Carrier Gas Hot Extraction Research Articles

Electrochemical hydrogen permeation measurement during cyclic stress testing of a ferritic chromium steel

AbstractIn the present work, the hydrogen diffusion during cyclic stress testing in the ferritic chromium steel 1.4521 (X2CrMoTi18‐2) is investigated. Therefore, electrochemical hydrogen permeation measurements are carried out on a hollow cylinder geometry. Simultaneously an alternating stress is applied using a stress ratio of R=‐1. At low stress amplitudes, no influence on the diffusion behaviour of hydrogen is found. At a higher stress amplitude, close to the macroscopic yield strength, a delayed hydrogen permeation by a factor of two is observed. It is postulated that this observation is a result of the nucleation of new hydrogen traps during the measurement. Hydrogen content measurements using carrier gas hot extraction support this hypothesis.

Parameters and challenges for reliable hydrogen determination in welded joints by carrier gas hot extraction

Parameters and challenges for reliable hydrogen determination in welded joints by carrier gas hot extraction

In situ measurement of hydrogen concentration in steel using laser-induced breakdown spectroscopy (LIBS)

AbstractThe ISO 3690 standard “Determination of hydrogen content in arc weld metal” requires a thermal activation of the diffusible hydrogen in a piece of weld metal for the subsequent ex situ concentration measurement by carrier gas hot extraction CGHE or thermal desorption spectroscopy (TCD). Laser-induced breakdown spectroscopy (LIBS) offers a time and spatially resolved, almost non-destructive, in situ measurement of hydrogen at surfaces without sample preparation. We measured hydrogen in steels, which were charged either electrochemically or by high-pressure hydrogen gas, and compared the results. Further, the feasibility of quantitative hydrogen line scan measurements with LIBS was demonstrated by measuring hydrogen at water jet cut surfaces. The hydrogen concentrations measured with the help of LIBS were compared with CGHE measurements. It was observed that hydrogen can be reliably measured with LIBS for concentrations larger than 2 wt.-ppm. The maximum hydrogen concentration achieved using electrochemical charging was 85.1 ppm. The results show that LIBS is a promising technique for time- and spatially resolved measurements of hydrogen in steels.

Detection of diffusible hydrogen during laser beam welding under water

A critical damage pattern in underwater welding processes is the formation of hydrogen-induced cold cracks, which do not occur immediately after welding. The electrolytic dissociation of the water during arc welding creates a large supply of hydrogen. As an alternative, laser beam welding could be used in the underwater industry in the future. A low hydrogen concentration is expected due to thermal dissociation only. In this paper, laser beam welds are performed with Yb:YAG-laser on 10 mm S235JR-steel and the diffusible hydrogen content of this samples is determined using carrier gas hot extraction. The laser beam power is varied between 3 kW and 4 kW and the feed rate between 0.25 m/min and 0.50 m/min to analyse their influence on the hydrogen content. In addition, the process zone is analysed about hydrogen formation using a high-speed camera and spectrometer. The results are compared with the hydrogen content of shielded metal arc-welded samples.

Investigation of Diffusible Hydrogen Concentration in Gas Metal Arc Brazing by Carrier Gas Hot Extraction Method Referring to ISO 3690

Arc brazing is an alternative joining technology well-suited for processing thermally sensitive materials and to produce mixed material connections. Due to the technological similarities of gas metal arc brazing to gas metal arc welding, it can be assumed that the process-related hydrogen input is of similar magnitude for both joining technologies. Since diffusible hydrogen is known to cause embrittlement in metallic materials, it is necessary to know the amount of diffusible hydrogen introduced by different manufacturing processes. Regarding the qualification of welding procedures, hydrogen ingress is an important factor to evaluate the risk of hydrogen-assisted cold cracking, especially when processing high-strength steels. For arc brazing, there is a lack of knowledge about the process-related hydrogen input. Hence, to study the influence of different brazing filler materials and varying levels of heat input on the diffusible hydrogen concentration in arc braze metal, a methodology to determine hydrogen content in arc weld metal in accordance with international standard ISO 3690 based on carrier gas hot extraction was applied to arc brazed specimens. Very low diffusible hydrogen concentrations of about HD = 0.1 to 0.3 mL/100 g were found for GMAB without significant influence of arc energy or filler metal used.

