Rotor Steel Research Articles

High-temperature low-cycle fatigue characteristics of the 12Cr10Co3MoWVNbNB turbine rotor steel

Low-cycle fatigue (LCF) experiments of the 12Cr10Co3MoWVNbNB steel at 598 °C and a strain amplitude ranging from 0.27 % to 0.55 % were executed to reveal the fatigue characteristics, fatigue fracture mechanism, and further to predict the high-temperature fatigue life of the new-style turbine rotor steel. The results indicate that the steel exhibits a high-temperature fatigue softening characteristic. During the high-temperature LCF, the cracks are generated at surface or subsurface defects of the specimens, and the fatigue striations and dimples are apparent on the fracture surfaces of the crack propagation zone and the final fracture zone, respectively. The Smith-Watson-Topper (SWT) and Manson-Coffin models are suitable to predict the high temperature fatigue life of the steel at high/low strain amplitudes, respectively. In terms of finite element analysis of the turbine set rotor, it is determined that the steady state strain of the rotor steel is approximately 0.2 %. And thus, the fatigue life of the steel rotor is estimated as 172,500 cycles at the strain amplitude, based on the Eq. (6) in this work. After 100,000 cycles at the strain amplitude, the yield and tensile strengths of the steel specimens are only reduced by 1 % and 3 %, respectively, indicating a long residual life, which is consistent with the above predicting result.

Cogging torque reduction by utilizing the unequal rotor slot arc method for FSPM Machines

Cogging torque is a crucial and offending problem for electric machines that consist permanent magnets. Flux switching permanent magnet (FSPM) machines have magnets embedded inside the stator coils. Due to the interaction between the stator magnetic flux and the rotor steel, cogging torque occurs. To express this interaction, Magnetic Permeance Distribution Model is implemented in this study. To minimize the cogging torque, unequal rotor slot arc (URSA) technique is developed and cogging torque is calculated by using finite element method. An optimization study is performed to get the minimum value of the cogging torque under reasonable output torque. Results prove that, the proposed URSA method have promising outcomes for cogging torque reduction.

Effect of W and Mo contents on the microstructural evolution and properties in the 9% Cr martensitic rotor steel during long-term aging at 630 °C

The evolution of tensile properties and microstructure in three 9% Cr martensitic rotor steel with varying contents of W and Mo elements is investigated after aging at 630 °C for different durations, with a primary focus on the characteristics of martensite laths and blocks, grain boundaries, dislocations, and precipitates. The results indicate that the presence of W and Mo elements not only influences the precipitation and coarsening behavior of Laves phases but also that of M23C6 carbides, which differs from conventional beliefs. The elevated content of Mo and reduced content of W in the steel leads to a reduction in the equilibrium Cr concentration, promoting enhanced precipitation of Cr atoms and increases nucleation sites of M23C6 carbides. During the aging treatment, the M23C6 carbides in the 2.0W0.5Mo steel exhibits the highest content and the lowest coarsening rate. The strength of 9% Cr martensitic rotor steel is primarily enhanced through dislocation strengthening and grain boundary strengthening. The presence of fine M23C6 carbides effectively immobilizes dislocations and interfaces, resulting in the exceptional stability of the microstructure in the 2.0W0.5Mo steel and a minimal reduction in yield strength.

Damage mechanism of 30Cr1Mo1V rotor steel during thermal fatigue at 540°C

AbstractThis paper reports the damage mechanism of 30Cr1Mo1V rotor steel during thermal fatigue at 540°C. Detailed information on macro‐failure characteristics and micro‐failure mechanisms was revealed here. The results showed that both intragranular micro‐defects and intergranular micro‐crackings were formed during the fatigue process. The formation of intragranular micro‐defects was related to the dislocation tangle inside the ferrite sub‐grains, while the formation of intergranular micro‐cracking could be related to the weakening of atomic cohesion at the matrix/cementite interface. Eventually, the transgranular crackings occurred in the 30Cr1Mo1V steel. The morphology of these crackings presented an edge‐shaped discontinuity, which may propagate perpendicularly to the loading direction and cause the material to fracture.

