Materials Science Forum Vol. 782

Abstract: Carbide spheroidisation is a significant metallurgical process, which contributes to profound changes in microstructure and mechanical properties. Carbide spheroidising occurs through diffusion, which is a long-term and energy-demanding process. The holding times, sometimes up to tens of hours make soft annealing one of the most expensive heat treatment processes. The process was newly designed at the company COMTES FHT shortens carbide spheroidising several times and therefore delivers considerable time and cost savings. The heat treatment was performed using induction heating. The purpose of this annealing process is to obtain globular carbides uniformly distributed in the matrix and to achieve overall softening. The present paper explores the effect of the newly-designed thermal schedules on the cementite lamellae fragmentation, on the decrease in hardness in bearing steel grade 100CrMnSi6-4 and on processing times.

Abstract: The microstructure development in cold rolled electrical steel under dynamic heat treatments was subjected to investigation. Significantly distinguish types of microstructures were obtained in the investigated steels confirming the different character of grain boundary motion. Application of annealing temperature within two phase region (austenite+ferrite) leads to abnormal grain growth in silicon steels. Moreover, in the optimum temperature range, there was a particular temperature leading to the most optimal microstructure and texture[1]. The effect of Si content on the phase transition temperature of the electrical steel (0.6, 1, 2.5, 2.9 % Si) was studied by using differential scanning calorimetry (DSC) analysis. The result indicated that DSC analysis could be used to detect the shift temperature of phase transformation in the electrical steel with different Si addition. DSC have been used in thermochemical studies and as complementary to the study of phase transformation. It can be used as a compliment to optical and electron microscopy.

Abstract: Steel 7CrMoVTiB10-10 is a Cr-Mo medium alloy steel with addition of vanadium and titanium as microalloying elements and with further addition of boron. The resulting microstructure after the heat treatment takes the form of a tempered bainite. In hot rolling of seamless tubes using this particular steel grade, it is very important to find the optimum heat treatment parameters in order to obtain the final mechanical properties according to STN EN 10216-2+A2. In this paper, various optimization concepts regarding temperature and time of tempering are being presented. Results obtained were utilized in production of seamless steel tubes in Železiarne Podbrezová.

Abstract: Steel grade 14MoV6-3 is a low-carbon microalloyed steel with addition of chromium and molybdenum. This medium-strength steel exhibits a ferritic-bainitic microstructure after the heat treatment. This grade is designed mainly for power industry applications, withstanding operating temperatures up to 580 °C; in Železiarne Podbrezová, this particular grade is used for production of hot rolled seamless boiler tubes. In this paper we present the basic chemical concept of 14MoV6-3 steel along with its mechanical properties after the heat treatment. Further, analysis of the final microstructure, carbide phases and precipitation of vanadium is being presented as well. For this purpose, the yield stress theory has been proposed along with predictive nomograms for selected ferritic-bainitic phases. According to the results of DTA analyses, necessary conditions for heat treatment after rolling have been proposed. Finally, CCT diagrams for required ferritic-bainitic structure are presented as well.

Abstract: This paper deals with the characterization of hardness, microstructure and precipitation processes in homogeneous T24 welds in the as-welded state and after laboratory PWHT at 745°C for 1 and 6 hours, respectively. Tempering of a T24 steel during quality heat treatment is accompanied by intensive precipitation of MX, M₇C₃ and M₂₃C₆ particles in the bainitic matrix. It was found that SAW welding resulted in a pronounced dissolution of minor phases in the base materials close to the joined faces. In the as-welded state microstructure of the CGHAZ was bainitic and the HV10 level in this part of the HAZ reached nearly 400 units. The dominant strengthening mechanism in the CGHAZ was attributed to transformation strengthening due to decomposition of austenite to bainite. Also the HV10 level in the bainitic filler metal (ca 320 HV10) was very close to the maximum acceptable hardness. PWHT at 740°C for 1 hour resulted in a significant decrease of hardness in the weld metal and especially in the CGHAZ. TEM studies revealed that recovery of bainite in the CGHAZ and the weld metal was accompanied by intensive precipitation of MX, M₇C₃ and M₂₃C₆ minor phases. The hardness level in the CGHAZ dropped to approximately 255 HV10. Prolonging of tempering at 740°C to 6 hours resulted in a modest softening in all parts of the weldments due to more pronounced both precipitation processes and recovery of bainite.

