The Effect of the Welding Parameters on the Properties of Thermomechanically Rolled High Strength Steels

In our days, the high strength steels play an important role in the design and construction of the welded structures. Under the different demands of applications (e.g. frames, cranes, bridges), complex expectations are developed with the materials, and the material innovation must follow these tendencies; for example beside the increased strength, preservation of the more formability. These constructions are often under cyclic loading conditions, so their typical damage form is the fatigue, which means even more complex expectation with the base material. The application of the modern high strength steels obtains an important role in these areas. In such cases, the thickness of the structural elements can be decreased altogether with the mass of the structure and the welded joints. Beside of the economic benefits, it is advantageous to the mechanical characteristics. In general, we can state that the increasing of the yield strength, the applied thickness decreases, so the amount of the filler material and the time of the welding decrease, as well. On the other hand, it is important to note, that these steels cost are two or three times higher than the average steels, furthermore the filler material costs for these steels are higher, and the weldability of these materials are more complex [1]. At the same time, the typical manufacturing technology is the welding, which – because of the heat input – modifies the original microstructure of the base material. Therefore, we achieve different mechanical characteristics (e.g. strength, toughness, hardness distribution), which is not acceptable in many cases in terms of the construction. Consequently, there is great significance of testing the welded joints of these steels, and the development of the suitable manufacturing technology. Accordingly, the properties of VOESTALPINE ALFORM 960M (S960M) thermomechanically rolled high strength steel were summarized. The weldability of these steels were introduced too, especially the effect of the linear energy. During our experiments, 15 mm thick base materials were used. The main reason was that these steels are frequently used in mobile crane, scraper and bulldozer structural elements, where the typical thickness is more than 15-20 mm. The direct investigation of these heavy plate thicknesses is not practical, because of the high costs and the long testing times, so we made a compromise with 15 mm thickness.