Effects of Alloying Elements on Microstructure and Hardness in Wear-resistant Fe-based Alloys

Abstract

In the present study, Fe-based alloys used for powder injection molding (PIM) parts having various qualities and hardness ranges by varying chemical compositions according to thermodynamically calculated phase diagrams. Their microstructure and hardness were analyzed and compared with those of PIM specimens made from conventional Fe-based alloy powders or stainless steel powders. The Cr to B ratio (XCr/XB) and the sum of Fe, Cr, and B content (XFe+XCr+XB) were varied to design nine Fe-based alloy compositions on the basis of the composition of ArmacorTM ‘M’ alloy powders. According to the microstructural analysis results of the cast and heat-treated Fe-based alloys, large amounts of (Cr,Fe)2B were formed in the tempered martensite matrix. The volume fraction of (Cr,Fe)2B was varied from 42 pct to 91 pct with alloy compositions, and these results were well matched with the thermodynamically calculated volume fractions of (Cr,Fe)2B. The hardness of the fabricated alloys was varied from 300 VHN to 1600 VHN with alloy compositions, and linearly increased with increasing volume fraction of (Cr,Fe)2B. From the correlation data between the volume fraction of (Cr,Fe)2B and hardness, the high-temperature equilibrium phase diagram, which could be used for the design of Fe-based alloys having various fractions and hardness values of (Cr,Fe)2B, was made. A powder injection molding (PIM) product containing (Cr,Fe)2B borides was fabricated with Fe-based alloy powders, and its microstructure and hardness were investigated in relation with volume fraction of (Cr,Fe)2B. When Fe-based alloy powders were injection-molded and sintered at 1165oC, a densified microstructure with almost no pores was obtained. In the sintered microstructure, 56 vol.% of (Cr,Fe)2B borides, together with a few pores (porosity

0.5%), were relatively homogeneously distributed in the tempered martensite matrix, which resulted in the very high hardness over 600 VHN. Such a high hardness suggested that the present Fe-based alloy powders could be readily adopted for fabricating PIM products or for replacing conventional stainless steel PIM products.This study aimed at developing new cost-effective ferrous amorphous alloys in order to apply these alloys to the thermal spray coatings. To achieve chemical compositions of alloys having high amorphous forming ability, thermodynamically calculated phase diagrams of Fe-Al-P-C-B five-component system, from which compositions of super-cooled liquid having lowest driving force of formation of crystalline phases such as Fe3C, α-Fe, γ-Fe, Fe3P, and Fe23C6 could be obtained, were used. Several representative alloys having highest amorphous forming ability were fabricated by using a suction casting technique, and their microstructure, hardness, and corrosion resistance were analyzed. Using correlation between driving force of phase formation and glass formation ability, multi-element Fe-based alloys were designed. Particularly, The Fe78Al2P10.83C7.47B1.7 alloy could be fabricated fully amorphous microstructure with a maximum thickness of 1.2 mm. This alloy had better corrosion resistance than that of conventional thermal spray coating alloys, and the production cost could be lowered due to the cheaper price of alloying elements.