Hydrogen Flakes Research Articles

On the Change in Hydrogen Diffusion and Trapping Behaviour of Pearlitic Rail Steel at Different Stages of Production.

To avoid hydrogen flaking in rail production, it is of crucial importance to understand the differences in hydrogen diffusion and trapping between different production steps. Therefore, as-cast unfinished material was compared with two finished rails, hot-rolled and head-hardened, using electron backscattered diffraction (EBSD), electrochemical permeation, and thermal desorption spectroscopy (TDS). A significant increase in dislocation density was in the head-hardened rail compared with the other material states. This leads to an effective hydrogen diffusion coefficient of 5.8 × 10-7 cm2/s which is lower by a factor of four than the diffusion coefficients examined in the other states. Thermal desorption spectroscopy analyses show a clear difference between unfinished and finished rail materials. While a peak in activation energy between 32 and 38 kJ/mol is present at all states, only as-cast unfinished material shows a second peak with an activation energy of 47 kJ/mol, which is related to microvoids. The results show that in the investigated material, the effect of increasing dislocation density has a stronger influence on the effective diffusion coefficient than the presence of a second active trapping site.

Fracture failure for the teeth of sprocket wheels in mineral conveying equipment

In this work, fracture failure for the teeth of sprocket wheels in a mineral conveying equipment was analyzed. The teeth fractured during the unloaded testing after the assembly of the sprocket structure. The evidence showed that failure of the sprocket tooth was a brittle fracture. It was caused by the rapid crack propagation from the inside defective region to the outer of the material under a combination action of the residual stress inside the material and the bending stress during the short-term operation. According to the analysis results, the initial region of the fracture was located at the areas of hydrogen flakes, which was the main reason for the fracture of the sprocket teeth.

Electrochemical hydrogen charging to simulate hydrogen flaking in pressure vessel steels

The current study investigates hydrogen flaking in large forgings. Two industrially forged pressure vessel materials, exhibiting different segregation behavior are compared for this purpose. Electrochemical hydrogen charging is used to simulate the flakes, present in real-life applications, on a small scale. As such, the sensitivity of the materials to hydrogen flaking is assessed by varying the charging conditions in terms of time and applied current density. The resulting cracks are subsequently evaluated by optical and scanning electron microscopy. Additionally, X-ray micro-computed tomography scans are performed to generate non-destructive data on the interior performance of the tested samples. MnS inclusions are found to act as crack initiation sites. An increased number of inclusions results into a larger number of small hydrogen induced cracks, whereas less inclusions lead to a smaller number of larger hydrogen induced cracks for the same applied hydrogen charging parameters. The artificial hydrogen flakes, as introduced by electrochemical charging on lab-scale, are induced at the same locations as real-life flakes, i.e. in segregated areas along the MnS inclusions. However, differences in the exact geometry and size of the flakes are observed.

Hydrogen Anti-flaking Heat Treatment in VAR89S Rail Steel

Hydrogen content in the diffusible range and beyond a critical level in steel is detrimental to the formation of hydrogen flakes that show up in ultrasonic testing. It is observed that the hydrogen content in any hot-rolled steel has a gradient with lowest hydrogen at the surface and higher towards the core. This is associated with the secondary phase transformation of high hydrogen-soluble austenite phase to a low-soluble ferrite phase, occurring last towards the core. Anti-flaking heat treatment was carried out in a flake-sensitive pearlitic rail steel grade, VAR89S, with an initial hydrogen level of 1.86 ppm at the core and 1.58 ppm in the surface of a 160-mm diameter hot-rolled bar. The heat treatment involved holding at 650 °C for times varying between 8 and 36 h, which decreased the hydrogen content. The decrease in hydrogen during anti-flaking heat treatment was correlated using Fick’s law model which took into account the initial hydrogen gradient across the bar cross section. The model predicted the experimental trend for hydrogen distribution at surface, mid-radius, and core. The hydrogen distribution at surface showed higher than predicted values, which could be attributed to the carbide spheroidization at the surface.

