Rotor Steel Research Articles

Characterization of Low Cycle Fatigue Parameters of Rotor Steel using Sub-sized Specimens

The paper is dealing with strain controlled cyclic testing method employing a novel strain-control technique based on digital image correlation (DIC) in low-cycle fatigue (LCF) region. The cyclic behaviour of 22CrMoNiWV 8-8 rotor steel was investigated on sub-sized round specimens with a diameter of 2 mm in gage length and total length of 20 mm. These results were compared with results obtained using conventional specimens designed in accordance with the ASTM E606 standard. The attention was paid to confirm the suitability of the proposed sub-sized geometry, testing set up and procedure. The test procedure and results obtained enabled to record hysteresis loops, construct Manson-Coffin curves and obtain cyclic material properties in LCF region.

Evaluation of component‐similar rotor steel specimens with a local probabilistic approach for LCF

AbstractLately, a successful validation of a combined size and gradient effect modelling approach in a local probabilistic framework for LCF was published. A second validation study is here presented using LCF data of component‐similar specimens made of forged X12CrMoWVNbN10‐1‐1. It is found that a reasonable LCF crack initiation life prediction can be achieved by the local probabilistic model using single batch material parameters. However, the material parameters obtained from mixed batch LCF test data overestimate the actual material inherent LCF life scatter. Consequently, the size effect shift is pronounced too much, and the LCF life is predicted beyond crack initiation. Estimating the material parameters from a single batch data set instead results in a much less dispersed LCF life distribution and an approximately correct size effect shift. The predicted median Wöhler curve then corresponds to borehole‐specimen lives for initial cracks at the detection threshold. A reasonable prediction of technically relevant crack sizes in such inhomogeneous components requires additional modelling of stress gradient effects.

Influence of Solid Rotor Steel on Dynamic Response and Losses of Turbogenerator With Loss of Excitation

The rotor of turbogenerator is made of solid steel, instead of laminated steel, for its high-speed rotation. The asynchronous torque resulting from the eddy current in the rotor core plays an important role in the asynchronous operation of turbogenerator after loss of excitation. On the other hand, the rotor loss and temperature rise restrict the operation of turbogenerator. The selection of solid steel is therefore a key factor that affects the operation limit and should be carefully considered in the design of turbogenerators. For a turbogenerator with the rotor core made of different solid steel, this paper presents time-stepping finite element calculation of the asynchronous performances and the experimental validation. The influences of different rotor solid steel on the dynamics, saturation, asynchronous operation ability, and rotor loss of turbogenerator with the loss of field are studied. The losses in different areas of rotor are calculated and compared. The results provide steel manufacturers and generator designers with the theoretic basis for improvement of the asynchronous operating ability of turbogenerator.

Microstructural characteristics and low cycle fatigue properties at 230 °C of different weld zone materials from a 100 mm thick dissimilar weld of ultra-supercritical rotor steel

The low cycle fatigue properties at 230 °C and the microstructures of the material in different zones of 100-mm thick dissimilar joint made of ultra-supercritical rotor steels were studied. The microstructures, failure path and the fatigue fracture surfaces were characterized by scanning electron microscope and transmission electron microscopy. The relationship between strain level and fatigue life of 30Cr1Mo1V (BM-1), weld-seam (SAW) and 30Cr2Ni4MoV (BM-2) at 230 °C were obtained, respectively. The effects of microstructural characteristics on low cycle fatigue performance at 230 °C were discussed based on the morphology of dislocation cells in fatigued samples.

