HP Steel Research Articles

Study of creep and creep crack growth fatigue of aging ASTM A297/A297M-19 HP steel modified with niobium

Study of creep and creep crack growth fatigue of aging ASTM A297/A297M-19 HP steel modified with niobium

Non-destructive ultrasonic testing and machine learning-assisted early detection of carburizing damage in HP steel pyrolysis furnace tubes

During the operation of HP steel furnace tubes, structural deterioration occurs due to carburization mechanisms. Herein, machine learning models were employed to detect carburization damage in furnace tubes using ultrasonic signals. The microstructural and elemental analysis revealed phases like austenite, chromium carbide, and niobium carbide. Our volumetric fraction analysis showed that the harmful chromium carbide phase increased toward the tube wall thickness. Three machine learning models, namely Gaussian Naive Bayes (GNB), Kernel Naive Bayes (KNB), and Subspace Discriminant (SD), were used to analyze the ultrasound signals. The GNB model demonstrated the highest accuracy rate (99.2%) and high sensitivity for the dataset with 26 features and a K-fold cross-validation with K value = 5, arising as the most effective classifier for detecting carburization damage in HP steel. Our results underscore the efficacy of the combined use of ultrasonic testing and machine learning for detecting carburization in HP steel furnace tubes.

Gas-phase aluminizing of HP40Nb steel used in reformer tubes: Synthesis, characterization, and its implications for the microstructure and high-temperature creep resistance of the base alloy

This study investigates the microstructure and creep behavior of gas-phase aluminized High-Performance (HP40Nb) steel. Microstructures of coated and base alloy were examined by electron and optical microscopes. Gas-phase aluminizing of HP steel resulted in the formation of a top AlNi layer over an interdiffusion zone (IDZ) on the base alloy. Results also showed that there were hardly any cavities or discontinuities formed in top layer and IDZ during the aluminizing process. The creep test results showed that the gas-phase aluminized steel had significantly a higher minimum creep rate and consequently a shorter time-to-rupture, as compared to uncoated HP40Nb steel. Microscopic examinations revealed that the main reason for the fracture of the steel after the creep test was formation of micro-cavities in the vicinity of carbides and growth of micro-cracks along the grain boundaries and alongside inter-dendritic areas. Also, fractography analysis showed that a quasi-cleavage fracture had occurred for both coated and uncoated steel. The implication of gas-phase aluminizing on creep resistance of coated HP40Nb steel, and its interrelation between the microstructure and observed mechanical properties are discussed and explained through the paper.

Study of monotonic and cyclic mechanical behavior and microstructural evolution at a high temperature of ASTM A297 HP steel modified with niobium

The present work studies the effects of niobium on the mechanical properties and the elastoplastic/viscoelastic behavior at 927 °C of ASTM A297 grade, HP cast stainless steel in the as-cast material, to simulate the actual work condition. Tensile tests were carried out at different temperatures, differential scanning calorimetry (DSC), relaxation tests, the fatigue of continuous cycles under the triangular waveform, and fatigue with a trapezoidal waveform of the type “Dwell” with dwell times of 120 and 300 s as specified in the E2714-13 standard and hot torsion tests, to analyze the variations in the mechanical properties. The results of the mechanical tests and studies of the microstructural evolution, as well as of its later analyzes, provided data, not only in the form of data sheet's for the application of the values obtained in projects but concurrently after comparing property and microstructure, to deepen the phenomenology of mechanical stress behavior versus monotonic and cyclic deformation, high-temperature deformation mechanisms, dynamic recovery, of the type of microstructure resulting after the tests and also of the fracture mechanisms associated with the types of mechanical stresses imposed on the material, obtained by through the mechanical tests mentioned above. The high-temperature fatigue tests showed expected stress versus strain behavior, where cycles without dwell times showed lower hysteresis area values, from where the energy consumed in a strain cycle is calculated. The increase in the consumed energy values is due to the insertion of the dwell times, but it is important to note that for the tests that contain dwell times of 120 s and 300 s they did not show significant differences between the consumed energy, showing a saturation effect of tension that does not promote more significant inelastic strains. The cyclic mechanical fatigue stresses with dwell times and the low deformation rate led to intense precipitation of secondary carbides Cr23C6 chromium alloys added to the intrinsic properties of stainless steel dislocation slip were factors that resulted in the dynamic recovery phenomenon. Fractographic analyzes performed on monotonically fractured specimens and specimens after fatigue tests revealed that the material is prone to fracture in intergranular regions.

