Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Multi-Layer Fe-Based Overlay Welding Zones
Literature Overview
The paper by Xiao Xinhua and Xing Zhigang (2015, Hot Working Technology, Vol. 44, No. 11, pp. 233–235) investigates the microstructural evolution and hardness distribution across multiple overlay welding layers deposited on a failed 5CrNiMo die using Fe-based flux-cored wire. The study employs air cooling after welding and characterizes each layer through optical microscopy and microhardness testing. The research was supported by the Hubei Provincial Key Discipline of Mechanical Engineering (Grant T201408).
Core Technical Findings
The central observation is that each overlay layer exhibits distinctly different microstructural morphologies and hardness values, despite being deposited under nominally identical welding parameters. From the first layer (adjacent to the substrate) to the third and outermost layer, fine lath tempered martensite progressively decreases while coarse lath tempered martensite increases. The outermost layer is dominated by coarse plate-like martensite, and the average microhardness reaches 540 HV, forming a clear gradient distribution from the innermost to the outermost layer.
Mechanism of Layer-to-Layer Variation
The primary cause is identified as the tempering effect of subsequent passes on previously deposited layers. When the second layer is deposited, the first layer is reheated to a temperature sufficient to cause tempering of its martensitic structure, softening it and coarsening the retained carbides. This process repeats for each successive layer, creating a systematic gradient in both microstructure and hardness.
| Layer | Dominant Microstructure | Approximate Hardness Trend | Reheat Temperature Effect |
|---|---|---|---|
| Layer 1 (innermost) | Fine lath tempered martensite | Lowest (tempered by subsequent passes) | Highest cumulative reheat cycles |
| Layer 2 | Mixed fine and coarse lath tempered martensite | Intermediate | Moderate reheat cycles |
| Layer 3 (outermost) | Coarse plate-like martensite | Highest (~540 HV) | Minimal reheat exposure |
Process Implications and Engineering Practice
This gradient phenomenon has direct consequences for overlay welding design in industrial applications. When the goal is to achieve maximum surface hardness for wear resistance, the outermost layer naturally provides the highest hardness. However, this also means that the bonding interface (Layer 1) is the softest and most susceptible to adhesive wear or plastic deformation under high contact stress.
Practical Recommendations
- Layer thickness optimization: For high-wear applications, maintaining a minimum of 2–3 layers ensures adequate transition from base material toughness to surface hardness.
- Heat input control: Reducing interpass temperature helps minimize the tempering effect on underlying layers, potentially preserving finer microstructures throughout.
- Post-weld heat treatment: A controlled tempering cycle applied after all layers are deposited can homogenize the hardness gradient while maintaining acceptable surface hardness levels.
- Dilution management: The first layer is most susceptible to substrate dilution, which can further alter the alloy composition and resulting microstructure.
Study Insights and Reflections
This work clearly demonstrates that multi-layer overlay welding is not simply a repetition of the same welding operation but rather a complex thermal-metallurgical interaction where each pass modifies the previous ones. The 540 HV achieved at the outermost layer is consistent with high-carbon martensitic steels, suggesting that the Fe-based flux-cored wire contains sufficient carbon and alloying elements to form hard phases. However, the coarse plate-like martensite at the surface raises concerns about potential brittleness and susceptibility to crack initiation under impact or thermal cycling conditions. In engineering practice, this finding supports the common industry approach of applying a final "capping" layer with slightly different composition to balance hardness and toughness, rather than relying on a single alloy throughout all layers.