Characterization of the Native Oxide Shell of Copper Metal Powder Spherical Particles.

The native oxide layer that forms on copper (Cu) metal spherical particle surfaces under ambient handling conditions has been shown to have a significant effect on sintering behavior during microwave heating in a previous study, where an abnormal expansion was observed and characterized during sintering of Cu compacts using reducing gases. Because microwave (MW) heating is selective and depends greatly on the dielectric properties of the materials, this thin oxide layer will absorb MW energy easily and can consequently be heated drastically starting from room temperature until the reduction process occurs. In the current study, this oxide ceramic layer was qualitatively and quantitatively characterized using the carrier gas hot extraction (CGHE) method, Auger electron spectroscopy (AES), and a dual-beam focused ion beam (FIB)/scanning electron microscope (SEM) system that combines both FIB and SEM in one single instrument. Two different commercial gas-atomized spherical Cu metal powders with different particle sizes were investigated, where the average oxygen content of the powders was found to be around 0.575 wt% using the CGHE technique. Furthermore, AES spectra along with depth profile measurements were used to qualitatively characterize this oxide layer, with only a rough quantitative thickness approximation due to method limitations and the electron beam reduction effect. For the dual-beam FIB-SEM system, a platinum (Pt) coating was first deposited on the Cu particle surfaces prior to any characterization in order to protect and to preserve the oxide layer from any possible beam-induced reduction. Subsequently, the Pt-coated Cu particles were then cross-sectioned in the middle in situ using an FIB beam, where SEM micrographs of the resulted fresh sections were characterized at a 36° angle stage tilt with four different detector modes. Quantitative thickness characterization of this native oxide layer was successfully achieved using the adapted dual-beam FIB-SEM setup with more accuracy. Overall, the native Cu oxide layer was found to be inhomogeneous over the particles, and its thickness was strongly dependent on particle size. The thickness ranged from around 22–67 nm for Cu powder with a 10 µm average particle size (APS) and around 850–1050 nm for one with less than 149 µm.

Recent studies to the impact of a ceramic breeder environment on the mechanical properties of EUROFER97 under operating conditions

The reduced activation ferritic-martensitic steel EUROFER is a prime candidate for fusion structural applications. In the advanced helium-cooled pebble bed blanket concept of DEMO, this steel is in direct contact with Li-ceramics pebbles and experiences cyclic thermomechanical loading under operating conditions. In this work, the effect of ceramic breeder environment on both the microstructure and particularly the fatigue properties of the RAFM steel was investigated. For this purpose, EUROFER samples were embedded in ceramic pebbles and annealed at 550 °C under a purge gas atmosphere for durations ranging 8–128 days. Microstructural studies show that annealing at the given conditions leads to the formation of a complex corrosion multilayer consisting of ferrite and iron oxide in an outer layer, as well as chromium oxide in both an intermediate and an inner layers. For annealing durations up to 32 days, the thickness of the oxide layer is continuously increasing. However, for more prolonged annealing, the thickness of the corrosion layer reaches saturation at about 20 µm. Similar to the growth behavior of the oxide layer, the strain-controlled LCF tests showed a remarkable reduction of the LCF lifetime with increasing in the annealing duration up to 32 days. However, beyond an annealing duration of 64 days, an additional decrease of lifetime is not observed. When performing additional experiments (annealing with ceramic pebbles in vacuum and annealing in purge gas without pebbles) it is found that corrosion is mainly caused by purge gas impurities, namely oxygen and water. Furthermore, the decrease in fatigue lifetime is related to the degradation of surface quality, causing an accelerated progress of damage. Finally, Charpy impact tests combined with carrier gas hot extraction investigations revealed that hydrogen embrittlement of the samples, annealed in a breeder environment containing 0.1 vol.% H2, can be excluded.

