Rotor Steel Research Articles

Investigation on the fracture behaviour of wire arc additive manufactured rotor steel weldments after heat treatment

This paper discusses the additive welding of Cr-Mo steel weldment carried out with Cold Metal Transfer Technology (CMT). The mechanical characterization including fracture behaviour and metallography of the Wire Arc Additive Manufacture (WAAM) weldments in the As Welded (AW) and Post Weld Heat Treated (PWHT) conditions were investigated. The longitudinal and transverse orientations parallel to building and deposition directions respectively were found to influence the fracture behavior of weldments. The specimens of longitudinal direction showed lower toughness and tensile strength when compared to the specimens of transverse direction both in the case of AW and PWHT weldments. Orientation was also observed to influence the elastic–plastic fracture toughness (J integral) values both in the as welded and heat-treated specimens. Fracture topographical analysis of specimens carried out by Scanning Electron Microscope (SEM) in association with Energy Dispersive Spectroscopy (EDS) showed the presence of soft ductile features and Mn deposition respectively in the specimens of transverse direction and justified the findings.

Improving cleanliness of 30Cr2Ni4MoV low-pressure rotor steel by CaO–SiO2–MgO–Al2O3 slag refining

Improving cleanliness of 30Cr2Ni4MoV low-pressure rotor steel by CaO–SiO2–MgO–Al2O3 slag refining

Stress corrosion crack propagation affected by microstructures for nuclear steam turbine rotor steels in the simulated environment

The propagation behavior of stress corrosion cracking (SCC) for two CrNiMoV steels in the simulated environment of low-pressure nuclear steam turbine rotor was systematically investigated by microstructural characterizations, electrochemical measurements, slow strain rate tensile tests, and crack growth rate (CGR) measurements. The results showed that both the corrosion susceptibility and stress corrosion susceptibility increased with the increase of localized residual strain, yield strength, and the decrease of grain size. With the higher stress corrosion susceptibility, the SCC CGR of Steel A was higher than Steel B in the corrosive solution at 80 °C, however, at 180 °C the SCC CGR of Steel A was lower than Steel B under the lower applied K. The change of SCC CGR was strongly affected by crack branches, crack deflections and plastic deformation during the SCC propagation, which may have some correlations with the microstructural distribution.

Narrow Gap MAG Welding and Joint Performance Analysis of 25Cr2NiMo1V Thick Plate

The narrow gap MAG welding system was used to successfully weld the 50mm thick butt joint of 25Cr2NiMo1V rotor steel. After 15-layer bead welding, heat treatment is performed on the welded joint. Compare the changes in the microstructure, tensile strength and impact energy of the welded joints and the heat-treated joints at 580°C (20h). The results show that after the heat treatment of the structure, the side lath ferrite in the coarse-grained region grows up, and the eutectoid ferrite grows up in the fine-grained region first. The strength of the welded joint is about 605MPa, and the fracture is characterized by ductile fracture. After heat treatment at 580°C (20h), the strength is about 543MPa, the fracture is characterized by ductile fracture, and there are also a large number of discontinuous small surface platforms, and the characteristic of brittle fracture appears slightly. The impact energy of the weld center of the welded joint is about 141J, the fusion line area is about 113J, and the toughness of the fusion line is slightly lower than that of the weld center. After heat treatment, the impact energy at the center of the weld is about 183J, the fusion line area is about 95J, the toughness of the weld center increases, and the toughness of the fusion line decreases.

Rotor steels crack resistance and fracture behavior for hydrogen targeted materials ever-widening database

