Thomas Böllinghaus

Hot Cracking Phenomena in Welds III

Hot tearing remains a major problem of casting technology despite decades-long efforts to develop a working hot tearing criterion and to implement it into casting process computer simulation. Existing models allow one to calculate the stress–strain situation in a casting (ingot, billet) and to compare it with the chosen hot tearing criterion. Two kinds of hot tearing criteria are available in literature: mechanical and non-mechanical one. The mechanical criteria of hot tearing are derived based on mechanical behaviour semi-solid, and the non-mechanical one is based on other properties of semi-solid. In most successful cases, the simulation shows a relative probability of hot tearing and the sensitivity of this probability to such process parameters as casting speed, casting dimensions, and casting practice. None of the existing criteria, however, can give the quantitative answer on whether the hot crack will appear or not and what will be the extent of hot cracking (position, length, shape). This chapter outlines the requirements for a modern hot tearing criterion as well as the future development of hot tearing research in terms of mechanisms of hot crack nucleation and propagation. Introduction – Mechanisms of Hot Tearing Various defects of as-cast product are still frequently encountered in casting practice. One of the main defects is hot tearing or hot cracking, or hot shortness. Irrespective of the name, this phenomenon represents the formation of an irreversible failure (crack) in the still semi-solid casting. in Aluminium Alloys Delft University of Technology, Netherlands Institute for Metals Research, Delft, The Netherlands 4 L. Katgerman, D.G. Eskin From many studies [1, 2, 3, 4, 5, 6, 7, 8] started already in the 1950’s, and reviewed by Novikov [9] and Sigworth [10], it appears that hot tears initiate above the solidus temperature and propagate in the interdendritic liquid film. In the course of solidification, the liquid flow through the mushy zone decreases until it becomes insufficient to fill initiated cavities so that they can grow further. The fracture has a bumpy surface covered with a smooth layer and sometimes with solid bridges that connect or have connected both sides of the crack [7, 8, 11, 12, 13, 14, 15, 16]. Research studies show that hot tearing occurs in the late stages of solidification when the volume fraction of solid is above 85–95% and the solid phase is organized in a continuous network of grains. It is also known that fine grain structures and controlled casting (without large temperature and stress gradients) help to avoid hot cracking. During direct-chill (DC) casting of aluminium alloys, primary and secondary cooling cause strong thermal gradients in the billet/ingot, resulting in uneven thermal contraction in different sections of the billet/ingot. As a result, macroscopic stresses cause distortion of the billet/ingot shape (e.g. butt curl and swell, rolling face pull-in) and/or may trigger hot tearing and cold cracking in the weak sections. The terms “hot” or “cold” refer to the temperature range where the cracking occurs – in the semi-solid mushy zone or below the solidus, respectively. In DC casting, the name “mushy zone” is frequently applied to the entire transition region between liquidus and solidus, which is misleading, as the semi-solid mixture in the top part of the transition region is actually a slurry. Only after the temperature has dropped below the coherency temperature, a real mush is formed. On the microscopic level solidification shrinkage and thermal contraction impose strains and stresses on the solid network in the mushy zone. The deformation behaviour of the mush is very critical for the formation of hot tears. The link between the appearance of hot tears and the mechanical properties in the semi-solid state is obvious and has been explored for decades; see for example reviews [9, 17]. Another important correlation – between the hot cracking susceptibility and the composition of an alloy – has been established on many occasions. A large freezing range of an alloy promotes hot tearing since such an alloy spends a longer time in the vulnerable state in which thin liquid films exist. A lot of efforts have been devoted to the understanding of the hot tearing phenomenon. Compilations of research in this field have been done by Novikov [9], Sigworth [10], and Eskin et al. [17]. Several mechanisms of hot tearing are already suggested in literature. Some of those are outlined in Table 1. In Search of the Prediction of Hot Cracking in Aluminium Alloys 5 Table 1. Summary of hot tearing mechanisms Mechanism Suggested and developed by Ref. Cause of hot tearing Thermal contraction Heine (1935); Pellini (1952); Dobatkin (1948) [18, 2, 19] Liquid film distribution Vero (1936) [20] Liquid pressure drop Prokhorov (1962); Niyama (1977) [37, 39] Vacancy supersaturation Fredriksson et al. (2005) [21]

Modeling Of Hydrogen Distributionin A Duplex Stainless Steel

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.

