Steam Turbine Materials Research Articles

Effect of chloride and sulfate ions on crevice corrosion behavior of low-pressure steam turbine materials

Crevice corrosion behavior between homogeneous and dissimilar materials of 13Cr and 3.5NiCrMoV steels was investigated in simulated boiler waters containing chloride ions (Cl−) and sulfate ions (SO42−), using open circuit potential (OCP) measurements in addition to morphology analysis. Pitting occurred inside the crevice not only on 13Cr steel but also on 3.5NiCrMoV steel in the test water containing 100 ppm Cl−, while it was suppressed by the coexistence of 50 ppm SO42− in the water with the 100 ppm Cl−. However, the presence of 50 ppm SO42− in the test waters promoted uniform corrosion on 3.5NiCrMoV steel inside the crevice.

Numerical modelling of the fatigue crack shape evolution in a shot-peened steam turbine material

In this study, the short crack initiation and growth behaviour in a notched sample under low-cycle fatigue (LCF) was investigated in a low-pressure steam turbine material. Different crack initiation mechanisms and crack shape evolution processes were experimentally observed in samples subjected to different surface treatments: polished, T0 (industrially applied shot peening process) and T1 (a less intense shot peening process). To better understand the effects of shot peening on fatigue, a 3D finite element (FE) model was developed to investigate the interaction between crack growth and the effects induced by shot peening. Firstly, residual stress redistribution caused by both mechanical loading and the presence of a crack was numerically investigated. This model was also used to successfully predict the differences in crack shape evolution between varying surface conditions, and quantified the retardation of short crack growth behaviour resulting from shot peening. Finally, the 3D model introduced in this study was compared with a previously developed 2D model with plane strain assumptions to demonstrate the limitation of the 2D model in simulating the crack growth behaviour, and to emphasise the importance of taking the 3D crack shape into account when evaluating the short crack growth behaviour.

Crevice Corrosion of Low-Pressure Steam Turbine Materials in the Boiler Water Contained Impurity Ions

Crevice corrosion of 3.5NiCrMoV and 13Cr steels, which are used as low-pressure (LP) steam turbine materials, was investigated by electrochemical corrosion tests in the simulated boiler water contained chloride and sulfate ions. For 3.5NiCrMoV steel, by comparison with the surfaces outside crevice, the surfaces inside crevice of the specimens coupled with both of the same steel and 13Cr steel showed no remarkably corroded pattern even though pitting corrosion was observed. The specimen of 13Cr steel coupled with the same steel plate exhibited pitting corrosion inside the crevice, and a lower open cycle potential (Ocp) than the single plate of 13Cr steel. On the other hand, the specimen of 13Cr steel coupled with 3.5NiCrMoV steel plate showed the lowest Ocp, as the anodic dissolutions of 3.5NiCrMoV steel became the dominate corrosion mechanism.

A numerical study of the effects of shot peening on the short crack growth behaviour in notched geometries under bending fatigue tests

The current paper presents a numerical analysis of the effects of shot peening on short crack growth in a low pressure (LP) steam turbine material, FV448. The fatigue behaviour of this material has been experimentally evaluated using a U-notched specimen (representing the fir tree root geometry of the turbine blade) under 3-point bend tests. Two different shot peening intensities were considered in this study: an industrially applied shot peening process and a less intense shot peening process.In the modelling work, a 2-D finite element (FE) model with static short cracks has been developed, incorporating both compressive residual stress and strain hardening distribution effects caused by shot peening. Both linear-elastic (LEFM) and elasto-plastic (EPFM) fracture mechanics were used to characterise the crack driving force in the un-peened and shot-peened conditions, taking into account the effects of stress redistribution caused by residual stress relaxation and crack opening. The stress intensity factor used in the LEFM approach was calculated using the weight function method, and the equivalent stress intensity factor used in the EPFM approach was calculated from the J-integral, which was evaluated using the cracked FE model. These results could explain the mechanism of (experimentally observed) retardation of crack growth through the shot-peening-affected layer and also quantified this influence on fatigue life. The relative contributions of compressive residual stresses and strain hardening were assessed by investigating them separately. The sub-surface compressive residual stress distribution produced by shot peening could effectively reduce crack propagation but the strain hardening distribution, in contrast, can accelerate it. However, strain hardening is expected to hinder the crack initiation process by restricting the plastic deformation during cyclic loading. Predictions of the fatigue life of the shot-peened notched specimens were made based on this numerical analysis. Acceptable results were obtained using both the LEFM and EPFM approaches and the difference between them is discussed.

