Hydrogen embrittlement (HE) poses the risk of premature failure for many metals, especially high-strength steels. Due to the utilization of hydrogen as an environmentally friendly energy source, efforts are made to improve the resistance to HE at elevated pressures and temperatures. In addition, applications in hydrogen environments might require specific material properties in terms of thermal and electrical conductivity, magnetic properties as well as corrosion resistance. In the present study, three high-strength Cu-base alloys (Alloy 25, PerforMet® and ToughMet® 3) as well as austenitic stainless AISI 321, Ni-base alloy IN 625 and ferritic steel 1.4511 are charged in pressurized hydrogen and subsequently tested by means of Slow Strain Rate Testing (SSRT). The results show that high-strength Cu-base alloys exhibit a great resistance to HE and could prove to be suitable for materials for a variety of hydrogen applications with rough conditions such as high pressure, elevated temperature and corrosive environments.

By utilizing hydrogen as an eco-friendly energy source, many metals are exposed to gaseous (pressurized) hydrogen. High-strength steels with an ultimate tensile strength of 800 MPa and above are especially susceptible to hydrogen-induced fracturing, also referred to as hydrogen embrittlement (HE). Both the microstructure and phase fractions within the steel, as well as lattice distortion, carbide precipitation, residual stress, etc., significantly affect the susceptibility to HE. Among others, one important cause for this observation is found in the locally varying hydrogen solubility within different microstructural phases such as martensite, bainite, pearlite, and ferrite. Both a thorough understanding of the HE mechanisms and taking countermeasures in the form of alloying design require an accurate analysis of local diffusive hydrogen concentrations within the material. Thermal analysis methods such as Thermal Desorption Mass Spectrometry only display an integral hydrogen concentration throughout the whole sample volume. To analyze the local diffusive hydrogen concentration, novel measuring techniques with a high special resolution must therefore be utilized. The current research presents first-of-its-kind hydrogen analyses by means of the electrochemical microcapillary cell. Using a 10 µm tip opening diameter allows for conducting local diffusive hydrogen measurements within individual grains of multi-phase carbon steel C60E (1.1221). The results confirm that hydrogen is distributed heterogeneously within multi-phase steels. Considering the individual phase fractions and the respective local diffusive hydrogen concentrations, a total diffusive hydrogen concentration can be calculated. The obtained value is in good agreement with reference thermal hydrogen analyses. Our results suggest that electrochemical microcapillary cell measurements offer great potential for further studies, which will provide a better understanding of HE and local hydrogen accumulation.

AbstractThe widespread use of hydrogen as an energy carrier is considered one of the most important keys to achieving the decarbonization necessary for the energy transition in numerous areas of technology and society. Not least due to the associated contact of metallic components with (pressurized) hydrogen, there is a latent risk of hydrogen-induced cracking (“hydrogen embrittlement”). The cause of damage is the hydrogen absorbed by the material, which is mobile via interstitial lattice diffusion. In high-strength steels with a tensile strength of more than 800 MPa, even very low diffusive hydrogen contents of less than 1 ppm (parts per million) can have a crack-inducing effect. Hence, dedicated, highly accurate analytical and testing methods are required for the detection of hydrogen and its effect on the mechanical properties of metals. This paper summarizes the current state of knowledge regarding hydrogen embrittlement and reviews the analytical, mechanical, and fractographic investigation methods for detecting hydrogen in metals.

Hydrogen embrittlement (HE) is a crucial problem for many advanced high strength steels (AHSS) used in the automotive industry. To contribute to the microstructural understanding on the resistance against hydrogen embrittlement a complex phase (CP) steel and dual phase (DP) steel were investigated systematically in the present work. Both steels have the same tensile strength, but different microstructures. Thermal desorption spectroscopy (TDS) and electrochemical permeation (EP) were used to study the effect of plastic pre-deformation on the chemical diffusivity and solubility. The results were used to parametrize a fully coupled diffusion-mechanical finite element (FE) model. Constant load tests (CLTs) were applied on electrochemical pre-charged notched samples to evaluate the role of hydrogen charging on the mechanical properties. A local evaluation of the CLTs with the fully coupled FE model revealed local limit strain curves (LSC). Comparing these curves yields a very strong effect of the microstructure on the resistivity on hydrogen embrittlement.

Characterization of crystal orientation and grain boundaries is of great interest in the understanding of the mechanisms taking place during the degradation process originated by the exposure of materials to erosive media. In the present study, an austenitic stainless steel previously analyzed by electron backscatter diffraction was exposed to ultrasonic cavitation erosion. Several parameters such as crystal orientation, misorientation, coincidence site lattice, Schmid factor and twin orientation were examined in order to determine their influence on the local susceptibility to early damage induced by cavitation. Crystallographic orientation was found to play a significant role in the deformation mechanisms occurring inside grains. Furthermore, high misorientation between adjacent grains was recognized as a determining factor leading to early damage observed at random grain boundaries, while the difference in Schmid factor was identified to have a significant effect on the cavitation resistance of twin boundaries. Moreover, the effect of the deviation from the ideal orientation predicted for coherent twins was analyzed in connection with cavitation resistance.

