Thomas Boellinghaus

Cold cracking tests—an overview of present technologies and applications

This study provides an in-depth survey of current technologies and applications for assessing the cold cracking susceptibility of welded joints. From the large variety of existent cold cracking test procedures, the most important and internationally established tests are presented and evaluated in terms of their usefulness and application limits. According to the type of loading, the test procedures are classified into self-restraint and externally loaded tests. Apart from the merely metallurgical weldability tests for determining the cracking susceptibility of base materials, filler materials and weld metals, advanced test methods are presented for evaluating the cold cracking susceptibility of welded components. A salient point brought out in this respect is the fact that the level of external loading in such component weld tests can be applied independently of the welding parameters, reproducing as realistically as possible the practical application case, i.e. the level of the restraint intensity. This study includes a summarized comparison of the cold cracking procedures. It is emphasized that highly accurate consideration and reproduction of the design-specific stiffness conditions is essential in the tests for assessing the cold cracking behaviour of welded joints. Therefore, various numerical analyses are presented in a final chapter for calculating the restraint intensity as a definitive factor affecting cold cracking.

Corrosion Testing of Welds, A Review of Methods

ForewordThis document has been produced by experts in the field of corrosion testing and corrosion resistant alloys under the auspices of sub-commission IX-H of the International Institute of Welding (IIW). It is intended to be a useful reference guide to standardised corrosion tests, listed in numerous national and international standards. Most corrosion tests were originally developed to assess various corrosion properties of parent metals and alloys. Many of these tests have been modified or adapted for use with weld metals and welded joints and this document is designed to act as a summary of the features of the various tests in the context of welds and welded joints. It is not intended to be a text book on corrosion testing but it is hoped that it will prove to be a useful aid to welding engineers and materials engineers who are not corrosion specialists in their own right. It is important to recognise that this document is a summary of published standards and should not be used as a substitute for such standards. The user is strongly advised to refer to the original standards to confirm precise and specific details.

Heat treatment Effects on The Reduction of Hydrogen in Multi-Layer High-Strength Weld Joints

High-strength structural steels with yield strengths up to 1 100 MPa are used in various industrial sectors such as for the construction of cranes, pipelines and offshore structures. However, with increasing strength Cthe ductility and deformation capacities of these materials are reduced and thus, they show an enhanced sensitivity against degradation due to hydrogen with increasing yield strength. It means they become susceptible to hydrogen-assisted cold cracking (HACC) during fabrication welding. In order to avoid such defects, Qthe existing standards recommend preheating and/or interpass temperature, as well as post heat treatments, However, the standards relate only to steels with a maximum yield strength of Rp.0.2 = 960 MPa. Hence, in welding these high-strength structural steels with yield strengths up to 1 100 MPa, it is very important to have practical guidelines for determining suitable heat treatment procedures to avoid HACC in welds, in particular in safety-relevant components. As a contribution to the further establishment of sufficient Hydrogen-Removal Heat Treatments (HRHT), two dimensional numerical models of butt and lap joints of various thicknesses were developed. Hydrogen diffusion and the effect of different post heat treatments upon hydrogen reduction in high-strength structural steel were studied. It turned out that the hydrogen diffusion behaviour in the lap and the butt joints are quite different and that the hydrogen concentration in the lap joint can be reduced significantly faster in comparison to the butt joint.

EFFECTS OF THE LOAD HISTORY ON THE RESIDUAL STRESS DISTRIBUTION IN WELDED COMPONENTS

The lifetime of a welded component is finally influenced by the various service conditions. The stress redistribution effect of subsequent load histories is fundamental to the load-bearing capacity of a structure later in service. Since every welded structure additionally experiences external shrinkage restraint, the resulting structural loads are very important for mechanical assessment. Forward-looking development of materials furthermore requires particular consideration of in-service load histories and of their effects on the global structural loads of a construction under shrinkage restraint. Using a large-scale test facility, stress and residual stress investigations have been performed relating to the subject area of residual stress redistribution resulting from a simulated load history. Based on the presented results, which were all carried out with matching filler materials, further research work concerning the influence of the variation of the mismatch factor on the residual stress distribution has to be done.

Mesoscale modeling of hydrogen-assisted cracking in duplex stainless steels

Quite a number of numerical models for hydrogen-assisted cracking in different kind of steels are existing reaching from simple analytical models to more complex two- and three-dimensional finite element simulations. These numerical models have been used to simulate the processes of hydrogen-assisted cracking in homogeneous microstructure. This paper contributes to numerical simulation of hydrogen-assisted cracking in heterogeneous microstructure, e.g., in a duplex stainless steel microstructure consisting of two phase fractions. If hydrogen is absorbed during welding or during service, i.e., due to cathodic protection, hydrogen is leading to material embrittlement and leads to hydrogen-assisted cracking. In order to improve understanding of the mechanisms of hydrogen-assisted cracking in duplex stainless steels, a numerical model has been created that operates at the mesoscale and enables simulation of stress–strain distribution as well as cracking in the various phases of a metallic material. Stress–strain distribution and hydrogen-assisted cracking in the duplex stainless steel 1.4462, consisting of approximately equal portions of ferrite and austenite, was simulated using the finite element program ANSYS. It was shown by numerical simulation that higher local stresses and strains are present at ferrite and austenite than the global stresses and strains in the duplex stainless steel, while the highest plastic deformations occur at austenite and the highest stresses can be found in small ferrite bars surrounded by ductile austenitic islands. By analyzing the stress–strain distribution in the duplex microstructure, crack critical areas in the ferrite can be identified. Hydrogen-assisted cracking was modeled assuming high hydrogen concentrations and regarding the local mechanical load in each phase of the duplex stainless steel. The mesoscale model qualitatively reflects the crack initiation and propagation process in the ferritic and austenitic phase of the duplex stainless steel.