A physically-based constitutive model and processing map of as-cast 30Cr2Ni4MoV low-pressure rotor steel

The hot deformation behaviour of as-cast 30Cr2Ni4MoV steel was systematically investigated within a temperature range of 1173–1473 K and strain rate range of 0.001–1 s−1 using uniaxial compression tests with a Gleeble-1500D thermal simulation tester. Flow curves were obtained under these conditions and the influences of deformation parameters on their morphologies are discussed. The characteristic parameters of the flow curves were determined and expressed in the form of simple power-law Zener-Hollomon parameters. A physically-based constitutive model was developed and its predictions of flow stress agree well with experimentally-measured flow stresses. Moreover, in order to optimize the forging parameters, processing maps of as-cast 30Cr2Ni4MoV steel were established at different stains based on a dynamic material model. By analysing the processing maps and microstructure at a strain of 0.7, the optimum forging process parameters for as-cast 30Cr2Ni4MoV steel were determined to comprise temperatures ranging from 1323–1473 K and strain rates of 0.1–1 s−1.

Critical Conditions for Dynamic Recrystallization of Hot Deformed 1 wt% Chromium - 1 wt% Molybdenum Rotor Steel

A medium carbon 1 wt% Chromium and 1 wt% Molybdenum low-alloy steel is an important material for making of rotors of turbines. Its hot forging requires precise control of temperature, strain, and strain rate for defect-free forging. In a view of this, this steel was subjected to a hot compression test in a thermomechanical simulator, Gleeble 3800®. Hot compression was carried out at temperatures of 900°C, 1,000°C, and 1,100°C. At each temperature four strain rates, i.e., 0.001 s−1, 0.01 s−1, 0.1 s−1 and 1 s−1, were used. Constitutive equations were used to analyze hot flow behavior, material constants, and activation energy. A mathematical model developed by Poliak and Jonas has been used to determine critical condition parameters like critical stress (σc), critical strain (εc), peak stress (σp), and peak strain (εp). For dynamic recrystallization (DRX), the variation of strain hardening rate up to steady state stress (σss), steady state strain (εss), and volume fraction of DRX (XDRX), with the help of Avrami-type models, have been discussed. Zener Hollomon parameters (Z) explained in terms of strain rate and steady state that flow stress (σss) has been expressed. Microstructures at different hot deformation conditions have been reported and explained.

Investigation of the turbogenerator’s rotor stress-strain state when the steel is damaged

Purpose. Investigation of the stress-strain state of the rotor of turbogenerator, which has damage in the form of cracks in the array of rotor barrel. Methodology. In the article mechanical stresses and deformations in the turbogenerator rotor's steel in a three-dimensional formulation are investigated by methods of mathematical modeling. Results. It is determined that the most mechanically region tensed is around the inner surface of the central opening of the turbogenerator rotor. In the nominal mode of operation, the crack in the central opening area of about 10 mm is critical for the turbogenerator. The appearance of a crack in the area of a small tooth contributes to breaking the tooth, which can damage the entire turbogenerator. Originality. The article continues analytical research and uses field methods based on the numerical solution of differential equations of structural mechanics using the finite element method. Practical value. The obtained results can be taken into account at the design stage of the turbogenerator, as well as when developing a system of diagnostic features for the diagnosis of the rotor damages. It is recommended in the process of turbogenerator rotor defectoscopy primarily to investigate the surface of the central hole of the rotor, because there is a high probability of cracks.

Влияние повторного нагрева при многопроходной сварке под флюсом на сопротивление сварных соединений роторной стали замедленному разрушению

Влияние повторного нагрева при многопроходной сварке под флюсом на сопротивление сварных соединений роторной стали замедленному разрушению

Effect of reheating in multipass submerged-arc welding on delayed fracture resistance of rotor steel welded joints

Effect of reheating in multipass submerged-arc welding on delayed fracture resistance of rotor steel welded joints

Study on Stress Corrosion Cracking Sensitivity of CrNiMoV Steam Turbine Rotor Steels

The stress corrosion sensitivities of 25Cr2Ni2MoV, 26NiCrMoV10-10 and 30Cr2Ni4MoV low-pressure rotor steels in simulated nuclear steam turbine operation condition were investigated by slow strain rate test (SSRT), and the stress corrosion cracking (SCC) mechanisms were studied by optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Results revealed that the SCC sensitivity of 25Cr2Ni2MoV steel was highest in 3.5wt.%NaCl solution at 180°C, while the SCC sensitivity of 26NiCrMoV10-10 steel and 30Cr2Ni4MoV steel are similar. The SCC sensitivity of CrNiMoV steam turbine rotor steels could be decreased by the increase of Ni element and the decline of mechanical intensity. Cracks initiate from metal surface and then propagate to the inner metal, which showed a form of transgranular cracking.

