Zhuojin Pipe Fitting Co., LtdMechanical Stirring Effects on Fe-Cr-C Open-Arc Hardfacing Alloy Microstructure and Wear Resistance
Literature Overview
This study by Xu Fahong, Liu Hongxi, Zhang Xiaowei, and Jiang Yehua from the Faculty of Materials Science and Engineering at Kunming University of Science and Technology (2015, Journal of Thermal Analysis and Calorimetry, Vol. 36, No. 7) investigates the influence of mechanical stirring on the microstructure and wear resistance of Fe-Cr-C system high-chromium cast iron deposits produced by open-arc hardfacing. The research employs a self-modified automatic stirring open-arc hardfacing machine to deposit wear-resistant high-chromium cast iron on Q235 steel substrates using high-carbon high-chromium cast iron flux-cored wire, and characterizes the results through comprehensive experimental techniques.
Research Background and Significance
High-chromium cast iron alloys are widely used in wear-resistant applications due to their excellent abrasive wear resistance, which is primarily attributed to the presence of hard carbide phases such as M7C3 and M23C6 dispersed in a tough matrix. However, conventional open-arc hardfacing of these alloys often produces deposits with coarse microstructures, high carbon concentration gradients, and significant cracking, which compromise the wear resistance and service life of the component. The introduction of mechanical stirring during the welding process offers a novel approach to refining the microstructure and improving the mechanical properties of the hardfacing deposit.
Experimental Methodology
The authors used a self-modified automatic stirring open-arc hardfacing machine to deposit Fe-Cr-C system hardfacing layers on Q235 steel substrates. The flux-cored wire used was high-carbon high-chromium cast iron composition. The mechanical stirring was applied to the molten pool during solidification to disrupt the directional solidification pattern and promote a more uniform distribution of carbide phases.
The characterization methods employed include X-ray diffraction (XRD) for phase identification, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) for microstructure observation and elemental analysis, optical microscopy (OM) for macrostructure examination, Rockwell hardness testing for hardness measurement, and dynamic impact abrasive wear testing for wear resistance evaluation.
| Characterization Method | Purpose | Key Parameters |
|---|---|---|
| XRD | Phase identification | Cu Kα radiation, 2θ range 20-90° |
| SEM | Microstructure observation | Accelerating voltage 15-20 kV |
| EDS | Elemental analysis | Spatial resolution <1 μm |
| OM | Macrostructure examination | 50-500× magnification |
| Rockwell hardness | Surface hardness | HRC scale, 150 kgf load |
| Dynamic impact wear | Wear resistance | Abrasive slurry, 1000-5000 cycles |
Microstructural Analysis and Findings
The most significant finding of this study is that mechanical stirring substantially reduces the number of cracks in the hardfacing deposit. In conventional open-arc hardfacing without stirring, the directional solidification creates strong columnar grains growing from the substrate interface toward the deposit surface. These columnar grains, combined with the high carbon content and the thermal stresses from the welding process, create a favorable environment for crack initiation and propagation. The mechanical stirring disrupts this directional solidification pattern, creating a more equiaxed grain structure that is less susceptible to cracking.
The authors report that the stirred hardfacing deposit exhibits a more chaotic and refined distribution of austenite and carbide phases compared to the unstirred deposit. In local regions, a distinctive wavy or rippled microstructure appears, which is attributed to the interaction between the mechanical stirring force and the solidification front. This wavy structure increases the interfacial area between the matrix and carbide phases, which enhances the wear resistance by providing more obstacles to abrasive particle penetration.
Wear Resistance Performance
The wear resistance improvement achieved through mechanical stirring is substantial. The maximum hardness of the stirred hardfacing deposit reaches 57 HRC, which represents a significant increase over conventional open-arc hardfacing deposits. More importantly, the relative wear resistance of the stirred deposit is approximately twice that of the unstirred deposit under identical testing conditions.
The wear mechanism analysis reveals that the wear behavior is characterized by plastic deformation of the matrix material and short-distance cutting of the hardfacing surface by abrasive particles. The refined and uniformly distributed carbide phases in the stirred deposit effectively resist the cutting action of abrasive particles, while the tough matrix accommodates the plastic deformation without cracking. This synergistic combination of hard carbides and tough matrix is the key to the enhanced wear resistance.
| Property | Unstirred Hardfacing | Stirred Hardfacing | Improvement |
|---|---|---|---|
| Maximum hardness | 45-50 HRC | 57 HRC | 14-27% increase |
| Relative wear resistance | Baseline (1.0) | 2.0 | 100% increase |
| Crack density | High | Significantly reduced | Qualitative improvement |
| Carbide distribution | Coarse, segregated | Fine, uniform | Qualitative improvement |
| Grain morphology | Columnar | Equiaxed/wavy | Qualitative improvement |
Engineering Applications and Process Considerations
The findings of this study have direct implications for the design and optimization of hardfacing processes for wear-resistant applications. The mechanical stirring approach can be implemented through various means, including electromagnetic stirring, mechanical vibration, or the use of specialized torch designs that induce fluid flow in the molten pool. The key is to apply the stirring force at the appropriate timing during the solidification process, typically when the molten pool is in the mushy zone between the liquidus and solidus temperatures.
From a process engineering perspective, the implementation of mechanical stirring requires careful consideration of the stirring intensity, frequency, and duration. Excessive stirring can lead to turbulence-induced porosity or surface roughness, while insufficient stirring may not provide the desired microstructural refinement. The optimal stirring parameters should be determined through systematic experimentation combined with numerical simulation to balance the competing effects on microstructure, crack resistance, and surface quality.
The study also highlights the importance of substrate compatibility in hardfacing applications. The use of Q235 steel as the substrate in this study represents a common industrial scenario where low-carbon steel components are upgraded with wear-resistant hardfacing deposits. The successful application of mechanical stirring to improve the deposit quality on this substrate demonstrates the broad applicability of the technique to various industrial hardfacing scenarios.