Effect of electroslag remelting and homogenization on hydrogen flaking in AMS-4340 ultra-high-strength steels

Hydrogen flakes and elemental segregation are the main causes of steel rejection. To eliminate hydrogen flaking, the present study focuses on the manufacture of AMS-4340 ultra-high-strength steel through an alternate route. AMS-4340 was prepared using three different processing routes. The primary processing route consisted of melting in an electric arc furnace, refining in a ladle refining furnace, and vacuum degassing. After primary processing, the heat processes (D1, D2, and D3) were cast into cylindrical electrodes. For secondary processing, electroslag remelting (ESR) was carried out on the primary heats to obtain four secondary heats: E1, E2, E3, and E4. Homogenization of ingots E1, E2, E3, and E4 was carried out at 1220°C for 14, 12, 12, and 30 h, respectively, followed by an antiflaking treatment at 680°C and air cooling. In addition, the semi-finished ESR ingot E4 was again homogenized at 1220°C for 6–8 h and a second antiflaking treatment was performed at 680°C for 130 h followed by air cooling. The chemical segregation of each heat was monitored through a spectroscopy technique. The least segregation was observed for heat E4. Macrostructure examination revealed the presence of hydrogen flakes in heats E1, E2, and E3, whereas no hydrogen flakes were observed in heat E4. Ultrasonic testing revealed no internal defects in heat E4, whereas internal defects were observed in the other heats. A grain size investigation revealed a finer grain size for E4 compared with those for the other heats. Steel produced in heat E4 also exhibited superior mechanical properties. Therefore, the processing route used for heat E4 can be used to manufacture an AMS-4340 ultra-high-strength steel with superior properties compared with those of AMS-4340 prepared by the other investigated routes.

Behaviour of Hydrogen in Industrial Scale Steel Melts

The behaviour of hydrogen during controlled industrial scale secondary steel making process has been examined in a variety of low alloy steels, sensitive to hydrogen flaking. The study examines the role played by the moisture in input raw materials such as the ferro-alloys, type of carbon additive and fluxes in enhancing the hydrogen content in the ladle furnace. Post alloying, the influence of vacuum degassing parameters such as the vacuum level, vacuum holding time, Ar flow rate, type of porous plug used, slag chemistry and the steel grade was examined. The vacuum degassing process was analysed using a kinetic model, which could justify the trends seen in the vacuum level, holding time and Ar gas flow rate. Finally, the hydrogen pick-up post vacuum degassing through slag cover and the casting tundish was found to be influenced by parameters such as the quality of the tundish spray mass, and casting sequence. The influence of steel grade in hydrogen removal was also examined.

Combination Criterion for Multiple Laminar Flaws in Steel Components

A laminar flaw is a planar subsurface flaw parallel to the rolling direction of the plate, where the applied stress is typically parallel to the rolling direction. The laminar flaw oriented within 10 degree of a plane parallel to the component surface is defined as a laminar flaw, in accordance with the definition of the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel (B&PV) Code Section XI. The ASME Code provides combination criterion for multiple laminar flaws. If there are two or more laminations, these laminations are projected to a single plane and, if the separation distance of the projected laminations is less than or equal to 25.4 mm, the laminations shall be combined into a single large laminar flaw in the assessment. The combination criterion was established on the basis of the non-destructive examination capabilities in the 1970’s. However, this methodology did not consider the offset distance of the laminations nor the mechanical interaction between the flaws. Therefore that combination methodology is not suited in case of a large number of laminar flaws. This may occur e.g. in case of hydrogen flaking in steel forging components. Actually, when multiple discrete laminar flaws are close to each other, interaction between the flaws has to be taken into account and these flaws shall be combined to a single laminar flaw for assessment. Stress intensity factor interactions for inclined laminar flaws were analyzed in the frame of hydrogen flaking issue in reactor pressure vessels of Doel 3 and Tihange 2 Belgian nuclear power plants. Based on the mechanical interaction between flaws, new combination criterion was developed and was presented in this paper.

