CrMoV Rotor Steel Research Articles

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.

クリープ予ひずみおよび繰り返し予ひずみを与えられたＣｒＭｏＶ鋼におけるクリープ，疲労き裂進展速度

The effect of mechanical degradation in a long-term-operated component on the creep- and fatigue-crack growth rates was studied. Creep-crack-growth tests and fatigue-crack growth tests were performed on creep-pre-strained and cyclic-strained CrMoV rotor-steel specimens.It was found that under constant load, creep-crack growth rates, da/dt, of the creep-pre-damaged specimens were higher than those of virgin specimens. It was also found that under the same stress intensity factor K, da/dt in the case of about 10%-crept and creep-void-induced specimens was increased 5 times, while in the case of 0.8%- and 2.8%-crept specimens, it only increased a little. However, all the data fall in a narrow scatter band in the relationship between creep J integral C* and da/dt.Another result showed that fatigue-crack growth rates, da/dN, of creep- and fatigue-pre-damaged specimens were higher than those of virgin specimens. Under the same stress intensity factor range ΔK, da/dN in an about 10%-crept specimen was increased 20 times. And da/dN in fatigue-pre-damaged specimens were accelerated 10 times. However, the results of the strain-controlled crack growth test fall in a narrow scatter band in the relationship between J integral range ΔJ and da/dN.These results suggest that the creep remaining lives and fatigue remaining lives of the mechanically damaged CrMoV rotor steel can be estimated by using C* & ΔJ of the damaged material and the da/dt and da/dN values of the virgin material.

Factors influencing the service‐like thermomechanical fatigue test cycle endurance of 1% CrMoV rotor steel

ABSTRACTService‐like thermomechanical fatigue tests have been performed in order to characterize the endurance of 1% CrMoV rotor steel under such transient thermal conditions. The key features of these tests are low strain rates (≤10−5 s−1) and longer hold periods. In all testpieces, ratcheting with progressive section reduction is observed in the central portion of the gauge length accompanied by local amplification of the strain range. The finite‐element‐based analysis of this phenomenon allows the measured endurances to be rationalized with those determined from isothermal tests. Post‐test inspection has revealed the development of two concurrent damaging mechanisms in the testpieces: (i) fatigue at the surface and (ii) creep in the interior. Different methods of damage calculation for creep–fatigue interaction are applied and compared in their predictive capabilities.

A nondestructive method for estimation of the fracture toughness of CrMoV rotor steels based on ultrasonic nonlinearity

The objective of this paper is to develop a nondestructive method for estimating the fracture toughness ( K IC) of CrMoV steels used as the rotor material of steam turbines in power plants. To achieve this objective, a number of CrMoV steel samples were heat-treated, and the fracture appearance transition temperature (FATT) was determined as a function of aging time. Nonlinear ultrasonics was employed as the theoretical basis to explain the harmonic generation in a damaged material, and the nonlinearity parameter of the second harmonic wave was the experimental measure used to be correlated to the fracture toughness of the rotor steel. The nondestructive procedure for estimating the K IC consists of two steps. First, the correlations between the nonlinearity parameter and the FATT are sought. The FATT values are then used to estimate K IC using the K IC versus excess temperature (i.e., T–FATT) correlation that is available in the literature for CrMoV rotor steel.

304 力学的な予損傷を与えられた CrMoV 鋼におけるクリープ, 疲労き裂進展速度

The effect of mechanical degradation in a long-term-operated component on the creep-and fatigue-crack growth rates was studied. Creep-crack-growth tests and fatigue-crack growth tests were performed on creep-pre-strained and cyclic-strained CrMoV rotor-steel specimens. It was found that under constant load, creep-crack growth rates, da/dt, of the creep-pre-damaged specimens were higher than those of virgin specimens. Another result showed that fatigue-crack growth rates, da/dn, of fatigue-pre-damaged specimens were higher than those of virgin specimens. The results suggest that the creep remaining lives and fatigue remaining lives of the mechanically damaged CrMoV rotor steel can be estimated by using C^*&ΔJ of the damaged material and the da/dt and da/dn values of the virgin material.

Simple modelling of the constitutive behaviour of a 1%CrMoV rotor steel in service-like thermo-mechanical fatigue tests

Thermo-mechanical fatigue tests have been performed to determine the cyclic constitutive behaviour of a 1%CrMoV rotor steel. The mechanical strain and temperature control profiles adopted in these tests were selected to represent loading conditions which could occur at critical rotor locations in a high temperature turbine rotor in service. The cyclic evolution of the elastic-plastic and creep stress/strain states is analysed. Cyclic deformation influences creep behaviour as well as the yield characteristics of this steel, the resistance to time independent and time dependent plasticity being reduced by approximately 20% at midlife. A constitutive model based on monotonic data (with time dependent and time independent plasticity components) is developed to determine midlife cycle behaviour and predictions are compared with measured TMF stress-strain curves. Simple rules are proposed which are suitable for application in turbine component lifetime calculations.

