Microstructure of high (45 wt.%) chromium cast irons and their resistances to wear and corrosion

High chromium cast irons (HCCIs) are widely used in many industrial processes that require materials possessing high resistance to wear and corrosion. In the Alberta oil sands industry, HCCIs are extensively used in slurry pumping systems as well as other processing and handling equipments. However, due to the very harsh mining environment and severe working conditions, conventional HCCIs do not always perform satisfactorily. Great efforts have been made to identify optimum microstructures and chemical compositions in order to effectively tailor HCCIs for improved performance under various operating conditions and minimize maintenance costs. One proposal is to extend the chromium concentration of HCCIs to higher levels.In this work, six HCCIs containing 45 wt.% of chromium and carbon concentrations ranging from 1 to 6 wt.% with small amounts of silicon and manganese were cast, solution-treated and aged, referred to as 45-series of HCCIs. Their microstructures were characterized using XRD and SEM/EDX. Corresponding resistances to wear, including abrasive wear and erosion–corrosion, and corrosion were evaluated. Microstructures of the 45-series of HCCIs were determined to be hypoeutectic (%C < 2), eutectic (%C ∼ 2) and hypereutectic (%C > 2). The ferrous matrix of 45-series of HCCIs was in martensite state. The volume fraction of total carbides increased with increasing the nominal carbon concentration. The formed carbides in the HCCIs with low carbon content (1–3 wt.%) were identified as cubic-face centred M23C6, while hexagonal close packed M7C3 mainly existed in HCCIs with higher carbon contents (5–6 wt.%). In HCCI 45-4, both M23C6 and M7C3 were detected. The wear resistance of 45-series of HCCIs was affected by the volume fraction, types and size of carbides, while their corrosion resistance was dominated by the free chromium content in the matrix and the ratio of volume fraction of carbides to ferrous matrix as well. Further examination showed that primary carbides in HCCI 45-4 had a shell-core structure (shell-M23C6, core-M7C3). Such a configuration could have a positive effect on the wear resistance of this alloy.

► Extending the %Cr in HCCIs to higher levels (e.g., 45wt.%) has been demonstrated to be effective in improving their resistances to wear and corrosion.
► The 45-series of HCCIs are in three states: hypoeutectic, eutectic and hypereutectic. The carbides in HCCIs with low %C (1-3wt.%) are cubic-face centered M23C6, while hexagonal close-packed M7C3 exists in HCCIs with high %C (5-6 wt.%). Both M23C6 and M7C3 have been found in HCCIs containing 4wt.%C. The ferrous matrix was in a martensitic state.
► A duplex configuration was observed in primary carbides in HCCI with 4wt.%C. The core is M7C3 and the shell is relatively softer M23C6, which may help reducing the mechanical mismatch between carbides and the ferrous matrix. This could be one of factors responsible for the best performance of HCCI 45-4.