Creep Of Steel Research Articles

Long-term creep life prediction of P91 steel using domain knowledge and back propagation artificial neural network

ABSTRACT In this work, the long-term creep life of P91 steels with the limited experiment data was predicted using the domain knowledge and back propagation artificial neural networks (BP-ANN). Based on the traditional creep life models of Larson-Miller (L-M) parameter and θ projection, the domain knowledge was incorporated to expand the creep dataset. Then, the fuzzy C-means (FCM) clustering algorithm was introduced for training data collection. Consequently, the creep life prediction was conducted with the corresponding dataset and BP-ANN. The obtained results demonstrated that in contrast to the conventional prediction models of creep life, the prediction accuracy for long-term creep life of P91 steels can be improved significantly by the present model. For instance, the predicted creep lives 200,000 h exhibit the average errors of 6.0%. In addition, the present study offers a convenient tool to solve the issue of limited experiment data, particularly the long-term creep of P91 steels.

A Multiscale Model of Creep in Steels with Account for the Microstructure

A Multiscale Model of Creep in Steels with Account for the Microstructure

Nanoindentation creep behavior of vacuum diffusion-bonded stainless steel and Inconel 718 joints using a Cu interlayer

Exhaust components combining Inconel and stainless steel offer the most cost-effective hybrid design for high-performance engines. Owing to the formation of brittle compounds, the most critical zone in such components is the joint interface, which must have a suitable creep life. The present study investigated the nano creep and nanomechanical behavior of diffusion-bonded Austenitic stainless steel (AISI 304) and Inconel (IN-718) with a Cu interlayer. Maximum nano hardness of 3.25 GPa in AISI 304 and 3.12 GPa in IN-718 was achieved at the Cu/IN-718 and Cu/AISI 304 interfaces, respectively. Additionally, the least elastic modulus of 159 GPa in the Cu interlayer, and 160 GPa in the bonding interface was occurred at the Cu/IN-718 and Cu/AISI 304 interfaces, respectively. Moreover, the creep displacement of the Cu/AISI 304 interface was 327% higher than that of the Cu/IN-718 interface. Lastly, the Cu/IN-718 interface exhibited diffusional creep, while the Cu/AISI 304 interface exhibited both diffusional and dislocation creep as active mechanisms.

Effect of Load Cycling on High Temperature Creep of 316L Stainless Steel

For components in concentrating solar thermal plants, the creep mechanism will cause a significant portion of material damage to receivers, storage tanks, turbines and pipework. These components will undergo conditions where loads, temperature, or both load and temperature are not constatnly applied. This creep damage process will not resemble the constant load creep tests used to characterise creep of materials. Therefore, creep tests incorporating a load cycle were undertaken to obtain a better understanding of how high temperature materials respond to these cyclic conditions. These tests showed that when time under load was considered, creep undertaken under load cycling conditions were accelerated relative to constant load conditions. A modified Larson-Miller approach was used to assess this effect and determine an equivalent stress where constant load test data agrees with the cycled test data. This found that cycling the load was equivalent to if the system were under 3 and 7 MPa higher stress, for tests which were loaded for 12 hrs and 6 hrs per 24 hrs respectively. This could be a potential approach to simply and easily consider these load cycling effects for design and life analysis.

Significant creep-strength improvement in modified 9Cr-1Mo steel via microstructural control through laser powder bed fusion

Modified 9Cr-1Mo steel was manufactured using laser powder bed fusion (LPBF). The as-built LPBF-sample microstructure comprised columnar δ-ferrite grains and fine martensite grains. The δ-ferrite was a unique phase that formed under the significantly rapid LPBF-induced solidification. Creep testing was performed for the LPBF sample at 923 K for up to 10,000 h. The LPBF-sample time-to-rupture and minimum creep rate were at least 10 times longer and 100 times smaller, respectively, than those of conventional modified 9Cr-1Mo steels at 923 K under 100 MPa. Microstructural characterization of the creep-ruptured samples revealed that creep deformation preferentially occurred in the martensite. Thus, the δ-ferrite contributed significantly to the enhanced creep strength. The intrinsic creep resistance of the δ-ferrite phase exceeded that of the martensite. The microstructural stability of the δ-ferrite grains (attributed to dense MX-phase precipitation at the grain boundary) was a likely strengthening factor. Overall, the modified 9Cr-1Mo steel creep strength was successfully improved via unique microstructural control through LPBF. This achievement is expected to extend LPBF application as a novel microstructural control process for steels and alloys, avoiding the diffusional transformation kinetics that occur in conventional heat-treatment processes.

