The relation of microstructure and fracture properties of electron beam melted,modified SAE 4620 steels

A method is described for the transmission and scanning electron microscope study of the relationship between the microstructure and the fracture properties of two quenched and tempered, electron beam melted, modified SAE 4620 steels consisting of tempered low carbon martensite. Among all the microstructure constituents considered, the constituentR (randomly oriented, “tempered low carbon martensite, TLCM”) achieved the highest probability for dimple fracture. The thick TLCM laths (designated as the microstructure constituent II) exhibited higher probability of dimple plus quasi-dimple mode of fracture than the thin laths (I). It is concluded that the steel EB1035 derived the high toughness from a) the high concentration of the “high toughness” microstructure constituentsR and II, b) “non-embrittled” prior austenite grain boundaries with 50 pct probability for smooth plus quasi-smooth mode and 50 pct dimple plus quasi-dimple mode of intergranular fracture. In contrast, besides having low content ofR and II, the steel EB1014 displayed “completely embrittled” prior austenite grain boundaries with 100 pct probability for smooth plus quasi-smooth intergranular fracture. The conclusions derived from the microconstituentsR, II and I seemed to reflect the “embrittling” effect of decreased spacings between the pseudo twin related laths and between the lath boundary cementite films, and the “toughening” effect of the randomly oriented laths. Auger spectra obtained from the fracture surface before and after sputtering is analyzed to determine the presence of grain boundary sulfur segregation.