Steam Turbine Steel Research Articles

Numerical prediction of warm pre-stressing effects for a steam turbine steel

In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS’s beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.

Stress Corrosion Cracking of Steam Turbine Steel: The Influence of Organic Acid Characteristics

This work evaluates the impact of different organic acids on the corrosion sensitivity and stress-corrosion cracking (SCC) of NiCrMoV steam turbine steel. For all organic acids, potentiodynamic measurements shows linear relationships between corrosion rate and hydrogen proton concentration between pH 2.4 and 3.9. For solutions with the same pH, i.e., similar conductivity, the corrosion rate differs depending on the type of organic acid. The anodic dissolution in formic acid is the highest, followed by acetic, propanoic and nonanoic acid. The acid dissociation reaction is identified as the rate determining step in the corrosion process. Nonanoic acid, alternatively, clearly acts as a corrosion inhibitor. In situ four-point constant-extension tests in formic acid, acetic acid and nonanoic acid, at a pH value of 3.4 were performed to evaluate their impact on the SSC sensitivity. The general degradation followed the trend of the corrosion rate, although the synergetic effect of corrosion and stress resulted in a higher degradation depth. Though nonanoic acid induced little visible corrosion, still stress-corrosion cracks were still detected. It was shown that solutions of different organic acids with the same pH do not have the same influence on stress-induced degradation.

The effect of dwell times and minimum temperature on out-of-phase thermomechanical fatigue crack propagation in a steam turbine steel—Crack closure prediction

Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.

Out-of-phase thermomechanical fatigue crack propagation in a steam turbine steel — Modelling of crack closure

Understanding of crack growth behaviour is necessary to predict accurate fatigue lives. Out-of-phase thermomechanical fatigue crack propagation tests were performed on FB2 steel used in high-temperature steam turbine sections. Testing results showed crack closure where the compressive part of the fatigue cycle affected crack growth rate. Crack closing stress was observed to be different, and had more influence on the growth rate, than crack opening stress. Crack growth rate was largely controlled by the minimum temperature of the cycle, which agreed with an isothermal crack propagation test. Finite element models with stationary sharp cracks captured the crack closure behaviour.

Effect of Film-Forming Amines on the Acidic Stress-Corrosion Cracking Resistance of Steam Turbine Steel

This work evaluates the effect of film-forming amines (FFA) on the acidic stress-corrosion cracking (SCC) resistance of NiCrMoV turbine steel. Contact angle measurements show an increased hydrophobicity of the surface when coating the steel with oleyl propylene diamine (OLDA). According to potentiodynamic measurements and post-mortem scanning electron microscopy (SEM) analysis, anodic dissolution and hydrogen embrittlement still occur when the steel is FFA coated. In situ constant extension rate testing (CERT) in acidic aqueous environment at elevated temperature of FFA-coated steel shows a ductility gain compared to non-coated steel, explained by a decrease in both corrosion rate and hydrogen uptake.

A coupled mechano-chemical peridynamic model for pit-to-crack transition in stress-corrosion cracking

We introduce a coupled mechano-chemical peridynamic model to describe stress corrosion cracking. In this model, two mechanisms, stress-dependent anodic dissolution and diffuse corrosion layer-assisted fracture, are considered to influence pitting and crack propagation in stress corrosion cracking. Diffusion peridynamic bonds (acting as dissolution bonds at the solid/liquid interface) and mechanical peridynamic bonds are used to represent the interactions between material points. Mechanical bonds can be damaged by mechanical stretching or by anodic dissolution. The magnitude of the dissolution fluxes for diffusion peridynamic bonds depends on both mechanical deformation and the applied electrical potential. The coupling between anodic dissolution and mechanical damage leads to cracks that initiate in the corrosion damage layer and propagate into the bulk. A 2D three-point bending/corrosion test demonstrates the concept. We verify the model in 3D using an experimental test from the literature for the case of stress-corrosion cracking process in a steam turbine steel sample. The model's results capture the pit-to-crack transition time, the pit size and shape at fracture, as well as the morphology of cracks that spring from, and connect the pits.

Mechanistic interpretation on acidic stress-corrosion cracking of NiCrMoV steam turbine steel

The present work evaluates the acidic stress-corrosion cracking (SCC) of NiCrMoV steam turbine steel. Between a pH of 4.2 and 7, iron oxidises to Fe2O3, whereas iron dissolution (Fe2+) and hydrogen proton (H+) reduction are favoured at lower pH. The corrosion rate increases with the H+ concentration via a linear relationship as well as by a temperature increase. During in-situ constant extension rate testing, both the ductility and strength losses, caused by anodic dissolution and hydrogen embrittlement, increase with a higher acetic acid concentration. Hydrogen embrittlement causes larger embrittled zones with higher acetic acid concentrations as well as stress-corrosion cracking.