Determination of creep behavior of vessel steel Q345R at elevated temperatures

Vessel steel Q345R is widely used in pressure applications in China due to its martensitic structure, often operating in high-temperature, high-stress environments. This study investigates its susceptibility to creep strain, which can lead to significant deformation and structural damage, compromising safety. Thermal creep tests for Q345R steel were conducted within the temperature range of 400 °C–600 °C under various stress ratios. Results indicate that, below 500 °C, stages I and II predominate creep curves, with stage II evolving slowly. However, as temperature rises, stage II becomes predominant, with stage III emerging earlier, especially under high stress, leading to accelerated creep and potential fracture. Temperature significantly influences creep rates, with even minor stress increments at elevated temperatures inducing substantial creep effects. Through fitting by creep strain curves, parameters for three different time hardening creep models were determined. Subsequently, material CREEP subroutines based on these models were developed and integrated into ABAQUS to simulate the entire creep test process. The simulation results validate the ability of calibrated creep models to accurately characterize Q345R steel's creep properties, facilitating the evaluation of safety in high-temperature service structures.

High-temperature creep and corrosion behavior of 316LN stainless steel in oxygen-saturated sodium

The high-temperature performance and sodium corrosion of in-core components are the main problems of liquid sodium-cooled reactors, and low-carbon nitrogen-controlled 316 stainless steel (316LN) is the main research and development direction of the main materials for the core of sodium-cooled fast reactors. In this work, the creep behavior of 316LN in oxygen-saturated liquid sodium at high temperatures (650 ℃, 700 ℃ and 750 ℃) was tested and compared with the results of creep tests in air at the same conditions. It is found that the creep life in the sodium environment is reduced by 63.44%, 67.93% and 40.07%, respectively. The specimens in oxygen-saturated sodium exhibit reduction in ductility, selective dissolution as well as oxidative corrosion. The corrosion mechanism of 316LN under external loading in oxygen-saturated sodium is illustrated.

Diagnosis of early creep degradation in 12Cr1MoVG steel based on a hybrid magnetic NDE approach

Accurate identification of microstructural degradation that occurs prior to crack coalescence and growth is crucial for continuous online monitoring of creep damage in main stream pipelines. The magnetic non-destructive testing technique is well-established for quantitative property measurement of creep degradation in steels. During the interrupted creep of 12Cr1MoVG steel at 580 ℃ and 110 MPa, the magnetic properties of crept specimens were evaluated using a combination of magnetic hysteresis loop and Barkhausen noise techniques, the microstructural evolution was also examined by means of scanning electron microscopy combined with EBSD method. The microscopic coercive force and Barkhausen noise characteristic values were chosen as the main parameters of diagnosis, as the most sensitive to the microstructural evolution such as dislocations, precipitates and voids, reflects the changes in material condition indicative of early degradation. Besides, these magnetic signatures even can identify the locations that potentially susceptible to future creep damage in the material, which were validated by microhardness measurement, grain misorientation and dislocation density distributions. It suggested that this work could be used as the technical basis for proactive management of material early degradation in the steam pipelines.