Short Term Metallurgy and Hot Cracking During Laser Beam Welding of Austenitic Stainless Steels

Industrial application of high alloyed austenitic stainless steel laser welding has grown significantly in the recent time due to the continuous improvement of compact and high power density lasers systems. The application of such processes meanwhile ranges from pipeline or railway car body manufacturing to the production of household wares. The largest advantages of the laser application to welding production are much higher welding speeds, reduction or complete exclusion of welding consumables, easy design of the weld joints, decrease of thermal distortions and thus, costs saving. In contrast to arc welding, laser beam welding might particularly be associated with metallurgical defects, like the formation of hot cracks. Such phenomena are related to an order of magnitude higher temperature gradients and cooling rates in the solidification zone, providing rapid solidification kinetics which may cause significant segregation of alloying elements such as Ni and Cr and respective undercooling of the solute at the solidification front. In specific metastable austenitic stainless steels alloys in vicinity of the so called eutectic rim of the Fe-Cr-Ni constitutional diagram, such effects might entail a change of solidification mode from primary ferrite to austenite, providing an increased risk of solidification cracking. Previous studies has shown that the primary solidification mode change during laser beam welding of Cr-Ni austenitic stainless steels such alloys could be effectively influenced by nitrogen absorption as well as by the laser plasma type and also proved the occurrence of metastable primary ferritic solidification. In the present contribution, such results are compared to recent investigations of laser welding newer austenitic Fe-Cr-Mn-Ni steel grades by identification of respective hot cracking critical welding parameter intervals and strain rates in the Controlled Thermal Weldability (CTW) Test.

Use of time-of-flight secondary ion mass spectrometry for the investigation of hydrogen-induced effects in austenitic steel AISI 304L

During the energy transformation from fossil fuels to renewable energy sources, the use of hydrogen as fuel and energy storage can play a key role. This presents new challenges to industry and the scientific community alike. The storage and transport of hydrogen, which is nowadays mainly realized by austenitic stainless steels, remains problematic [L. Zhang et al., Int. J. Hydrogen Energy 39, 20578 (2014)], which is due to the degradation of mechanical properties and the possibility of phase transformation by hydrogen diffusion and accumulation [P. Rozenak, Metall. Mater. Trans. A 45, 162 (2014)]. The development of materials and technologies requires a fundamental understanding of these degradation processes. Therefore, studying the behavior of hydrogen in austenitic steel contributes to an understanding of the damage processes, which is crucial for both life assessment and safe use of components in industry and transportation. As one of the few tools that is capable of depicting the distribution of hydrogen in steels, time-of-flight secondary ion mass spectrometry was conducted after electrochemical charging [O. Sobol et al., Surf. Interface Anal. 48, 474 (2016)]. To obtain further information about the structural composition and cracking behavior, electron-backscattered diffraction and scanning electron microscopy were performed. Gathered data of chemical composition and topography were treated employing data fusion, thus creating a comprehensive portrait of hydrogen-induced effects in the austenite grade AISI 304L. Specimens were electrochemically charged with deuterium instead of hydrogen. This arises from the difficulties to distinguish between artificially charged hydrogen and traces existing in the material or the rest gas in the analysis chamber. Similar diffusion and permeation behavior, as well as solubility, allow nonetheless to draw conclusions from the experiments [Y. Fukai and H. Sugimoto, Adv. Phys. 34, 263 (1985)].During the energy transformation from fossil fuels to renewable energy sources, the use of hydrogen as fuel and energy storage can play a key role. This presents new challenges to industry and the scientific community alike. The storage and transport of hydrogen, which is nowadays mainly realized by austenitic stainless steels, remains problematic [L. Zhang et al., Int. J. Hydrogen Energy 39, 20578 (2014)], which is due to the degradation of mechanical properties and the possibility of phase transformation by hydrogen diffusion and accumulation [P. Rozenak, Metall. Mater. Trans. A 45, 162 (2014)]. The development of materials and technologies requires a fundamental understanding of these degradation processes. Therefore, studying the behavior of hydrogen in austenitic steel contributes to an understanding of the damage processes, which is crucial for both life assessment and safe use of components in industry and transportation. As one of the few tools that is capable of depicting the distribution of hydr...

Corrosion Fatigue Of X46Cr13 in CCS Environment

During CCS components are exposed to a corrosive environment and mechanical stress which results in corrosion fatigue and is inevitably followed by the lifetime reduction of these components. The lifetime reduction of the cyclically loaded high alloyed stainless injection-pipe steel AISI 420C (X46Cr13, 1.4034) constantly exposed to highly corrosive CO2-saturated hot thermal water is demonstrated in in-situ-laboratory experiments (T = 60 °C, brine: Stuttgart Aquifer, flow rate: 30 l/h, CO2) in an environment similar to the on-shore CCS-site in the Northern German Bassin. In-situ tension-compression experiments were established simultaneously along with electrochemical measurements using a newly designed corrosion chamber in a resonant testing machine at a frequency as low as 30 – 40 Hz. In addition technical CO2 was introduced into the closed system at a rate close to 9 L/h. S-N plots, micrographic analysis and surface analysis of the fracture surface are demonstrated. X46Cr13 (surface roughness Rz = 4) reached the maximum number of cycles (12.5 x 106) at stress amplitude of 173 MPa producing a low scatter range of 1:3.5. Hydroxide and siderite layers were found on pits and crack surfaces. No typical fatigue limit exists. Pit corrosion prior to crack initiation may be identified as failure cause.

