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[en] We have studied here the distribution of carbon and alloying elements in the martensite-austenite (M-A) constituent in intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) of a high-strength pipeline steel using atom probe tomography (APT). Notable enrichment of C (0.49 wt%) and Mn (2.32 wt%) was observed within the M-A constituent, which induced the formation of lath martensite and deteriorated the toughness. Elemental segregation in the interfacial region between M-A constituent and matrix may contribute to the debonding mechanism of M-A constituent and assist nucleation of cleavage cracks. Distribution of solute Nb indicated no apparent difference between the matrix and M-A constituent.
[en] This investigation is concerned with the toughness and microstructure of martensitically simulated HAZ in 12% Cr steel. Unnotched specimens were subjected to weld thermal cycles in a weld simulator. The parameters-peak temperatures, cooling rate, influence of PWHT and plastic deformation were cosidered. After weld simulation, the specimens were heat-treated, V-notched and impact tested. An optical metallographic examination was performed to correlate the HAZ toughness with microstructure. Also a fractographic examination was done to obtain information on the fracture mode. The toughness of the coarse grained zone and the part of HAZ subjected to a peak temperature range 700-800 deg C are lower than the other parts. However, they are still high enough. The double PWHT cycle could not improve the HAZ toughness in present study. However, if the first PWHT is conducted before the work piece is cooled below Mf, it is expected that the double PWHT may be beneficial to the toughness of the HAZ. It is also expected that martensitic welding can be used on production welds. (Author)
[en] In this work, the microstructure, macro-and micro-mechanical properties of dissimilar A302/Cr5Mo were investigated by metallographic experiments, tensile and nanoindentation tests under room temperature. Based on inversion analysis, the plastic parameters of micro-domains (small regions across weld joint) were estimated, and the comparison with the experimental results on parent metal (PM) yielded a good agreement. Therefore, by means of this inversion analysis method, elastoplastic properties of PM, welding metal(WM), fine grain heat affected zone(FGHAZ), and coarse grain heat affected zone(CGHAZ) were obtained, which could provide a straightforward way of determining plastic properties on different regions across welded joints. (paper)
[en] The paper briefly demonstrates the development of weldable construction steels in the FRG exemplified by the development of steel grade St 37 to StE 960. Improvements of steel quality, especially weldability, is expected from the ladle metallurgy process, thermomechanical rolling, intensive cooling after rolling and direct annealing by using rolling heat. Positive effects were achieved above all in lamellar tearing strength, cold cracking behaviour and in the heat-affected zone. (DG)
[de]Die Entwicklung schweissgeeigneter Baustaehle in der Bundesrepublik Deutschland wird am Beispiel des St 37 zum StE 960 kurz dargestellt. Verbesserungen der Stahlqualitaet, vor allem der Schweisseignung, werden durch pfannenmetallurgisches Verfahren, thermomechanische Walzen, Intensivkuehlung nach dem Walzen und Direkthaerten aus der Walzhitze erwartet. Positive Auswirkungen wurden insbesondere bei Terassenbruchsicherung, Kaltrissverhalten und in der Waermeeinflusszone erzielt. (DG)
[en] The static performances and failure characteristics of resistance spot welds (RSW) of steel sheets were studied in this work. Both similar and dissimilar joints of press-hardened steel grade 22MnB5 and as-rolled high-strength steel grade 1000 were examined. From metallographic analyses and micro-hardness measurements, it was shown that the microstructures of base metal (BM), heat affected zone (HAZ) and fusion zone of steel joints were noticeably different. In particular, the HAZ close to BM or subcritical HAZ (SCHAZ) of 22MnB5 steel exhibited greatly decreased hardness due to occurring tempered martensite, in which carbide precipitation was clearly observed. Furthermore, tensile shear test and cross-tension test of the welded steel sheets were carried out. The determined maximum loads, energy absorbability and corresponding failure mechanisms were analyzed and compared. The RSW samples of similar steel grade 1000 provided the highest load-carrying capacity and absorbed energy in both tests. Due to the large hardness difference and soft zone in weldment of dissimilar steel specimen, failure loads and static absorbed energies were deteriorated up to 17 and 68%, respectively. The observed fracture occurrences of all investigated joints were pullout failure mode, in which BM and HAZ were the crack initiation sites of grades 1000 steel and 22MnB5 steel, respectively. The SCHAZ of welded boron steel considerably caused the failure onset in tension shear test, but seemed to have negligible effect on cross-tension test.
[en] Defect assessment of a weld zone is important in fitness-for-service evaluation of plant components. Typically a J and C* estimation method for a defective homogeneous component is extended to a mismatched component, by incorporating the effect due to the strength mismatch between the weld metal and the base material. The key element is a mismatch limit load. For instance, the R6/R5 procedure employs an equivalent material concept, defined by a mismatch limit load. A premise is that if a proper mismatch limit load solution is available, the same concept can be used for any defect location (either a weld centre defect or a Heat Affected Zone (HAZ) defect) and for any material combination (either two-material or multi-material combinations; either similar or dissimilar joints). However, validation is still limited, and thus a more systematic investigation is needed to generalise the suggestion to any geometry, any defect location and any material combination. This paper describes the effect of structural geometry on the C* integral for defective similar welds, based on systematic elastic-creep 2-D and 3-D Finite Element (FE) analyses, to attempt to elucidate the questions given above. It is found that the existing 'equivalent material' concept is valid only for limited cases, although it provides conservative estimates of C* for most of cases. A modification to the existing equivalent material concept is suggested to improve accuracy
[en] There is described a method for strengthening surfaces of heat-treated steel parts, which includes technological procedures of carburizing by electroerosive alloying process (CEEA) and also nitriding processes. The nitriding process is simultaneously carried out with the CEEA operation for a period of time sufficient to saturate the surface layer of a part with nitrogen to the depth of a heat-affected zone. Thus, the process of so-cold nitro-carburizing process performed by the EEL method (NCEEA) takes place. To reduce a surface roughness, the NCEEA process is carried out in at least two stages, with a decreasing a discharge energy value at each subsequent stage. (paper)
[en] This study is concerned with effects of complex oxides on Charpy impact toughness of Heat Affected Zone (HAZ) of API X80 linepipe steels. Three kinds of steels were fabricated by varying alloying elements such as Ti, Al, and Mg and hot-rolling conditions to form complex oxides, and their microstructures and Charpy impact properties were investigated. The number of complex oxides present in the steel containing excess Ti, Al, and Mg was twice larger than that in the conventional steels, while their size ranged from 1 to 3 μm in the three steels. After the HAZ simulation test, the steel containing a number of oxides contained about 20 vol.% of acicular ferrite in the simulated HAZ, together with bainitic ferrite and martensite, whereas the HAZ microstructure of the conventional steels consisted of bainitic ferrite and martensite with a small amount of acicular ferrite. This formation of acicular ferrite in the oxide-containing steel was associated with the nucleation of acicular ferrite at complex oxides, thereby leading to the great (five times or more) improvement of Charpy impact toughness over the conventional steels