The steel grade 20MnMoNi5-5 (according to German DIN standard or 16MND5 according to French AFNOR standard) is widely applied in (weld) fabrication of reactor pressure vessel components. Thus, a wide range of welding technologies (like submerged arc welding (SAW) or tungsten inert gas (TIG)) is used resulting in different heat affected zone (HAZ) microstructures. During weld fabrication, the weld joints may take up hydrogen. Especially, the HAZ shows an increased susceptibility for a degradation of the mechanical properties in presence of hydrogen. In addition, the hydrogen-assisted degradation of mechanical properties is influenced by three main local factors: hydrogen concentration, microstructure, and load condition. Hence, the base material (BM) and two different simulated non-tempered as-quenched HAZ microstructures were examined using hydrogen-free and hydrogen-charged tensile specimens. The results indicate that the effect of hydrogen on the degradation is significantly increased in case of the HAZ compared to the BM. In addition, hydrogen has remarkable effect in terms of reduction of ductility. It was ascertained that the degradation of the mechanical properties increases in the order of BM, bainitic HAZ, and the martensitic HAZ. Scanning electron microscope (SEM) investigation showed a distinct change of the fracture topography depended on the microstructure with increasing hydrogen concentration in case of the as-quenched HAZ microstructures.
High-energy synchrotron study of the stress-strain behavior of hydrogen-charged high strength steel
(2012)
High strength structural steels are susceptible to hydrogen embrittlement. A critical combination of stress, amount of diffusible hydrogen and microstructure is believed to cause cold cracking. Especially during welding of high strength structural steels high tensile residual stresses may develop. Therefore, a feasibility study was conducted using synchrotron X-ray diffraction in order to analyze the stress–strain behavior during tensile loading. For that purpose two types of steel showing different hardening mechanisms were used. On the one hand a thermo-mechanically treated S1100MC and on the other hand a quenched and tempered S1300Q were chosen. The samples were electrochemically charged with hydrogen and subsequently stored in liquid nitrogen to prevent effusion. Tensile tests of the samples were conducted in a special load frame allowing for tilting the samples while applying constant loads. High energy synchrotron radiation was used for energy dispersive X-ray diffraction (EDXRD) analysis in transmission geometry. This method offers the possibility for measuring several diffraction lines of all contributing crystalline phases of the material. Strains as well as stresses applying the sin²ψ-method were determined for varying load situations. This feasibility study shows how the interaction of hydrogen and the stress/strain response may be assessed by diffraction methods. Examples are presented showing that hydrogen alters the load distribution as well as the strain behavior between different lattice planes in high strength steels.
Since the mid-nineties, supermartensitic stainless steels (SMSS) have increasingly been applied to welded subsea-pipeline systems in the North Sea oil and gas fields, especially to flowlines at mild sour service conditions. However, in 2001 cracking and leaks occurred during installation and service start-up of two SMSS flowlines in the Norwegian Tune gas condensate field, welded with a new developed matching filler wire. Brittle transgranular cracking started especially at inter-run lack of fusion and propagated brittle, predominantly through the weld metal. The present paper provides a brief overview of the original failure case and respective sequence of events leading to complete replacement of the SMSS by carbon steel flowlines in 2002. Then, detailed investigations of a circumferential weld sample of the failed Tune flowline are highlighted, targeted at comparison of the failure appearance to previous investigations of this filler material type and to search for possible explanations for the brittle fracture at the crack initiation area. SEM investigations of the fracture surface revealed brittle areas only in the direction towards the top side of the weld while the major part of the investigated surface exhibited ductile fracture. As an approach to clarify, if the fracture was a consequence of hydrogen assisted cracking, five small sized specimens have been cut out of the original sample. Cracking has been introduced parallel to the original fracture surface in these specimens at respective saw cuts and bending. The results show that brittle transgranular cracking appeared only in the specimen cooled down to very low temperatures by liquid nitrogen and in the sample charged with hydrogen to an average concentration of about 15 ml/100 g. However, a fracture similar to the original surface was observed only in the hydrogenized specimen. As a further result, very similar fracture surfaces of supermartensitic stainless steel weld metals had been observed on specimens subjected to hydrogen assisted cold cracking (HACC) as well as to hydrogen assisted stress corrosion cracking (HASCC). In total, the results indicate that brittle fracture starting at the inter-run lack of fusion were not initiated by high notch tip deformation rates, but rather influenced by hydrogen, probably taken up during welding.
Three high strength Nb-, Ti- and Ti+V- bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on HAZ toughness. Charpy V notch impact tests from three microalloyed welds under different cooling rates have been performed. Fractographic examination shows that several factors including large-sized grain, upper bainite or hard second phase, interact to determine the brittle fracture mode and impaired toughness of Nb bearing weld at high heat input. In contrast to this reduced toughness, Ti bearing welds exhibits satisfied toughness regardless of at a fast or slow cooling rate as a result of limited austenite grain and refined favourable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the fracture process and crystallographic misorientation results also confirms that high angle boundaries between fine ferrites plates provide much effective barrier for crack propagation and contribute to improved toughness.