Ispitivanje mikrostrukture parovoda visokog pritiska nakon 200 000 sati rada

This study examined the microstructure of the main steam pipes in thermal power plant using the method of metallographic replicas. The aim of the research was to analyze and assess the condition of the pipes after 200 000 hours of operation, as well as to evaluate their remaining service life and endurance under long-term loads according to the VGB-S 517 standard. This standard establishes rigorous guidelines for inspecting and assessing the microstructural composition and damage due to creep in creep-resistant steel for high-pressure piping, boiler components, welded joints, considering the parameters such as material, wall thickness and construction. In addition to microstructure analysis, particular emphasis was placed on studying the time-dependent strength of the steam pipes, especially in the context of creep. Creep or deformation under constant load over time at high temperatures is a key factor influencing the long-term performance of the pipes. The results of the conducted research will provide a broader understanding of the condition of the steam pipes after 200 000 hours of operation, identify potential defects or weaknesses in the material and enable timely implementation of appropriate maintenance or replacement measures, thereby reducing the risk of accidents and failures.

Anchors Post-installed in High Strength Concrete

Nowadays, the great constructions of Colombia and the world, especially road infrastructure, have encouraged the use of high strength concrete. The use of post-installed anchors has also grown simultaneously in all concrete structures. Therefore, an experimental procedure was developed in this investigation to assess post-installed anchor systems based on tensile tests with high strength concrete specimens. The evaluation considers the analysis of the tensile tests results of test specimens with an anchoring system for three lengths (6, 9 and 12 times the diameter of the rod) with two rod diameters (3/8" and 5/8") in two grades of high strength concrete (5000 Psi and 6000 Psi) to determine the effective length and system load. The analysis carried out on the experimental test is to correlate the result of the maximum strength of steel reinforcement used (fy = 4200 Psi) with the variable post-installed anchoring systems. The results obtained are the percentage of system strength depending on anchor length and the effective length that ensures the steel creep and the correlations between the percentage of effective strength achieved for each anchorage length and the use of high strength concrete. With the above parameters, the contribution of high strength concrete to the efficiency of the post-installed anchorage systems was analyzed.

Creep of High-Strength Steel Coated with Plasma Sprayed Self-Fluxing Alloy

This article compares the creep testing behavior of AISI 4340 high-strength steel in the as-received and coated conditions. The coating material used was a NiCrBSi self-fluxing alloy. The microstructural characterization was carried out using optical and scanning electron microscopy. The creep tests were conducted at a temperature of 550 °C and with loads of 200, 250, and 300 MPa. The microstructure analysis of the deposited layer revealed some inclusions, very low porosity, and good adhesion to the substrate. The results of the creep tests indicated a decrease in the time to rupture under loads of 250 and 300 MPa for the coated steel. At a load of 200 MPa, the coated steel presented longer times to rupture and higher yield strength, demonstrating an improvement over the uncoated steel under these test condition. The fracture surface inspection showed a failure by a ductile fracture in both samples, with and without coating.

Evaluation of microstructure degradation during creep of P91 steel using electrochemical detection technique

Evaluation of microstructure degradation during creep of P91 steel using electrochemical detection technique

Characterization of geometrically necessary dislocation evolution during creep of P91 steel using electron backscatter diffraction

In this study, the change of geometrically necessary dislocation (GND) has been investigated during the elevated temperature creep of P91 steel. Firstly, the crept microstructures of P91 steel at 873 K and 165 MPa were characterized by electron backscatter diffraction (EBSD). Then, the resultant GND distributions were analyzed in detail. Meanwhile, the GND density was quantitatively estimated by two methods of kernel average misorientation (KAM) and Nye's dislocation density tensor, respectively. The obtained results indicated that during creep of P91 steel, the values of GND density calculated by the KAM method are highly consistent with those of Nye's dislocation density tensor method. Besides, the GND density increases significantly during the initial creep stage and attains the maximum approximately at the transition from initial creep to steady creep. During the subsequent creep, it gradually decreases. In addition, it suggested that the internal stress predicted by the obtained GND density is consistent with previous experimental data. Accordingly, it further demonstrated that the present characterization of GND evolution offers a convenient tool for the microstructure degradation analysis of P91 steel during creep. • The microstructure of P91 steel during creep has been investigated by EBSD method. • The change of GND density was quantitatively characterized by two methods of KAM and Ney's dislocation density tensor, respectively. • The GND densities obtained by the above methods are highly consistent. • The GND densities vary closely with the creep stage, which mainly results from the interaction between dislocation and precipitates.