Characterization of Hydrogen Diffusion in Offshore Steel S420G2+M Multi-layer Submerged Arc Welded Joint

As onshore installation capacity is limited, the increase in the number of offshore wind turbines (OWT) is a major goal. In that connection, the OWTs continuously increase in size and weight and demand adequate foundations concepts like monopiles or tripods. These components are typically manufactured from welded mild steel plates with thickness up to 200 mm. The predominant welding technique is submerged arc welding (SAW). In accordance with the standards, the occurrence of hydrogen-assisted cracking is anticipated by either a minimum waiting time (MWT, before non-destructive testing of the welded joint is allowed) at ambient or a hydrogen removal heat treatment (HRHT) at elevated temperatures. The effectiveness of both can be estimated by calculation of the diffusion time, i.e., diffusion coefficients. In this study, these coefficients are obtained for the first time for a thick-walled S420G2+M offshore steel grade and its multi-layer SAW joint. The electrochemical permeation technique at ambient temperature is used for the determination of diffusion coefficients for both the base material and the weld metal. The coefficients are within a range of 10−5 to 10−4 mm2/s (whereas the weld metal had the lowest) and are used for an analytical and numerical calculation of the hydrogen diffusion and the related MWT. The results showed that long MWT can occur, which would be necessary to significantly decrease the hydrogen concentration. Weld metal diffusion coefficients at elevated temperatures were calculated from hydrogen desorption experiments by carrier gas hot extraction. They are within a range of 10−3 mm2/s and used for the characterization of a HRHT dwell-time. The analytical calculation shows the same tendency of long necessary times also at elevated temperatures. That means the necessary time is strongly influenced by the considered plate thickness and the estimation of any MWT/HRHT via diffusion coefficients should be critically discussed.

Induction Heating in Underwater Wet Welding-Thermal Input, Microstructure and Diffusible Hydrogen Content.

Hydrogen-assisted cracking is a major challenge in underwater wet welding of high-strength steels with a carbon equivalent larger than 0.4 wt%. In dry welding processes, post-weld heat treatment can reduce the hardness in the heat-affected zone while simultaneously lowering the diffusible hydrogen concentration in the weldment. However, common heat treatments known from atmospheric welding under dry conditions are non-applicable in the wet environment. Induction heating could make a difference since the heat is generated directly in the workpiece. In the present study, the thermal input by using a commercial induction heating system under water was characterized first. Then, the effect of an additional induction heating was examined with respect to the resulting microstructure of weldments on structural steels with different strength and composition. Moreover, the diffusible hydrogen content in weld metal was analyzed by the carrier gas hot extraction method. Post-weld induction heating could reduce the diffusible hydrogen content by −34% in 30 m simulated water depth.

Thickness and microstructure effect on hydrogen diffusion in creep-resistant 9% Cr P92 steel and P91 weld metal

Martensitic 9% Cr steels like P91 and P92 show susceptibility to delayed hydrogen assisted cracking depending on their microstructure. In that connection, effective hydrogen diffusion coefficients are used to assess the possible time-delay. Limited data on room temperature diffusion coefficients reported in literature vary widely by several orders of magnitude (mostly attributed to variation in microstructure). Especially P91 weld metal diffusion coefficients are rare so far. For that reason, electrochemical permeation experiments had been conducted using P92 base metal and P91 weld metal (in as-welded and heat-treated condition) with different thicknesses. From the results obtained, diffusion coefficients were calculated using to different methods, time-lag, and inflection point. Results show that, despite microstructural effects, the sample thickness must be considered as it influences the calculated diffusion coefficients. Finally, the comparison of calculated and measured hydrogen concentrations (determined by carrier gas hot extraction) enables the identification of realistic diffusion coefficients.

