CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Quenching on Microstructure and Properties of Weld Overlay Cladding Deposits
Literature Overview
This paper, published in the Journal of Shenyang University of Technology in 2013 by Chen Jiexiang and Liu Jian from the Academy of Armored Force Engineering, investigates how the quenching rate during cladding welding influences the microstructure evolution and mechanical properties of weld overlay deposits. The work was supported by the National Natural Science Foundation of China (Grants 50975286 and 51205408), indicating its significance in understanding the solidification behavior of cladding layers under different thermal regimes. The research is particularly relevant to engineers working on armor plate cladding, where rapid cooling is often inherent to the welding process and must be understood to control the resulting hardness, toughness, and crack susceptibility of the deposit.
Core Technical Content
The fundamental premise of this study is that the cooling rate at the weld interface and within the cladding deposit itself governs the phase transformations that occur during solidification and subsequent cooling. In cladding applications, especially those involving hardfacing or high-alloy overlay layers, the thermal gradient can be extreme, leading to non-equilibrium solidification. The authors examined how varying the quenching conditions — achieved through different backing materials, water cooling, or ambient air cooling — affected the grain morphology, phase composition, and hardness distribution of the cladding layer.
Solidification Behavior and Phase Formation
Under high cooling rates, the cladding deposit tends to exhibit finer grain structures and may retain metastable phases that would otherwise transform at slower cooling rates. For Cr-Mo-V based hardfacing alloys, rapid quenching promotes martensitic transformation, resulting in high hardness but potentially reduced fracture toughness. The study highlights that the dendritic arm spacing decreases with increasing cooling rate, following the relationship:
- $\lambda = A \cdot \dot{T}^{-n}$, where $\dot{T}$ is the cooling rate, and $A$ and $n$ are material-dependent constants.
This refinement of the dendritic structure directly correlates with increased hardness values, as the finer spacing increases the number of strengthening interfaces. However, the authors also note that excessively rapid quenching can lead to residual stresses and microcracking, particularly at the dilution zone where the base metal and filler metal interact.
Microstructural Characterization
The paper employs optical microscopy and scanning electron microscopy to characterize the microstructure at different cooling rates. Key observations include:
| Cooling Condition | Approximate Cooling Rate (°C/s) | Dominant Phase | Hardness (HV30) | Grain Size |
|---|---|---|---|---|
| Ambient air cooling | 5–15 | Pearlite + Ferrite | 250–320 | Coarse (200–400 μm) |
| Water quenching | 100–300 | Martensite + Bainite | 450–620 | Fine (50–150 μm) |
| Induced quenching (thin backing) | 50–100 | Mixed (Martensite + Bainite) | 350–480 | Medium (100–250 μm) |
The transition from ferritic-pearlitic to martensitic structures is the most critical microstructural change observed. The presence of retained austenite at the highest cooling rates was also noted, which can be beneficial for toughness but detrimental if it transforms during service.
Dilution Effects
The dilution zone, where the base metal and cladding layer interact, is particularly sensitive to cooling rate. At higher cooling rates, the diffusion of alloying elements between the base metal and the deposit is limited, resulting in a sharper compositional gradient. This can create a brittle intermetallic layer if the alloying elements form compounds such as Cr₂₃C₆ or Mo₂C. The study recommends post-weld heat treatment (PWHT) to relieve these dilution-zone stresses and stabilize the microstructure.
Engineering Practice Implications
For engineers involved in cladding armor plates or hardfacing critical components, this study provides a clear framework for understanding how to control cooling rates through process design. The use of thermal mass backing plates, water cooling sprays, or controlled cooling fixtures can all be employed to achieve target cooling rates. In practice, the following process controls are recommended:
- Preheat temperature: Maintaining a preheat of 150–250°C reduces the effective cooling rate at the weld interface, minimizing the risk of cold cracking in high-carbon equivalents.
- Interpass temperature: For multi-pass cladding, maintaining interpass temperatures between 200–350°C allows partial recovery of the underlying layers while maintaining adequate cooling for the new pass.
- Post-weld heat treatment: A tempering cycle at 550–650°C for 1–2 hours can relieve residual stresses and convert retained austenite to stable phases.
Key Questions and Reflections
A critical question arising from this work is the optimal balance between hardness and toughness in cladding applications. In armor applications, the cladding layer must resist penetration while the base plate must maintain structural integrity. The quenching rate directly influences this trade-off. The authors suggest that a controlled cooling approach — neither too fast nor too slow — provides the best compromise, but the exact parameters depend on the specific alloy system and service conditions.
Another reflection is the applicability of these findings to modern cladding processes such as laser cladding or plasma transferred arc (PTA) welding. These processes inherently produce higher cooling rates due to their concentrated heat input and rapid travel speeds. Understanding the fundamental relationship between cooling rate and microstructure, as established in this study, is essential for optimizing these advanced processes.
Study Insights and Conclusions
This study reinforces the principle that microstructure in cladding deposits is not merely a function of composition but is equally determined by the thermal history during solidification. Engineers must treat cooling rate as a controllable process variable, not an afterthought. The data presented provide a valuable reference for selecting backing materials, preheat levels, and PWHT parameters in cladding operations. For future work, the authors suggest investigating the effects of combined cooling and magnetic field application, which could further refine the microstructure without the drawbacks of excessive residual stress. The findings are directly applicable to the design of cladding processes for armor plates, wear-resistant overlays, and bimetallic components where the interface microstructure governs long-term performance.