Comparison in microstructure, mechanical properties, and fatigue behavior of 9Cr3W3Co turbine rotor steel and its welded joint

AbstractThe microstructure, mechanical properties, and high/very‐high cycle fatigue behaviors of 9Cr3W3Co turbine rotor steel and its welded joint were investigated. The welded nugget zone (WNZ) exhibited the highest microhardness value, followed by the heat‐affected zone (HAZ), while the base material (BM) showed the lowest hardness value. The WNZ exhibited a higher yield strength compared to the BM with reduced elongation, while maintaining similar ultimate tensile strength. The fatigue strength of the BM at 5 × 107 cycles is roughly 53.7% of the yield strength, whereas that of the WNZ increases to approximately 55.0%. In comparison to the fatigue strength of the BM at 5 × 107 cycles, the WNZ exhibits a notable increase of 10.5%. Smaller grain size and higher dislocation density were observed in the WNZ, which contributed to improved microhardness, yield strength, and fatigue resistance through grain boundary strengthening and dislocation hardening.

Dynamic recrystallisation behaviour and intrinsic workability of 12%Cr USC rotor steel during hot deformation

ABSTRACT Hot deformation of 12%Cr USC rotor steel was investigated by hot compression testing over a temperature range of 900 °C to 1,200 °C at strain rates of 0.001 s−1–1 s−1 with the use of a Gleeble–1500D thermal-mechanical simulator. The true stress–strain curves were obtained and showed that the stress increased with a decrease in temperature, an increase in strain rate, or both. Based on the stress–strain curves, processing maps of 12%Cr USC rotor steel under various strains were established, and the Zener-Hollomon (Z) parameter was calculated, it increased with decreasing temperature or increasing strain rate. Combining the values of the Z parameter and the microstructure after thermal compression, it was found that dynamic recovery was the main softening mechanism at lnZ > 39.09, and dynamic recrystallisation (DRX) became the main softening mechanism at 39.09 > lnZ > 32.56. When lnZ < 32.56, DRX occurred completely, and a small amount of recrystallised grains formed. The deformed microstructures showed that 12%Cr USC rotor steel incurred a continuous dynamic recrystallisation mechanism, a discontinuous dynamic recrystallisation mechanism, and a geometric dynamic recrystallisation mechanism during the hot deformation process. The intrinsic workability of 12%Cr USC rotor steel was explored by analysing the processing maps. It was found that the best forging process parameters range of the material in the actual production process was a deformation temperature range of 1,100 °C to 1,200 °C and a strain-rate range of 0.01 s−1–0.1 s−1.

High/very-high fatigue properties and microstructure evolutions of 9Cr3W3Co turbine rotor steel at room temperature and 650 °C

The study aimed to investigate the high and very-high fatigue properties of 9Cr3W3Co turbine rotor steel at room temperature (RT) and 650 °C. The resulting S–N curves demonstrated a continuous downward trend, without exhibiting a conventional fatigue limit. The fatigue strength corresponding to 5 × 107 cycles is approximately 53.9% of yield strength at RT, increasing to around 55.4% at 650 °C. Meanwhile, compared to the fatigue strength at 5 × 107 cycles at RT, it at 650 °C decreased by 53.3%. Surface crack initiation was found to be the main fatigue fracture failure model, observed at both RT and 650 °C. Microscopic analysis revealed that severe dislocation motion and grain deformation occurred within the material matrix during the cyclic loading process at both RT and 650 °C, resulting in the formation of abundant low-angle grain boundaries, dislocation lines and sub-grains. At RT, the presence of the M23C6 precipitated phase imposed a strong pinning effect, while the MX phase within the martensite laths effectively impeded the movement of dislocation lines, thereby enhancing fatigue properties. However, at 650 °C, the emergence of newly precipitated Cu-rich phases was observed, which can reduce the free path of dislocations, hinder slip motion of dislocations, and thus compensate for the negative effect caused by high-temperature microstructural degeneration.

