Coarse Grained Heat-Affected Zone Microstructure and Brittleness of Ti-Nb-B Microalloyed High Toughness and Wear Resista

    
    Ti-Nb-B microalloyed high toughness and wear resistant steel has become the new favourited in the engineering applications due to its superior mechanical properties and super-toughness at elevated temperatures. The microalloying combined with the grain refinement is responsible for delivering the ideal strength values and toughness of the steel. However, the heat treatment process of steel can often cause serious brittleness of the material as a result of undesired precipitation, coarse grain structure, and increased hardness.
    
    This article discusses the effect of coarse grained heat-affected zone (CGHAZ) microstructure on the brittleness of Ti-Nb-B microalloyed high toughness and wear resistant steel during heat treatment. In particular, it reviews the effects of grain size, the presence of boron, and the presence of nitrides on the resistance of the steel to embrittlement.
    
    First of all, when heat treating Ti-Nb-B microalloyed steel, the grain size of the material increases significantly due to the formation of new grains and recrystallization. The larger grain size induces a number of undesirable effects, including higher microhardness and a decrease in intergranular fracture toughness, resulting in an increased susceptibility for brittle failure. Moreover, the larger grain size reduces the spheroidizing rates and influence their final composition. In addition, since grain size has an influence on both mechanical and hardening behaviors, it is essential to maintain a proper grain size when heat treating Ti-Nb-B microalloyed high toughness and wear resistant steel.
    
    Secondly, boron has been found to have an important role in the microstructure of Ti-Nb-B microalloyed high toughness and wear resistant steel. It has been observed that boron can reduce the formation of nitrides and borides during heat treatment, resulting in improved toughness. Additionally, the release of nitrogen bound in steel by the addition of boron can increase the carbonitride ledage phase and improve the resistance to brittle fracture. It has also been suggested that boron can act as an alloying element to reduce the grain size of the steel.
    
    Finally, the microstructure of Ti-Nb-B microalloyed high toughness and wear resistant steel is also affected by the formation of nitrides during heat treatment. During the heat-treatment process, nitride form within the ferrite matrix which decreases the hardness and toughness of the steel. It has been observed that the nitrides present in the microstructure of Ti-Nb-B microalloyed high toughness and wear resistant steel increase the ductility of the steel, thus reducing brittleness.
    
    From the discussion above, it is clear that coarse grained heat-affected zone microstructure of steel can have a significant effect on its brittleness. The larger grain size reduces both its strength and toughness and increases the susceptibility for brittle fractures. Furthermore, the presence of boron and nitrides can reduce brittleness by influencing the grain size and hardness respectively. It is therefore necessary to optimise the microstructural characteristics of the steel while heat treating Ti-Nb-B microalloyed high toughness and wear resistant steels to ensure improved properties and better fatigue resistance.

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