Welding of High-Strength Steels for the Automotive Industry

Transport based on lighter and more durable vehicles is significant for different branches of industry. In automotive sector, high-strength martensitic steels play an essential role, because of yield stress and ultimate tensile strength. The article is focused on examining and comparing the behaviour of S960QL (Q-Quenching and Tempering, L-Low notch toughness testing temperature) steel as a representative of HSS grade steel and Docol 1200M (M-martensitic) steel as a representative of AHSS grade one. HSS and AHSS steels are relatively not very well weldable, as evidenced by the significant difference in the mechanical properties of the base material and the weld. The HAZ (Heat Affected Zone) and weld reflect cracks. Another serious disadvantage is related to the two times smaller value of ultimate tensile strength of a weld than in the case of base material. The differences in strengths are the consequence of chemical composition and microstructure of the base material and weld, which results as effect of electrode wires with increased nickel and molybdenum content and much lower sulphur and phosphorus content that affects much better mechanical properties. The weld metal deposit contains mainly martensite and bainite with coarse ferrite, while the parent material contains mainly martensite and fine ferrite. Non-destructive inspections on macro-specimens were corresponded with further destructive test results (tensile strength, hardness, fatigue, metallographic structure analyses). Main aim of the article is to select one of the tested materials for the construction of a movable landing, taking into account the properties of the joint after welding with the use of various parameters.