The Modeling and Simulation of Austenite Grain Growth in 25Cr2Ni4MoV Nuclear-Power Rotor Steel

The modeling of austenite grain growth of 25Cr2Ni4MoV steel for super-large nuclear-power rotors was investigated during the common heating process including the continuous heating and isothermal heating process. Based on the isothermal grain growth model considering the steady-state grain size and the rule of additivity, a new grain growth model during the continuous heating process was established. The comparison between experimental and predicted results indicates the model has good predictability. To describe the anisotropic and isotropic grain growth during the different isothermal heating stages of the super-large nuclear-power rotor, a cellular automaton model considering anisotropic grain boundary energy for grain growth of 25Cr2Ni4MoV steel was developed. It is found that the anisotropic grain boundary energy mainly exists in the early isothermal heating stage at lower temperatures, and the normal grain growth occurs under anisotropic grain boundary energy conditions. When the temperature is not less than 1273 K and the cellular automaton step is not less than 15, the normal grain growth containing only isotropic grain boundary energy occurs. The analysis of the morphology, energy variance, topology and growth kinetics further indicates that normal grain growth of 25Cr2Ni4MoV steel can be simulated fairly well by the present CA model.

Modeling of the circumferential crack growth under torsional vibrations of steam turbine shafting

Long-term operation of steam turbines has one of its consequences the formation of fatigue cracks of various types in their structural elements. Evidence of this are accidents and catastrophic destructions of steam turbines.Maine reason of long-term accumulation of fatigue damage in steam turbines shafting is torsional vibrations caused by the connection of turbogenerator to an electrical network with an inaccurate synchronization. A methodology is proposed to study the process of circumferential crack growth in the turbine shafting because of many such connections. The methodology is based on the use of the 3D finite element model of the steam turbine K-200–130 shafting and a fracture mechanics approach.There was demonstrated that the inaccuracy of turbogenerator connection to an electric network is the most influential factor affecting the crack growth. The conditions for the crack to reach a critical size has been evaluated. In the problem being considered damping capacity of the rotor steel influences the intensity of crack growth insignificantly.

Electrochemical corrosion analysis of wire arc additive manufactured rotor steel weldments subjected to heat treatment

Electrochemical corrosion analysis of wire arc additive manufactured rotor steel weldments subjected to heat treatment

Stress corrosion cracking growth rate prediction model for nuclear power turbine rotor steel in a simulated environment

Considering the electrochemical reaction and chemical reaction near the crack in the local corrosion environment, the stress corrosion cracking growth rate prediction model for nuclear power turbine rotor steel was established based on the slip dissolution mechanism and finite element method. The calculated parameters were defined based on the material and electrochemical properties of steam turbine rotor in the simulated environment, and the calculated results can agree well with the experiment results. Results revealed that the stress intensity factor has an important influence on the local corrosion environment at the crack tip, the electrode potential is decreased and the process of anodic dissolution is accelerated with the increase of the stress intensity factor. Furthermore, the prediction model can describe stage-I and stage-II of stress corrosion cracking propagation well, and provide a possible trend prediction for practical production practice.

Increasing Turbine Hall Safety by Using Fire-Resistant, Hydrogen-Containing Lubricant Cooling Liquid for Rotor Steel Mechanical Treatment

This paper is devoted to the development of hydrogen-containing, environmentally safe, fire-resistant, and corrosion-protected lubricant cooling liquids (LCLs) from vegetable oils with improved sanitary and hygienic parameters for the machining of parts and equipment made from high-strength steels for application during the interoperation period in turbine halls. The use of plant raw materials as ecologically and fire-safe LCLs increased the efficiency of LCLs when evaluating drilling steel in terms of the dependence of the stability of the cutting tool on the drilling speed. Chips formed from LCLs during turning had a compact, cylindrical appearance, and the addition of both water and coolant during turning significantly changed the morphology of the cutting particles. Using water and LCL intensified the physical and chemical destruction processes. After the use of water and LCL, the concentration of hydrogen in the cutting products of 38KHN3MFA steel increased, which indicated its participation in facilitating the destruction during machining. In the chips formed when using LCL, the amount of hydrogen increased by 2.25 times compared to the chips obtained with the dry treatment, while with coolants, it increased by 2.6 times, indicating the intense flow of decomposition products of LCL through diffusion processes in the cutting zone. Hydrogen reduces the energy costs for the destruction of structural and phase components and promotes their dispersion. The creation of 2D and 3D images allowed for a more detailed approach to the study of the influence of LCL on surface treatments.

