Effects of Laser Welding on the Microstructure and Fatigue Behavior of Strenx 700 CR Steel

Abstract This study examines the cold-rolled microalloyed steel Strenx 700, with a minimum yield strength of 700 MPa, used in the manufacture of agricultural machinery and implements. The aim is to improve the weight/mechanical strength ratio and apply new advanced welding processes, such as laser welding. In the context of the agricultural machinery industry, laser welding is gaining prominence in the welding of thin microalloyed steels, mainly due to its small heat-affected zone and welding speed, which allow for superior mechanical properties, longer fatigue life, and excellent surface finish of welded joints compared to GMAW and GTAW welding processes. In this context, the behavior of laser-welded joints in Strenx 700 microalloyed steel was evaluated in the welding of plates used in agricultural implement components subjected to cyclic loads. The microstructural properties, mechanical properties, and fatigue life are not yet clearly documented in the literature. This work aimed to evaluate the performance of manual welds produced using a CO₂ laser process and to elucidate their effects on the microstructure, mechanical properties, and fatigue behavior of 2 mm thick Strenx 700 cold-rolled steel sheets. Based on technical evaluations through pre-tests, it was determined that the welds would be performed autogenously with welding powers of 1100, 1300, and 1500 Watts, using joints with parallel edge preparation without root opening, in order to optimize the processes and reduce industrial costs. After welding, joint characterization was performed using macrographs and micrographs obtained by optical and electron microscopy, in addition to microhardness mapping, tensile testing, and evaluation of uniaxial fatigue life in tension-tension conditions. The results indicate satisfactory macroscopic weld formation. In the fusion zone, the microstructure consisted predominantly of martensite, Widmanstätten ferrite and bainite. In the coarse-grained heat-affected zone (CGA), bainite, Widmanstätten ferrite, and martensite were observed, while in the fine-grained heat-affected zone (FGA), ferrite and martensite were identified. The base metal was composed of ferrite, bainite, and martensite. The mechanical strength of the welded joints was comparable to that of the base metal; however, the elongation values were lower than those of the base metal. The mean fatigue life of the base material was 450,128 cycles, significantly higher than that of the welded conditions, with average values of 50,055 cycles for the welded group (c), followed by group (b) with 44,962 cycles, while the welded group (a) presented a mean fatigue life of 30,338 cycles. As main conclusions, we can observe the good microstructural formation of the joints for all welding conditions, with adequate mechanical strength values, but with reduced elongation and predominant fracture in the heat-affected zone and in the fusion region. It is also possible to conclude that the average fatigue life values were lower for the welded conditions compared to the base material, this result being strongly impacted by the presence of welding defects related to the process.