Characterisation of deformation and damage in a steam turbine steel subjected to low cycle fatigue

The increased use of renewable energy pushes steam turbines toward a more frequent operation schedule. Consequently, components must endure more severe fatigue loads which, in turn, requires an understanding of the deformation and damage mechanisms under high-temperature cyclic loading. Based on this, low cycle fatigue tests were performed on a creep resistant steel, FB2, used in ultra-supercritical steam turbines. The fatigue tests were performed in strain control with 0.8-1.2 % strain range and at temperatures of 400 °C and 600 °C. The tests at 600 °C were run with and without dwell time. The deformation mechanisms at different temperatures and 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 fracture surface, regardless of whether the specimen had been exposed to dwell time or been purely cycled. In addition, the amount of low angle grain boundaries was lower at 600 °C than at 400 °C. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed to being all plastic strain.

Influence of mean stress and notches on the very high cycle fatigue behaviour and crack initiation of a low‐pressure steam turbine steel

AbstractThe aim of the present work is to investigate the influence of different stress concentration factors (1.09, 1.31, and 2.42) and several R‐values on the fatigue behaviour of martensitic steel X10CrNiMoV12‐2‐2 up to 2 · 109 load cycles at ambient temperature. The tests were performed using an ultrasonic fatigue testing system developed at the authors' institute. The S‐N curves of specimens with the lowest investigated stress concentration factor of 1.09 show for both R = −1 and R = 0.5 a flat slope with transition from surface to volume crack initiation at about 1–2 · 107 load cycles. The maximum number of load cycles where fracture occurs increases with increasing R‐value. The √area approach by Murakami describes lifetime behaviour within about 4 decades of lifetime and a wide range of load ratios. In case of internal crack initiation at R = −1, the fracture surface contains a fine granular area (FGA) around the inclusion within the typical “fish‐eye” fracture. With increasing notch factor, the fatigue strength up to 2 · 109 load cycles decreases and the slope of the S‐N curve becomes steeper in the low cycle fatigue and high cycle fatigue regime, especially for αk = 2.42. In this case, damage at internal defects is suppressed and fatigue cracks are generally initiated at the notch root.

Novel concepts on the growth of corrosion fatigue small and short cracks

Corrosion fatigue small, short and long crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300 ppb Cl‐ + 300 ppb SO42‐ solution and in air at 90 °C. The crack growth rate for short and long cracks was monitored by direct current potential drop (DCPD) and for the small cracks by combining high resolution optical microscopy and DCPD. Comparison of the fatigue growth rate demonstrated that in solution the short crack growth rate was remarkably enhanced in comparison to long cracks, when the crack size is smaller than 250 μm. This enhancement was attributed to the electrochemical crack size effect associated with greater anodic polarisation of the short crack in such low conductivity solution. However, such enhanced growth was not observed for small cracks, which was rationalised on the basis of additional contribution of current from the pit limiting crack‐tip polarisation.

Small and short crack growth and the solution-conductivity dependent electrochemical crack size effect

Corrosion fatigue small crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300ppb Cl− and 300ppb SO42− under simulated two-shifting conditions. The remarkably enhanced crack growth rate observed previously for a short crack (relative to that for a long crack) was not evident for small cracks initiated from pits. In contrast, the stress corrosion small crack growth rate determined in aerated 35ppm Cl− was markedly increased with respect to long crack growth. The concept of the solution-conductivity dependent electrochemical crack size effect is introduced to provide a fundamental framework to rationalise the disparate results. The engineering implications for two-shifting operation are discussed.

Novel Concepts on the Growth of Corrosion Fatigue Small and Short Cracks

Corrosion fatigue small, short and long crack growth rates have been determined for a 12Cr steam turbine steel in aerated 300 ppb Cl- + 300 ppb SO42- solution and in air at 90 °C. The crack growth rate for short and long cracks was monitored by direct current potential drop (DCPD) and for the small cracks by combining high resolution optical microscopy and DCPD. Comparison of the fatigue growth rate demonstrated that in solution the short crack growth rate was remarkably enhanced in comparison to long cracks, when the crack size is smaller than 250 μm. This enhancement was attributed to the electrochemical crack size effect associated with greater anodic polarisation of the short crack in such low conductivity solution. However, such enhanced growth was not observed for small cracks, which was rationalised on the basis of additional contribution of current from the pit limiting crack-tip polarisation.

Environmentally Assisted Small Crack Growth

Corrosion fatigue small crack growth rates have been determined for a 12Cr steam turbine steel in a simulated condensate solution under conditions representing repetitive turbine start-up and shut-down. Cracks were grown from a single corrosion pit generated electrochemically using the droplet technique and the crack length monitored optically and by direct current potential drop (DCPD). A comparison is made with growth rates for short and long cracks determined using fracture mechanics specimens. The enhanced crack growth rate observed for a short crack (relative to that for a long crack) but not realised for the small crack is rationalized on the basis of an electrochemical crack size effect.

