INVESTIGATION OF THE CAUSES OF UNSATISFACTORY RESULTS OF ULTRASOUND CONTROL OF EXPERIMENTAL FORGES FROM AUSTENITE HIGH-ALLOY STEEL

The purpose of the work is to establish the main causes of defects by ultrasonic testing (UST) of experimental forgings made of high-alloyed austenitic steel 08Kh18N10T on the basis of comprehensive comparative studies of samples of defective and suitable forgings. Techniques. Chemical analysis of forgings was performed on a high-sensitivity spectrometer "SPECTROMAX" company "SPECTRO", Germany; studies of macro- and microstructure (liquation heterogeneity, grain boundary structure, non-metallic inclusions, excess phases, etc.) were carried by the methods: metallographic and electron microscopic with micro-X-ray spectral analysis; tests of forgings for durability against intergranular corrosion (IGC) were carried out by the methods of AMU, GOST 6032. Results. It was established that experimental forgings rejected by UZK, in contrast to suitable forgings, were characterized by: the presence of areas of liquation inhomogeneity of steel, including high content of δ-ferrite; local defects of the macrostructure in the form of flocs; areas with anomalous multi-grained microstructure with a grain size of –2 to 8 points according to GOST 5639; release of chromium carbides at the boundaries of austenitic grains; susceptibility to intergranular corrosion (ICC). All forgings contained non-metallic inclusions within the permissible limits according to GOST 1778, as well as titanium carbides. Scientific novelty. For the first time on the basis of complex researches it is proved that the main reason of unsatisfactory results of UZK of experimental forgings from high-alloyed austenitic steel, is the anomalous multigrain structure connected with the unstable temperature-deformation mode of forging of ingots. Practical significance. Recommendations for improving the structure and improving the quality characteristics of industrial forgings made of high-alloy austenitic steels in terms of industrial production have been developed.

Structural integrity assessment and lifetime prediction for the control rods couplings of the WWER-440 reactor. Part 3. Optimi- zation of post-irradiation recovery annealing of the control rods couplings

The optimization of post-irradiation recovery annealing of metal of control rods couplings (marten- sitic-ferritic stainless steel 14Kh17N2 grade, analogue of AISI 431 steel) is carried out. It is shown that the optimized recovery annealing leads to complete recovery of the mechanical properties of coupling metal embrittled under neutron irradiation. The recovery annealing does not reduce corrosion resistance of control rod tube made of austenitic stainless steel 08Kh18N10T grade (analogue of AISI 321 steel).

On corrosion resistance of nitrogen-containing steel in abrasion conditions

This work presents tests’ results on the corrosion and erosion resistance of nitrogen-containing steel 04Kh20N6G11M2AFB in comparison with steel 08Kh18N10T, AB2-2 and VT1-0 titanium alloy. It is difficult to determine steel wear resistance against ice with abrasive (20%) due to the rapid melting of ice. It has been shown that the most effective methodology for assessing the corrosion-erosion resistance to simultaneous impact of abrasive and a 3.5% solution of NaCl to the metallic surface consists in the determination of the repassivation rate of the oxide film. It was found that the electrochemical potential of 04Kh20N6G11M2AFB stainless steel recovers 1.5 times faster than that of 08Kh18N10T steel, so it is very promising to use nitrogen-containing steel as a cladding layer of bimetallic alloy for icebreakers belts manufacturing.

Swelling of VVER-1000 Core Baffle: Numerical Modeling and Direct Measurement of its Geometrical Dimensions

Traditionally, the brittle strength evaluation of reactor pressure vessel was the central issue in lifetime assessment of Ukrainian nuclear power plants (NPPs). The problem of swelling of the reactor core baffle only recently got due attention from the side of operator. Here the most efforts were given on numerical modeling of austenitic steel 08Kh18N10T swelling and its effect on induced stresses in core baffle and distortion of its geometry. The calculation shows that essential changing of core baffle dimensions is expected after 35–40 years of operation. Eventually this can lead to the contact with the core barrel. Yet, these predictions contain the big number of uncertainties related to the input data used in analysis: fluence distribution; temperature variation due to heat release induced by neutron and gamma radiation; thermal-hydraulic boundary condition between the baffle and coolant; and, especially, the adopted law of swelling in dependence with above factors as well as mechanical stresses. So, the second task was to measure the real geometry of baffle after 27 years of operation, to determine its change and compare these results with the numerically calculated data with accounting for the design tolerances. Thus, the spatial measurement system (SMS) equipped with ultrasonic gages was designed. It contains the central vertical beam which can move in vertical direction and rotate. To the lower end of the beam four horizontal levels are attached, which are equipped with device resistant to the hot water and radiation. The gages are used to measure the shortest distances to the edges of baffle. Two types of results were obtained. The first one are the measurements in the different horizontal planes obtained by rotation the SMS around the vertical axis with angular steps equal to 1 degree. These results were difficult to handle with and required a special mathematical treatment due to the possible shift of the centre of measurement. The second set of measurements was performed by moving the SMS in vertical direction. These data demonstrate the change of distance with the height. The results clearly show that problem of swelling do exists, and, in general, the measured patterns of the distortions along the vertical and angular coordinates correspond to numerically obtained results. Further work on baffle integrity is however needed.