged condition. It turned out that both microstructures show distinct tendency for gradual degradation of mechanical properties in the presence of increasing hydrogen concentration. Already for a hydrogen concentration about and above 2 ml/100 g Fe, a significant ductility reduction has been observed. SEM investigations revealed that the fracture topography changes from ductile topography in uncharged condition to intergranular topography for the CGHAZ and to ductile-brittle mix for the weld metal (WM) in hydrogen charged condition. Ti-rich inclusions were identified as central regions of quasi-cleavage fracture areas in the WM. An approximation procedure is applied to quantify the degradation intensity.
Quite a number of models for hydrogen distribution in steels and welds have been developed in the past 20 years. They reach from simple analytical models to more complex two and three dimensional finite element simulations. So far, these models have been used to simulate hydrogen distribution in homogeneous microstructure. This paper contributes to numerical simulation of hydrogen distribution in heterogeneous microstructure, e. g. in a duplex stainless steel microstructure consisting of two phase fractions. Under appropriate conditions, such as cathodic protection, it is possible that hydrogen is absorbed leading to material embrittlement and possibly initiating hydrogen assisted cracking. In order to avoid hydrogen assisted cracking in duplex stainless steels, it is of great interest to know more about the diffusion behavior of the ferrite and austenite phase. A numerical model has been developed that operates on the mesoscale and enables simulation of hydrogen transport in the various phases of a metallic material. As a first application of this model, hydrogen distribution in a duplex stainless steel 1.4462, consisting of approximately equal portions of ferrite and austenite, was simulated using the finite element program package ANSYS. The results reflect the dependency of hydrogen distribution on the microstructural alignment of the ferrite and austenite phase fractions. Crack-critical areas can thus be identified, provided the critical strain-hydrogen combination is known for the respective microstructural phase.
The relation between microstructure and corrosion behavior of GTA welded AZ31B magnesium sheet
(2007)
Welding of AZ31B magnesium alloy was carried out using gas-tungsten arc (GTA) welding. The microstructure and the corrosion behavior of welded magnesium AZ31B alloy were investigated. ac and dc polarization tests were carried out on the welded Mg sheet. The microstructure was examined using optical and electron microscopy (TEM and SEM), X-ray analysis and EDS. Scanning Kelvin probe force microscopy (SKPFM) was used in order to measure the Volta potential of different phases relative to the matrix. The results showed that the GTA process effected both the microstructure and the corrosion behavior. These results can be explained by the effects of the process on microstructure of AZ31B Mg alloy sheet such as grain size and precipitates caused by the change in precipitation and recrystallization behavior.
More than 400 iron hydrogen storage containers (also called bottles or cylinders exploded on the air strip Berlin-Tempelhof on May 25, 1894, leaving immense destruction. The Royal Prussian Materials Testing Institute was requested to investigate the material properties and to furnish an expertise, how an increased safety of such cylinders might be achieved for the future under protection of the interests of the air ship service, as for instance by improvement of delivery specifications or respective material inspections.
The studies conducted personally by the director Prof. Adolf Martens and his deputy Prof. M. Rudeloff represent one of the first comprehensive failure case investigations in history and initiated BAMs long tradition in failure analysis. Martens and his colleague elaborated quite detailed specimen plans and investigated original failure parts with a special emphasis on conspicuous fracture appearance, but also made comparison experiments with hardened as well as annealed samples. Experienced investigators might identify some first routines how to conduct failure analyses and the importance of Adolf Martens as a pioneer in this field becomes evident.
Martens publications about the original expertise Martens, 1896 [1] and [2] include detailed descriptions about the experimental procedures and specimen preparation. Also, quite modern materials testing technologies and machines have been utilized, as for instance light microscopy as well as the tensile testing machines developed by Werder and Pohlmeyer. As special features developed by Martens, precision strain measurements have been applied during respective tensile tests and the so-called micro-photographic apparatus has been adopted to produce photos of the investigated microstructures. Additionally, the publications contain at that time very valuable advices regarding appropriate materials selection for gas storage cylinders.
The present contribution provides a nearly complete and as exact as possible translation of the original report Martens, 1896 [1] written in Old German language. Only little changes have been made in the text for a better understanding.
Welded components of P91 9% Cr steel demand for careful welding fabrication with necessary post weld heat treatment (PWHT). Before the PWHT, a hydrogen removal heat treatment is necessary for avoidance of hydrogen assisted cracking (HAC). In this context, the microstructure and temperature-dependent hydrogen diffusion is important, and reliable diffusion coefficients of P91 weld metal are rare. For that reason, the diffusion behavior of P91 multi-layer weld metal was investigated for as-welded (AW) and PWHT condition by electrochemical permeation experiments at room temperature and carrier gas hot extraction (CGHE) from 100 to 400 °C. Hydrogen diffusion coefficients were calculated, and the corresponding hydrogen
concentration was measured. It was ascertained that both heat treatment conditions show significant differences. At room
temperature the AW condition showed significant hydrogen trapping expressed by to seven times lower diffusion coefficients. A preferred diffusion direction was found in perpendicular direction expressed by high permeability. The CGHE experiments
revealed lower diffusion coefficients for the AW condition up to 400 °C. In this context, a hydrogen concentration of approximately 21 ml/100 g was still trapped at 100 °C. For that reason, a certain HAC susceptibility of as-welded P91 weld metal cannot
be excluded, and hydrogen removal should be done before PWHT.