Papers by Keyword: Hydrogen Embrittlement

Abstract: The use of duplex stainless steels (DSSs) is steadily increasing. For many uses where joining is needed, gas tungsten arc welding (GTAW) is one of the most important joining methods for DSSs. Since hydrogen embrittlement (HE) occasionally occurs in DSSs, understanding the relationship between the extent of HE and the welding condition is crucial to prevent HE. In this research, the effect of the heat input of GTAW process on the microstructure and the extent of HE in a UNS S31260 (JIS SUS329J4L) has been investigated. For this purpose, three samples have been prepared with diffrent velosity. All the samples have been cathodically hydrogen-charged, and then subjected to tensile test at a strain rate followed by fractography observation. Thermal desorption spectroscopy (TDS) has been carried out on the samples welded at low and average velosities. The results showed that tensile properties of the welded specimens were lower than those of base metal due to coarsening of the matrix ferrite grains and loss in the fraction balance of ferrite and austenite phases in the weld metal zone, where fracture took place.

Abstract: Samples extracted from flow formed tubes made of 18% nickel maraging steel grade C18Ni1750 were subjected to tensile testing at room temperature in laboratory environment at two different strain rates. Testing was carried out in as flow formed as well as flow formed and aged conditions. Aging was carried out adopting four different cycles. Distinct loss of ductility was observed at the lower strain rate in all tested conditions. The embrittlement occurring during low strain rate testing is explained in terms of hydrogen induced damage, hydrogen coming from the moisture in the environment. It is also concluded that the heavy cold work imparted to the material during flow-forming is importantly responsible for the ductility loss observed at low strain rate.

Abstract: We investigated the effect of electrolytic hydrogen-charging on regularities of plastic flow, strength and fracture mechanisms of AISI 316L and 321 austenitic stainless steels. In the steels, an ultrafine-grained structure of various morphologies was formed using methods of warm abc-pressing and thermomechanical treatment (cold rolling and annealing). Hydrogen-charging of ultrafine-grained steels reduces their yield strength and elongation. The high dislocation density and low-angle boundaries inhibit the effects of hydrogen embrittlement in 316L and 321 steels.

Abstract: Hydrogen is increasingly considered as fuel for future mobility or for stationary applications. However, the safe distribution and storage of pure hydrogen is only possible with suitable materials. Interstitially dissolved hydrogen atoms in the lattice of numerous metals are responsible for hydrogen embrittlement (HE). If hydrogen is introduced by an external source, it is called hydrogen environment embrittlement (HEE). Commonly, steels like AISI 316L with a high resistance to HEE include a large number of alloying elements and in high amount. High alloying levels result in a decrease of cost-efficiency. Therefore, the systematic investigation of lean-alloyed austenitic stainless steels is necessary in order to understand the mechanism of HEE. For that purpose, the steel grades AISI 304L and AISI 316L are selected in this work. Tensile tests in air and 400 bar hydrogen gas atmospheres are performed. After tensile testing in H, AISI 304L revealed secondary cracks at the specimen surface, which are related to the local austenite stability, which in turn is affected by the level of micro-segregation. The microstructural investigations of the crack environment directly contribute to the understanding of the micro-mechanisms of HEE. Property-maps generated from experimentally measured distributions of alloying elements allow to correlate the impact of micro-segregations on the local austenite stability. It is shown, that local segregation-bands affect the initiation and propagation of secondary cracks. In this context, the local austenite stability which is significantly affected by the Ni distribution will be discussed in detail by comparison of the metastable austenitic steel grades AISI 304L and AISI 316L.

Abstract: The hydrogen-induced damage behavior of ultra-high strength steels (UHSS) has been predicted by a combination of experimental and numerical investigations. Firstly, the resistance against hydrogen-induced failure was examined by slow strain rate tests (SSRT) using various sample geometries and hydrogen contents. Secondly, the hydrogen distribution and loading conditions during the tensile test were calculated by means of the finite element method (FEM). Finally, a combination of various damage models was applied and validated by further SSRT. The main result of this study is a failure prediction model, which considers local stress and strain conditions, as well as hydrogen content.

