Wear Characteristics of Hardfacing Coatings for Internal Mixer Rotors

Literature Overview

This paper, published in 2004 by Zhang Huichen and colleagues from Dalian Maritime University, investigates the tribological performance of three hardfacing coatings designed for internal mixer rotors in rubber processing equipment. The internal mixer rotor operates under severe sliding and abrasive conditions, with continuous contact against rubber compounds containing reinforcing fillers such as carbon black and silica. The authors employed a ball-on-disc tribometer to simulate the wear conditions and compared automatic hardfacing coatings against manual (hand-applied) hardfacing coatings, both based on high-chromium alloy systems.

Core Findings and Metallurgical Analysis

The central conclusion of this study is that the welding method profoundly influences the microstructure and, consequently, the wear resistance of the hardfacing layer. The automatic hardfacing process, while offering consistency and throughput advantages, produced excessively high local temperatures during deposition. This thermal overexposure led to the formation of coarse dendritic microstructures in the high-chromium alloy layer. Coarse dendrites create weak inter-dendritic boundaries, reduce the effective volume fraction of hard carbide phases, and lower the overall hardness of the deposit.

In contrast, manual hardfacing produced fine equiaxed grains with a higher hardness and superior wear resistance. The slower thermal cycling and more controlled heat input in manual welding allowed for better nucleation and growth control of the carbide phase, resulting in a more homogeneous and harder microstructure.

Engineering Practice Implications

This study carries significant practical relevance for engineers specifying hardfacing repairs on critical rotating equipment. In rubber mixing operations, rotor life directly affects plant availability and product quality. The finding that automatic hardfacing can degrade performance due to overheating challenges the common assumption that automated processes always yield superior results. Engineers should carefully evaluate:

• The specific heat input parameters of automatic hardfacing equipment, particularly current density and travel speed
• Whether interpass temperature control is adequate to prevent cumulative thermal damage
• The trade-off between deposition rate and microstructural quality

From a process optimization standpoint, this paper suggests that hybrid approaches may be beneficial: using automatic hardfacing for building up base layers and finishing with manual passes to achieve the desired microstructure. Alternatively, adjusting automatic welding parameters (reducing current, increasing travel speed) could mitigate the overheating issue while retaining some automation benefits.

Reflections and Key Questions

This research highlights a fundamental principle in welding metallurgy: microstructure governs properties, and process parameters govern microstructure. The distinction between dendritic and equiaxed grain formation in high-chromium hardfacing alloys is well-established in the literature, but its practical impact on mixer rotor service life deserves more attention in industrial settings. A critical question remains unanswered: what is the optimal heat input window for high-chromium hardfacing alloys that balances productivity with microstructural quality? Future work should quantify the relationship between linear energy input and grain size in these systems, providing engineers with clear process specification limits.

Summary

This paper provides a clear demonstration that hardfacing process selection is not merely a productivity decision but a metallurgical one with direct consequences for component durability. Engineers responsible for maintenance and repair of internal mixer rotors should prioritize manual hardfacing or carefully parameterized automatic processes when microstructural integrity is paramount. The work reinforces the importance of metallographic verification after hardfacing operations and serves as a cautionary case study against assuming that automated processes universally outperform manual ones in terms of functional performance.