Beneficial Effects of the Core–Shell Structure of Primary Carbides in High-Cr (45 wt%) White Cast Irons on Their Mechanical Behavior and Wear Resistance

Abstract

High-chromium cast irons (HCCIs) are widely used in many industrial processes involving wear and corrosion, such as oil sands mining and slurry handling in the oil sands industry. In very harsh mining environment and severe working conditions, conventional HCCIs, however, often exhibit a short service life. Great efforts have been made to improve the performance of HCCIs. One of the trials is to extend the chromium content to a higher level, e.g., 45 wt%, and widen the carbon content to a range of 1-6 wt%. In this study, we investigated the wear behavior of 45-series (Fe-45 wt%Cr with 1-6 wt%C) and observed that 45-4 HCCI (Fe-45 wt%Cr-4 wt%C) had the highest hardness and exhibited remarkably high resistance to abrasion and erosion. Microstructure analysis showed that the primary carbides in 45-4 had a core-shell structure. The outer shell was M 23 C 6 , while the inner core was harder M 7 C 3 , confirmed by the EBSD technique. The ferrous matrix was eutectic colony with martensite and fine M 23 C 6 carbides. Micro-mechanical probing in combination with finite element analysis demonstrated that the core-shell structure helped reduce the interfacial stress, thus enhancing the hardness and overall wear resistance of the material.

Simulation of structure formation in the Fe–C–Cr–Ni–Si surfacing materials

“…Significant differences in the chemical composition of carbides in the analyzed alloys may cause a difference in the microhardness of carbide phases in favor of the alloy 500Cr40Ni40Si2BZr. This may be due to the higher content of chromium with a corresponding increase in the covalent component in the intra-atomic bonds [20].…”

Promoting in situ formation of core‐shell structured carbides in high‐Cr cast irons by boron addition

“…Increasing the carbon content can increase the volume fractions of carbides, leading to higher hardness. Cr helps stabilize carbides, intervene graphite formation, strengthen the ferrous matrix and form a protective passive film to resist corrosion and erosion 4,6,7 …”

“…This situation can be changed if the interfacial mismatch associated with the interfacial stress can be minimized. Recent studies have shown that such minimization can be achieved by the formation of core(M 7 C 3 )‐shell(M 23 C 6 ) structured carbides in HCCIs 5,7,9,10 . Tang et al 7 investigated HCCIs having 45 wt%Cr and observed that Fe‐45Cr‐4C HCCI with 4 wt% C had the core‐shell structured carbides, which exhibited the higher resistance to wear and erosion than its other family members without the core‐shell structured carbides.…”

“…Recent studies have shown that such minimization can be achieved by the formation of core(M 7 C 3 )‐shell(M 23 C 6 ) structured carbides in HCCIs 5,7,9,10 . Tang et al 7 investigated HCCIs having 45 wt%Cr and observed that Fe‐45Cr‐4C HCCI with 4 wt% C had the core‐shell structured carbides, which exhibited the higher resistance to wear and erosion than its other family members without the core‐shell structured carbides. Direct investigation on the interfacial properties by wear modeling and finite element analysis 11 shows that the shell of M 23 C 6 in the core(M 7 C 3 )‐shell(M 23 C 6 ) structured carbides, whose hardness is between those of the core (M 7 C 3 ) and the matrix, generates a transitional layer at the M 7 C 3 /matrix interface and thus helps reduce the interfacial stress, leading to increased resistance to wear.…”

“…However, it is unclear how such core‐shell structured carbide forms and why CSSCs were observed only in Fe‐45Cr‐4C alloy in the Fe‐45Cr‐C system (Carbon content: 1%C–6%C) 7 . Understanding the driving force for the formation of the core‐shell structured carbide would help control the in situ formation of such carbides towards harder and tougher HCCIs in wider compositional ranges.…”