Hydrogen Interaction with Residual Stresses in Steel Studied by Synchrotron X-Ray Diffraction

The residual stress state in a material has an important role in the mechanism of cracking, induced or assisted by hydrogen. In this contribution, the beamline EDDI in BESSY II instrument in Berlin was used in order to investigate the influence of hydrogen upon the residual stresses state existing in a Supermartensitic stainless steel sample. The method used for investigating the residual stresses is the “sinus square ψ” method. This method involves the usage of high energy X-ray diffraction in order to measure the residual stress state and magnitude. It was found that hydrogen presence has a significant influence upon the magnitude of the residual stresses, as its value decreases with high hydrogen content. This effect is reversible, as hydrogen desorbs from the sample the residual stress magnitude gains its initial value before hydrogen charging.

Principles and Concepts of Technical Failure Analysis

This chapter introduces conceptually Part C of the Handbook Technical Diagnostics of Machines and Plants. Although various prescriptions for organization and performing failure analyses are existing in literature, up to the present no approach is consistent to the concept of technical systems in the Sects 2.2 and 2.3 (Fig. 2.6) and thus, a comprehensive concept is outlined in this section, with a special attribute to the VDI-Guideline 3822 Failure Analysis.

Hydrogen Behavior in GTA Welded Ti-6Al-4V and Beta-21S Aerospace Applicative Titanium Alloys

Ti-6Al-4V and β-21S (Ti-15Mo-3Al-3Nb-0.3Si, wt%) titanium alloys were exposed to a hydrogen-containing environment, introduced by Gas-Tungsten Arc welding via a mixed Ar + 5% H2 shielding gas. The different characteristics of hydrogen absortion/desorption behavior and trapping in the welded Ti-6Al-4V and β-21S alloys were studied by means of thermal desorption spectroscopy (TDS). Thermal spectra analysis is supported by data from a variety of other experimental techniques, e.g., Leco hydrogen determinator and microstructure investigations. In the specimens welded in hydrogen-containing environment, no cracking was observed. However, the complex process of hydrogen desorption was found to be significantly affected by the microstructure developed in the alloys after welding. The unique microstructural morphology, the presence of potential irreversible trapping sites and phase transitions (oxide dissociation) were considered to be the potential parameters affecting the hydrogen desorption behavior from the alloys.

Hydrogen determination in welded specimens by carrier gas hot extraction—a review on the main parameters and their effects on hydrogen measurement

Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in weld joints using a thermal conductivity detector (TCD) for hydrogen measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries (ISO 3690 type B and small cylindrical samples), and factors that additionally influence hydrogen determination. They are namely specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PI-furnace controller, as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up to the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by evaluation of the recorded data. Generally, independent temperature measurement with dummy specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).

Hydrogen's Absorption/Desorption Behavior in Gaseous-Phase Charged Duplex-Annealed Ti-6Al-4V Alloy

Ti-6Al-4V alloy has proven to be technically superior and cost-effective materials for a wide variety of aerospace, industrial, marine and commercial applications. The mechanical properties of Ti-6Al-4V are very sensitive to the microstructure obtained after the thermo-mechanical treatment. The duplex structures provide good tensile ductility, fatigue strength, resistance to microcrack growth and crack initiation, and are often used in demanding fatigue critical tasks. However, although Ti-6A-4V is considered to be reasonably resistance to chemical attack, severe problems can arise when it comes in contact with hydrogen-containing environments due to its susceptibility to hydrogen embrittlement. The objective of this paper is to investigate the absorption and desorption behavior of external hydrogen on a duplex-annealed Ti-6Al-4V alloy. While investigating the desorption profile, we seek to better understand the thermodynamics and the kinetic nature of the interaction between traps and hydrogen atoms, with specific emphasis on the investigation of the impact of these interactions on the microstructure of the studied aerospace applicative titanium alloy. In order to achieve these goals, thermal desorption spectroscopy (TDS) was applied and the data obtained from this analysis was supported by a variety of other experimental techniques, such as LECO hydrogen determinator, XRD and microstructure investigations by means of optic and electronic microscopy. Hydrogen was found to influence significantly the microstructure of the alloy. The process of hydrogen evolution was found to be a very complex process, being affected mainly by the phase transformations that may occur during the thermal analysis.