MAG Welding Tests of Modern High Strength Steels with Minimum Yield Strength of 700 MPa

Abstract

The modern high strength steel plates have an excellent combination of strength and toughness based on micro-alloying and complex microstructure. Retaining this combination of properties in the weld zone is a major challenge for applications in high-demanding structural construction. This work investigates the weldability of three different modern high strength steel plates, with a thickness of 8 mm. Two of the test materials were produced by a thermo-mechanically controlled process (TMCP) and one by a quenching and tempering method (Q&T). Two-passes MAG (metal active gas) welding was used with four different heat inputs. The tests implemented on all the materials included tensile, hardness profiles (HV5), Charpy-V impact toughness tests, and microstructure analysis using scanning electron microscope (SEM). For one of the TMCP steels, some extended tests were conducted to define how the tensile properties change along the weld line. These tests included tensile tests with digital image correlation (DIC), and 3-point bending tests. The most notable differences in mechanical properties of the welds between the materials were observed in Charpy-V impact toughness tests, mostly at the vicinity of the fusion line, with the Q&T steel more prone to embrittlement of the heat affected zone (HAZ) than the TMCP steels. Microstructural analysis revealed carbide concentration combined with coarse bainitic structures in HAZ of Q&T steel, explaining the more severe embrittlement. During the tensile tests, the DIC measurements have shown a strain localization in the softest region of the HAZ. Increasing the heat input resulted in earlier localization of the strain and less maximum strength. The tensile properties along the weld line were investigated in all welding conditions, and the results emphasize relevant and systematic differences of the yield strength at the transient zones near the start and end of the weld compared with the intermediate stationary domain.

Determination of Grain Growth Kinetics and Assessment of Welding Effect on Properties of S700MC Steel in the HAZ of Welded Joints

“…The methodology described in Section 2.4 provides the possibility to prepare samples where both the values of the mechanical and impact properties in a HAZ can be determined. Various authors applied temperature cycles to [9][10][11][12][13][14][15][16] fine-grained test specimens, but different limits appeared in their works. As a first, there is the T max of the temperature cycle.…”

“…Similar results were provided by study [13], where the author focused on the evaluation of S700MC weldability by various welding methods. Lahtinen et al [14] dealt with the weldability of 8 mm sheets from two types of high-strength fine-grained steels-namely, S700MC and S690QL. Two-layer welded joints made by MAG technology with four different heat input values from 0.7 to 1.4 kJ•mm −1 , which corresponded to values t 8/5 = 5, 10, 15, and 20 s. Tests carried out on all materials included the determination of ultimate tensile strength, hardness profiles (HV5), Charpy impact tests, and microstructure analysis by scanning electron microscopy (SEM).…”

Effect of the t8/5 Cooling Time on the Properties of S960MC Steel in the HAZ of Welded Joints Evaluated by Thermal Physical Simulation

“…Such a structure is sensitive to the reduction of toughness. Samardžić et al [16] and Lahtinen et al [17] analyzed the effect of the temperature input and cooling rate on the toughness of individual HAZs.…”

Influence of Welding Parameters and Filler Material on the Mechanical Properties of HSLA Steel S960MC Welded Joints

“…As the thickness of the material increases, the heat transfer from the weld is sufficient (higher cooling rate) and the resulting width of the soft zone does not result in a decrease in strength. This was demonstrated by Lahtinen et al [28] and Silva et al [29].…”