Diamond Coating Reduces Nuclear Fuel Rod Corrosion at Accidental Temperatures: The Role of Surface Electrochemistry and Semiconductivity.

If we want to decrease the probability of accidents in nuclear reactors, we must control the surface corrosion of the fuel rods. In this work we used a diamond coating containing <60% diamond and >40% sp2 “soft” carbon phase to protect Zr alloy fuel rods (ZIRLO®) against corrosion in steam at temperatures from 850 °C to 1000 °C. A diamond coating was grown in a pulse microwave plasma chemical vapor deposition apparatus and made a strong barrier against hydrogen uptake into ZIRLO® (ZIRLO) under all tested conditions. The coating also reduced ZIRLO corrosion in hot steam at 850 °C (for 60 min) and at 900 °C (for 30 min). However, the protective ability of the diamond coating decreased after 20 min in 1000 °C hot steam. The main goal of this work was to explain how diamond and sp2 “soft” carbon affect the ZIRLO fuel rod surface electrochemistry and semi conductivity and how these parameters influence the hot steam ZIRLO corrosion process. To achieve this goal, theoretical and experimental methods (scanning electron microscopy, Raman spectroscopy, electrochemical impedance spectroscopy, carrier gas hot extraction, oxidation kinetics, ab initio calculations) were applied. Deep understanding of ZIRLO surface processes and states enable us to reduce accidental temperature corrosion in nuclear reactors.

Diffusible Hydrogen Concentration in Drawn Arc Stud Weldments

AbstractThe mechanized drawn arc stud welding is a well established process in steel construction for joining technique for pin shaped elements, especially in the field of composite constructions. Therefore, the process is commonly applied on construction sites under different climatic conditions. The process is characterized by its relatively high heat input due to high current and a weld pool protection that is realized by a ceramic ferrule holding the metal vapor as a shielding gas. However, turbulences inside the ferrule during welding cause contaminations with atmospheric humidity. Furthermore, a high cooling rate results in a strongly hardened heat‐affected zone and a considerable residual stress state in the parent material. Thus, there is a risk of hydrogen assisted cold cracking, especially for stud welding on high strength steels or under adverse welding conditions. The knowledge about the expectable hydrogen concentration is fundamental for evaluating the cold cracking susceptibility. In this study, the effect storage conditions of the consumables and atmospheric conditions are investigated and evaluated regarding their effect on the resulting hydrogen concentration in weld metal. For determination of diffusible hydrogen concentration in stud weld metal a method based on the carrier gas hot extraction is used.

Calibration capacity of hot-pressed hydrogen standards for glow discharge optical emission and mass spectrometry

Mixed copper and titanium hydride powder was hot-pressed and characterized by Carrier Gas Hot Extraction, X-Ray Diffraction, Thermal Gravimetric Analysis coupled with Mass Spectrometry, and Scanning Electron Microscopy. The hot-pressed and five conventional samples were applied for calibration of hydrogen in Glow Discharge Optical Emission and Mass Spectrometry. Up to the introduction of 15 ng/s hydrogen the emission yield model is useful in Glow Discharge Optical Emission Spectrometry. A correlation between saturation and even reversal of the emission yield of the spectral lines H121, H486 and H656 and low sputtering rates was found. Hydrogen effects exist for the spectral lines of Cu(II) 219 and Ti(I) 399. In Glow Discharge Mass Spectrometry, a linear dependency of the 1H ion current on the sputtered mass per time exists over the total range of hydrogen content investigated. Hydrogen effects also exist for the sensitivity of 48Ti and 63Cu. The sputtering rate of two-phase materials depends linearly on the sputtered mass per time of one phase, which allows the sputtering rate of two-phase materials with known composition to be predicted.