Abstract: This paper is focused on the research of a dissimilar metal weld (DMW) behavior during long-term NPP (VVER) operation. Investigated material comes from decommissioned non-operated VVER 440 reactor (NPP Nord, Greifswald) in Germany. The weld joining of ferritic 22K and austenitic 08Ch18N10T steels was performed in VÍTKOVICE a. s. The buttering first and second DMW layers are made from Sv-10CH16N25AM6 and Sv-04CH19N11M3 materials, respectively; the filler metal is EA-400/10T. The DMW microstructure and microhardness were evaluated in original state and after simulation of 30, 40 and 60 years of operation. Accelerated ageing at 450°C was designed to simulate long-term operation of the material. The aim of the research was to compare the original DMW with the aged ones and to determine the possible long-term operation effect on their microstructure and microhardness. Light optical microscope Nikon EPIPHOT 300 equipped with analyzer NIS Elements 3.0 and microhardness tester MHT Anton Paar 4 were used for the evaluation. The research program funded by ČEZ Company was ordered by ÚAM Brno in ÚJV Řež, a. s.

Abstract: Surface coatings protection is one of the most important processes ensuring efficient and economic use of basic materials, mostly of lower-quality. At interface of clad and basic material intermetallic phases are formed, representing quite different matrix with dissimilar properties unlike the welded materials. One type of surface coating is explosive bonding which belongs to group of pressure welding. The work is focused on some mechanical properties, micro-and nanohardness controlled by AFM and interface shape line, in homogeneities in vicinity of the wave joint both in basic material and in vicinity of the Ti and Cr/Ni stainless steel matrix weld line. Investigated weld was both in as-received state and after heat treatment carried out at 600°C/90 minutes/air. Phases has been identified X-ray diffraction performed BW-5 beamline applying synchrotron radiation, and Tiα, Fe-fcc and Fe-bcc were detected at interface area, whereas intermetallic phases were not revealed.

Abstract: Welding and pad welding are widely used methods of repair and regeneration of defective magnesium alloys castings. AZ91 is one of the most popular magnesium alloys used at temperature up to 120°C. Therefore there is need to study the influence of welding and elevated temperature exposure on the microstructure and properties of AZ91 weld joints. This paper discusses microstructural stability of AZ91 gas tungsten arc weld joint in T6 condition (solution treated and aged). In order to examine the weld joints microstructural stability, the long-term annealing has been carried out at 120°C. Hardness of the base metal and fusion weld was measured after 250, 500, 750 and 1000h exposure respectively. Additionally, the base metals and the welds microstructure after 1000h annealing was investigated. Both in the base metal and in the weld, long-term annealing caused decrease in the intermetallic phases volume fraction. However, neither phase composition nor hardness of the weld joint were influenced by the heat treatment. Based on these results, it could be stated that the microstructure of investigated weld joint is stable at working temperature.

Abstract: Welds of thick plates (304 SS) cladded with Ti of commercial purity both in as received state and subsequently heat treated (HT, 600°C/1.5 h/air), charged by hydrogen, were investigated after fatigue tests (tension-pressure) with amplitude of 20 Hz. Simultaneously, samples of above mentioned welds were also exposed in H₂S in accord with NACE Standard TM0284-2011 to find hydrogen induced cracking (HIC) response. Charged welds after the HT showed by 15 MPa higher threshold level than the welds without the HT. The HIC tests generally demonstrated favourable results both after as weld state and after subsequent HT. Short and thin cracks were observed, exclusively located in intermetallic phase areas, where approx. 16-20 wt. % of Ti was revealed. Fracture surfaces of fatigue bars showed maximal failure in areas with 42-97 wt. % of Ti.

Abstract: Liquid metal embrittlement ( LME ) is a phenomenon , where liquid (molten ) metal is mostly intergranulary (but not only ) penetrated into solid metal and causes its brittle fracture. The LME is usually related to low melting metals (zink, tin , cadmium ...) ,which are in the contact with higher molten ones ( steels, Ni alloys...). Cases of LME as a Cu penetration into pipeline welded joint of 15G2S steel ,a hot temperature corrosion in the cement works shell plate made of AISI 310 stainless steel attacted by Ni₃S₂ (Ts= 644°C) and a vanadium corrosion in the boiler vessel (10CrMo9-10 steel) due to V₂O₅ + Na₂SO₄ eutecticum ^(~600°C ) are concerned.