高温フレッティング疲労特性に及ぼす接触面圧の影響

The fretting fatigue tests under various levels of contact pressure were carried out at both room and elevated temperatures using 12Cr-Mo-W-V steam turbine steel in order to investigate the effect of contact pressure on fretting fatigue properties such as fatigue life and fatigue strength. Two step tests, where fretting action is removed at certain cycles, were also carried out at elevated temperature to investigate the effects of contact pressure on fretting fatigue behavior and propagation rate of fretting fatigue crack. Although the fretting fatigue strength decreased with increasing contact pressure, it attained a constant value when the contact pressure was higher than 100MPa. The fretting fatigue crack initiated at the early stage of life and its growth rate was accelerated by fretting action. This acceleration of fatigue crack was significant in the case of the higher contact pressure of 300MPa. The effect of fretting on fatigue life seemed to appear within 30% or 40% of fretting fatigue life regardless the contact pressure level and test temperature. This result was explained by comparing two fatigue crack growth curves with and without fretting.

Effect of mean stress on fretting fatigue strength at elevated temperature.

In order to investigate the effect of mean stress on fretting fatigue strength at elevated temperature, high-temperature fretting fatigue tests with various levels of mean stress were carried out using 12Cr-Mo-W-V steam turbine steel. The main results obtained are as follows. The fatigue strengths were decreased by fretting by a factor of three at both room and elevated temperatures regardless of the mean stress level. The modified Goodman relation held for fretting fatigue as well as for plain fatigue. The relationships between relative slip amplitude, friction coefficient and stress amplitude were independent of the mean stress level. The proposed method of fatigue life prediction based on fracture mechanics analysis, where the crack closure behavior was taken into consideration, was successfully applied to fretting fatigue with various levels of mean stress at elevated temperature.

Improvement of high-temperature fretting fatigue strength by shot peening.

Fretting fatigue tests at elevated temperature were carried out using shot-peened specimens of 12Cr-Mo-W-V steam turbine steel. The shot peening process increased the fretting fatigue strengths by a factor of 1.8 at an elevated temperature (773K) as well as at room temperature. This is mainly due to the compressive residual stress induced by shot peening. In order to investigate the effectiveness of shot peening for long-term practical use, the fretting fatigue tests of the specimens shot-peened and subsequently exposed for long periods up to 30000hrs at 773K were carried out. Although the compressive residual stress was reduced by a factor of 3 after this process, the fretting fatigue strength at 773K increased by a factor of 1.8. The effectiveness of the shot peening was found for long-period practical use at 773K. Prediction of fretting fatigue lives was made on the basis of elastic-plastic fracture mechanics, where the compressive residual stress by shot peening and the frictional force between the fretting pad and the specimen were taken into consideration. The predicted lives were in good agreement with the experimental results.

Fretting fatigue at elevated temperatures in a 12Cr-Mo-W-V steam turbine steel.

Fretting fatigue tests at elevated temperatures were carried out using a 12 Cr-Mo-W-V steam turbine steel. The fretting reduced the fatgiue strengths by a factor of three at an elevated temperature (773 K) as well as at room temperature. The friction coefficients between the specimen and the contact pads increased in proportion to the appropriate stress amplitude, but attained saturation. No significant difference in this behavior of the friction coefficient was Observed between room and elevated temperatures. Predictions of fretting fatigue lives were made on the basis of the cyclic J-integral analysis proposed in the previous paper, where the frictional force between the fretting pad and the specimen was taken into consideration. The predicted lives were in good agreement with the experimental results both at room and elevated temperatures.

Fretting fatigue at elevated temperature in a Cr-Mo-V steam-turbine steel.

Fretting fatigue tests at room and elevated temperatures were carried out using a Cr-Mo-V steam-turbine steel. The fretting reduced the fatigue strengths by a factor of two at an elevated temperature (773 K) as well as at room temperature. The friction coefficients between the specimen and the contact pads increased in proportion to the appropriate stress amplitude. No significant difference in this behavior of the friction coefficient was observed between room and elevated temperatures. Predictions of fretting fatigue lives were made on the basis of the cyclic J-integral analysis proposed in the previous paper, where the frictional force between the fretting pad and the specimen was taken into consideration. The predicted lives were in good agreement with the experimental results at room temperature. At the elevated temperature, the predicted lives were shorter than the experimental ones, as a resulted of crack closure due to significant exidation. The predicted lives at the elevated temperature, where the oxidation-induced crack closure was taken into consideration, were in good agreement with the experimental results.