Asymptotic creep deformation behavior of modified HP steel after long-term service

Creep behavior of modified HP steel was investigated through the tensile creep tests under the stress conditions of 25–55 MPa at 950 °C with four reformer tubes of different service periods: Unused (virgin), 8.50, 9.67, and 16.17 year aged. The minimum creep strain rates, creep rupture life, as well as elongation and reduction of area of the ruptured specimen were measured. It was observed that all the measured properties did not vary with different service periods. The creep deformation behavior was modeled based on the experimentally measured creep data utilizing the secondary creep (SC) model, the tertiary creep (TC) model, and the combined model of two latter ones (SC-TC model). The results indicated the asymptotic creep deformation behavior of the secondary creep strain rate. The tertiary creep also demonstrated asymptotic behavior, however, its strain rate slightly increased, as the service period increased. The creep curves predicted by the SC-TC model were more accurate than those obtained using the SC or TC model. Microstructural observations supported the observed asymptotic behavior.

Method to Determine the Thickness of the Carburized Layer of Pyrolysis Furnace Tubes Using Ultrasound Technique

Carburization that takes place in HP steel tubes in pyrolysis furnaces coils is an undesirable event, as it negatively affects mechanical and resistance to corrosion properties. This has been an important factor for the study and development of non-destructive methodologies aiming at maintaining structural integrity. In this work, non-destructive testing using ultrasound was employed for specimen with varying carburization degrees, in order to characterize them. The results allowed for the development of a correlation between the thickness of the carburized layer and of the half height bandwidth of the backwall echo frequency spectra.

Evaluation of electric and magnetic signals in heat-resistant HP alloys using scanning probe microscopy

Abstract HP steels are commonly used in steam reforming tubes, which are exposed to harsh operational conditions causing microstructural changes. These microstructural changes cause variations in the magnetic properties, which can be measured by different techniques. This work aims at the measurement of the magnetic and electric variations in HP-Nb modified alloys with different chemical composition using scanning probe microscopy (SPM) techniques. The results show that, despite the microstructural change due to variation of chemical composition, samples with aging state I exhibit a magnetic response at the carbides boundary, while for advanced aging states, the magnetic response is not evidenced.

Carburization level identification in industrial HP pipes using ultrasonic evaluation and machine learning

Ultrasound nondestructive testing is commonly applied in industry to guarantee structural integrity. HP steel pyrolysis furnaces are used in petrochemical industry for lightweight hydrocarbon production. HP steel chromium content may be reduced in high-temperatures due to carbon diffusion. This characterizes the carburization phenomenon, which modifies magnetic properties, reduces mechanical resistance and may lead to structural rupture. For safe operation it is required to frequently determine carburizing level in pyrolysis furnace pipes. This is traditionally performed manually using magnetic evaluation. This work proposes a novel procedure for carburizing level estimation using ultrasonic evaluation associated to signal processing and machine learning techniques. Experimental data from pulse-echo ultrasonic tests performed in HP steel pipes are used. Discrete Fourier transform was applied for feature extraction and different classification systems (neural networks, k-nearest neighbors and decision trees) are applied and compared in terms of carburizing level identification efficiency.

Characterization of the oxide scale formed on external surface of HP reformer tubes

This work presents the characterization of the external oxide scale of a centrifugally cast HP steel tube, modified with Nb and Ti, after 90,000h of operation in a steam reforming furnace, showing different aging states as a function of the column height and exposure temperature. The objective was to relate the oxide microstructural morphology to its magnetic response for three different microstructural aging states. The morphological and compositional characterization was performed by high resolution scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction. The magnetic response of the external surface was analyzed by magnetic force microscopy and scanning magnetic susceptometer. The results showed the formation of a multilayer oxide scale with different compositions along its thickness. In addition, a microstructural modification was identified in the scale-matrix interface, due to the depletion of chromium. It was also observed that the magnetic response is dependable both on the oxide scale thickness and the chromium carbides depleted zone. Magnetic force microscopy was able to identify the origin of the magnetic response resulted from the steel tube surface.

Characterization of the aging state of modified HP steels by ultrasonic signal processing and the effect of creep voids in the interdendritic region

HP austenitic stainless steels are extensively used in the oil and gas industry as radiant tubes in reformer and pyrolysis furnaces. These steels exhibit high resistance to oxidation, thermal stability, and creep resistance at elevated temperatures.Their microstructure changes as a function of temperature, leading to different aging states. In addition, creep voids can be formed due to furnace operating conditions such as time and stress. Therefore, the characterization of the microstructural conditions is necessary for service life assessment. This study aims at characterizing two modified HP steel samples with different aging states and volumetric fraction of creep voids via scanning electron microscopy and ultrasonic testing. Fast Fourier transforms were applied to the second backwall echo of the acquired ultrasonic signal. Spectral analysis established a correlation between the microstructural conditions and the signal response. According to the methodology developed, the ultrasonic parameters and spectral analysis results were sensitive to microstructural changes caused by aging and the presence of creep voids.

