Role of Microstructure in Wear Resistance of Chromium Carbide Overlay
Creators: Li, Jing
Publisher: name: University of Alberta Library
Year: 2023

<p>Iron-based alloy overlays are extensively utilized in industry to increase the longevity of components that are exposed to wear and corrosion. Welding is a common overlay technique due to its cost-effectiveness and high efficiency. To expedite the overlay design and development process, the button melting method has been compared with wire fabrication and welding. Both methods produce similar microstructures and hardness for the same overlay alloy. Using button melting as a replacement for wire fabrication is feasible. However, button melting does not result in dilution from the base metal, and the microstructure comprises more directionally solidified M7C3 grains due to the predominant directional cooling. Moreover, button melting produces faster cooling rates that promote the formation of martensite in the steel matrix.<br>
An accelerated method for alloy design was proposed using a combination of button melting and machine learning. By leveraging existing data, a closed-loop design process was developed that enables the prediction of future experiments and suggests new ones. Through an active database, the influence of Nb, V, and Ti on the hardness of the overlay was established. XRD analysis revealed that Nb promotes the formation of austenite in the matrix, while Ti fosters the formation of martensite. The effectiveness of this method is demonstrated by successfully predicting the combined effects of carbide formers on overlay hardness. This showcases the potential of machine learning in the design of chromium carbide overlays.<br>
The microstructures of two different overlays, namely an as-welded chromium carbide overlay and a novel Fe-Cr-C-B overlay with multiple alloying elements, have been thoroughly examined. The microstructure of the chromium carbide overlay consists of large primary carbides (M7C3) along with the presence of austenite and carbide eutectic phases. In contrast, the microstructure of the new overlay is composed of granular primary carbides (MX-type) containing elements such as Nb, Ti, and Mo, along with a dendritic structure of δ-ferrite/austenite. Eutectic phases of austenite and M¬2B boride (M = Fe and Cr) are also observed. The overlay&#39;s high hardness is attributed to the presence of fine MX-type hard particles, as well as the refined eutectic and matrix microstructure. The non-equilibrium solidification process responsible for the complex microstructure is discussed utilizing Thermo-Calc.<br>
By adding small quantities (0, 0.1, 0.5, and 1 wt.%) of aluminum to a chromium carbide overlay, the size of the large M7C3 carbides was effectively reduced. Comparative scratch tests revealed multiple cracks in the large primary M7C3 carbides of the 0% aluminum sample, while fewer cracks were observed in the refined M7C3 carbides. This refinement process is attributed to the heterogeneous nucleation of carbides on Al2O3, which forms prior to M7C3 nucleation. The presence of aluminum is found to expedite the growth time of primary M7C3 carbides by elevating the eutectic temperature. In terms of mechanical properties, the addition of 1% aluminum resulted in a slightly softer overlay. This is attributed to a decrease in the amount of martensite and an increase in retained austenite within the overlay matrix. Notably, the proposed approach of incorporating a small quantity of aluminum offers a novel method for refining primary M7C3 carbides without introducing new hard phases.</p>