Jader Furtado

EBSD Phase Identification and Modeling of Precipitate Formation in HP Alloys

Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used as reformer tubes in petrochemical industries. Their composition includes Nb and Ti as strong carbide formers. In the ascast condition, alloys exhibit an austenite matrix with intergranular MC, M23C6 and/or M7C3 eutectic carbides. During exposure at high temperature, phase transformations occur: chromium carbides of M7C3 type transform into the more stable M23C6 type, intragranular M23C6 carbides precipitate, and a silicide, the G-phase (Ni16Nb6Si7), forms due to the instability of MC carbides (NbC). Thermodynamic simulation is of great help for understanding precipitate formation and transformations. Thermo-Calc and Dictra are used to simulate the precipitation of carbides in the austenite matrix during service. However, from an experimental point of view, M23C6 and M7C3 are not easy to distinguish in bulk alloys. Indeed, backscattered scanning electron microscopy does not bring any contrast between the two phases, and energy dispersive spectroscopy (EDS) analysis does not lead to carbon content and consequently to the distinction between M23C6 and M7C3. With transmission electron microscopy (TEM), sample preparation is difficult and the observed area is extremely small. In the present work, HP alloys are investigated by electron backscatter diffraction (EBSD) coupled to EDS. Carbides are identified on the basis of crystal structure, in the bulk, within their microstructural context, and the experimental procedure is both simpler and cheaper than TEM. Precipitates (M23C6, M7C3) could be identified by orientation mapping and single spot analysis.

Hydrogen-enhanced fatigue of a Cr-Mo steel pressure vessel

The current international standards and codes dedicated to the design of pressure vessels do not properly ensure fitness for service of vessels used for gaseous hydrogen storage and subjected to hydrogen enhanced fatigue. In this context, the European project MATHRYCE intends to propose an easy to implement vessel design methodology based on lab-scale tests and taking into account hydrogen enhanced fatigue.In the present document the lab-scale experimental developments and results are presented. The material considered was a commercially available Q&T low alloy Cr-Mo steel from a seamless pressure vessel. Due to the high hydrogen diffusion at room temperature in such steel, all the tests were performed under hydrogen pressure to avoid outgassing. Different types of lab-scale tests were developed and used in order to identify the most promising one for a design code. The effect of mechanical parameters, such as H2 pressure, frequency and ΔK, on fatigue crack initiation and propagation was analyzed. In particular, special attention was paid on the influence of H2 on the relative parts of initiation and propagation in the fatigue life of a component.The second part of the work was dedicated to cyclic hydraulic and hydrogen pressure tests on full scale vessels. Three artificial defects with different geometries per cylinder were machined in the inner wall of each tested cylinder. They were specifically designed in order to detect fatigue crack initiation and fatigue crack propagation with a single test.The final goal of this work is to propose a methodology to derive a “hydrogen safety factor” from lab-scale tests. The proposed method is compared to the full-scale results obtained, leading to recommendations on the design of pressure components operating under cyclic hydrogen pressure.Copyright

Reduction in Fretting Fatigue Strength of Austenitic Stainless Steels due to Internal Hydrogen

Fretting fatigue is one of the major factors in the design of hydrogen equipment. The effect of internal hydrogen on the fretting fatigue strength of austenitic stainless steels was studied. The internal hydrogen reduced the fretting fatigue strength. The reduction in the fretting fatigue strength became more significant with an increase in the hydrogen content. The reason for this reduction is that the internal hydrogen assisted the crack initiation. When the fretting fatigue test of the hydrogen-charged material was carried out in hydrogen gas, the fretting fatigue strength was the lowest. Internal hydrogen and gaseous hydrogen synergistically induced the reduction in the fretting fatigue strength of the austenitic stainless steels. In the gaseous hydrogen, fretting creates adhesion between contacting surfaces where severe plastic deformation occurs. The internal hydrogen is activated at the adhered part by the plastic deformation which results in further reduction of the crack initiation limit.

Crack Initiation and Propagation Under Hydrogen-Enhanced Fatigue of a Cr-Mo Steel for Gaseous Hydrogen Storage

