Impact Toughness of Welded Joints on 34KhN2MA Steel Produced from Electrodes with Various Phase Compositions
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IMPACT TOUGHNESS OF WELDED JOINTS ON 34KhN2MA STEEL PRODUCED FROM ELECTRODES WITH VARIOUS PHASE COMPOSITIONS A. R. Dzyubyk
UDC 621.791.011; 539.512; 620.181
We determine the brittle-fracture resistance of the metal taken from various zones of welded joints of 34KhN3MA steel produced from electrodes with different phase compositions or their combinations. It is shown that, regardless of the zone of welded joint in which a notch is made in the specimen, the highest values of impact toughness are observed for the weld metal obtained from the austenitic electrode. At the same time, the lowest value of impact toughness are observed for the combination of austenitic (weld root) and ferrite-pearlite electrodes. For any applied electrode, the lowest values of impact toughness were always observed for the metal of the heat-affected zone. Therefore, this zone determines the serviceability of welded joints under dynamic loads. The results of fractographic investigations demonstrate that austenitic electrodes are optimal for the purpose of welding of sheets made of high-strength steels because they guarantee the highest fracture energy of the joints under the action of dynamic loads. Keywords: welded joints, impact toughness, fractographic features of fracture.
Introduction It is known that high-strength rolled products prove to be efficient in welded structures encountered in machine-building. However, the hazard of their failures increases in the presence of the stress concentrators caused by the defects of welded joints (WJ). The appearance of these defects is explained by the imperfect technology and conditions of welding, low quality of electrodes, coatings, and fluxes, insufficient qualification of the welders, etc. [1]. However, this does not necessarily lead to the loss of serviceability of welded elements [2]. Indeed, the mechanical properties of WJ depend on the applied materials, sizes and shapes of defects, the influence of hydrogenation, operating conditions, loads, etc. [3–6]. All these factors should be taken into account under the conditions of the periodic action of high dynamic loads upon the welded structures (especially on the hull structures). In this case, most of the materials become susceptible to brittle fracture [7, 8]. Indeed, the impact toughness is an integral characteristic of the material that depends both on the strength and plasticity of the metal and, hence, it is more sensitive to the inhomogeneity of the material [9]. In general, for the weld metal (WM), the impact toughness decreases as the area of defects in the WJ becomes larger. However, even in the case where the ratio of the area of defects (pores, faulty fusions, and slag inclusions) to the cross sectional areas of the specimen becomes as high as 30%, the impact toughness of the WM under dynamic loading is not lower than the admissible level 0.26 MJ/m 2 [10]. The technological aspects of the process of welding (especially the procedures aimed at the prevention of the appearance of cold cracks) also remain in the field of vision of th
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