Thermally induced soil structure interaction in the existing integral bridge

While Cross’ method enabled scientifically based structural design of integral bridges (IB) a similar progress in understanding and analyzing the relevant complex soil structure interaction has not been made yet. This hampers a wider adoption of IB systems, whose geo-structural system inherently brings multiple sustainability and performance benefits to transportation infrastructure. To this end, a full 3D finite element model of an existing three-span integral bridge was assembled and subjected to a combined thermal and gravity loads. The bridge superstructure consists of the two sets of concrete piers, two abutments, and fourteen HP steel piles (seven at each abutment), whose strong axis of bending is oriented parallel to the longitudinal direction of the bridge. Upon a successful validation and the verification of the computational model, several loading scenarios simulating different amounts of temperature increase in the presence of different soils adjacent to the abutment were simulated. Further analyses indicated that effects of the compaction level of the soil adjacent to the abutments, and of a magnitude of the thermal load on the substructure are opposite from the effects of these agents on the superstructure.

Influence of chemical composition on microstructure and phase evolution of two HP heat resistant stainless steels after long term plant-service aging

The present work is aimed on the microstructure characterisation of two different HP steels after over 80 000 h of service in petrochemical plants of steam reforming. By means of optical and electron microscopy, as well as energy dispersive X-ray elements mapping and X-ray diffraction techniques, the alloys have been deeply investigated in order to make clear the influence of chemical composition, in particular titanium and tungsten additions, on the microstructural evolution after long service aging. The most evident features of aged materials can be summarised in terms of coalescence and coarsening of interdendritic precipitates, precipitation of secondary carbides in the austenite matrix and transformation of niobium-rich carbides in the G-phase silicide. We have observed that titanium and tungsten additions have played a synergistic role in preventing and reducing chemical evolution of the secondary phases present in the matrix.

Variation of Creep Properties in HP Steel by Influence of Temperature

The structure and equipment of a plant can fail due to excessive plastic deformation and progressive degradation of their materials. Such is the case of petrochemical plants where, for example, pyrolysis furnaces operating at high temperatures are used and, as a consequence, materials are subject to creep conditions, which is one of the main mechanisms causing their failure.The aim of this paper is to study the creep behaviour of the HP type austenitic steel (Fe, 35Ni, 25Cr) microalloyed with Nb, which has high thermomechanical strength associated with the internal microstructure of the matrix. Tests were performed at temperatures and under stresses similar to service ones in order to identify acting degradation mechanisms. Simultaneously, microstructural evolution under the same time and temperature conditions as those of the tests was characterized.Torsional creep-rupture tests were conducted working in a temperature range of 1073 to 1273K and under effective stresses between 40 and 90MPa, keeping both variables constant during each test.

Effects of working temperature and carbon diffusion on the microstructure of high pressure heat-resistant stainless steel tubes used in pyrolysis furnaces during service condition

In the present study, high pressure heat-resistant cast stainless steels (HP steels) modified with niobium and titanium were investigated in as-cast conditions and after being used in pyrolysis furnaces. Life span of the studied specimens obtained from pyrolysis furnace was 5 years. Microstructural changes were studied via scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS), optical microscopy (OM), and X-ray diffraction (XRD). The effect of temperature and carbon diffusion on the microstructure, chromium-rich carbides, the NbC transformation to G-phase and other precipitates formed during service condition were discussed. The results showed that two major phases, namely chromium and niobium carbides, existed in the as-cast specimens. Temperature and carbon diffusion influenced the composition and volume fraction of secondary precipitates. Chromium and niobium carbides were transformed to M 23C 6 and G-phase respectively during service. Higher working temperatures do not always cause coarsening of precipitates. However, factors such as decarburization and carbon diffusion have important roles, too.

Investigation of weldability and property changes of high pressure heat-resistant cast stainless steel tubes used in pyrolysis furnaces after a five-year service

High pressure heat-resistant cast stainless steel (HP steel) tubes produced by centrifugal casting are used in petrochemical industries for pyrolysis furnaces. They have appropriate ductility and weldability in as-cast conditions. These steels lose their ductility and weldability after being used in service and, hence, require repair. In the present study, the effect of metallurgical changes on weldability and ductility was investigated. The life span of the studied tubes was 5 years. Using electrodes with a chemical composition close to the base metal analysis, welding was done by gas-tungsten arc welding (GTAW) process. Solution treatment was used to recover the properties of tubes which can be useful, depending on metallurgical changes.

Microstructural characterization of dissimilar welds between alloy 800 and HP heat-resistant steel

In this study, dissimilar welds between HP heat-resistant steel and Incoloy 800 were made with four different filler materials including: 309 stainless steel and nickel-based Inconel 82, 182 and 617. The microstructure of the base metals, weld metals and their interfaces were characterized by utilizing optical and scanning electron microscopy. Grain boundaries migration in the weld metals was studied. It was found that the migration of grain boundaries in the Inconel 82 weld metal was very extensive. Precipitates of TiC and M 23C 6 (M = Cr and Mo) in the Inconel 617 weld metal are identified. The necessary conditions for the formation of cracks close to the fusion line of the 309-HP joints are described. Furthermore unmixed zone near the fusion line between HP steel base metal and Inconel 82 weld metal is discussed. An epitaxial growth is characterized at the fusion line of the 309-Alloy 800 and Inconel 617-Alloy 800 joints.

