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NDT Characterisation of Thermal Ageing by Barkhausen Signal AnalysisSoraia Pirfo
ASI - Applied Structural Integrity Consulting - Hungary
Atomic Energy Research Institute - KFKI/AEKI - Hungary
The in-service exposure of a wide range of engineering components leads to deterioration of certain mechanical properties, such as strength, physical and chemical properties. Among the factors, which change material properties is the temperature exposition resulting thermal ageing. Equipment where reliability is a crucial issue is exposed to temperature degradation, such as reactor vessel and turbine components. It is a key to proper maintenance or extended service life to evaluate material conditions. An important tool for evaluation is the nondestructive testing. Different testing devices can determine the electromagnetic properties of the materials and some of the properties are sensitive to material structure changes. The permeability and the spikes on the magnetisation of ferromagnetic materials known as Barkhausen signal are among them.
This work studies the effect of thermal ageing on the Barkhausen signal and relative permeability using two materials. The two selected materials were CrMoV steel used in WWR type reactor vessels and NiCoFe alloy, which is typical in turbine components. The measured Barkhausen noise and the relative permeability were compared to mechanical properties.
Material damages caused by environmental and operational effects are the life limiting factors of structures in different industries. Proper and low cost maintenance requires the knowledge of material service degradation. The structural integrity assessment of components uses of among others the nondestructive evaluation to determine the residual life. Mechanical testing can follow the ageing of structural materials, but it generally destroys the structure. Nondestructive testing (NDT) is widely used for defect detection, but not for evaluation of ageing.
There are several factors, which can contribute to the ageing of materials, changing their properties, and result in possible failure of components and systems. These include temperature resulting in thermal ageing. To study the thermal ageing effect tests were performed on laboratory aged CrMoV steel and NiCoFe alloy.
Barkhausen signal and another electromagnetic parameters were measured and compared with mechanical properties and metallography results in order to investigate the correlation with material degradation. The results were analysed with different methods to provide the best fittings.
PC controlled equipment (Stresstest type 20.01) used to generate and pick-up Barkhausen noise signals. The following outputs are available on the device:
A particular feature of thermal ageing is that it is temperature and time dependent, developing over much longer periods at lower temperatures. In addition, the effect varies slightly with material composition. The mechanisms occurring in the microstructure can be precipitation of particles, transformation of phases, growth of precipitated phases and the grains of the matrix and dissolution of precipitates. In case of some material, e.g. steel, the precipitation of carbides can restrain the movement of dislocations resulting material embrittlement.
Thus, it is important to evaluate the thermal ageing effect on reactor vessel and turbine materials due reliability needed on such systems. By that CrMoV steel to the first and NiCoFe alloy to the second system were chosen material to studies.
All ageing was performed in laboratory. Accelerated ageing at elevated temperature was used. The ageing time and temperature were calculated by using the G parameter (Arrhenius law):
G = log T + log (20 + log t)
where T is temperature in °K and t is time in hours. In case of equivalent G parameters the accelerated ageing has the same effect on material properties as service ageing, if the temperature during accelerated ageing is not higher than the temperature of service ageing + 100 °C, and it is well below the annealing temperature of the material.
3.1 Testing of Cr-Mo-V materials
The WWER type reactor vessel steel 15H2MFA was used for low temperature thermal ageing study. The purpose of the study was to understand the low-temperature thermal ageing, and to check whether thermal ageing occurs at low temperature (270-300 °C). To simulate long term of ageing at 2700 C accelerated ageing at 3500C was used. The calculated ageing times are shown in table 5.1.
|Ageing time at 350 °C [h]||0||800||1600||2000||2500|
|Simulated operational time at 270 °C [h]||0||32000||64000||80000||100000|
|Table 5.1: Low temperature isothermal accelerated ageing program for 15H2MFA|
The results of the metallography test shown that the grain structures of the 15H2MFA steel samples aged at 3500C manifest only slight changes compared with the original material samples. After ageing the grain boundaries became slightly thicker, showing the initiation of the diffusion processes. The same effects can also be observed on the electron microscope pictures. The microsonde (FEM) analysis didn't reveal significant changes at the grain boundaries, showing that the rate of the segregation at the boundaries and even the thickness of the grain boundaries are small enough to be beyond the microsonde sensitivity.
Before ageing and after 800, 1600, 2000 and 2500 hours of laboratory ageing, the hardness of the 15H2MFA specimens were measured. The hardness testing were microhardness and Brinnel used according to ASTM E-10-84. The results show that the initial hardness is to some degree higher at the surface of the forging as compared to the middle section, and the change during the accelerated ageing is very slight. The results verified by metallography, showing that the ageing - though slow at low temperature exists.
Previous studies have proved that the material properties (especially the nil-ductility transition temperature) of the thick walled forging are dependent on distance from the surface. Therefore the nil-ductility temperature have been measured by Charpy test at 10-11 locations across the wall thickness. The effect of the low temperature thermal ageing on the nil-ductility transition temperature can be seen in fig. 1.
Fig 1: The effect of thermal ageing on the transition temperature
Fig 2: Polynomial fit on the Barkhausen results
The Barkhausen testing results had shown considerable scattering, but the distribution of the data also shows correlation with the change of the transition temperature. This correlation can be observed better by polynomial fitting of the results as shown in fig. 2.
3.2 Tests on isothermally aged nickel-iron-cobalt alloys
The material was NiCoFe alloy used on turbine components. It exposed to elevated temperatures for different time periods in order to simulate thermal degradation mechanisms. The selected ageing temperature was 700° C and time periods of temperature exposition were 100, 269.5, 469.5, 738 and 838 hours.
Hardness was checked by a Vickers (HV) test, a diamond pyramid indentor was used and the applied load was 5 Kg to standard size specimens and 20 gr. (0.02 Kp = 0,1962 N) to metallography samples.
During the first period of the heat treatment the cold worked material annealed. From annealing stage 2 or 269.5 hours exposition only material ageing occurred and the Barkhausen RMS signal and hardness measurements had shown a good correlation through the later stage of ageing as it can be seen on fig.3.
|Fig 3: Correlation among Barkhausen RMS signal and hardness Vickers due to effect of thermal ageing on NiCoFe material.||Fig 4: Correlation among the yield strength and relative value to permeability (called GAP) in differently aged NiFeCo alloy.|
In fig. 3 and 4 the R is related to as received condition and the numbers 1, 2, 3, 4, 5 belongs to 100, 269.5, 469.5, 738 and 838 hours isothermal ageing.
Yield strength measured on the differently aged specimens was also correlated with the so called GAP parameter. GAP is relative to the permeability and flux leakage values provided by the equipment. As the flux leakage is constant the GAP changes becomes relative to the permeability alterations. The relative permeability or GAP and the yield strength have shown good correlation as it presented on Fig.4.
Low temperature thermal ageing on the 15H2MFA type steel affected the nil-ductility transition temperature. The Barkhausen testing results shows correlation with the change of the transition temperature.
The Barkhausen signal also traced the degradation in NiFeCo alloy exposed to thermal ageing. Several examined characteristics of the magnetic Barkhausen noise provided good correlation with hardness values in the different phases of thermal ageing The permeability and conductivity was also related to the tensile properties of the new and differently aged.
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