Delayed fracture resistance of welded joints of rotor steel 25Kh2NMFA after welding reheating

Delayed fracture resistance of welded joints of rotor steel 25Kh2NMFA after welding reheating

Сопротивление замедленному разрушению сварных соединений роторной стали 25Х2НМФА после повторного сварочного нагрева

Сопротивление замедленному разрушению сварных соединений роторной стали 25Х2НМФА после повторного сварочного нагрева

Hot Deformation Behavior and Dynamic Recrystallization Characteristics of 12Cr Ultra-Super-Critical Rotor Steel

In this paper, the constitutive model and dynamic recrystallization characteristics of 12Cr ultra-super-critical rotor steel were investigated quantitatively during hot deformation. A series of axisymmetric hot compression tests at temperatures from 900 to 1200 °C under strain rates of 0.001–1 s−1 were conducted on a Gleeble−1500D thermal simulator. Based on the experimental true stress–strain curves varying with temperature and strain rates, a complete constitutive model was established and all material parameters in the model could be expressed as a function of strain using a fifth order polynomial fit. The proposed model was verified so as to have the capability of accurately predicting the flow behaviour with an average absolute relative error of < 2.82%. Meanwhile, after hot deformation the microstructure was observed via electron backscatter diffraction technology. Then, the dependence of the characteristic parameters on the Zener–Hollomon parameter were confirmed. Furthermore, the kinetics equation of dynamic recrystallization was obtained, which included the flow stress calculated based on the evolution equation of the dislocation density during the work hardening-dynamic recovery stage. The result indicated that the predicted values for dynamic recrystallization volume fraction and flow stress were in line with the experimental values.

Study on Dynamic Recrystallization Behaviors in a Hot-Deformed FB2 Ultra-supercritical Rotor Steel

In the present work, hot deformation behaviors of FB2 ultra-super-critical rotor steel are investigated by isothermal compression tests under the deformation temperature range of 1323–1473 K and strain rate range of 0.01–1 s−1. The microstructure evolution of the deformed samples and the nucleation mechanism of dynamic recrystallization are studied by using electron backscatter diffraction and transmission electron microscopy. The results show that: (1) when the strain rate is higher than 0.1 s−1 and the temperature is lower than 1373 K, it easily results in a large number of substructures with relatively low-angle boundaries due to the intense work hardening effect. (2) Discontinuous dynamic recrystallization characterized by grain boundary bulging is the dominant nucleation mechanism for the studied rotor steel. (3) Geometrically necessary dislocations are sensitive to strain rate for the studied rotor steel but is less sensitive to the deformation temperature. (4) The low-angle boundaries fraction slightly increases with the increase in the Zener–Hollomon (Z) parameter under the test conditions.

Creep life assessment method for online monitoring of steam turbine rotors

ABSTRACTThe principal objective of work was to develop fast and accurate method for online creep life monitoring of steam turbine rotors. For this purpose, a characteristic strain model of creep was investigated and validated using test data of 2%CrMoV rotor steel. The model was then used to determine creep damage functions which are introduced to compute creep damage at a constant temperature based on the Robinson time fraction rule. Finite element (FE) creep calculations of a turbine rotor were performed using the same creep model to obtain reference damage results for validating the proposed method. Comparisons of the results obtained using both methods showed a very good accuracy of the online creep damage predictions.