Effect of Grain Size and Compression Direction on the Hot Deformation Characteristics of High-Cr Ultra-Super-Critical Rotor Steel with Columnar Grains

Effect of Grain Size and Compression Direction on the Hot Deformation Characteristics of High-Cr Ultra-Super-Critical Rotor Steel with Columnar Grains

A Probabilistic Model for Forging Flaw Crack Nucleation Processes for Heavy Duty Gas Turbine Rotor Operations

Abstract We present a probabilistic model for quantifying the number of load cycles for nucleation of forging flaws—for a 3.5NiCrMoV high strength low alloy rotor steel—into a crack under gas turbine operating conditions. The model correlates low cycle fatigue data, ultrasonic testing indication data, flaw morphology, and type with the nucleation process. This paper is the third of a series of publications presenting this modeling approach progressively. It focuses on the effect of temperature variation on the nucleation life of forging flaws. We quantified the number of cycles to crack nucleation was for specimens that included forging flaws at elevated temperatures. Flaws of different sizes and shapes are effectively described at respective temperature and stress levels by either an ellipsoidal finite element model or an analytical area-based model. A local probabilistic low-cycle fatigue model analyzes the resulting stress distributions accounting for statistical size effects. Via Maximum Likelihood Estimation of these probabilistic low cycle fatigue results, a probabilistic model for crack nucleation of forging flaws is obtained. This proposed probabilistic model is based on experimental data for realistic heavy duty gas turbine rotor temperature and stress conditions. It can be utilized in the energy sector for component life time quantification. Our suggested approach can support component assessment under flexible gas turbines operation conditions driven by increased availability of intermittent renewable energy sources.

Viscosity and surface tension of CaF2–CaO–Al2O3-based slag with varying SiO2 and B2O3 contents for ESR of rotor steel

Viscosity and surface tension of CaF2–CaO–Al2O3-based slag with varying SiO2 and B2O3 contents for ESR of rotor steel

Effect of CaF2 and Li2O on structure and viscosity of low-fluoride slag for electroslag remelting of rotor steel

The structure and viscosity of CaF2-CaO-Al2O3-B2O3-SiO2-Li2O slag with varying CaF2 and Li2O contents were investigated by Raman spectroscopy, 27Al, 29Si and 11B magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy, and rotating cylinder method, respectively. The relative fraction ofQAl2 significantly increases, the fractions ofQAl3 andQAl4 decrease with increasing the CaF2 or Li2O content. Aluminum exists in the form of four-coordinated aluminum (AlⅣ) corresponding to AlO4 and a small amount of AlO3F. The fraction ofQSi0 species increases at the expense ofQSi1,QSi2(1Al) andQSi2 with increasing the CaF2 or Li2O content. B is present in the form of BⅢ, besides 0.57% of BIV in the glass with 14.69mass% CaF2. The relative fraction of T0 increases and T1 decreases as the CaF2 or Li2O content is increased. The degree of polymerization of aluminosilicate and borates decreases with increasing the CaF2 or Li2O content, which is consistent with the results of viscosity and viscous activation energy.

Improvement of the Mechanical Characteristics, Hydrogen Crack Resistance and Durability of Turbine Rotor Steels Welded Joints

This article is devoted to the following issues: calculating the values of temperatures obtained by simulating welding heating and the subsequent implementation of the welding process at the given mode parameters made it possible to obtain a welded joint of the rotor with an improved initial structure and increased mechanical properties, hydrogen resistance and durability by up to 10–15%; simulating welding heating in the areas of fusion, the overheating and normalization of the HAZ and the formation of austenite grains; specified welding heating creates the conditions for the formation of new products of austenite decomposition in the form of sorbitol in the area of the incomplete recrystallization of the HAZ. In air and gaseous hydrogen, the destruction of the combined joints took place on the weld metal, as well as on the fusion areas, the overheating and the incomplete recrystallization of the HAZ of 20H3NMFA steel as the base metal. Structural materials have a relatively low strength and high fracture toughness in air. This is manifested in a significant reduction in the elongation (δ), the area (ψ) and critical stress intensity factor (KIc) of welded joints and the endurance limit of cylindrical smooth rotor steel specimens, which were cut from transverse templates. Welded joints in the whole range of load amplitudes are sensitive to the action of hydrogen.