Preparation of bulk Ti[sbnd]15Mo alloy using cryogenic milling and spark plasma sintering

Gas atomized Ti15Mo alloy powder was processed by cryogenic milling. Both initial gas atomized and milled powders were consolidated by spark plasma sintering at temperatures ranging from 750 °C to 850 °C for 1 and 3 min. Microstructure of the sintered samples was studied by scanning electron microscopy (SEM), chemical contamination was determined by carrier gas hot extraction (CGHE) method and phase composition was investigated by X-ray diffraction.Cryogenic milling resulted in severe plastic deformation of initially spherical particles, changing their shape, but without a significant decrease of particle size. The milled powder achieved the full density after sintering at 750 °C, while 850 °C was required for a full densification of the initial powder. The amount of α phase was higher in the sintered milled powder, while the sintered non-milled powder contained significant amount of ω phase. It was shown that an ultrafine-grained two-phase microstructure can be produced by a combination of cryogenic milling and spark plasma sintering.

Influence of the microstructure of Zn–Ni coatings on hydrogen effusion characteristics

This study reports about the influence of the microstructure of electroplated Zn–Ni alloy coatings, especially the extent and characteristic of existing microcracks within, on hydrogen effusion characteristics at room temperature (RT) and 200 °C. For comparison, the behavior of an electroplated Zn coating was also investigated. The microstructures of the Zn and Zn–Ni coatings were examined by means of scanning electron microscope (SEM) and optical microscope (OM). These results were correlated with the hydrogen effusion profiles, which were determined by using a carrier gas hot extraction (CGHE) technique. In addition, a silver decoration method was applied to visualize the hydrogen effusion paths within the coatings. First both Zn–Ni electroplating processes exhibit less hydrogen absorption compared to Zn. In addition the microstructure of the Zn–Ni coatings, especially the characteristic and magnitude of the microcracks, significantly influences the hydrogen effusion characteristics. By using the silver decoration method according to (Schober in MTA 14:2440-2442, 1983) it could be proofed that for Zn–Ni hydrogen primary effuses along the microcracks and the interfaces of pustules within the coating. But the microcracks seem to promote the hydrogen effusion much more than the interfaces of the pustules.

Study on Hydrogen Content in Cord Steel by Carrier gas hot extraction

Cord steel has very strict requirements on the content of impurities. Hydrogen is a harmful gas element in steel and is the cause of hydrogen embrittlement. In this work, the Carrier gas hot extraction method is used to measure the hydrogen content of 77A steel. It was found that the measured value of hydrogen content was related to cooling capability of the mold. It is found that with the increasing of cooling speed of the mold, the hydrogen content increases. The measured hydrogen content can roughly reflect the hydrogen content in the liquid alloy by using the cast iron mold with faster cooling speed.

Manufacturing of fine-grained titanium by cryogenic milling and spark plasma sintering

Commercially pure (CP) gas-atomized titanium powder was processed by wet cryogenic milling in liquid argon and compacted by spark plasma sintering. The time-dependent sintering evolution at different temperatures was evaluated by using the master sintering curve (MSC) approach with the aim of achieving a material with maximum relative density and minimum grain size. Carrier-gas hot extraction (CGHE) confirmed a purity consistent with ASTM standard of Grade 4 CP Ti. Grain size and texture were determined by EBSD. An apparent activation energy of sintering of 115 kJ/mol was found based on the MSC approach. It is significantly lower than the activation energy of self-diffusion in Ti. This is attributed to an enhanced diffusion rate due to high concentration of defects in the powder after milling. The relative density was correlated with the resulting grain size and a general trade-off relationship between achieving high relative density and maintaining small grain size was found. The distribution of oxygen after milling and subsequent sintering at low temperatures is heterogeneous as determined by complementary XRD, CGHE and microhardness measurements. The distribution of oxygen becomes homogeneous with increasing sintering temperature. The microhardness of the material was shown to depend on residual porosity, the content and distribution of oxygen and also on texture. Processing parameters of milling and sintering were optimized to achieve fully dense, fine grained material with a low contamination by nitrogen and oxygen.

