The Comparison between Mechanical Properties of Laser-Welded Ultra-High-Strength Austenitic and Martensitic Steels

    
    Introduction
    
    The increasing demands and complexity of modern engineering projects have given impetus to the development of strong, ultra-high strength materials for structural applications. These materials are not only adopted in structural components and components for special applications, but also for lightweight structural applications. The high strength and excellent corrosion resistance of martensitic steel, particularly ultra-high-strength martensitic steel, have been making their way into the field of ultra-high-strength steels for ultimate structural applications.
    
    However, the high-strength and excellent corrosion resistance of martensitic steel is accompanied by challenging issues such as low weldability, relatively poor ductility and fracture toughness, and high notch-sensitivity. The low fracture toughness of martensitic steels can be a result of retained austenite, which is caused during the quenching phase. Due to this reason, the optimum strength and toughness of martensitic steels are not easily achievable with conventional fabrication techniques. Similarly, austenitic steels, while having excellent weldability, lack the strength of the martensitic steel and have relatively lower equivalent strain capacity and yield strength compared to their martensitic counterparts.
    
    Therefore, the aim of this article is to properly compare the mechanical properties of austenitic and martensitic steels laser welded together. This article will look into the differences in the mechanical properties of the two materials, and offer suggestions for improving the mechanical properties of the welded joint.
    
    Difference in Mechanical Properties
    
    The mechanical properties of laser-welded austenitic and martensitic steels differ due to various factors like presence of grain boundary and surface characteristics of the as-welded joint. It is well known that austenitic steels, due to their higher workability, show better plasticity and ductility than their martensitic counterpart when laser welded together. This is why austenitic steels are preferred for high stress applications such as special structural members.
    
    On the other hand, martensitic steels possess the highest yield strength and tensile strength, which makes it a preferred choice for high strength applications. However, their brittle character makes them a poor choice for welded joints. As laser welding of steels is a complex process, the microstructure of the weld is affected by the starting microstructure of the steels. It is well known that austenitic steels have smaller grains, whose weld joins with the larger grains of the martensitic steel can result in increased brittleness of the joint.
    
    Figure 1. Microstructure of the laser-welded steels
    
    Furthermore, the presence of retained austenite in the martensitic steel can also lead to decreased ductility and weldability in the as-welded joint. This is also due to the increased strain hardening in the austenitic steels, which can result in a decrease in joint strength.
    
    Therefore, it can be clearly seen that laser welding of ultra-high-strength austenitic and martensitic steels is an intricate process, which may cause significant changes in the mechanical properties of the steels when welded together.
    
    Improving Mechanical Properties of Laser-Welded Joints
    
    Due to the differences in mechanical and metallurgical properties in the two materials, it is essential to employ strategies that improve the mechanical properties of the laser-welded joints. For instance, a pre-heating cycle prior to welding can alleviate the differences in the microstructures of the materials, which, in turn, results in better mechanical properties of the joint. Additionally, using processing parameters such as higher welding speed and lower welding current can help to suppress the heat-tinting of the welded joint and improve its microstructure.
    
    Furthermore, a proper selection of welding consumables, such as low hydrogen consumables, can also be used to reduce the brittleness of the weld and improve its mechanical properties. Additionally, different weld geometries can also be explored to improve the mechanical properties of the joint.
    
    Conclusion
    
    This article has compared the mechanical properties of laser-welded austenitic and martensitic steels, while taking into account the various factors that contribute to the mechanical properties of the joint. It has been shown that laser welding of austenitic and martensitic steels is a complex process that can cause a decrease in the weld properties if not properly managed and controlled. To ensure that the joint maintains its strength, appropriate strategies such as pre-heating, selection of proper welding consumables, and different weld geometries can be employed.
    
    

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