The influence of treating welded joints edges on the strength and fatigue of S640Q steel

Weldable steels of high strength were discussed. The factors influencing the achievement of low grain structure that ensures high strength of these steels was explained. Traditional and modern methods of producing high strength steel were shown. The scheme of production line was shown, the line which is used to manufacture steel of high strength in thermomechanical way. The methods of achieving strength to fatigue used for welded joints were presented. The factors influencing the increase of strength to fatigue of welded joints were elaborated. The thermally hardened steel sheets of high strength weldable S640Q steel underwent research. Basic mechanical properties of this steel were indicated. The results of static tensile test and technological bend test were given. Then the sheets were welded by semiautomatic MAG method. The examinations of welds quality were conducted and their usefulness for further tests was stated. Mechanical properties of welded joints were determined. The mechanical properties of native material and welded joints were compared. The microstructure of welded joints in the weld axis, in heat influence zone as well as in native material was examined. Fatigue tests in a non-symmetrical cycle of lengthening – compressing for the welded joints were carried out. The computational strength to fatigue was determined and compared with fatigue strength of welded joints. Tests results were elaborated by the statistical analysis of linear regression. After conducting fatigue tests, the Wöhler diagrams were made. Afterwards, the joints welded with TIG method, were penetrated without adding filler metal. The results were presented for the fatigue tests as well as the Wohler graph for welded joints after penetration of the joints edges with TIG method without adding filler metal. The results were compared.