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.
Hydrogen assisted stress corrosion and cold cracking represent still a major topic regarding the safety of welded steel components against failure in many industrial branches. Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or at cathodic protection. Additionally, understanding and avoidance of hydrogen entry into weld microstructures from gaseous pressurized environments becomes increasingly important for renewable energy components. There are two types of metallurgical mechanisms associated with hydrogen assisted cracking, i.e. the cracking as well as hydrogen transport and trapping mechanisms. For numerical modelling, it has to be considered that both types are not independent of each other, that the mechanisms are not yet completely clarified and that validation of such models strongly depends on implementation of the correct hydrogen related materials properties. However, quite significant achievements have been made in modelling of hydrogen assisted cracking by indirect coupling of thermal, stress-strain as well as hydrogen uptake and diffusion analyses. After a brief introduction into the subject and by revisiting various proposed cracking mechanisms, the present contribution focuses on recent developments of a numerical model based on a comparison of actual hydrogen concentrations and mechanical loads with respective hydrogen dependent material properties as crack initiation and propagation criteria. The basic procedure for numerical simulation of crack initiation and propagation is outlined and it is shown how such numerical simulations can be validated experimentally. Furthermore, it is highlighted how such a procedure has been extended to a comprehensive model for life time prediction of welded steel pipeline components and experimentally verified. Finally, it is outlined how the model can be extended to simulate cracking in heterogeneous steel microstructures on the different scales.
Hydrogen can have an extreme degradation effects in steels, particularly concerning the mechanical properties. These effects can lead to hydrogen-assisted cracking in microalloyed high-strength steels during fabrication and/or operation in industrial applications. In order to study these effects, electrochemically charged tensile specimens were tested to elucidate the degradation of their properties. The carrier gas hot extraction (CGHE) method, which functionally combines a mass spectrometer with a thermal desorption analysis (TDA) process, was used for the detection of ultra-low diffusible hydrogen concentrations in the material specimens. The mass spectrometer provided rapid and automatic determination of hydrogen concentration, whereas the TDA presented the activation energy within the respective test specimen at the specific temperature. Additionally, specimen temperature was carefully monitored to reduce the evaluation error for local effusion peaks. A quenching and deformation dilatometer was used for the analysis of typical heat-affected zones during the welding process for a high reproducibility of the homogenous microstructures that were studied. The present work shows the interaction between hydrogen and lattice defects in different microalloyed materials and heat-affected zones of weldable fine-grained steels. These steels were prepared in a quenched and tempered condition and in a thermo-mechanically rolled condition. These preparations were made according to German standard DIN EN 10025-6 and to DIN EN 10149-2, respectively. The trapping characteristics of two steel grades, S690QL and S700MC, were studied with respect to the activation energy dependent on carbon content and microalloying elements such as Ti, Nb, Mo, Cr, and V. The two steel grades exhibited several types of traps: carbide formations, dislocations, and/or grain boundaries were common, which can influence activation energy and hydrogen solubility. The type and dimension of inclusions or particles also affected the hydrogen trapping behavior. A decrease of carbon and specific alloying elements in thermo-mechanically hot rolled steels led to a change in the activation energy binding the trapped hydrogen. This thermo-mechanically hot rolled steel revealed an increased interaction between hydrogen and precipitations. The higher carbon content in the quenched and tempered steel led to a higher interaction between hydrogen and iron carbide, specifically in the martensitic phase. Furthermore, the trapping behavior in heat-affected zones showed a significant increase in activation energy, especially in the coarse-grained microstructure. These previously mentioned various effects were studied to better understand the degradation of mechanical properties in these two steels.
Hydrogen can have an extreme degradation effects in steels, particularly concerning the mechanical properties. These effects can lead to hydrogen assisted cracking in micro-alloyed high strength steels during fabrication and/or operation in industrial applications. The Carrier Gas Hot Extraction method, which functionally combines a mass spectrometer with a Thermal Desorption Analysis process, was used for the detection of ultra-low diffusible hydrogen concentrations in the material specimens. The work shows the interaction between hydrogen and lattice defects in different micro-alloyed materials and HAZ. These steels were prepared in a quenched and tempered condition and in a thermo-mechanically rolled condition. The trapping characteristics of two steel grades, S690QL and S700MC, were studied with respect to the activation energy dependent on carbon content and micro-alloying elements such as Ti, Nb, Mo, Cr and V. The two steel grades exhibited several types of traps: carbide formations, dislocations and/or grain boundaries were common, which can influence activation energy and hydrogen solubility. The type and dimension of inclusions or particles also affected the hydrogen trapping behavior. A decrease of carbon and specific alloying elements in MC steels led to a change in the activation energy binding the trapped hydrogen. This thermo-mechanically hot
rolled steel revealed an increased interaction between hydrogen
and precipitations. The higher carbon content in the
quenched and tempered steel led to a higher interaction
between hydrogen and iron carbide, specifically in the
martensitic phase. Furthermore,the trapping behavior in HAZ showed a significant increase in activation energy, especially in the coarse grained microstructure.