Liquid phase sintering mechanism and densification behavior of boron-alloyed Fe-Ni-Mo-C-B powder metallurgy steel

For powder metallurgy (PM) iron-based materials, boron is an effective element for facilitating liquid phase sintering (LPS). However, the roles of the various alloying elements in the LPS of boron-alloyed PM steel have not yet been fully identified. The main objective of this study was thus to clarify the effects of alloying elements on the LPS and microstructure of Fe-xNi-0.5Mo-0.5C-0.4B (x=0-4 wt%) PM steel using SEM, EPMA, EBSD, DSC, and density analyses in combination with thermodynamic simulation. The results demonstrated that the 0.5 wt% C addition plays a decisive role in the LPS behavior because it induces the formation of secondary liquid and increases the equilibrium liquid volume at 1250 degrees C by similar to 5 vol%, resulting in improved densification. The incorporation of C atoms in the liquid can change the structure of the compound phase from M2B boride to M-3(B,C) borocarbide, where M represents the metallic elements. Moreover, the 4 wt% Ni or 0.5 wt% Mo additive does not apparently affect the LPS behavior, although Ni can slightly lower the liquid generation temperature and increase the theoretical liquid volume. Based on the findings of this study, a detailed LPS mechanism is proposed, and the sintered density of Fe-xNi-0.5Mo-0.5C-0.4B can be optimized. (C) 2017 Elsevier Ltd. All rights reserved.