Analisis Kehilangan Gaya Prategang Girder Pada Jembatan Proyek Pembangunan Jalan Tol Tebing Tinggi – Inderapura

Currently, the construction of toll roads in several regions of Indonesia is being carried out. One of them is the construction of the Tebing Tinggi-Inderapura Toll Road. In this project there are several bridge constructions as a link between roads or as a pedestrian bridge. This bridge uses prestressed beams in which there are steel wires. The steel wire is tensioned by pulling it using a hydraulic jack. When stress is applied, the beam loses its prestressing force. The purpose of this study was to determine the percentage loss of prestressing forces that occurred in the bridge beam. Loss of prestressing force must not exceed the maximum that has been determined. There is some loss of prestressing force which is taken into account. Short-term prestress losses include elastic shortening of the concrete, friction along the tendons and anchor slippage. Furthermore, the loss of long-term prestressing forces is creep in steel, shrinkage in concrete and relaxation of steel. After all the loss of prestressing force is calculated, the total loss of prestressing force is 21.62%. To find out the maximum percentage of loss of prestressing force using calculations from the book Design of Prestressed Concrete Structures by T. Y. Lin H. Burns with the formula Pe/Ac and the value is 8.98 MPa. Where 8.98 MPa is 25%, so 21.62% is smaller than 25%, so it is still safe. In addition, an implementation method for installing girders on bridges is introduced..

Physics-based model of irradiation creep for ferritic materials under fusion energy operation conditions

Irradiation creep is known to be an important process for structural materials in nuclear environments, potentially leading to creep failure at temperatures where thermal creep is generally negligible. While there is a great deal of data for irradiation creep in steels and zirconium alloys in light water reactor conditions, much less is known for first wall materials under fusion energy conditions. Lacking suitable fusion neutron sources for detailed experimentation, modeling, and simulation can help bridge the dose-rate and spectral-effects gap and produce quantifiable expectations for creep deformation of first wall materials under standard fusion conditions. In this paper, we develop a comprehensive model for irradiation creep created from merging a crystal plasticity representation of the dislocation microstructure and a defect evolution simulator that accounts for the entire cluster dimensionality space. Both approaches are linked by way of a climb velocity that captures dislocation-biased defect absorption and a dislocation strengthening term that reflects the accumulation of defect clusters in the system. We carry out our study in Fe under first wall fusion reactor conditions, characterized by a fusion neutron spectrum with average recoil energies of 20 keV and a damage dose rate of ≈3×10−7 dpa/s at temperatures between 300 and 800 K.

Experimental analysis of high-temperature creep in FV566 steel based on digital image correlation.

Digital image correlation (DIC) is an optical measurement method of material strain/displacement based on visible light illumination, which can be used for the measurement of long-term mechanical behavior. In this paper, an experimental method for analyzing high-temperature creep in FV566 steel material based on DIC was independently designed. Aiming at the problems of glass observation window medium refraction and thermal airflow disturbance in high-temperature testing, the corresponding correction methods were proposed to improve the measurement accuracy. Based on the above methods, high-temperature creep tests were carried out on three specimens with different shapes, and the strain concentration area at 600°C was calculated. Then, the influences of shape and other properties on material creep failure, stress distribution, and actual strain were investigated. Finally, the DIC calculation results were analyzed and compared with results of finite element analysis and the final fracture position of the specimen. The three results had a high degree of consistency, which verified that the proposed method can accurately measure and analyze the creep behavior of materials.

