Influence of Tempering in Different Melting Routes on Toughness Behavior of AISI 4340 Steel

Martensitic microstructure of AISI 4340 steel can be heat treated to achieve the desired mechanical properties. However, mechanical properties degrade owing to the impurities and cleanliness during the steel production. In this work, AISI 4340 steel was produced through three different routes, such as vacuum degassing (VD), electro slag remelting (ESR), and vacuum arc remelting (VAR), followed by austenizing, hardening, and tempering. Further, mechanical characterization such as tensile, hardness, and toughness were carried out in a wide range of tempering temperatures (171-649 degrees C). A variation in mechanical properties was observed due to the evolution of precipitated carbide with the tempering temperature in all three routes. A thicker carbide layer along the martensitic lath boundary led to higher embrittlement in VD and VAR for tempering regime 171-427 degrees C. The absence or lesser embrittlement in ESR attributes to the homogeneously distributed fragmented carbides. Martensitic lath coarsening, ferritic phase formation along with the precipitated carbide distribution, significantly enhances the fracture toughness over the impact toughness at higher tempering temperature (> 316 degrees C). The difference in the mechanical properties in all the three routes is found to be sensitive toward the chemical composition causing a marked difference in the carbide precipitation and its distribution along the martensitic lath boundaries.