High-temperature deformation mechanisms and physical-based constitutive modeling of ultra-supercritical rotor steel

This study presents an investigation that characterizes the evolution of the dynamically recrystallized structure and flow behavior of FB2 rotor steel during hot deformation in the temperature range from 900 °C to 1200 °C at strain rates from 0.001s−1 to 0.1s−1 using Gleeble-3500 thermo-simulation machine. Dynamic recrystallization (DRX) plays a key role in softening mechanism for hot deformed FB2 rotor steel. The fraction of LABs greatly increases with the decrease of deformation temperature. Low angle boundaries (LABs) always have higher local misorientation angle, which means the lower deformation temperature is capable of providing not enough driven force for the migration of LABs and thereby decelerates the DRX process to consume the dislocations. Furthermore, the critical stress and critical strain for initiation of DRX are calculated by setting the second derivative of the third order polynomial. Based on the classical stress–dislocation relation and the kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of FB2 steel. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformations for the studied steel.

Low cycle fatigue modelling of a steam turbine rotor steel

Materials in steam turbine rotors are subjected to cyclic loads at high temperature, causing cracks to initiate and grow. To allow for more flexible operation, accurate fatigue models for life prediction must not be overly conservative. In this study, fully reversed low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done isothermally, within temperature range of room temperature to 600 °C, under strain control with 0.8-1.2 % total strain range. Some tests included hold time to calibrate the short-time creep behaviour of the material. Different fatigue life models were constructed. The life curve in terms of stress amplitude was found unusable at 600 °C, while the life curve in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life model, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges. This allowed the formulation of life curves based on plasticity or creep domination, which showed creep domination at 600 °C, while at 500 °C, creep only dominates for higher strain range.

Superconducting AC Homopolar Machines for High-Speed Applications

This paper presents a novel high-speed alternating current (AC) homopolar motor/generator design using stationary ReBCO excitation windings. Compact, lightweight, high-efficiency motors and generators are sought for a multitude of applications. AC homopolar synchronous machines are an ideal choice for such applications as these machines enable very high rotational frequencies. These machines include both AC armature winding and direct current (DC) excitation winding within the stationary part of the machine. The stationary excitation winding magnetizes a solid steel rotor, enabling operating speeds limited only by the mechanical stress limit of the rotor steel. The operating speeds are many multiples of conventional power 50/60 Hz machines. Significant cooling requirements limit machines of this type utilizing copper excitation windings to only a few kilowatts. However, megawatt ratings become possible when superconductor coils are used. This paper describes the design and analysis of an AC homopolar machine in the context of developing a 500 kW flywheel system to be used for energy recovery and storage in commuter rail subway systems. Different approaches are discussed for an AC armature employing conventional copper coils. Challenges of building and cooling both armature and field coils are discussed and preferred approaches are suggested. Calculations of the machine performance are then made.

Some Aspects of Microstructure Damage to Cr–Mo–V Rotor Steel as a Consequence of Extremely Long Operation

The observed plasticity degradation of Cr–Mo–V heat resistant steel as a consequence of more than 20-year operation at 510 and 300°C is caused by an increase in microdistortions of the crystalline lattice, density of dislocations, and sizes of mosaic blocks. The greatest variation of these microstructure properties in comparison with those for the initial state was observed for material operating at 300°C. According to the data of acoustic emission in the variant of linear location with an increase in stresses of the second kind, there occurs accelerated formation of clusters, regions of local explosive plastic deformation. The clusterization should directly result in an increased probability of nucleation of an equilibrium mesocrack of millimeter range.

Some Aspects of Cr–Mo–V Rotor Steel Damage due to Long-Term Service from Acoustic Emission Data

It is shown that damages accumulating in a rotor steel after more than 20 years in service under creep conditions at high temperatures reveal themselves in the change of acoustic emission (AE) parameters as compared with the initial structural state. The formation of clusters of AE activity is observed in zones 102–104 μm in size inducing a similar process by the shear mechanism in a neighbor zone with a larger effective elastic modulus. Hence, AE revealing of clusters of plastic deformation takes place.