Simple modelling of the constitutive behaviour of a 1%CrMoV rotor steel in service-like thermo-mechanical fatigue tests

Simple modelling of the constitutive behaviour of a 1%CrMoV rotor steel in service-like thermo-mechanical fatigue tests

Evaluation of fracture toughness degradation of CrMoV rotor steels based on ultrasonic nonlinearity measurements

The objective of this paper is to develop a nondestructive method for estimating the fracture toughness (K IC) of CrMoV steels used as the rotor material of steam turbines in power plants. To achieve this objective, a number of CrMoV steel samples were heat-treated, and the fracture appearance transition temperature (FATT) was determined as a function of aging time. Nonlinear ultrasonics was employed as the theoretical basis to explain the harmonic generation in a damaged material, and the nonlinearity parameter of the second harmonic wave was the experimental measure used to be correlated to the fracture toughness of the rotor steel. The nondestructive procedure for estimating theK IC consists of two steps. First, the correlations between the nonlinearity parameter and the FATT are sought. The FATT values are then used to estimateK IC, using theK IC versus excess temperature (i.e.,T-FATT) correlation that is available in the literature for CrMoV rotor steel.

Creep Rupture Properties of High-Temperature Bainitic Steels After Weld Repair

Welded high-temperature power plant components can experience a greater risk of failure by creep during service, when compared to similar as-wrought components. The heat-affected zone (HAZ) of alloy steels is usually the region of a weldment exhibiting poor mechanical properties. The arc welding of an ASTM A470, Class 8-rotor steel in this study identified the intercritical heat affected zone (ICHAZ) as the weakest region in terms of creep rupture life. The type of welding procedure significantly affects this region, but most important are the welding parameters utilized. Because of the microstructural heterogeneity of the HAZ and sensitivity of these microstructures to changes when exposed to elevated temperatures, their performance at later times is difficult to predict. Extrapolation techniques are limited in value for predicting service lives of homogeneous materials, because these do not incorporate the microstructure changes of the materials during high temperature operation. They are even less useful for predicting the operating lives of weldments. This paper considers the creep performance and structure stability of the ICHAZ of 12 percent Cr and 214-1Mo vanadium modified weldments produced on a retired CrMoV rotor steel. [S0094-4289(00)00303-0]

Effects of Inclusions on Fracture Toughness for 1%CrMoV Rotor Steel

This thesis studied that seven kinds of residual elements(inclusions) had influenced on fracture toughness() obtained by Begley-Logsdon and Rolfe-Novak model equation using tensile an d impact test data of I%CrMoV HP(high pressure) rotor steel. design curve of ASME and fracture surface by SEM were also considered, obtained results are summarized as follows was linearly increased with increase of temperature, effect of the inclusions was significantly over FATT. at lower shelf temperature was quantitatively related to yield strength and was agreed well with Begley's equation. It was difficult to determine because of specimen size and tester capacity at upper shelf temperature, but for this view point Rolfe-Novak's equation was useful. The degree of brittle fracture was dependent on FATT fundamentally, adding S, Sb to matrix decreased impact energy and adding Cu, As increased yield(tensile) strength, and the influence of the others minority inclusion was comparatively insignificant.

Influence of high temperature low cycle fatigue deformation on the microstructure of a CrMoV rotor steel subjected to long-term service exposure at 425°C and retempering at 677°C

A study has been made of the influence on the microstructure of a CrMoV rotor steel of low cycle fatigue (LCF) deformation at 538°C with or without a 1-hour hold time applied at the peak tensile load of the fatigue cycle, following service exposure at 425°C for 23 years and retempering at 677°C for 24 hours. Samples for transmission electron microscopy were taken from the shoulder and gage sections of the broken fatigue specimens. The shoulder section of this steel revealed massive M 23C 6 along the grain boundaries, M 3C in globular or ellipsoidal form, fine spheroidal MC type carbide, and long needles of M 2C. LCF deformation at 538°C led to a tendency toward spheroidization of M 3C, promoted the nucleation of M 2C in ferrite, and slightly coarsened the MC carbide. Superimposing a 1-hour hold period at the peak tensile load resulted in nucleating M 2C preferentially at cementite-ferrite interfaces.