The Combination of Modeling Techniques to Predict the Service Behaviour of Heat Resistant Alloys

It is here demonstrated that combinations of various modeling techniques can be used to predict the service behaviour of heat resistant alloys, and sometimes to design “made-to-measure” alloys for specific high temperature applications. A first example is given, where an affordable creep-resistant nickel-base superalloy for operation around 750°C has been designed without any experiment. Based on the analysis of huge databases on existing alloys, Gaussian processes were used to predict its thermomechanical properties. Thermo-Calc allowed to design its fabrication process and to assess its weldability and its thermodynamical stability at service temperature. The alloy has then been tested and the validity of the modeling approach verified a posteriori, in particular a creep rupture life of 100 000 h at 750°C under 100 MPa. A second example is given, in the case of high-carbon Fe-Ni-Cr based alloys for reformer tube applications (HP steels). Their mechanical properties are predicted through the analysis of existing data with artificial neural networks. Parallelly, their thermodynamical stability in operating conditions is assessed using Thermo-Calc in combination with Dictra, to simulate the precipitation of carbides in the austenite matrix during service. It is therefore tried to understand microstructural evolution in service, damage mechanisms, and durability.

Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels

Centrifugally cast heat-resistant HP stainless steels are particularly suitable for applications where service conditions comprise high temperatures and aggressive environments; thus, they are extensively used in reformer furnaces, in which hydrogen production takes place. The demand for better performance has motivated developments in these steels. The addition of Nb and Ti as microstructural modifiers has proved successful in providing a more stable microstructure. In this work yttrium was added to centrifugally cast NbTi-modified HP steels. It was observed that its presence increased the level of fragmentation of the chromium carbides, a positive aspect for creep resistance. The main cause of the fragmentation is the formation of yttrium carbides, which serve as heterogeneous nucleation sites for the other carbides. One of the tubes, with a lower titanium content, showed the best creep performance among those tubes studied owing to the presence of a smaller volume fraction of the deleterious G phase.

Microstructures, properties and high-temperature carburization resistances of HVOF thermal sprayed NiAl intermetallic-based alloy coatings

In this paper, the microstructure, mechanical properties and high-temperature carburization resistance of HVOF thermal sprayed NiAl intermetallic-based coatings were investigated by scanning electron microscopy, X-ray diffraction and micro-indentation testing. It was found that the addition of CeO 2 and Cr improves the wetting and bond strength of the coatings to the substrate, which decreases the tendency of brittle peeling during thermal spraying. The addition of CeO 2 and Cr is also beneficial in forming coating layers with less cracks and pores. The NiAl intermetallic coatings containing CeO 2 and Cr also exhibit higher hardness, improved elastic modulus and increased η ratio. The hardness and elastic modulus of the NiAl intermetallic-based alloy coatings were all decreased after carburizing at high temperature. The η ratio was also decreased after carburization for coatings without the addition of CeO 2 but to a minor degree for those containing CeO 2. These observations are related to the reduced depletion of Al and inter-diffusion between the coating layer and the substrate due to the presence of CeO 2 and/or Cr. Compared with uncoated HP steel, the samples with different NiAl coatings exhibited excellent carburization resistance in 2 vol.% CH 4 balance hydrogen gas mixture with a carbon activity of a c=3 at 1100 °C. This may result from the diffusion barrier role of the NiAl coatings and the presence of oxide films such as Al 2O 3 and CeAlO 3.

The influence of accumulated strain on the temper embrittlement response of a low alloy bolting steel during service

The present paper describes part of a larger study which investigated the effects of accumulated service strain (e) on the reverse temper embrittlement (RTE) response of a series of large HP and IP turbine CrMoV steel bolts which had been in service for 122 000 h at elevated temperatures or around 490°C. It is shown that accumulated service strains exert significant effects on toughness losses in the CrMoV bolting steel, especially at low levels of strain, and that such effects can be adequately described by the expressionsCn = 7.6 × 10−2 e−0.8andFATT(K) = 418–13.28 e−0.8where Cn is the normalized Charpy energy and FATT is the Charpy fracture appearance transition temperature. It is suggested that the extent of the RTE response in low alloy steels can be separated into a pure RTE component and a strain-induced or synergistic RTE component. Also, from limited Auger electron spectroscopy data it is suggested that the latter may not be the result of strain-induced enhanced grain boundary phosphorus segregati...