Banded Microstructure Investigation in CCO's by Dilution and Cooling Rate Control

Chrome carbide overlays (CCOs) are widely used in Oil Sands applications to provide protection against the abrasive sand particles. CCOs commonly have a banded type microstructure, with uneven distribution of wear resistant carbide particles towards the top of the deposit. This creates problems of uneven wear throughout the lifetime of the overlay. This uneven wear can be costly as the overlays need replacing more often to ensure failure does not occur.

The Fe-Cr-C ternary phase diagram and the setup of a thermal-control substrate were investigated using both computational and fundamental thermodynamics and experimental melts. This project was part of a larger investigation with the ultimate goal of understanding the fundamental reasons behind the banded microstructure typically observed in CCOs welded with the submerged arc welding (SAW) process. The computational thermodynamics tasks involved ThermoCalc predictions for various compositions across the austenite and M7C3 eutectic phase boundary. A thermal-control substrate was designed to avoid melting of the substrate whilst simultaneously completely melting the powders which is an issue encountered with conventional carbon steel substrates. The substrate consists of a cooled copper plate and was designed so that through a heat transfer analysis to ensure that the heat generated from the arc would be completely transferred to the water flowing in direct contact with the base of the copper plate. The experimental melts consisted of the deposition of molten powders on the substrate using the plasma transferred arc welding (PTAW) process chosen for its lower heat input and minimal dilution. The composition chosen for the powders correspond to those studied with ThermoCalc. The preliminary results of this project indicate that the banded microstructure present in CCOs is at least in part due to segregation during the weld cooling.