Microstructural Changes in a Service Exposed CrMoV Rotor Steel Subjected to High Temperature Low Cycle Fatigue

Microstructural Changes in a Service Exposed CrMoV Rotor Steel Subjected to High Temperature Low Cycle Fatigue

Progress in the Design of an Improved High-Temperature 1 Percent CrMoV Rotor Steel

Silicon-deoxidized, tempered bainitic 1 percent CrMoV steel is currently used extensively for high-temperature steam turbine rotor forgings operating at temperatures up to 565°C due to its excellent creep rupture properties and relative economy. There is impetus to improve the creep rupture strength of this steel while maintaining its current toughness level and vice versa. The excellent creep rupture ductility of the low Si version of this steel allows the use of a higher austenitizing temperature or tensile strength level for improving creep rupture strength without loss in creep ductility or toughness. When the tensile strength of this steel is increased from 785 to 854 MPa, the creep rupture strength exceeds that of the more expensive martensitic 12CrMoVCbN steel currently used for high-temperature rotor applications where additional creep rupture strength is required. The toughness of 1 percent CrMoV steel is improved by lowering the bainite start (Bs) temperature in a “superclean” base composition which is essentially free of Mn, Si, P, S, Sb, As and Sn. The Bs temperature can be lowered through the addition of alloying elements (i.e., C, Ni, Cr, and Mo) and/or increasing the cooling rate from the austenitizing temperature. Using these techniques, the 50 percent FATT can be lowered from approximately 100°C to below room temperature, which provides the opportunity to eliminate the special precautionary procedures currently used in the startup and shutdown of steam turbines. The most promising steels in terms of creep rupture and toughness properties contain 2.5 percent Ni and 0.04 percent Cb (for austenite grain refinement and enhanced tempering resistance). In general, the creep rupture strength of the superclean steels equals or exceeds that of the standard 1 percent CrMoV steel. In addition, the superclean steels have not been found to be susceptible to temper embrittlement, nor do they alter the room temperature fatigue crack propagation characteristics of the standard 1 percent CrMoV steel. These new steels may also find application in combination high-temperature-low-temperature rotors and gas turbine rotors.

Evaluation of electrochemical properties for CrMoV steel in acidic solutions to determine embrittlement—II. Electrochemical reaction in 2,4-dinitrobenzoic acid solution

Electrochemical reaction for a CrMoV rotor steel in 2,4-dinitrobenzoic acid solutions has been investigated to explain a characteristic correlation between the open circuit potential and the embrittlement level of the steel. Despite relatively low pH of the solution (pH < 3), the cathodic reaction was not hydrogen evolution, and the observed limiting current had no linear relation with hydronium ion concentration. From analytical investigations on the reaction product, it was proved that 2,4-dinitrobenzoic acid was reduced to 2-nitro-4-aminobenzaldehyde on the steel surface. It was shown that this reaction was controlled by diffusion of both 2,4-dinitrobenzoic acid (or its dissociated ions) and hydronium ion, and was not influenced by the embrittlement level of the steel. On the other hand, anodic dissolution was largely affected by the embrittlement level and became inactive with embrittlement progress. Microstructural observation showed that the dislocation density for embrittled steel was much lower than that for unembrittled steel. That is, the aging of the steel caused the reduction of dislocation density, concurrently with embrittlement. Thus, it was concluded that the shift in the open circuit potential with the progress in the embrittlement level was caused by the inactivation in the anodic dissolution reaction due to the microstructure change such as dislocation density reduction.

Crystallography of M 23C 6 and M 6C precipitated in a low alloy steel

The orientation relationship of M 23C 6 and M 6C precipitated in a CrMoV rotor steel has been studied by means of transmission electron microscopy. A new orientation relationship, similar to the Nishiyama-Wassermann relationship for M 23C 6 α , has been found for γ α . M 6C can either form directly in ferrite or grow on the existing M 23C 6, both with a parallel orientation relationship. These are discussed in the light of matching of individual atoms on the interface between two phases.

Upper shelf temperature dependence of fracture toughness for four low to intermediate strength ferritic steels

To examine the inadequacy and expose the potential consequences of utilizing transition temperature range fracture toughness values when performing fracture mechanics analyses on structures operating at upper shelf temperatures, the upper shelf fracture toughness behavior of three rotor forging steels (ASTM A471 NiCrMoV, ASTM A470 CrMoV and AISI 403 modified 12 Cr) plus an ASTM A217 2 1 4 Cr -lMo cast steel was investigated. Elastic-plastic fracture toughness values were obtained via the J IC resistance curve test technique. Except for the CrMoV rotor steel, upper shelf fracture toughness peaked near the temperature where zero percent brittle fracture first occurred in the Charpy V-notch impact tests after which decreases of 25–45% in toughness were realized. Fracture toughness tests were conducted up to 800°F.

The high temperature creep and fracture behavior of a 1-pct CrMoV rotor steel

The creep and fracture behavior of a low alloy ferritic steel has been studied between 220 and 340 MN/m2 at 823 and 838 K. Over most of the creep life the creep strain could be represented by: e-eo + eT(l-e-mt ) + est + χLep(t-tt) where eo is the instantaneous strain on loading, eT the total transient strain,m a parameter relating to the rate of exhaustion of transient creep,t the time, es the steady creep rate,tt the time to the onset of tertiary creep and eL andp are tertiary creep parameters. The steady-state creep rate was related to applied stress and temperature by: e=AΣa17 exp(RT/-397,000)