Abstract
Effects of phosphorus content and boron addition on impact toughness of medium carbon steels with an ultra-high strength level over 150 kgf/mm2 have been investigated. The toughness is deteriorated by two independent fracture modes, that is, intergranular fracture and Tempered Martensite Embrittlement (TME).Intergranular fracture is greatly enhanced with the increase in phosphorus content of steel, whereas the boron addition is effective in reducing it and increases toughness over the whole range of tempering temperatures below 400°C. However, phosphorus content and boron addition have no significant effect on TME.Auger Electron Spectroscopic (AES) observation reveals that boron addition reduces the intergranular fracture by reducing the phosphorus segregation at the grain boundaries. The estimation by diffusion calculation and McLean's grain boundary equilibrium segregation model shows that boron segregates to grain boundaries much faster than phosphorus does at austenitizing temperatures and reduces the boundary energy for phosphorus segregation.
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