Low cycle fatigue life prediction in shot‐peened components of different geometries—part I: residual stress relaxation

AbstractIn this study, the residual stress relaxation behaviour occurring during low‐cycle fatigue in shot‐peened specimens with either a flat or a notched geometry has been studied. A representative low‐pressure steam turbine material, FV448, was used. The residual stress and strain hardening profiles caused by shot peening were measured experimentally and were then incorporated into a finite element model. By allowing for both effects of shot peening, the residual stress relaxation behaviour was successfully simulated using this model and correlated well with the experimental data. Although more modelling work may be required to simulate the interaction between shot peening effects and external loads in a range of notched geometries, the model predictions are consistent with the specimens tested in the current study. The novelty of this study lies in the development of such a modelling approach which can be used to effectively simulate the complex interaction between shot peening effects and external loads in notched regions. Compared with the un‐notched geometry, the notched geometry was found to be more effective in retaining the improvement in fatigue life resulting from shot peening, by restricting the compressive residual stress relaxation during fatigue loading.

Effect of Corrosion on the Low-cycle Fatigue Strength of Steels used in Frequent Start-up Power Generation Steam Turbine

The practical importance of fatigue failure in steam turbine materials has directed many experimental research towards assessing the physical reason for material sensitivity to corrosion fatigue and providing design rules for engineers. Metallic materials used in steam turbine are exposed to cyclic loading at high temperature and steam environment, during their service life. In this study, an original fatigue testing setup was developed to investigate the effect of aqueous solutions and temperature on the fatigue strength on the martensitic stainless steel X19CrMoVNbN11-1 used for rotating blades and (ii) a cast G17CrMoV5-10 steel used for casing. Fatigue tests were carried out in two environments: (i) in air at 90°C and (ii) in distilled water at 90°C (pH = 7.2 and 02 = 3 ppm) at a loading frequency of 1Hz. Internal and surface crack initiation are observed in air at 90°C, whereas in purified water at 90°C, the crack initiated in the surface at corrosion defects. The decrease observed in the corrosion fatigue strength of specimens was more important at high plastic strain level of that on similar specimens tested in air. Based on fractography analysis, fatigue crack initiation mechanisms in air and in distilled water were identified. Two different scenarios for fatigue damage depending on the microstructure are proposed and will be discussed in this paper.

Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam Turbine Materials in the Simulated Boiler Water Added Chloride and Sulfate Ions

Using a rotor material, 3.5NiCrMoV steel, and a blade material, 13Cr steel, for low-pressure (LP) steam turbines of thermal power plants, electrochemical crevice corrosion tests were conducted in the simulated AVT (All Volatile Treatment) boiler water added chloride and sulfate ions. The crevice corrosion behaviors as well as the films formed on the specimen surfaces inside crevices were investigated. The 3.5NiCrMoV steel and the 13Cr steel in the test water showed crevice corrosions in a general type and a pitting type, respectively. For both the two steels, however, passive films formed on the specimen surfaces inside crevices. It was found that the passive film formed on 3.5NiCrMoV steel was composed mainly of Fe3O4, while that on 13Cr steel was composed mainly of Cr-oxides and partly of inward Fe-oxides. Especially, it was confirmed that CrOOH and CrO3 were concentrated in the outermost surface of the passive film formed on 13Cr steel. [doi:10.2320/matertrans.M2013202]

Void Nucleation Model in a Steam Turbine Material under Creep Condition

In order to establish a reliable remaining life assessment method for high temperature components in thermal power plants, it is necessary to understand the internal damage mechanism in heat resistant alloys and establish the damage prediction method. In the case of commercial heat resistant alloy used for thermal plants, nucleation, growth and coalescence of creep voids are dominant damage mechanism. So far, the void growth mechanism is well investigated and various growth models are suggested. However, rational modeling of void nucleation was remained to be difficult subject because basic mechanism has been unclear. Therefore, in this study, void nucleation behavior of a turbine rotor material under creep condition was investigated by a scanning electron microscope and development of void nucleation models was examined. Void nucleation model was developed based on the idea that creep voids nucleate by vacancy condensation at the interface between precipitated carbides and matrix on the grain boundary. In addition, consideration of creep property was included in the model via a relationship between strain and vacancy density obtained from the molecular mechanics simulation. In addition, model was expanded to predict the void number density, by a statistical treatment of the different physical property related to void nucleation in the every grain boundary. It was confirmed that the present void nucleation model can reproduce evolution of void number density.