Study of the creep cavitation behavior of P91 steel under different stress states and its effect on high-temperature creep properties

In this study, circumferential U-notches of different depths were machined on a plain cylindrical creep specimen to simulate different stress states during steam pipe operation. The effects of stress concentration on the creep cavitation behavior and high-temperature creep properties of P91 steel were investigated through creep experiments and microstructure analysis, as well as measurement and analysis of scanning electron microscopy (SEM) micrographs using ImageJ to obtain the equivalent diameter, circularity, area and distribution of the voids after high-temperature creep of P91 steel. In the analysis of 98 notched samples of 14 bars, it was found that a large number of voids with diameters of 0.78–3.56 μm existed at the notch root, the voids were distributed in a fan shape near the notch root, the size of the distribution area was related to the stress, and the larger the stress was, the larger the area. The number, density and average area of voids increased with stress and time. The stress level at the notch root was found to be positively correlated with the area of damage caused by the voids in the region and negatively correlated with the number of voids based on the FE analysis. By analyzing the experimental data, it is found that the circularity is closely related to the void diameter, and the void coalescence occurs when the void diameter reaches a critical value, which varies with temperature, stress and time.

Simulation of crystal plasticity of P92 heat resistant steel considering martensitic lath structure

Based on the theory of crystal plasticity, coupled with dislocation and lath hardening models, this paper establishes a crystal plasticity finite element model describing the high temperature creep of P92 steel. Open source software was used to generate lath models with an average size of 350nm, 650nm and 950nm to explore the effect of lath coarsening on the high-temperature creep behavior of P92 steel. The results show that the roughening of the slats increases the rate of creep deformation, resulting in a decrease in the service life of the material. Observing the slat model, it can be seen that the roughening of the slats enlarges the numerical gradient of stress and strain, and aggravates the overall plastic strain of the model. The coarsening of the slats accelerates the movement of dislocations, causing the density of movable dislocations to increase, and at the same time the shear strain amplitude of the slip system increases, thereby reducing the hardening behavior of the material.

Creep of MARBN-type 9Cr martensitic steel in gaseous CO2 environment

With an interest in moving from steam to supercritical CO2 as the working fluid in advanced energy systems, it is important to study the mechanical response of structural alloys to CO2-containing environments. MARBN-type 9Cr martensitic steel was originally developed for application in boilers for supercritical and ultra-supercritical power plants where the steam temperature reaches 650 °C. In this research, a MARBN-type 9Cr martensitic steel designed and manufactured at NETL was creep tested in a gaseous CO2 environment (i.e., 0.1 MPa at 650 °C), and the results are directly compared to creep tests conducted in air. It was found that environmentally-assisted cracking facilitated by carbide formation beneath the growing oxide accelerated failure in gaseous CO2. This work indicates that creep-oxidation interactions in advanced martensitic steels represent an important consideration for materials selection in supercritical CO2 power cycles. Furthermore, this work confirms that more oxidation-resistant alloys will be required for the highest temperature portions of these systems.

Effect of Neutron Irradiation on the Mechanical Properties, Swelling and Creep of Austenitic Stainless Steels.

Austenitic stainless steels are used for core internal structures in sodium-cooled fast reactors (SFRs) and light-water reactors (LWRs) because of their high strength and retained toughness after irradiation (up to 80 dpa in LWRs), unlike ferritic steels that are embrittled at low doses (<1 dpa). For fast reactors, operating temperatures vary from 400 to 550 °C for the internal structures and up to 650 °C for the fuel cladding. The internal structures of the LWRs operate at temperatures between approximately 270 and 320 °C although some parts can be hotter (more than 400 °C) because of localised nuclear heating. The ongoing operability relies on being able to understand and predict how the mechanical properties and dimensional stability change over extended periods of operation. Test reactor irradiations and power reactor operating experience over more than 50 years has resulted in the accumulation of a large amount of data from which one can assess the effects of irradiation on the properties of austenitic stainless steels. The effect of irradiation on the intrinsic mechanical properties (strength, ductility, toughness, etc.) and dimensional stability derived from in- and out-reactor (post-irradiation) measurements and tests will be described and discussed. The main observations will be assessed using radiation damage and gas production models. Rate theory models will be used to show how the microstructural changes during irradiation affect mechanical properties and dimensional stability.