Hydrogen embrittlement is one of the most crucial problems in the application of advanced high strength steel (AHSS) sheets for the automotive industry. Especially, the severe plastic deformation in punched edges makes the components susceptible to hydrogen assisted cracking (HAC). While small amounts of hydrogen are measured in the bulk material, hydrogen concentration increases in the micrometer-sized shear affected zone many times, along with severe plastic deformation. To contribute to the understanding of local microstructure and local stress states on the hydrogen accumulation in the shear affected zone, two industrial AHSS were investigated. Both steels had the same ultimate tensile strength of 1200 MPa, but different uniform elongations. High-pressure torsion (HPT) deformed samples were used to represent the material state in the shear affected zone. Thermal desorption spectroscopy (TDS), X-ray diffraction (XRD), magnetic retained austenite measurements and electron backscattering diffraction (EBSD) in a high resolution secondary electron microscopy (SEM) were applied among other techniques to gain more insights with microstructural resolution. The hydrogen analysis results of the HPT samples were correlated with the local hydrogen trapping capacity in punched edges. Finally, a simplified "two-zone" model considering the bulk and punched edge as separate zones was developed in order to estimate the local hydrogen concentration of punched AHSS sheets as a function of hydrogen bulk concentration. In a concluding remark it is shown, that the effect of the hydrostatic stress field as trap site is negligibly small compared to other traps. (C) 2020 Elsevier B.V. All rights reserved.

Sub-surface crack networks in areas of altered microstructure are a common cause for bearing failures. Due to its appearance under light microscopy, the damage pattern is referred to as White Etching Cracks (WEC). The root causes leading to the formation of WEC are still under debate. Nevertheless, it has already been shown that atomic hydrogen can have an accelerating effect on the formation and propagation of WEC. In addition to hydrogen pre-charging, hydrogen can be released and absorbed during rolling/sliding due to the decomposing of the lubricant and water. The current work focuses on the analysis of the hydrogen content of cylindrical roller thrust bearings after testing in a FE8 type test rig using two different lubricants. Within the framework of this work, two different hydrogen analysis methods were used and assessed regarding their applicability. The results show that the so-called Hydrogen Collecting Analysis (HCA) is more suitable to investigate the correlation between lubricant chemistry and hydrogen content in the test bearings than the Local Hydrogen Analysis (LHA). The measurements with the HCA show a continuously increasing freely movable and diffusible hydrogen content under tribological conditions, which leads to the formation of WEC. Comparative tests with an oil without hydrogen showed that the tendency of the system to fail as a result of WEC can be reduced by using a lubricant without hydride compounds.

Cavitation erosion is an undesired phenomenon in the majority of technical applications. It occurs when there is a change in pressure field and/or velocity of fluid and a local pressure drop causes the formation of gas bubbles, which collapse when the surrounding pressure rises again. The bubble collapse causes high intensity shock waves and micro jets that damage the material surface. In ship propellers, the bubbles cause damage to the surface of the propeller blades. Today ship propellers for almost all medium and large merchant ships are made of aluminium bronze. Aluminium bronze is a heterogeneous material and the complex microstructure has a significant influence on the resistance against cavitation erosion. For the present investigations, samples were taken directly from a large ship propeller to investigate the properties within the cast material and their influence on the cavitation erosion resistance in artificial seawater. Further investigations aimed at understanding the influence of the alloy composition on the materials properties by casting four alloy compositions.

In fractography, different fracture surfaces are examined and described with regard to their appearance and morphology. In this context, the fracture surface carries information on the fracture event and on the material, as the morphology reflects the microstructure as well as the physical properties. In the course of this work it is show that the fractographic examination may go beyond just viewing the fracture surfaces. In addition to that, some fracture appearances are presented that are rarely encountered in practice.

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.

The transport behavior of hydrogen in ultra-high strength steel grades (UHSS) has been analyzed by several test and evaluation methods. In particular, permeation and desorption measurements have been performed to evaluate material specific parameters such as the effective diffusion coefficient, the reversible trap density and the reversible trap activation energy. Subjects of this study were a dual phase steel grade (DP) with a ferritic-martensitic microstructure and a martensitic steel grade (MS). The results of the permeation measurements indicate that the influence of irreversible traps might be negligible for the investigated UHSS compared to other impact factors. The evaluated reversible trap densities were some orders of magnitude higher than those known for pure iron reflecting the more complex microstructure. The major influence on hydrogen trapping is attributed to reversible traps like grain boundaries and dislocations based on the results of desorption measurements.