Accelerated Corrosion Behavior due to Alternating Dry-Wet Conditions for LP Steam Turbine Materials of Fossil Power Plants

The materials of the attachment near the last stage of low-pressure steam turbines in power plants will be exposed to a severe corrosion condition due to the concentration of corrosive chemicals produced by the alternating dry and wet environment, a phenomenon caused by the frequent shutdown and the load change of power plants. The corrosion behavior of typical low-pressure steam turbine materials is evaluated mainly by tests under dry-wet conditions. An increase in the concentration of sulfate ion and chloride ion in waters enhanced the general corrosion particularly for the 3.5NiCrMoV rotor steel. As for 12Cr blade steel, the coexistence of sulfate ion and chloride ion accelerated the general corrosion. An alternating dry-wet condition increased the maximum corrosion pit depth of 3.5NiCrMoV steel remarkably with the application of stress. The pitting corrosion potential of 3.5NiCrMoV steel showed almost the same values in all water qualities tested. However, the coexistence of Cl- and DO (dissolved oxygen) lowered the pitting corrosion potentials of 12Cr steel. From the consideration between the corrosion and the Cr content in corrosion films, it is clearly evident that the Cr content in the films controls the corrosion resistance.

溶存酸素と腐食性化学種との共在下における低圧蒸気タービン材料のSCC挙動

溶存酸素と腐食性化学種との共在下における低圧蒸気タービン材料のSCC挙動

Effects of cyclic operation on advanced energy conversion systems

The impact of plant cycling is evaluated for near and medium term advanced energy conversion systems in terms of effects on equipment and implications for the choice of materials. A brief outline is given of current problems and their causes in steam and combined cycle gas turbine (CCGT) plants. In contrast to earlier years, it is now quite common for relatively new plants to have to cycle. The causes of this are increased competition in the power supply industry, unforeseen changes in the relative price of fuels, and regulatory changes which no longer have the effect of insulating certain types of energy conversion systems from the need to load follow and two shift. It is therefore vital that the issue of cycling be considered at the plant design stage.In advanced steam plants, where steam inlet temperatures will be well over 600°C, problems will involve the performance of transition joints and austenitic alloys under cycling conditions. The“ R” function is described and tabulated to show the likely susceptibility of a range of ferritic, austenitic and nickel based alloys to thermal shock, caused by quenching of pipework by slugs of condensate. It would seem that even modern high strength austenitics do not have the same capability as the modified 9 and 12 Cr ferritics. There may also be problems with steam turbine materials. The main concern in advanced CCGT plants is the potential for increased thermal fatigue, due to the use of more sophisticated blade cooling techniques. However a potential issue is shortcomings in blade life assessment models as applied to directionally solidified and single crystal materials, as it seems likely that current approaches, using isotropic data, may be giving a severe over estimate.It seems unlikely that coal gasification plant will be able to two shift, although load following will be possible. It is suggested that the critical area will be that of the synthesis gas exchanger where there is potential for corrosion assisted fatigue. This problem is likely to increase if more resistant coatings are used, as these tend to be less ductile than current alloys.Conversely recuperative gas turbines are designed to load follow and two shift and should be able to resist the effects of rapid start ups and shut downs. Modern designs using either primary or secondary surface recuperators are briefly described. The topical issue here is passage collapse in primary surface systems, which is likely to increase under cycling duty.

532 蒸気タービン鋳鋼経年劣化材の応力緩和特性

532 蒸気タービン鋳鋼経年劣化材の応力緩和特性

219 蒸気タービン鋳鋼経年劣化材の損傷評価データベース

219 蒸気タービン鋳鋼経年劣化材の損傷評価データベース

An analysis of stress waves in 12Cr steel, Stellite 6B and TiN by liquid impact loading

This research placed emphasis on the computer simulated stress distribution on the surface and in the bulk of the materials which are subjected to the water impact causing erosion damage. The erosion damage was predicted by evaluating the spatial and temporal stress wave distribution generated by water impact pressure on 12Cr steel and Stellite 6B as steam turbine materials and TiN as a hard coating material. There were two distinctive stress wave behaviors. Firstly, the large tensile stress at the surface was developed by the Rayleigh wave component which appeared between the water drop and the Rayleigh wave front. After the Rayleigh wave detached from the water drop, the materials were in the tensile stress state which could be related to fracture initiation. Secondly, the largest tensile stress in the bulk was near the surface due to the Rayleigh wave generated at the surface and decreased due to the enlargement of wave front as the radial distance increased. Rayleigh wave's shape was broadened due to the difference between the contact point velocity and the wave front velocity, while its value decayed exponentially in the depth direction. Also, there may be a tendency to produce a circumferential crack by σ rr near the surface and a lateral crack by σ zz in the sub-surface. The tensile stresses in TiN were much lower than those in 12Cr steel and Stellite 6B due to its higher wave velocity.