The current international standards and codes dedicated to the design of pressure vessels do not properly ensure fitness for service of such vessel used for gaseous hydrogen storage and subjected to hydrogen enhanced fatigue. Yet, hydrogen can reduce the fatigue life in two ways: by decreasing the crack initiation period and by increasing the fatigue crack growth rate. The European project MATHRYCE aims are to propose an easy to implement vessel design methodology based on lab-scale tests and taking into account hydrogen enhanced fatigue.The study is focused on a low alloy Cr-Mo steel, exhibiting a tempered bainitic and martensitic microstructure, and classically used to store hydrogen gas up to 45 MPa. Due to hydrogen diffusion at room temperature in such steel, tests have to be performed under hydrogen pressure to avoid outgassing.In the present work, experimental procedures have been developed to study both crack initiation and crack growth. The specimens and tests instrumentation have been specifically designed to quantitatively measure in-situ these two stages under high hydrogen pressure. We developed and tested crack gages located close to a small drilled notch. This notch simulates the presence of steel nonmetallic inclusions and other microstructural features that can affect fatigue crack initiation and propagation. The experimental results addressing the effects of the testing conditions, such as stress ratio, frequency and hydrogen pressure will be compared to the local strain and stress fields obtained by Finite Element Method and correlated to the possible hydrogen enhanced fatigue mechanisms involved.Copyright

Phase transformations in Fe–Ni–Cr heat-resistant alloys for reformer tube applications

ABSTRACT Fe–35Ni–25Cr–0.4C alloys with different compositions are aged between 750 and 1150°C up to ∼10,000 h. As-cast microstructure contains interdendritic carbides of type M7C3 (‘Cr7C3’) and MC (‘NbC’). At service temperatures, M7C3 transform into M23C6 (‘Cr23C6’) within hours. Then, a hardening precipitation of secondary intragranular M23C6 occurs over hundreds of hours, the nose of the ‘temperature-time-hardening’ curve being around 1000°C. G phase forms after long aging; its solvus temperature and formation kinetics depend on silicon content. Z phase is observed after long aging at 950°C or above. G and Z phases form at the expense of MC. Very long aging causes nitridation under air, with first a transformation of M23C6 into chromium-rich M2X carbonitrides (X = C,N), then of MC into chromium-rich MX carbonitrides.

Failure Mechanisms of an AISI 4135 Steel Submitted to the Disk Pressure Test under Helium and Hydrogen Gas

The present work focuses on the experimental multi-scale characterization of fracture of an AISI 4135 steel by using the Disk Pressure Test (DPT). In order to precise the specific features of hydrogen embrittlement, comparison was made between disks burst under helium and hydrogen gas. SEM - EBSD analysis of disks samples before and after the test allowed to analyze and to compare the main microstructural mechanisms of the failure process. The location of the main crack initiation was consistent with Finite Element (FE) simulations of the DPT.

A Numerical and Experimental Study of the Disk Pressure Test

The Disk Pressure Test is used to select metallic materials for hydrogen storage and transportation and consists of a thin metallic disk bulged out until fracture by applying a gas pressure on its lower face. The ratio of the fracture pressures obtained with a neutral gas and with hydrogen gas defines a phenomenological index for material hydrogen sensitivity. In order to use the Disk Pressure Test for the selection of materials exposed at high pressure (50–70 MPa), a thorough study of this method has been carried out. Experimental investigations of damage mechanisms at different scales and finite element computations of the test have been carried out to exhibit the main features of hydrogen embrittlement in low alloy Fe-Cr-Mo tempered steels during the disk rupture test. Finite Element computations of the test permitted to predict the global response and to understand the effect of boundary conditions and of material behavior on stress gradients and plastic strain distribution throughout the disk. Using hydrogen sensitive cohesive elements to model the global crack path, good agreement with experiment was obtained on the effect of disk thickness and of hydrogen pressure rate on the failure pressure. The relative influence of loading conditions and material behavior on the hydrogen embrittlement during the disk rupture test are discussed.Copyright

The Influence of Phase Transformations on Creep Resistance in Fe-Ni-Cr Alloys for Reformer Tube Applications

Heat resistant steels of the HP-series have widespread uses in the petrochemical industry in pyrolysis and reformer furnaces. The alloys are carbon-rich Fe-Ni-Cr alloys, with additions like Mn, Si, Nb, Ti, W... The typical microstructure of as-cast HP alloys is an austenite matrix with intergranular eutectic-like primary chromium carbides of the M7 C3 type and niobium carbides of the MC type. Upon ageing, phase transformations occur. Intragranular secondary M23 C6 carbides precipitate, which is thought to restrict dislocation motion, and intergranular M7 C3 transforms into M23 C6 . Under certain thermal conditions, a partial transformation of the primary niobium carbides into a nickel-niobium silicide called G phase can occur. These phases may play a critical role during creep, but neither their role on mechanical properties nor the mechanisms of phase transformations are clearly identified. The aim of this study is to understand the role of each phase or phase transformation in the creep resistance of HP alloys. Consequently, a critical review of phase formation and transformations in such alloys is presented using a set of experimental and modelling techniques (electron microscopy, Castaing microprobe, creep tests at high temperature and neural networks modelling of mechanical properties...).Copyright