It has been investigated the influence of hydrogen at pressure up to 10 MPa (and absorbed hydrogen with concentration up to 8.1 ppm) on the strength, ductility, crack resistance of 35NiCrMoV12-5 (38KhN3MFA) rotor steel specimens with different chemical composition and tempering modes. Due to hydrogen embrittlement, the high-pressure gaseous hydrogen (up to 10 MPa) has the most significant influence on rotor steel properties under static tensions, namely relative transverse elongation, reduction of area in smooth cylindrical specimens, fracture toughness in compact samples, and less - on the critical stress intensity factor (SIF) Kfc at cyclic loading. The hydrogen influences the plasticity, static, and cyclic fracture toughness parameters reach the maximum for the ambient temperature. The hydrogen impact on the listed parameters becomes negligible at 530 K. With the increase of the annealing temperature from 923 up to 1023 K structure with larger structural components has formed, lower local stresses increases, the flow boundary decreases and the sensitivity to hydrogen embrittlement under all types of the load is reduced. The SIF diagrams (at static and cyclic loading) were constructed, the crack growth rates on plateau sections were determined. The Paris equation parameters were obtained; all above are valuable tools for the rotors' lifetime prediction. As deducted from stress and strain-controlled S-N curves, hydrogen decreases the high-cycle durability of rotor steel. Hydrogen also decreases the low-cycle durability of rotor steel. The degree of hydrogen effect increases with strain amplitude decrease.

Crack nucleation at forging flaws studied by non-local peridynamics simulations

We present a computational study and framework that allows us to study and understand the crack nucleation process from forging flaws. Forging flaws may be present in large steel rotor components commonly used for rotating power generation equipment including gas turbines, electrical generators, and steam turbines. The service life of these components is often limited by crack nucleation and subsequent growth from such forging flaws, which frequently exhibit themselves as non-metallic oxide inclusions. The fatigue crack growth process can be described by established engineering fracture mechanics methods. However, the initial crack nucleation process from a forging flaw is challenging for traditional engineering methods to quantify as it depends on the details of the flaw, including flaw morphology. We adopt the peridynamics method to describe and study this crack nucleation process. For a specific industrial gas turbine rotor steel, we present how we integrate and fit commonly known base material property data such as elastic properties, yield strength, and S-N curves, as well as fatigue crack growth data into a peridynamic model. The obtained model is then utilized in a series of high-performance two-dimensional peridynamic simulations to study the crack nucleation process from forging flaws for ambient and elevated temperatures in a rectangular simulation cell specimen. The simulations reveal an initial local nucleation at multiple small oxide inclusions followed by micro-crack propagation, arrest, coalescence, and eventual emergence of a dominant micro-crack that governs the crack nucleation process. The dependence on temperature and density of oxide inclusions of both the details of the microscopic processes and cycles to crack nucleation is also observed. The results are compared with fatigue experiments performed with specimens containing forging flaws of the same rotor steel.

Low cycle fatigue properties assessment for rotor steels with the use of miniaturized specimens

The article deals with the development of advanced techniques of low cycle fatigue testing (LCF) using miniature test samples, which has the potential to be used to assess the degree of material degradation of large in-service components in the power industry. For this purpose, the results of a wide portfolio of test samples geometries have been investigated. Using the advanced techniques involving a special sampling device, a DIC virtual extensometer and test rig, a new possibility in strain-controlled LCF testing is presented. The results indicated that cyclic-strain and strain-life curves of proposed minisample geometries are almost of the same shape for both investigated rotor steels. Also the cyclic parameters obtained indicate a good consistency of results despite of a wide range of sample volumes and utilization of two different strain-control methods.

Effect of 580 °C (20 h) Heat Treatment on Mechanical Properties of 25Cr2NiMo1V Rotor-Welded Joints of Oscillating Arc (MAG) Narrow Gap Thick Steel.

The thick plate narrow gap welding of 25Cr2NiMo1V rotor steel is achieved by metal active gas arc welding, in which the weld gap was 18.04–19.9 mm. After welding, the weldment was heat treated at 580 °C (20 h). The impact and tensile properties in the as-welded and heat-treated were studied. The results show that after heat treatment, the coarse carbides in the center of the weld were transformed into fine granular carbides distributed along the grain boundaries, and the quantity of carbide precipitates in the weld near the fusion line was reduced. The tensile fracture mode changed from a ductile fracture to a combination of brittle and ductile fractures, and the tensile strength of the weld metal changed from 605 MPa to 543 MPa. After heat-treated, the radiation zone of the weld center changed from a brittle fracture to a combination of brittle and ductile fractures, and the impact energy changed from 141 J to 183 J; the characteristics of the brittle fracture in the radial zone of the fusion line were more obvious, and the impact energy changed from 113 J to 95 J. Therefore, after heat treatment, the toughness of the welded metal was improved, without reducing the strength and hardness of the welded metal to a large extent.