Abstract: One of the most common types of metal destruction in the oil and gas industry is hydrogen embrittlement. Hydrogen corrosion is a complex of negative effects of hydrogen on steel, leading to the destruction of metal structures. Hydrogen passes through a defect-free metal, without lingering in it. In the presence of defects, hydrogen is retained in the metal forming a brittle solid solution, metal stratification along the segregation streamer, blistering. Studies of the metal of a gas pipeline made of steel 09G2S are presented in the article. The sample was selected from the local zone of destruction in the condensate collector, the metal of the pipe had typical for hydrogen corrosion stratifications. The scope of the study was identification of the most dangerous part of hydrogen corrosion on the sample taken from the local zone of destruction. Studies on the chemical composition and mechanical properties of 09G2S steel were also carried out. Stress-related characteristics of the metal microstructure of the failed gas pipeline were obtained and the character of the destruction progress was revealed. The presence of sulfides cluster in the metal of studied pipe was determined applying metallographic method for determining nonmetallic inclusions.

Abstract: In this article, the causes of damage of a cover were investigated by metallographic and fractographic analysis. The component part was made from non-alloy quality steel for cold forming DC04. The failure occurred during high temperature pulsation (90 °C, 7b, 1 Hz) after 180,000 cycles. This component part was fabricated by deep drawing. After this process’s step, outlets were soldered at 1100 °C/5 min. to this part and then whole component part was coated using method without any specification. The coating layer was formed of Zn-Ni. The last step in this process was hemming where all component parts were assembled together. This case study was solved using light and scanning electron microscopy. The chemical composition was detected by energy dispersive X-ray analysis.

Abstract: Acicular ferrite (AF) and upper bainite (UB) are microstructural constituents commonly found in ferritic weld metals. Both microstructures are formed within a similar temperature range and by the same type of transformation mechanisms. They have however, substantially different morphologies and microstructural features that govern both their mechanical properties and hydrogen embrittlement susceptibility. This work shows that despite substantial microstructural differences, the mechanical properties of both microstructural constituents were quite similar. However, the microstructural differences were found to significantly affect the hydrogen crack propagation resistance. Hydrogen assisted cold cracking (HACC) propagates along a path of least resistance through the surrounding microstructure. The unit crack path was significantly shorter for AF than for UB, which implied more frequent changes in direction and thus increased dissipation of energy from the crack driving force. These results suggest that AF, possessing fine interlocking grains and high angle grain boundaries (HAGB), increases the localised resistance to HACC propagation more than UB due to the impediment of brittle, cleavage-like crack propagation at HAGB’s.

Abstract: This paper deals with hydrogen embrittlement of cold-drawn pearlitic steel wires to be used in prestressed concrete structures in civil engineering. Special attention is given to the micro-level of hydrogen degradation, i.e, the hydrogen-assisted micro-damage (HAMD) that takes place in pearlitic steels in the form of the so-called tearing topography surface (TTS). It is shown that the appearance of this special topography evolves with the degree of cold drawing in the steels (level of cumulative plastic strain undergone by the wires) from standard TTS in hot rolled pearlitic steels (not cold-drawn at all) to a special hydrogen damage topography (HDT) consisting of a sort of enlarged and oriented TTS in heavily cold-drawn pearlitic steels (prestressing steel wires), thereby resembling Donatello wooden sculpture texture (DWST).

Abstract: The fracture mechanics assessment of materials exposed to harmful environments requires the understanding of the interaction between the soluted species and the affected mechanical behaviour. With the introduction of a mass transport mechanism the entire problem is subjected to a time frame that dictates the time-dependent action of soluted species on mechanical properties. A numerical framework within the phase field approach is presented with an embrittlement-based coupling mechanism showing the influence on crack patterns and fracture toughness. Within the phase field approach the modeling of sharp crack discontinuities is replaced by a diffusive crack model facilitating crack initiation and complex crack topologies such as curvilinear crack patterns, without the requirement of a predefined crack path. The isotropic hardening of the elasto-plastic deformation model and the local fracture criterion are affected by the species concentration. This allows for embrittlement and leads to accelerated crack propagation. An extended mass transport equation for hydrogen embrittlement, accounting for mechanical stresses and deformations, is implemented. For stabilisation purposes a staggered scheme is applied to solve the system of partial differential equations. The simulation of showcases demonstrates crack initiation and crack propagation aiming for the determination of stress-intensity factors and crack-resistance curves.