Papers by Author: Vlastimil Vodárek

Abstract: Applications of advanced heat resistant ferritic steels in boiler repairs require detailed information on the creep behaviour of welds made of various combinations of steels. The paper deals with the results of hardness and microstructure characterization of a dissimilar circumferential weld of 14MoV6-3 and P91 tubes after about 10 years of service exposure in a boiler operated at 580 °C and steam pressure of 10.3 MPa. The P91 tube (f38x4 mm) was welded to the 14MoV6-3 tube (f38x6.3 mm) using the GTAW (141) technology. Bőhler-FOX IN 9-IG (3Cr0.5Mo0.3V) wires were applied as a filler material. Microhardness evaluation after long-term service exposure revealed two carburized zones, values in these zones did not exceed 350 HV0.5. The slowdown of recrystallization in partially decarburized areas of the 14MoV6-3 and the WM suppressed the formation of soft ferritic bands along fusion lines. This phenomenon is related to the high thermodynamic stability of V(C,N) particles in vanadium-bearing low alloy heat resistant ferritic steels at temperatures below 600 °C.

Abstract: This paper deals with results of detailed investigations on a failed cast upper ram (63 tons) of a die-forging hammer. Subsurface cast defects primarily represented the “weak areas” responsible for the initiation of fatigue cracks during industrial exploitation. These defects mostly corresponded to gas bubbles. The occurrence of Ti (C,N) particles on the surface of these bubbles indicated that these defects were probably a consequence of release of gas products from the casting mould and/or mould's painting during the pouring of liquid metal. The evidence of fatigue initiation on repair welds was obtained only in one case. It was revealed that in this case the cast defects on the surface of the guiding radius were not completely cut off before repair welding. The remnants of the cast defects in the interface weld/base material initiated the fatigue crack. Basic mechanical properties of the subsurface layer of the upper ram complied with the design requirements.

Abstract: This paper deals with the formation of sulphides in as-cast grain-oriented electrical steel (GOES) thin strip during solidification and subsequent cooling through the (δ + γ) field. Chemical composition of the strip was as follows, in mass %: 0.034 C, 2.81 Si, 0.06 Mn, 0.024 S, 0.20 Cr, 0.15 Cu, 0.055 Ni, 0.0011 Ti, 0.0056 N and 0.002 Al. It was found out that chemical composition of coarse sulphides, formed in the area of final solidification, was very different from the composition of fine sulphides precipitated in the two-phase region. Coarse sulphides were rich in iron. Fine complex sulphides were identified as the Cr₂CuS₄ phase. The effect of fine sulphides on the austenite decomposition at the end of the (δ + γ) field was studied.

Abstract: The paper deals with results of long-term stress rupture tests on „cross-weld“ specimens made of HR3C–P92 heterogeneous welds. Stress rupture tests were carried out in air at temperatures of 625 and 650 °C up to ca 20 000 h to rupture. Creep rupture strength values of HR3C–P92 welds for 10⁴ h at both 625 and 650 °C were calculated. The preferential location of failure was the intercritical part of the heat affected zone in P92 steel. Local changes of hardness during creep exposure were evaluated by hardness profiles across the welds. Metallographic studies were performed in individual parts of heterogeneous welds. A special attention was paid to precipitation reactions in both base materials and heat affected zones.

Abstract: This paper deals with the formation and decomposition of Widmanstätten austenite during solidification of the thin belt-casted strip made of a grain oriented electrical steel (GOES). Solidification of liquid steel starts with the formation of d-ferrite. Cooling in the delta + gama phase field results in the formation of a small fraction of Widmanstätten austenite by displacive transformation accompanied by carbon partition. Widmanstätten austenite laths have an orientation relationship with the ferrite grain into which they grow. Furthermore, they form a flat low energy interface along the ferrite grain boundary. In order to minimize the interfacial energy, ferrite grain boundaries in the vicinity of flat austenite/ferrite interface facets are forced to migrate which results in straightening of these grain boundaries. If parallel Widmanstätten austenite laths form in two adjacent ferrite grains, zig–zag ferrite grain boundaries arise. Precipitation of sulphides along ferrite/austenite interfaces make it possible to study the early stages of austenite decomposition under the delta + gama phase field. It starts with the formation of epitaxial ferrite accompanied by further partitioning of carbon into remaining austenite. The growth of epitaxial ferrite into the flat ferrite/austenite interface facets along ferrite grain boundaries results in a wavy shape of these ferrite grain boundaries. Finally austenite transforms either to pearlite or to plate martensite.