Identification of the Constitutive Model Parameters by Inverse Optimization Method and Characterization of Hot Deformation Behavior for Ultra-Supercritical Rotor Steel.

X12 (X12CrMoWVNbN10-1-1) ferritic heat resistant steel is an important material for the production of new-generation ultra-supercritical generator rotors. Hot compression tests of X12 ferritic heat-resistant steel were performed via a Gleeble-1500D testing machine under temperatures of 1050–1250 °C and strain rates of 0.05–5 s−1. In order to provide material model data for finite element simulations and accurately predict the hot deformation behavior, a reverse optimization method was proposed to construct elevated temperature constitutive models of X12 ferritic heat-resistant steel in this paper, according to the Hansel–Spittel constitutive model. To verify the accuracy of the model, the predicted and experimental values of the constitutive model were compared. The results indicated that the model had a high prediction accuracy. Meanwhile, the correlation coefficient between the experimental value and the predicted value of constitutive model was 0.97833. For further verification of the accuracy of the model, it was implemented in finite element FORGE@ software to simulate the compression tests of different samples under different conditions. Comparing actual displacement–load curves with displacement–load curves acquired through finite element simulations, the results indicated that displacement–load curves predicted by the model were very consistent with actual displacement–load curves, which verified the accuracy of the model. Moreover, to research the optimal processing parameters of the material, hot processing maps were drawn according to the dynamic material model. In terms of microstructure evolution, a characteristic area distribution map of the hot processing map was established. Therefore, the optimal hot forming parameters regions were in the range of 1150–1200 °C/0.05–0.62 s−1 for X12 ferritic heat-resistant steel.

Hot Workability of Ultra-Supercritical Rotor Steel Using a 3-D Processing Map Based on the Dynamic Material Model.

As a new-type of ultra-supercritical HI-IP rotor steel, X12CrMoWVNbN10-1-1 alloy steel has excellent integrative performance, which can effectively improve the power generation efficiency of the generator set. In this paper, uniaxial thermal compression tests were carried out over a temperature range of 950–1200 °C and strain rates of 0.05–5 s−1 with a Gleeble-1500D thermal simulation testing machine. Moreover, based on hot compression experimental data and the theory of processing diagrams, in combination with the dynamic material model, a three-dimensional (3-D) thermal processing map considering the effect of strain was constructed. It was concluded that optimum thermal deformation conditions were as follows: the temperature range of 1150–1200 °C, the strain rate range of 0.05–0.634 s−1. Through secondary development of the finite element (FE) software FORGE®, three-dimensional thermal processing map data were integrated into finite element software FORGE®. The distributions of instability coefficient and power dissipation coefficient were obtained over various strain rates and temperatures of the Ø 8 × 12 mm cylinder specimen by using finite element simulation. It is shown that simulation results are consistent with the microstructure photos. The method proposed in this paper, which integrates the three-dimensional processing map into the finite element software FORGE® (Forge NxT 2.1, Transvalor, Nice, France), can effectively predict the formability of X12CrMoWVNbN10-1-1 alloy steel.

Austenite grain growth kinetics and mechanism of grain growth in 12Cr ultra-super-critical rotor steel

ABSTRACT In this paper, the austenite grain growth behaviour of 12Cr ultra-super-critical (USC) rotor steel was investigated by a series of heat treatments. The heat treatments at heating temperatures of 900°C–1250°C and holding time of 1 h–20 h were conducted in an electric box-type heating furnace. Experimental results showed that the sizes of austenite grain were affected by heating temperatures and holding time, and heating temperature was the dominant factor. In addition, the grain growth rate changed significantly before and after the turning points of 1050°C and 1250°C. Meanwhile, an austenite grain growth mathematical model was established at different heating temperature stages, and possession of the capability to accurately predict austenite grain size was confirmed. Furthermore, the microstructure of austenite grain in the heating process was observed by optical microscope (OM) and transmission electron microscopy (TEM), which revealed the mechanism of austenite grain growth. Analysis indicated that the change of quantity of precipitate particles with increasing heating temperature was the main reason for the difference in austenite grain growth.

