CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Multi-Pass Submerged Arc Cladding
Literature Overview and Research Significance
This 2015 study by He Xiaofeng, Lu Qinghua, Peng Birong, Chen Xuan, and Zhang Jing from the School of Materials Science and Engineering at Shanghai University of Engineering Science, supported by the National Natural Science Foundation of China (Grant No. 51305253), presents a comprehensive investigation into the microstructure evolution and mechanical properties of multi-pass submerged arc cladding (SAW) overlays. Published in the Journal of Shanghai University of Engineering Science, this research provides valuable fundamental insights that directly inform engineering practice in bimetal product manufacturing.
Microstructural Evolution in Multi-Pass SAW Cladding
The microstructure of multi-pass SAW cladding deposits exhibits significant variation between the first pass (root pass) and subsequent passes, as well as between the bottom (near base metal) and top (free surface) of each individual pass. The primary microstructural features include:
- Columnar dendrites: Predominantly present in the first pass and at the bottom of subsequent passes, growing perpendicular to the fusion boundary. The dendrite arm spacing (DAS) is typically 50–200 micrometers, depending on the cooling rate.
- Equiaxed grains: Increasingly present toward the top of each pass and in later passes, as the preheated base metal and previous passes provide nucleation sites for equiaxed grain formation.
- Inclusion distribution: Sulfide and oxide inclusions are preferentially segregated at interdendritic regions, with higher concentrations at pass boundaries due to local chemical enrichment.
- Phase composition: For austenitic stainless steel overlays (e.g., 316L), the microstructure consists primarily of austenite with minor ferrite (typically 5–15% delta ferrite). For nickel-based alloy overlays (e.g., Inconel 625), the microstructure is predominantly austenitic with possible carbide precipitation at grain boundaries.
The cooling rate is the dominant factor controlling microstructural evolution. In the first pass, cooling rates can exceed 100°C/s, while in later passes, they may drop below 10°C/s due to the thermal mass of the preceding layers. This cooling rate variation leads to significant differences in grain size, phase fraction, and mechanical properties throughout the overlay thickness.
Mechanical Properties and Their Variation
The mechanical properties of multi-pass SAW cladding deposits vary systematically with position within the overlay:
| Property | First Pass (Bottom) | Middle Passes | Top Pass | Base Metal |
|---|---|---|---|---|
| Hardness (HV) | 200–250 | 180–220 | 160–200 | 150–180 |
| Yield Strength (MPa) | 450–550 | 400–500 | 350–450 | 345–450 |
| Tensile Strength (MPa) | 600–750 | 550–700 | 500–650 | 490–630 |
| Elongation (%) | 20–30 | 25–35 | 30–40 | 21–30 |
| Impact Energy (J, -20°C) | 40–80 | 60–120 | 80–150 | 34–67 |
The hardness gradient from bottom to top is primarily attributed to the decreasing cooling rate, which allows more time for diffusion and microstructural coarsening. The impact toughness generally improves from bottom to top due to the transition from columnar to equiaxed grain structures, which reduces cleavage fracture susceptibility.
Thermal Cycling Effects and Residual Stress
Multi-pass SAW cladding subjects the base metal and previous passes to repeated thermal cycling, which has profound effects on the final properties:
- Thermal softening: The base metal near the fusion boundary may experience tempering or softening, reducing its yield strength by 10–20%.
- Residual stress accumulation: Each pass introduces new residual stresses, while also partially relieving stresses from previous passes. The final residual stress state is a complex superposition of all passes.
- Phase transformation: In low-alloy steel base metals, repeated thermal cycling can cause phase transformations in the heat-affected zone (HAZ), potentially leading to hardness increases or decreases depending on the specific alloy composition and cooling rate.
Engineering Implications and Process Recommendations
The findings from this research have direct implications for engineering practice:
- Overlay thickness design: For applications requiring uniform mechanical properties through the overlay thickness, the number of passes should be minimized, and the first pass should be carefully controlled to establish a sound metallurgical bond.
- Post-weld heat treatment (PWHT): A PWHT cycle (typically 600–650°C for 2 hours per 25 mm thickness) can homogenize the microstructure, relieve residual stresses, and improve toughness, but may reduce hardness and corrosion resistance in some alloy systems.
- Welding sequence optimization: For large surface areas, the welding sequence should be planned to minimize thermal distortion and residual stress, using a back-step or skip welding pattern.
- Material selection: The choice of overlay alloy should consider not only corrosion resistance but also the metallurgical compatibility with the base metal, particularly regarding dilution effects on the top layer composition.
Study Reflections and Research Gaps
This research provides valuable fundamental data on microstructure-property relationships in multi-pass SAW cladding. However, several areas require further investigation, including the long-term creep behavior of overlay layers at elevated temperatures, the effect of interpass temperature on microstructural evolution, and the interaction between overlay microstructure and corrosion resistance under specific service conditions. For practicing engineers, the key takeaway is that overlay quality is not merely a function of the top layer composition but is fundamentally influenced by the thermal history and microstructural evolution throughout the entire overlay thickness.