Metallurgy for Erosion & Corrosion
CD4MCu is in the category of a high grade stainless steel. The chemical resistance is better than a Ni-Hard or HCI but the abrasion resistance is not as good as a Ni-Hard or a HCI white cast irons. The mechanical properties of this material is also quite different than that of the other two alloys. CD4MCu will work harden as it is subjected to abrasive wear. It has been observed that under extreme conditions the castings will appear smooth with small flakes washed away by the pumped media. CD4MCu does not reach the higher Brinnell hardness of the other alloys. This alloy is usually around 450 Brinnell in its hardened state.
When comparing these two white cast irons both are found to be hard Martensitic irons which differ in molecular structure. Ni-Hard is found to contain hard iron carbides which are reinforced by a hardened iron substrate or matrix. This would be analogous to a sea of hardened iron with many tightly packed hard iron carbide islands. Under a high powered microscope the material would tend to appear as having a island-like grain structure.
Ni-Hard tends to be used in applications where abrasion is the primary concern. In a corrosive environment the substrate is attacked leaving the hard iron carbides in tact with little or no foundational support. Upon closer inspection it is found that the surface of the casting has higher peaks and lower valleys than original cast part which causes the hard iron carbide particles to break off in the flow stream. When this occurs the hard iron carbides are undermined (at the molecular level) it appears that excessive "wear" may be occurring. This type of wear in many cases may not be visible to the unaided eye.
HCI (High Chromium Iron) tends to be more uniform in the overall formation of its' molecular structure. The molecular structure density is greater in HCI as compared to Ni-Hard. This is a result of having austenite and martensite steel substrate present in conjunction with hard chromium carbide islands. Since martensite and austenite are different forms of the same material there are less air gaps present which helps account for HCI being less susceptible to chemical attack. Under a high powered microscope the material would tend to appear as having a needle-like grain structure.
HCI is generally used when the application requires a material which will withstand a slight to moderate amount of corrosion and erosion. This material exhibits good wear characteristics as does Ni-Hard but with the added benefit of corrosion resistance.
CD4MCu is a material which should be considered when the application is more corrosive and where some abrasive material would be present. This material is a true steel casting and is grouped with Austenitic class materials. CD4MCu is more of a homogeneous mixture and when viewed under a high powered microscope a single matrix is observed which compared to HCI and Ni-Hard which show two distinct layering patterns. This helps with chemical resistance but does not allow for the best abrasion resistance. It is a good material for lower ph applications and high pH applications.
Each application should be reviewed for proper material selection. Consideration for erosion and corrosion must be taken into account. The following information should be gathered prior to final determination of material: