Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of MAG Surfacing Layer on E-Grade Steel
Literature Overview
This paper by Ding Yongfeng, Xu Wujiao, and Lei Fan from Chongqing University (published in Metal Heat Treatment, 2013, Vol. 38, No. 9) investigates the microstructure, hardness distribution, and chemical composition of a surfacing layer deposited on E-grade steel using flux-cored wire (FCW) with MAG (Magnetron Arc Gas) welding, specifically employing surfacing electrode 635E. The study focuses on understanding the metallurgical behavior at the fusion line and heat-affected zone (HAZ), with particular attention to carbon migration phenomena that occur during and after the surfacing process.
Core Metallurgical Findings
The study reveals several important metallurgical features that are critical for understanding the performance and durability of the surfacing overlay:
| Zone | Microstructure | Hardness Behavior | Carbon Distribution |
|---|---|---|---|
| Base metal (E-grade steel) | Tempered martensite | Baseline hardness | Normal carbon content |
| HAZ | Tempered martensite | Slightly reduced | Slight carbon depletion |
| Fusion line - base metal side | Decarburized layer | Significantly lower than base metal | Carbon depleted |
| Fusion line - filler side | Carbon-enriched layer | High hardness due to carbide precipitation | Carbon enriched |
| Surfacing layer (635E deposit) | Heterogeneous microstructure | Good strength and toughness | Alloy composition controlled |
The most significant finding is the carbon migration phenomenon at the fusion line. During the surfacing and subsequent tempering processes, carbon atoms migrate across the fusion boundary, creating a decarburized zone on the base metal side and a carbon-enriched zone on the filler metal side. This is a well-documented but critically important phenomenon that directly affects the mechanical integrity and wear resistance of the overlay.
Carbon Migration Mechanism Analysis
The carbon migration observed in this study can be explained through thermodynamic and kinetic considerations. During the surfacing process, the fusion zone experiences rapid heating and cooling. The filler metal (635E), being a high-carbon alloy, creates a steep carbon concentration gradient at the fusion boundary. During the subsequent tempering treatment, carbon atoms in the base metal side of the fusion line are driven toward the higher-activity region (the filler metal side) by the concentration gradient and the presence of strong carbide-forming elements in the filler metal.
The decarburized layer on the base metal side exhibits significantly reduced hardness because the loss of carbon reduces the ability of the microstructure to form hard martensite or bainite during cooling. This creates a potential weak zone that could be susceptible to wear or fatigue initiation under cyclic loading conditions. Conversely, the carbon-enriched layer on the filler metal side precipitates hard carbides (likely cementite Fe₃C and possibly alloy carbides), resulting in elevated hardness values.
Engineering Implications
For engineers designing surfacing systems for wear-resistant applications, the carbon migration phenomenon has several important implications:
- Wear resistance distribution: The decarburized zone at the fusion line represents a soft band that may initiate wear preferentially. In applications where the overlay is subjected to abrasive or erosive wear, this soft zone could serve as a wear initiation site, potentially leading to premature overlay failure.
- Bond strength considerations: While the study reports good fusion line bonding, the carbon migration creates a metallurgical discontinuity that could affect fatigue resistance. Under cyclic loading, the hardness gradient and microstructural discontinuity at the fusion line could act as stress concentration sites.
- Heat treatment optimization: The tempering process, while necessary for relieving residual stresses and improving toughness, exacerbates carbon migration. Engineers must balance the benefits of stress relief against the risk of excessive decarburization. Optimizing tempering temperature and duration is essential.
- Filler metal selection: The choice of filler metal composition directly influences the severity of carbon migration. Filler metals with high carbon content and strong carbide-forming elements (such as Cr, Mo, V, and W) will drive more aggressive carbon migration. In some applications, selecting a lower-carbon filler metal or adding grain refiners may help mitigate this issue.
Study Insights and Recommendations
This study provides valuable fundamental insights into the metallurgical behavior of MAG surfacing on E-grade steel. The identification of carbon migration and its effects on hardness distribution is particularly important for engineers who rely on overlay layers for wear protection. The findings suggest that a holistic approach to surfacing design is necessary, considering not only the properties of the deposited overlay but also the metallurgical interactions at the fusion line.
For practical implementation, engineers should consider performing hardness profiling across the entire cross-section (base metal through overlay) to identify the depth and severity of the decarburized zone. If the decarburized zone is found to be excessively deep or soft, process modifications such as reduced heat input, preheating optimization, or post-weld heat treatment adjustments may be warranted. Additionally, in applications where the fusion line integrity is critical, the use of a transition layer between the base metal and the final wear-resistant overlay may be beneficial to buffer the carbon migration effect and provide a more gradual property transition.