Hydrogen diffusion in creep-resistant 9% Cr P91 multi-layer weld metal

Welded components of P91 9% Cr steel demand for careful welding fabrication with necessary post weld heat treatment (PWHT). Before the PWHT, a hydrogen removal heat treatment is necessary for avoidance of hydrogen assisted cracking (HAC). In this context, the microstructure and temperature-dependent hydrogen diffusion is important, and reliable diffusion coefficients of P91 weld metal are rare. For that reason, the diffusion behavior of P91 multi-layer weld metal was investigated for as-welded (AW) and PWHT condition by electrochemical permeation experiments at room temperature and carrier gas hot extraction (CGHE) from 100 to 400 °C. Hydrogen diffusion coefficients were calculated, and the corresponding hydrogen concentration was measured. It was ascertained that both heat treatment conditions show significant differences. At room temperature the AW condition showed significant hydrogen trapping expressed by to seven times lower diffusion coefficients. A preferred diffusion direction was found in perpendicular direction expressed by high permeability. The CGHE experiments revealed lower diffusion coefficients for the AW condition up to 400 °C. In this context, a hydrogen concentration of approximately 21 ml/100 g was still trapped at 100 °C. For that reason, a certain HAC susceptibility of as-welded P91 weld metal cannot be excluded, and hydrogen removal should be done before PWHT.

Modeling hydrogen diffusion in a tribological scenario: A failure analysis of a thrust bearing

A coupled diffusion-mechanical finite element simulation model was developed to study the diffusion of hydrogen in a cylindrical roller thrust bearing (CRTB). The simulations enabled obtaining qualitative information pertaining to stress-assisted diffusion in tribological loading and under the influence of residual stresses. The mechanical behavior of bearing steel was obtained from experiments and supported by literature. In parallel, rolling-contact fatigue (RCF) tests on CRTBs lubricated with a fully additivated transmission oil were conducted for 25 h and 50 h to investigate their premature failure modes.Post-RCF rheological analysis indicated decrease of the lubricant viscosity due to degradation. Carrier gas hot extraction analysis indicated a significant increase in hydrogen content in the bearings tested for 50 h. Whereas, serial cross-sectional analysis revealed the formation of subsurface White Etching Crack (WEC) networks associated with White Etching Areas (WEA); the cracks breached the surface on multiple positions causing flaking and eventual failure. On the other hand, no signs of damage were observed in the bearings tested for 25 h.The simulations revealed insignificant hydrogen accumulation due to stress-assisted diffusion in comparison to concentration gradient driven diffusion; hydrogen trapping was pronounced in zones undergoing plastic deformation due to the formation of deformation induced trapping sites. However, residual stresses had an evident influence on the subsurface accumulation of hydrogen. A comparison between the zones of elevated hydrogen concentration due to residual stresses and RCF induced subsurface damage yielded a good correlation.

Processing of gas-nitrided AISI 316L steel powder by laser powder bed fusion – Microstructure and properties

This work investigated the processing of high nitrogen-alloyed austenitic stainless steels by laser powder bed fusion (L-PBF). Prior to L-PBF processing, the AISI 316 L steel powder was nitrided at a temperature of 675°C in a 3 bar nitrogen atmosphere, thus achieving a N content of 0.58 mass-%. By mixing nitrided 316 L powder with untreated 316 L powder, two different powder mixtures were obtained with 0.065 mass-% and 0.27 mass-% nitrogen, respectively. After nitriding and mixing, the powder was characterized in terms of its flow properties and chemical composition. The nitrided steel powder was then processed by L-PBF, and the microstructure as well as the chemical composition were investigated by means of scanning electron microscopy and carrier gas hot extraction. It was shown that nitriding of steel powders in an N2 atmosphere can be used to significantly increase the nitrogen content of the powder without impairing its flow properties. With increasing nitrogen content of the powder, the porosity within the L-PBF built specimens increased. However, both the yield strength and the tensile strength were greatly improved without a marked reduction in the elongation at fracture of the respective steels. This work shows that nitrogen-alloyed austenitic stainless steels can be processed by L-PBF and the mechanical properties can be improved.