B. Vlach

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

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.

The Effect of Microstructure on the Fracture Toughness of Structural Steels

Abstract Comprehensive research has demonstrated that fracture toughness, K IC , may be uniquely related to the mechanical properties describing a materials crack tip behaviour and microstructure. In the work described in the present paper the behaviour of cracks in seven weldable low alloy structural steels with tensile strengths, σ TS , in the range 455–765 MPa was investigated by measurement and observation of plane-strain fracture toughness, tensile properties and the microstructure. The information so obtained was used to establish a sophisticated K IC calculation model. It is shown that the process zone size is the dominant microstructural factor controlling the fracture toughness and that it varies from two to six grain sizes, depending on the particular microstructure. The model proposed matches the experimental data well and can be applied to find an optimum steel microstructure for given brittle fracture design requirements.

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.