M. Holzmann

Degradation of mechanical properties of crMoV and CrMoVW steam turbine rotors after long-term operation at elevated temperatures. Part II: fracture toughness, correlation of fracture toughness with Charpy V-notch results

Abstract The effect of long-term exposure at elevated temperatures on the ductile-to-brittle transition behaviour of the fracture toughness for CrMoV and CrMoVW rotor steels has been studied. The rotors were retired from various fossil power plants after about 1·2 × 105 h. The shift of the onset of transition region in fracture toughness versus temperature behaviour about 60°C and 30°C towards higher temperatures for CrMoV and CrMoVW steel, respectively, was noted when compared with the fracture toughness behaviour for hot and cold locations. Practically the same shift was observed for fracture appearance transition temperature (FATT) determined by the Charpy impact tests with samples taken from the same sites (Part I). The correlation of fracture toughness to Charpy V-notch results, based on the excess temperature (i.e. t-FATT) approach, has proved to be also valid for degraded conditions. The KIC curve defined by this correlation nearly coincides with lower bound (confidence limit 90%) of fracture toughness scatter band in the transition region

Measurement of fracture toughness transition behaviour of Cr–Ni–Mo–V pressure vessel steel using pre-cracked Charpy specimens

Abstract The potential application of small pre-cracked Charpy specimens for the prediction of the fracture toughness of the 1T-thickness specimens and the reference temperature T 0 has been examined. Transition fracture behaviour of plane sided, side-grooved and 1T SENB specimens, respectively, was investigated over a wide temperature range. The fracture toughness regions with various fracture initiation mechanisms were defined and ductile to brittle transition temperatures denoted. The fracture toughness transition region of small pre-cracked specimens was shifted to lower temperatures as compared with that of 1T SENB specimens. The fracture toughness data of small pre-cracked specimens have been size corrected (weakest link) to 1T thickness and used to establish the reference temperature T 0 and K J c (mean) fracture toughness vs. temperature curve. The calculated temperature T 0 has been in consistence with that of the 1T SENB specimen. However, some corrected fracture toughness data lay outside the scatter band of 1T thickness specimens and the shape of the K J c (mean) curve has been quite different from the K J c (med)(1T) curve. It was found out that the original measured fracture toughness results of corrected data points lying outside the scatter band violated the validity condition b 0 R p 0.2/ J c ≥30. Bearing in mind the work of Koppenhoefer and Dodds Jr. (Engng Fract Mechanics (1997);58:249–270), and the most recent analysis of Ruggieri et al. (Engng Fract Mechanics (1998);60:19–36), the fracture data of small pre-cracked specimens having the validity parameter lower than 50 have been first constraint adjusted using the cleavage fracture toughness scaling model of Dodds and coworkers (J Testing Evaluation (1991);19:123–134; Int J Fracture (1995); 74:131–161; Engng Fract Mechanics (1997);58:249–270), and only then size corrected. The K J c (mean) curve of such treated data was identical with K J c (med)(1T) .

R-Curves and fracture toughness transition behaviour at static, rapid and impact loading of Cr-N-Mo-V reactor pressure vessel steel

Abstract The fracture toughness of Cr-Ni-Mo-V reactor pressure vessel steel was investigated over a wide temperature range. In the upper shelf fracture toughness region, R -curves were determined using large SEN bending specimens and small side-grooved precracked Charpy sepecimens. Two approaches were used for the evaluation of the fracture toughness scatter in the transition and in the lower shelf fracture toughness region: the exponential function with a least-squares regression procedure for a fracture toughness-temperature data and Weibull statistics for a fracture toughness data set obtained at the given temperature. The investigation of the influence of loading rate, ranging from 1·0 MPa m 1 2 s −1 (static) to 3 × 10 4 MPa m 1 2 s −1 (most rapid), on the behaviour of fracture toughness has shown a shift of cleavage/fibrous transition temperature by 60°C. In contrast to that, at impact loading using a precracked Charpy specimen (K I = 2 × 10 5 MPa m 1/2 s −1 ) the shift of this transition temperature was only 30°C, and a sudden transition of fracture toughness occurred from the lower shelf to the upper shelf toughness region.

Degradation of mechanical properties of CrMoV and CrMoVW steam turbine rotors after long-term operation at elevated temperatures. Part I: tensile properties, intergranular fracture strength and impact tests

Abstract The influence of long-term operation at elevated temperature on tensile properties, brittle fracture initiation stress and on the Charpy V-notch transition behaviour of CrMoV and CrMoVW rotor steels has been investigated. A decrease in the 0·2% proof strength and tensile strength was observed in the CrMoVW steel after operation of 1·1 x 10 5 h at a temperature about 510°C. For the CrMoV steel only a small reduction of these characteristics was noted after operation at temperatures about 475°C. The initiation of brittle fracture of both steels after long-term operation was due to intergranular cracking, the intergranular fracture strength, established by testing smooth and notched-bars at low temperatures, was lower than the cleavage fracture stress of undegraded states (cold location). The fracture appearance transition temperature (FATT) determined by the Charpy impact test was increased by about 55°C and 37°C for CrMoV and CrMoVW steel, respectively. De-embrittling heat treatment (640°C/2 h/air-cooled) both of the service exposed and the undegraded state of CrMoV steel caused the shift in Charpy transition curve of exposed state by about 35°C to the lower temperature and the fracture was of a cleavage mode. On the contrary, no change in location of transition curve was noted for the undegraded state. The difference of a 20°C between these curves is likely to be a result of carbide induced embrittlement.