Numerical Investigations on Hydrogen-Assisted Cracking (HAC) in Duplex Stainless Steels

Duplex stainless steels have been used for a long time in the offshore industry, since they have higher strength than conventional austenitic stainless steels and they exhibit a better ductility as well as an improved corrosion resistance in harsh environments compared to ferritic stainless steels. However, despite these good properties the literature shows some failure cases of duplex stainless steels in which hydrogen plays a crucial role for the cause of the damage. Numerical simulations can give a significant contribution in clarifying the damage mechanisms. Therefore, a numerical model of a duplex stainless steel microstructure was developed enabling simulation of crack initiation and propagation in both phases. The phase specific stress strain analysis revealed that local plastic deformation occurs in both austenite and δ-ferrite already in the macroscopically elastic range. Altogether, phase specific hydrogen-assisted material damage was simulated for the first time taking into account all main factors influencing hydrogen assisted cracking process. The results agree well with experimental observations and thus allow a better insight in the mechanism of hydrogen-assisted material damage.

Imaging ToF-SIMS as a Chemical Metrology Tool to Support Material and Analytical Science

As a chemical metrology tool time-of-flight secondary ion mass spectrometry (ToF-SIMS) has become a very popular technique to monitor the elemental, isotopic and molecular distribution in two or three dimensions. Its reduced sampling depth, high sensitivity, great structural specificity and the direct detection of hydrogen thereby increase the emergence of ToF-SIMS for material and analytical surface science, particularly due to recent instrumental developments improving mass, depth and lateral resolution. For basic surface science, adsorption processes and surface reactivity thus can be investigated in high detail on organic as well as inorganic samples. The use of multivariate data analysis in addition can effectively assist to identify trends in the complex SIMS raw data set and define key co-variances between certain samples or mass spectra. In this contribution the essence of ToF-SIMS is illustrated by discussing two highly relevant energy applications. First, for piezoelectric electroceramics oxygen exchange active zones have been visualized to determine the impact of external field-load to the oxygen vacancy distribution between anode and cathode. As a second case study the interaction of hydrogen species with the microstructure of a duplex stainless steel was investigated. It was concluded that ToF-SIMS has a valuable essence for detailing hydrogen related degradation mechanisms.

Welding in the World—50 years

This year, Welding in the World celebrates its 50 anniversary as the leading technical publication of the International Institute of Welding. In 1963, there were 4 issues with 17 total papers. This year, we will publish over 80 papers in 6 issues. The original editor (1963–1966) was Guy Parsloe. He was succeeded in 1968 by Philip Boyd who held this position until 1991 and was followed by John Hicks (1991–1995). From 1996 through 2009, the International Institute of Welding CEO served as the editor of Welding in the World and included Michel Bramat, Daniel Beaufils, Andre Charbonnier, and Cecile Mayer. In 2009, Bruno de Meester, Thomas Bollinghaus, and John Lippold were appointed as coeditors. The original publication coordinator at the Secretariat was Andre Leroy (1963–1974). He was succeeded in 1975 by Henri Granjon, who held this post until 1986, followed by Michel Bramat (1986–2000), Noelle Fauriol (2001–2004), and Veronique Souville (2004–2012). The current coordinator is Pierre Tran.

Korrosion und Korrosionsschutz

Korrosion ist die physikochemische Wechselwirkung zwischen einem Metall und seiner Umgebung, die zu Veranderungen der Eigenschaften des Metalls fuhrt und die zu erheblichen Beeintrachtigungen der Funktion des Werkstoffes, der Umgebung oder des technischen Systems, von dem diese einen Teil bilden, fuhren kann [1]. Diese Definition ersetzt fruhere Betrachtungsweisen der Korrosion, in denen oft sehr unklar der Korrosionsprozess selbst, das Ergebnis in Form der Korrosionsprodukte oder der Korrosionsschaden gemeint waren. Insbesondere ergibt sich daraus, dass die Korrosion und Bestandigkeit gegen Korrosion keineswegs eine reine Materialeigenschaft, sondern eine System- bzw. Bauteileigenschaft ist.

Fatigue testing for spot welds: Problems, solutions and new development

Engineering design of spot welded components as well as computational analyse of the local stress in the weld zone require relevant fatigue test data. Those test data, however, are dependent on the applied specimens, on the specimen restrain, on the load introduction into the weld zone as well as on the definition of the specimen failure. The discussion of the advantages and drawbacks as well as of the influence of these factors shows that a deliberate selection of specimens is necessary for different test purposes. For the comparability of the test results it is essential in addition to specify comparable test conditions, e. g. specimen fixturing, load introduction and definition of the specimen failure. On account of the variety of variants, a standardization of suitable specimens and respective test conditions hence constitutes an approach to the determination of relevant and comparable test results.