Risk-Based Planning of Diagnostic Testing of Turbines Operating with Increased Flexibility

An increase in the share of renewable sources in the energy mix makes coal-fired power plants operate in new conditions that require more dynamic operation and adequate flexibility. The frequency of the power unit start-ups increases and so does the frequency of changes in loads. This intensifies some life consumption processes, such as low-cycle fatigue and crack propagation in the turbine components. Further operation of power unit elements that have already been in service for a long time has to be supplemented with new diagnostic and repair procedures that take into account the intensification of life consumption processes. This article gives predictions about the propagation rate of potential cracks in the turbine rotor for different scenarios of the power unit’s long-term operation. A method is presented of rational selection of the diagnostic testing time based on risk analysis. The method is used to estimate the optimal interval after which diagnostic testing of a 200 MW turbine rotor should be carried out. Changes in the rotor steel crack toughness are evaluated based on the results of testing of microspecimens cut out of the rotor. Turbines with more frequent start-ups and shorter start-up times necessitate performance of diagnostic testing of the rotor central bore after about 12 years of turbine operation.

Modeling of Metadynamic Recrystallization Kinetics and Recrystallization Mechanisms of V-Containing 12Cr Rotor Steel

In order to study the metadynamic recrystallization (MDRX) behavior of 12Cr rotor steel, the double-hit hot compression tests were performed on a Gleeble1500D thermal simulator with temperatures of 1273-1373 K, strain rates of 0.001-0.05 s−1 and inter-pass times of 5-60 s. Based on the true stress–strain curves obtained at different deformation conditions, the effects of different deformation parameters on the MDRX softening behavior are discussed. Meanwhile, the microstructure was observed using an optical microscope, electron backscatter diffraction and transmission electron microscopy. Analysis showed that recovery was the main softening mechanism, followed by recrystallization. And the bulging at the grain boundary was the fundamental nucleation mechanism for MDRX. Furthermore, the kinetics model for MDRX of 12Cr rotor steel was established, which was verified to have an accurate capability of predicting MDRX softening behavior with a correlation coefficient of R = 0.9956.

Comparative evaluation of LCF behavior of dissimilar steels and welds in an ultra-supercritical turbine rotor at 280 ℃

Buttering layer (BL) was used to weld two ultra-supercritical turbine rotor steels. Low-cycle fatigue behavior of different regions in the dissimilar metal welded joint (DMWJ) was compared and evaluated at 280 °C. Microstructure, fracture and weak link were observed by OM, SEM, EDS and TEM. BL had lower fatigue life than weld metal (WM) and base metals (BMs). Weld zone showed cyclic softening characteristic. Microstructures evolved into cell structures after LCF tests. Fatigue crack in weld zone propagated mainly through coarse carbides. The interface near BL was the weakest link of the DMWJ due to ferrite layer and strength mismatch.

A modified Bonora damage model for temperature and strain rate-dependent materials in hot forging process

The initiation and propagation of damage are sensitive to temperature and strain rate, especially for ultra-super-critical rotor steel with high strength at elevated temperature, which probably leads to premature failure in the hot forming process. To accurately describe the damage behavior, a novel damage model incorporating temperature and strain rate was proposed based on Bonora damage model. To establish this damage model, a GA (genetic algorithm)-based inverse method was designed for identifying four damage parameters in Bonora damage model by correlating experimental and simulation data in the tensile tests at elevated temperatures and different strain rates. And then these damage parameters as functions of temperature and strain rate were determined. The proposed damage model was integrated into a finite element software FORGE to simulate the damage evolution of a notched upsetting specimen and the hot tensile specimens. The damage from compressive to tensile states and the fracture behavior were properly predicted, respectively. Additionally, the digital image correlation (DIC) technique was employed to capture the crack initiation and propagation on a flate notched X12 ultra-super-critical rotor steel specimen, which were compared with simulation results to further verify the accuracy of the proposed damage model.