In Europe between 2006 and 2012 several ultra-super-critical (USC) coal-fired power plants were built employing T24 (7CrMoVTiB10-10 / DIN EN 10216-2:2014-03 / VdTUV sheet 533/2) in membrane walls. During commissioning stress corrosion cracking (SCC) on the tube-to-tube butt welds appeared. The widespread damages required the development of a new patented commissioning procedure to avoid recurring damages [1]. Although this commissioning procedure was employed successfully and the power plants are in operation since then, a debate about the implementation of a hardness limit for such butt welds was initiated. According to the European standards butt welds of T24 boiler tubes with wall thickness < 10 mm (0.3937 in) do not require any post-weld heat treatment (PWHT) and no hardness limits are given [2-3]. When looking at manufacturing related issues such as an imminent risk of cold cracking after welding of micro-alloyed steels a widely applied but coarse hardness limit is 350 HV [4]. Based on laboratory tests, some authors reallocated this 350 HV hardness limit for addressing SCC susceptibility of low-alloyed steels [5]. This article describes typical hardness levels of T24 boiler tube TIG butt welds and the SCC behavior in high temperature water. Further the effect of the stress relief heat treatment (SRHT) of the boiler membrane walls between 450 degrees C and 550 degrees C (842 degrees F and 1022 degrees F) on its hardness values and on the SCC behavior is discussed, showing that the hardness values should not be used as an indicator for SCC susceptibility of T24 boiler tube butt welds.

The abrasion behaviour of heat-treated 2.8C21Cr1Mo cast iron was studied. The specimens were destabilised at two temperatures, 980 and 1 050 degrees C, for 4 h, air hardened, and then tempered at five temperatures, 220, 320, 400, 500, and 620 degrees C, for 2 h followed by air cooling. Using a pinon- plate abrasion apparatus, the specimens were abraded on four types of bonded abrasives (silicon carbide, corundum, flint, and glass). The effect of work hardening on the abrasion resistance was investigated. It was found that the increase in alloy hardness produced by heat treatment had little effect on the abrasion resistance against silicon carbide or corundum; the inverse was true against flint or glass. The as-hardened structure containing 40% retained austenite gave the best abrasion resistance, whereas the hardened and tempered at 620 degrees C showed the worst. Both bulk hardness and matrix hardness before wear correlated poorly with the abrasion resistance. Therefore, a general model "equivalent hardness" was developed, in which the hardness of the abraded matrix was considered. With this model, the abrasion behaviour can be clearly analysed.

As a protective coating, TiCN hard PVD coating was deposited on magnetron sputtered NiTi thin films under various coating architectures. The microstructure, composition, mechanical properties, tribological performance as well as the cavitation resistance of deposited coatings were analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nanoindentation, ball–on-disc, scratch test, three dimensional (3D) optical microscopy, and the standard ultrasonic cavitation test (ASTM G 32). The obtained results revealed that under a specific coating architecture, the superelasticity of NiTi can be combined with high hardness and wear resistance of TiCN coatings. As a consequence of the combination of these properties, the composite NiTi based coatings are capable of presenting remarkable cavitation resistance and tribological performance.

The influence of cold deformation on liquid metal embrittlement of a steel in a liquid zinc bath was studied. A typical construction steel was cold deformed with different degrees. The microstructure of the different material conditions were investigated as well as their mechanical properties at 450 °C in hot air. Tensile tests with different strain rates were made in a 450 °C hot zinc bath. For each specimen the reduction in area was determined and the fracture surfaces were studied. It can be shown that the strain rate has a large effect on liquid metal embrittlement. An increasing degree of deformation leads to a decrease on liquid metal embrittlement which is caused by the change of microstructure through deformation.

Cavitation is a severe wear mechanism in technical applications where parts are in contact with rapidly flowing liquids. Examples are turbine blades in hydropower plants or pump components. Coating exposed surfaces with wear-resistant materials is an effective measure for extending lifetime in the case of cavitation attack. NiTi is an attractive material for such coatings considering its clearly pronounced damping behavior based on its pseudoelastic properties. A promising processing route for coating net-shaped components with NiTi is low-pressure plasma spraying (LPPS). In the present work, NiTi layers were produced by LPPS, starting from pre-alloyed NiTi powder. Cavitation resistance was investigated in relation to LPPS parameters, layer thickness and specific surface treatment. Increased cavitation resistance was demonstrated compared to UTP 730, an established cavitation protection material. The study was accompanied by comprehensive characterization of microstructure and phase transformation behavior of the NiTi coatings.

Metal matrix composites (MMC) have become important materials in many technical fields. But above all there is a high amount of failures due to alternating stresses because of temperature changes.In this investigation the thermal fatigue behaviour of two metal matrix composites with different particle contents and their base material without particles are compared. It is shown that there are three main parameters influencing lifetime: the maximum temperature of the cycles, the particle content and the particle size. An increasing of these parameters means a decreasing of the cycle numbers before damage occurs.