Abstract: The paper deals with microstructure characterization of a full-scale X14CrMoVNbN 10 1/ 27NiCrMoV 15-6 heterogeneous weld intended for IP/LP welded rotors. The welded rotors are being developed for modern turbines of coal fired power plants operating above 570 °C. The full–scale weld has been shown to be of good internal quality. Hardness surveys revealed the effects of post weld heat treatment parameters on weld metals and parent steels. Microstructure and precipitation of minor phases in parent materials, heat affected zones and weld metals in the state after the final stress relief annealing have been studied.

Abstract: The role of copper additions to GOES has not been fully understood yet. Several mechanisms have been proposed: stabilization of austenite during hot rolling in two phase α+γ region, precipitation of ε - Cu, dissolution and re-precipitation of Cu-bearing sulfides, segregation of copper at grain boundaries and support of deformation by twinning and shear during cold rolling.This paper deals with minor phase evaluation in Cu–bearing GOES after following manufacturing steps: a) the first cold rolling and decarburization annealing at 820 °C, b) the second cold rolling followed by a slow laboratory heating to the temperature of primary recrystallization (620°C). Microstructure analysis was carried out using EBSD and TEM techniques. Results of experimental studies were compared with Thermocalc predictions of the copper effect on equilibrium phases in GOES.

Abstract: The balance between strength and toughness of supermartensitic steels can be controlled by heat treatment parameters. Quality heat treatment usually consists of quenching and single or double tempering above the A_c1 temperature. Such a treatment results in stabilization of reverted austenite in the tempered martensite. Tempering can also be accompanied by intensive precipitation processes, especially in high alloyed grades. A detailed TEM characterisation of precipitation processes in a 13Cr6Ni2.5MoTi supermartensitic steel proved that single tempering at temperatures of 600°C and 690°C was accompanied by precipitation of three minor phases: MX (TiX), M₂₃C₆ and Laves phase (Fe₂Mo type). Precipitation processes were more intensive at 690°C. Volume fractions of MX and M₂₃C₆ phases were low. Laves phase precipitation was intensive and particles of this minor phase grew fast. However, thermodynamic calculations using the Thermocalc software suggest that Laves phase is not an equilibrium phase in the steel under consideration.

Abstract: Crystallization of metallic materials occurs during their casting and following solidification. The influence of directional material solidification, the size of grains, chemical heterogeneity and existence of any non – compactness, can negatively affect properties of the final product. Microstructural analysis of its longitudinal section was performed to study heterogeneity in a continuously cast steel billet. Three typical areas of the cast structure of the billet were monitored (the chill zone, columnar crystals area and central area of equiaxed crystals). EDX microanalysis and micro-structural analysis were used for evaluation of chemical heterogeneity. Via X-ray microanalysis, concentrations of the following elements were observed: manganese, chromium, silicon, phosphorus, molybdenum and iron. The microanalytical measurements were completed by microhardness measurements in the investigated areas. Based on the measured data it was proven that central areas were enriched with all the substitutional elements including a considerable increase in phosphorus concentration.

Abstract: The presented paper is devoted to the study of hydrogen diffusion characteristics in the C-Mn-Si TRIP 800 steel. The steel was tested in three different states: in as-received state after hot and cold rolling and subsequent heat treatment; and furthermore after 5% and 10% tensile deformation. Hydrogen diffusion characteristics were studied by means of electrochemical permeation method. Two build up transients corresponding to lower and higher charging current densities as well as a decay transient were recorded during experiment. The lowest values of hydrogen diffusion coefficient (from 1 to 3.5.10^-7 cm².s^-1) were observed during the first build up transient; the value of 3.5.10^-7 cm².s^-1 corresponded to 10% tensile deformation. During the 2^nd build up transient corresponding to the higher charging current density, hydrogen diffusion coefficients increased markedly reflecting thus the fact that hydrogen trapping was less pronounced. For decay transients hydrogen diffusion coefficients were situated between values obtained for the 1^st and 2^nd build up transients. In all studied states, a rather high sub-surface hydrogen concentration was observed during the 1^st build up transient rising to 12.6 ppm of hydrogen in as-received state. The obtained results are explained taking into account steel microstructure and hydrogen trapping.