Alloying of steel and graphite by hydrogen in nuclear reactor

In traditional power engineering hydrogen may be one of the first primary source of equipment damage. This problem has high actuality for both nuclear and thermonuclear power engineering. Study of radiation-hydrogen embrittlement of the steel raises the question concerning the unknown source of hydrogen in reactors. Later unexpectedly high hydrogen concentrations were detected in irradiated graphite.It is necessary to look for this source of hydrogen especially because hydrogen flakes were detected in reactor vessels of Belgian NPPs.As a possible initial hypothesis about the enigmatical source of hydrogen one can propose protons generation during beta-decay of free neutrons поскольку inasmuch as protons detected by researches at nuclear reactors as witness of beta-decay of free neutrons.

Alloying of Steel and Graphite by Hydrogen in Nuclear Reactor

In traditional power engineering hydrogen may be one of the first primary sources of equipment damage. This problem has high actuality for both nuclear and thermonuclear power engineering. Study of radiation-hydrogen embrittlement of the steel raises the question concerning the unknown source of hydrogen in reactors. Later unexpectedly high hydrogen concentrations were detected in irradiated graphite. It is necessary to look for this source of hydrogen especially because hydrogen flakes were detected in reactor vessels of Belgian NPPs. As a possible initial hypothesis about the enigmatical source of hydrogen one can propose protons generation during beta-decay of free neutrons Ð?оNÐoолNŒÐoNƒ inasmuch as protons detected by researches at nuclear reactors as witness of beta-decay of free neutrons.

Hydrogen effects on fracture of high-strength steels with different micro-alloying

Abstract Constant load tests of high-strength carbon steels with different micro-alloying using strengths in the range of 1000–1400 MPa were performed at ambient temperature under continuous electrochemical hydrogen charging. Hydrogen markedly affects delayed fracture of all the studied steels. Fractography of the studied steels shows that fracture mechanism depends on the chemical composition of the studied steels and hydrogen-induced cracking exhibits intergranular or transgranular character occurring often in the form of hydrogen flakes. The size and chemical composition of non-metallic inclusions are analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Hydrogen-induced cracking initiates at TiN/TiC particles in steels with Ti alloying. Crack paths are studied with electron backscatter diffraction mapping to analyze crack initiation and growth. The thermal desorption spectroscopy method is used to analyze the distribution of hydrogen in the trapping sites. The mechanisms of hydrogen effects on fracture of high-strength steels are discussed.

Phenomenology of Hydrogen Flaking in Nuclear Reactor Pressure Vessels*

Abstract Recently, the problem of hydrogen flaking resurfaced, when internal defects were detected in the reactor pressure vessels of two Belgian nuclear power plants. These defects turned out to be hydrogen flakes formed during the fabrication of these pressure vessels. The goal of this publication is to provide important insights into the phenomenon of hydrogen flaking, the different parameters that play a role in the mechanism, as well as the typical morphology and location of these flakes. Therefore an extensive literature study was combined with a detailed metallurgical characterization of a significant number of flakes. Hydrogen flaking is a fabrication problem, which is strongly linked with segregation phenomena. A combination of a sufficient amount of hydrogen, stresses and a sensitive microstructure causes hydrogen flaking. For these reasons hydrogen flakes inside large reactor pressure vessels are found in the so-called ghost lines, which originate from segregation processes during casting.

The effects of a nickel oxide precoat on the gas bubble structures and fish-scaling resistance in vitreous enamels

The effects of a NiO precoat on the interfacial microchemistry and the structure of gas bubbles at the steel–enamel interface were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), thermogravimetric analysis (TGA) and image analysis techniques. The experimental evidence demonstrates that nickel oxide applied to the steel substrate as a precoat accelerates the diffusion of Fe from the steel into the enamel and promotes decarburisation of the steel substrate, this latter reaction generating CO and/or CO 2 as gases. The resulting increase in FeO concentration reduces the viscosity of enamel. The apparent decrease in the viscosity of liquid enamel, along with formation of substantial quantities of CO and/or CO 2 gases control the distribution of gas bubbles in the enamel layer. The investigation also explains the reason for the association of large gas bubbles with Fe–Ni metal-rich particles with a dendritic appearance (termed ‘dendrites in the following text) at the enamel–steel interface. The role of these Fe–Ni metal-rich ‘dendrites’ in reducing the tendency for hydrogen flaking or cracking of the enamel layer, generally referred to as fish-scaling, is also elucidated.