DETERMINATION OF THE CAUSES OF LOW SHOCK VISCOSITY OF RING WELD
DOI:
https://doi.org/10.14529/met190207Keywords:
weld, toughness of pipe steels, steel 10Mn2VNbAl, steel 09Mn2Si, overheating, widmanstett-ferrite.Abstract
The reasons for the reduced toughness of the heat-affected zones and the weld metal in welded joints can be the heterogeneity of the chemical composition (segregation) and overheating of the metal. Segregation leads to non-uniformity of mechanical properties over the cross section of the welded joint. Overheating, accompanied by grain growth, significantly increases the tendency of the weld to brittle fracture. In this study, a stamped welded tee was investigated: the main magis-tral was made of 10Mn2VNbAl steel, the extension ring was made of 09Mn2Si steel. The aim of the study was to establish the reasons for the low toughness of the annular weld of the specified product. Macrostructural studies of the welded joint showed that the seam was applied with dis-placement. The unevenness that occurs leads to a displacement of the fracture line, as a result of which the fracture takes place in the most fragile structure of the weld sections.
In the study of the microstructure of the weld metal, the presence of widmanstett-ferrite in the structure was established. This structural component is observed both at the external surface of the seam and in the center. The presence of widmanstett ferrite has a negative effect on the mechani-cal properties of the metal. Since microstructural studies showed an uneven distribution of coarse-needle-ferrite across the weld thickness, it was proposed to increase the number of passes during welding from three to four. Comparative studies of the structure and properties of samples of pilot welds showed the feasibility of such changes. So, measurements of microhardness of welded joints after welding in 3 and 4 passes showed that when applying 3 rollers, the hardness of the weld metal is lower than when applying 4 rollers, but no more than permissible values of 260 HV10. Thus, ac-cording to a study to eliminate the causes of low toughness, the following measures were recom-mended: strict control of the welding technology of weld sections (to prevent displacement of the weld axis) and the establishment of the minimum number of passes during welding (to obtain the desired mechanical properties).
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