Fracture Resistance of Cast Ferritic C-MN Steel for Container of Spent Nuclear Fuel

Fracture resistance of cast ferritic steel predetermined for containers of spent nuclear fuel has been evaluated based on sets of different fracture mechanical test specimens and assessment procedures. Standard fracture toughness values were determined from IT SENB specimens and compared with data from pre-cracked CVN specimens (P-CVN). The other parameters that have been in focus of interests were: the effect of metallurgical technology (two melts followed), specimen location (midthickness vs. surface locations) in thick walled plate, the effect of loading rate (here followed on CVN and P-CVN specimens), statistical effects etc

The role of microstructure in brittle fracture behaviour of low alloy tempered bainitic steel

The validity of various micromechanistic models for the cleavage initiation in tempered bainite have been tested using two quite different model bainitic microstructures. The microscopic cleavage fracture stress (CFS) was measured for both microstructures using Charpy V-notch type specimens. Various micromechanistic models were applied to experimental results to explain the metallurgical nature of CFS. The validity and applicability of these models for a particular bainitic microstructure have been discussed.

Degradation of mechanical properties of cast Cr-Mo-V and Cr-W-V steam turbine casings after long-term service at elevated temperatures. Part I: Tensile properties, brittle fracture strength and Charpy impact properties

Abstract The effect of elevated service temperature on tensile properties, brittle fracture strength and on the Charpy V-notch transition curve of Cr-Mo-V and Cr-W-V cast steels is presented. A lowering of the yield stress and ultimate tensile stress (softening) was observed with both types of cast steels after long-term exposure at elevated temperatures. The brittle fracture strength of Cr-Mo-V steel established by testing both the smooth bars σ BF and notch specimens σ∗ BF at low temperatures was not influenced during exposure at elevated temperatures. The fracture appearance transition temperature (FATT) of this steel determined by impact Charpy test was also not affected by long-term exposure. Thus, it could be concluded that this type of cast steel was not embrittled during operation. On the contrary, a decrease in brittle fracture strength σ∗ BF (250 MPa) and an increase in FATT (50°C) were observed in the Cr-W-V steel after 2 × 10 5 h of service.

Degradation of mechanical properties of cast Cr-Mo-V and Cr-W-V steam turbine casings after long-term service at elevated temperatures. Part II: Fracture toughness, correlation of fracture toughness with Charpy V-notch toughness

Abstract The effect of long-term exposure at elevated temperatures on the transition behaviour of the fracture toughness temperature curve of cast Cr-Mo-V and Cr-W-V steels has been studied. The fracture toughness versus temperature behaviour after any operational period seems to be controlled by the initial fracture properties. Relationships linking the fracture toughness to Charpy V-notch impact properties are reported. The validity of these relationships for cast steels was confirmed.

Plastic deformation and fracture behaviour of 214Cr-1 Mo pressure-vessel steel

Abstract During the heat treatment of steel plates and forgings of large thicknesses, microstructures with various volume fractions of ferrite appear. Plastic properties and fracture behaviour of these mixed microstructures are a function of ferrite content. The influence of ferrite content in the range from 0% to 54% in the bainitic-ferritic microstructure on mechanical properties and fracture behaviour of 2 1 4 Cr -1 Mo steel was examined. The yield stress was found to decrease linearly with the volume fraction of ferrite. The tensile strength was independent of ferrite content up to 25%, after which the tensile strength decreased. Using the Charpy test it has been found that the critical ferrite content—25%—exists in a mixed microstructure, at which the propagation and initiation transition temperatures attain the highest values. The fracture toughness tests gave the same results. Increasing the volume fraction of ferrite, the cleavage fracture toughness/temperature curves were shifted to higher temperatures. Simultaneously, the ductile-brittle fracture toughness transition temperature was raised reaching the highest value for the critical ferrite content. The fracture behaviour could be tentatively explained through the influence of ferrite volume fraction on both the cleavage fracture stress and the stress level at the crack tip.

Evaluation of Strain Rate Effects on Transition Behaviour Applying the Master Curve Methodology

The master curve methodology has been used for an evaluation of strain rate effects on transition behaviour of cast ferritic CrMo steel. The physical aspects of strain rate effect on reference temperature has been analysed as a base for the prediction of this dependence. Statistical aspects of the strain rate effects on the reference temperature and the shift of master curve on temperature axis has been discussed showing capability of the method for the prediction of strain rate susceptibility of steel fracture behaviour