Microstructure Characterization of SAW and TIG Welded 25Cr2Ni2MoV Rotor Steel Metal

Low pressure turbine rotors are manufactured by welding thick sections of 25Cr2Ni2MoV rotor steel using tungsten inert gas (TIG) backing weld, and submerged arc welding (SAW) filling weld. In this study, the microstructure of columnar grain zones and reheated zones in weld metal was characterized meticulously by Optical Microscope (OM), Scanning Electron Microscope (SEM) and Electron Back-Scatter Diffraction (EBSD). The results showed that, compared with SAW weld metal microstructure, TIG weld metal microstructure was relatively fine and homogeneous, due to its lower heat input and faster cooling rate than SAW. The maximum effective grain size in TIG and SAW weld were 7.7 μm and 13.2 μm, respectively. TIG weld metal was composed of lath bainite (LB) and blocky ferrite (BF), while SAW weld metal was composed of acicular ferrite (AF), lath bainite (LB)and ferrite side plate (FSP). Tempered martensite (TM) was detected along columnar grain boundaries in both TIG and SAW weld metals, which was related to the segregation of solute elements during weld solidification. Electron Probe Micro-Analysis (EPMA) results showed that the contents of Ni and Mn at the dendritic boundaries were 50% higher than those at the dendritic core in TIG weld. Similarly, 30% of Ni and Mn segregation at dendritic boundaries was also found in SAW weld. In addition, the microhardness of the two welded joints was tested.

A modified kinetics model and softening behavior for static recrystallization of 12Cr ultra-super-critical rotor steel

In this paper, the static recrystallization (SRX) of 12Cr ultra-super-critical (USC) rotor steel was investigated by a series of hot compression tests on a Gleeble1500D thermal simulator. Double-hit hot deformation tests were conducted at temperatures of 1223K–1323 K and strain rates of 0.001 s−1–0.1 s−1 with inter-pass times of 10s–180 s. A conventional kinetics model of SRX was established based on flow curves obtained by regression analysis under different deformation conditions. However, a significant deviation resulted between predicted and experimental values for the SRX fraction, and therefore a modified kinetics equation was proposed by analysing the SRX characteristics and possession of the capability to accurately predict SRX softening behaviour was confirmed. Effects of hot deformation parameters on SRX softening behaviour and SRX grain size were discussed. Furthermore, microstructure deforming was observed at different inter-pass times with an optical microscope (OM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). Analysis showed that recovery was the main softening mechanism and the fundamental nucleation mechanism for SRX was bulging at the grain boundary.

High temperature strain heterogeneities tracking within hole-specimens of FV566 turbine steel via digital image correlation

This paper presents experimental and numerical investigations to study spatial-temporal distributions of strain heterogeneities within a FV566 turbine rotor steel subjected to high temperature conditions. Strain field were determined during monotonic tensile tests performed on flat specimens containing a circular hole to give rise to heterogeneous strain fields. A home-made digital image correlation system with an image restoration module was developed and employed to record and analyse the deformation process till the macroscopic fracture of the investigated specimens. Noticeable in-plane strain concentration zones and effects of the sample design were highlighted and deeply examined. Experimental results combined with finite element analyses based on a classical Johnson-Cook model indicated that the hole shape was the important factor affecting the strength and the spatio-temporal distribution of the strain fields. Microstructural investigations were also conducted to characterize and assess the microscale damage features of the FV566 steel. The ruptured area for all the investigated temperature tests revealed ductile rupture behaviour. Changes on grain boundaries and misorientation near strain localized regions were also investigated. It was found that the propagation of cracks surrounding the hole in FV566 specimens was mainly affected by the nucleation and propagation of strain localization zones.

Low cycle fatigue life modelling using finite element strain range partitioning for a steam turbine rotor steel

Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, 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 at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.

Critical condition parameters and kinetics of dynamic recrystallization for hot deformed 1 wt%Cr-1 wt%Mo rotor steel

Isothermal hot compression tests of low alloy 1 wt%Cr–1 wt%Mo medium carbon rotor steel were carried out over the temperature range from 850 °C–1050 °C under three strain rates varying from 0.001 s−1–0.1.1 s−1. The hot deformation behavior, activation energy and material constants were determined by constitutive analysis. Onset of critical condition parameters for dynamic recrystallization (DRX) of the present steel were determined with the help of mathematical models developed by Poliak and Jonas. DRX kinetics were studied to determine the fraction of dynamic recrystallization with the help of Avrami type model. Also, the values of Avrami power exponent and material constant were calculated for Avrami equation in each deformation condition. Optical and scanning electron (SEM) micrographs of the as-received and the deformed specimens were analyzed to understand the flow stress behavior of this steel.