An enhanced lemaitre damage model and verification method for X12 alloy steel in hot forming process

One major issue is accurate prediction and effective control of crack defects, which is one of the key scientific problems in developing high-quality rotor forgings. As a result, an enhanced Lemaitre continuous damage model considering the effect of stress triaxiality, strain rate and temperature was put forward in this paper, aiming at predicting the crack initiation of rotor steel during forging process at elevated temperature. The parameters of Lemaitre continuous damage model were inversely obtained by genetic optimization algorithm according to the data of tensile test. Then, the proposed damage model was integrated into the finite element software FORGE® to delineate the hot deformation of X12 alloy steel. In order to verify the accuracy of the model, V-notch cylindrical specimens and notched tensile specimens were designed for isothermal compression test and tensile test. Digital image correlation (DIC) technology was used to capture the strain field and displacement field of specimen under elevated temperature conditions. The accuracy of the model has been verified by comparing the test results with the finite element simulation results. Finally, the down-ratio forging test of the ultra-supercritical high and medium pressure rotor was carried out. The established elevated temperature damage model was used for researching the crack defects of ultra-pressure rotor steel in forging process. By comparing the location of crack defects between down-ratio forging test and simulation, which further verified the accuracy of the proposed damage model. The results indicate that the enhanced damage model can predict the crack defects of X12 alloy steel during its hot forging process.

Permanent Magnet Generator for a Gearless Backyard Wind Turbine

This paper presents the design of a permanent magnet generator for a gearless backyard wind turbine. The magnetisation characteristics of the rotor steel and the stator at different field strength ranges were considered at the design stage and mathematically described using a model in Matlab. The detailed calculations and the design of the planar model were carried out using FEMM software. The high-quality results obtained from the calculations shown in the paper made it possible to make a real model of the generator. This paper presents views of the stator package, the rotor, the entire generator and selected test results. The parameter of this turbine that distinguishes it from a wide range of manufactured generators is its low, non-standard rotational speed and low breakaway torque, which allows the power plant to start in winds of approximately 2 m/s. Other advantages of this generator is its low weight resulting from the use of a light rotor and light alloys for the generator housing.

SIMULATION OF FATIGUE CRACK GROWTH DURING TRANSVERSE VIBRATIONS OF A TURBINE SHAFT

In real operational conditions structural elements of steam turbines are subjected to a wide range of thermal and mechanical loading. Even substantial reserve of static and dynamic strength, laid down at the stage of turbine design, can not prevent the appearance of fatigue cracks in structural elements, which lead to catastrophic failures. One of the reasons of damage in structural elements of turbine is technological operations used in the process of manufacture (forging, turning, and milling, heat treatment), since they are accompanied with plastic deformation of material, which is the physical basis of the so-called distributed fatigue damage. It accumulates during long-term cyclic deformation and turns into local damage of a fatigue crack type. In addition, the appearance of cracks in turbine shafts is caused by complex geometry, that is, by the presence of fillets and grooves, which are stress concentrators and, therefore, potential areas of initiation and growth of fatigue cracks. The high pressure rotor of the K-200-130 steam turbine was used to simulate the process of crack growth at forced transverse vibrations of the rotor when it passes through the first critical speed. At this the amplitude-dependent energy dissipation typical for metallic materials was taken into account. There was estimated the maximum stresses arising in the rotor when it passed through the critical speed rotation and the number of loading cycles leading to the crack growth. It was assumed that a crack with a depth of about 1 mm has formed on the surface of the rotor, which is the maximum permissible depth according to the instructions for safe operation of the turbine. The growth rate of this crack is predicted based on the fracture mechanics approaches through the determined maximum stresses in the section with a crack and experimental dependences of the crack growth rate on the stress intensity factor range. Based on the model, the crack growth time is predicted until the rotor loses its bearing ability. Predictions are made for different scenarios of loading and mechanical properties of rotor steel.