Effect of the Cooling Rate of Thermal Simulation on the Microstructure and Mechanical Properties of Low-Carbon Bainite Steel by Laser-Arc Hybrid Welding

Abstract

A new kind of low-carbon bainite steel with excellent strength and toughness was developed, serving as the bogie of the next-generation high-speed train. However, the softening of the heat-affected zone (HAZ) in laser-arc hybrid welding (LAHW) needs to be overcome. In this study, the effect of the cooling rate of the LAHW process on the microstructure and mechanical properties in the HAZ was explored via thermal simulation. The results showed that with increased cooling rate, the grain size increased, the content of lath martensite decreased, and the lath bainite gradually changed to a granular shape in the thermal simulation specimen. With the decrease in the cooling rate, i.e., with the increase of t8/5, the strength–toughness matching of the material showed a downward trend. The thermal simulation specimen with a t8/5 of 6~8 s had higher strength and good toughness, which can be considered a potential welding parameter reference. The content of martensitic austenite (M-A) constituents was the main factor that determined the strength and toughness of the joint. During the tensile test, the axial force caused the material to tighten, and the transverse stress as obvious in the part of the M-A constituents that are prone to microcracks and many defects, resulting in cracks, paths, and multi-component layers in the center. As a result, the thermal cycle specimens had mixed fracture characteristics.

Charpy Absorbed Energy in Simulated Heat-affected Zone of Laser-arc Hybrid Welded Joints by High Strength Steel for Bridge Structures

“…A previous study of hybrid welding on steel plates 4.8 mm thick showed that the cooling rate in the fusion zone was calculated as 20-83.3°C/sec [14]. In the case of another previous study of hybrid welding on steel plates 6 mm thick, the cooling rate in HAZ was 25-35°C/sec [15]. The cooling rate selected in this study seemed to be within these values.…”

Investigations on the microstructure of drawn arc stud welds on structural steels by quantitative metallography

“…In contrast to the S355J2 + N and S690QL1, no LB/M but FS was found in GCHAZ. Therefore, a sufficient local C enrichment is present to form granular M/A microphases as found for linear welding [13]. A diffusion-controlled C segregation took place despite the higher cooling rates in DASW.…”

“…Whereas, microalloyed steels bring great advantages in weldability due to their lower hardenability and grain growth hindering and thus forming mainly bainitic structure [7,9,10]. Additionally, martensite-austenite constituents (M/A) are formed depending on cooling rates [13]. However, these investigations mainly focus on linear arc welding processes with typical thermocycles and heat inputs causing larger cooling times of t 8/5 > 5 s. DASW implies a specific thermocycle with extremely high heating and fast cooling [14].…”

Research and Progress of Laser Cladding: Process, Materials and Applications

“…Analyzing the molten pool flow field, temperature field and stress field during the LC process is of great help in understanding the macroscopic morphology, microstructure evolution and properties control of LC. Many scholars simulated the molten pool flow behavior, temperature field, stress field and microstructure based on fluid dynamics and physical phase field process [18][19][20][21][22].…”