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Industry faces a growing demand for high-strength structural steels with yield strengths of up to 1,300 MPa in order to cope with increasingly higher strength requirements in engineering. Higher strength levels are achieved by a special coordinated production process and an adapted chemical composition. Nevertheless, disastrous damage cases with high-strength steels have occurred in the past. The sensitivity to mechanical property degradation by hydrogen increases dramatically with strength. This phenomenon leads to hydrogen-assisted cold cracking. T-joints with fillet welds made from one side with an included angle of 60° were examined for their cold cracking behavior. Based on the T-joint, a modified heat input, even interpass temperature, plate thickness, and length ones were examined. The diffusion behavior and the effectiveness of different post-weld heat treatments in joints were simulated. The results of post-weld heat treatments are illustrated in practical hydrogen removal heat treatment diagrams. It is noticed that the T-joint is subject to a very high risk of hydrogen-assisted cold cracking (HACC). Contrary to other joints, its most critical area for cracking is not the weld metal but the heat-affected zone surrounding area of the root pass. The simulation shows that HACC in the T-joint can only be avoided by applying a sufficient post-weld heat treatment.
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 present contribution highlights recent trends in investigating hydrogen diffusion and cracking in steel welds. For such studies, supermartensitic stainless steels (SMSS) have exemplarily been selected. These materials have been used for offshore and marine constructions for about two decades now. They present a versatility of improved properties such as high strength to thickness ratio, good weldability and good corrosion resistance. However, as shown by respective failure cases, SMSS welds might become prone to hydrogen assisted cracking and the degradation phenomena are more easily to investigate due to the nearly homogeneous martensitic microstructure than in other materials. Generally, it has to be distinguished between cracking that occurs during or shortly after fabrication welding, or during subsequent operation of SMSS components. In order to achieve crack avoidance during fabrication and service, conclusive test sequences have to be applied, ranging from field tests at real components and full scale tests investigating the materials behavior under real service conditions to basic and small scale tests, such as tensile and corrosion tests, oriented more towards a materials ranking. Considerable testing of SMSS welds has been carried out and the present paper particularly summarizes spotlights on 1:1 scale component testing of welded tubulars, slow strain rate testing and basic tests oriented to elucidate the hydrogen behavior and degradation in SMSS weld microstructures. Also, permeation tests, hydrogen dependent degradation of mechanical properties and thermal desorption spectroscopy are adressed. As a specific item, first results of lately conducted investigations for tracking hydrogen movement in such weld microstructures by using high energy synchrotron radiation are elucidated.
Duplex stainless steels (DSS) are used in various industrial applications, e.g. in offshore construction as well as in chemical industry. But, at specific conditions, as for instance arc welding fabrication, cathodic protection or exposure to sour service environments, such materials can take up hydrogen which may cause significant property degradation particularly in terms of ductility losses which, in turn, may entail hydrogen assisted cracking (HAC). If HAC occurs in a duplex stainless steel the cracking mechanism is different from steels having only one phase, because hydrogen diffusion, stress-strain distribution and crack propagation are totally different in the austenite or ferrite phase. Whereas, the mechanism of hydrogen assisted crack initiation and propagation as well as hydrogen trapping in DSS have not been fully understood up to the present, as in most two-phase microstructures. In order to better understand the mechanisms of hydrogen assisted cracking in duplex stainless steels, knowledge of the diffusion behavior and of the stress-strain distribution in the ferritic and austenitic phase is of great interest. A numerical mesoscale model was created with a view to studying the hydrogen transport and the stresses and strains in each phase of duplex stainless steel. The material investigated in this work was DSS 1.4462, consisting of approximately equal portions of ferrite and austenite. Hydrogen diffusion in the duplex base metal was studied using the finite element program ANSYS. Stress-strain distribution as well as hydrogen assisted cracking in the ferritic and austenitic phase fractions were additionally investigated. The results of numerical simulation of the hydrogen diffusion process as well as structural analyses enable the identification of crack critical areas in the DSS microstructure. Numerical simulations qualitatively reflect the crack initiation and propagation process in ferrite. Crack critical combinations of hydrogen concentrations and local mechanical loads initiating HAC can be identified.
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.
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.
Modeling of Hydrogen-Assisted Cracking (HAC) in Duplex Stainless Steels (DSS):
- Hydrogen causes significant degradation in DSS and consequently HAC
- Mechanisms of HAC have not been fully understood in two-phase microstructures
- Mesoscale numerical modeling for HAC in DSS was created, coupled to the macroscale, enabling simulation of HAC in both phases independent from predefined crack paths
In the field of modelling hydrogen assisted cracking (HAC) phenomenon, hydrogen diffusivity is an important input parameter for numerical simulation. In terms of hydrogen diffusion coefficients, they have great impact on realistic assessment of the evolution of possible crack critical hydrogen concentrations. In addition, the chemical compositions of steels can have a strong effect on hydrogen diffusion. Unfortunately, literature provides a wide range of available hydrogen diffusion coefficients even for similar microstructures and equal temperatures. The scattering of the data can lead to significant deviations in the results of simulating the evolving hydrogen concentrations due to hydrogen uptake (by fabrication or service). Thus, the application of such data to crack-models or for component life tie predictions can be realized up to the present only by considering envelope curves of such value, corresponding to a work or bench case scenario, respectively. For improved reliability of numerical simulaitons, it is necessary to minimize the mentioned deviation of these data. Hence, this work focuses on the validation of hydrogen diffusion coefficients obtained from permeation experiments at room temperature. Two baintic steels with different alloying concepts were investigated, the creep-resistant 7CrMoVTiB10-10 and the reactor pressure vessel grade 20MnMoNi4-5. A numerical model is presented for simulation of the corresponding hydrogen diffusion during permeation experiments using the finite element software ANSYS. Three different diffusion coefficients (obtained from different common calculation methods) are considered and compared to numerical results. The vases of thes calculation methods are permeation transients which are a direct measure for hydrogen. The results of the simulated hydrogen diffusion coefficients show that only one procedure for calculation of diffusion coefficitnes is suitable in comparision to the experimental values. Thus, it is suggested to use this method for analysis of experimental results in case of hydrogen diffusion during permeation experiments. Furthermore, this work supplies validated values for the hydrogen diffusion coefficients of both steel grades.