CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Pre-Treatment on Microstructure and Properties of Yttrium-Containing Austenitic Weld Electrode Overlay Layer
Literature Overview
This study investigates how various pre-treatment methods applied to the base substrate influence the microstructure, mechanical properties, and corrosion resistance of overlay layers deposited using yttrium-containing austenitic stainless steel welding electrodes. Yttrium, as a rare-earth element, is known to refine grain structure, modify solidification behavior, and enhance the thermodynamic stability of overlay deposits. The work examines pre-treatment variables including surface cleaning methods, preheating temperatures, and interpass temperature control, correlating these with resulting microstructural features and performance indicators.
Core Technical Findings
The research identifies that pre-treatment conditions significantly alter the columnar-to-equiaxed transition (CET) behavior in the overlay weld microstructure. Without adequate pre-treatment, the overlay layer tends to develop coarse columnar dendrites extending from the fusion boundary, which compromise both mechanical integrity and corrosion resistance. With optimized pre-treatment, the overlay layer exhibits a refined equiaxed grain structure with reduced grain boundary area fraction, leading to improved intergranular corrosion resistance.
Key findings include:
- Preheating the base metal to 200–250 °C reduces the cooling rate at the fusion boundary from approximately 80 °C/s to 35 °C/s, promoting the formation of ferrite-austenite duplex microstructure rather than single-phase austenite.
- Yttrium addition at 0.02–0.05 wt% in the electrode coating acts as a grain refiner, reducing average grain size by 30–40% compared to yttrium-free counterparts.
- Pre-treatment involving mechanical grinding (Ra ≤ 3.2 μm) followed by solvent cleaning eliminates surface oxides and adsorbed contaminants that would otherwise nucleate porosity in the overlay layer.
Microstructural Analysis
The overlay deposit microstructure is characterized by a mixture of austenite (γ) and delta ferrite (δ) phases. The ferrite content, determined by the Schaeffler diagram approach and confirmed by metallographic examination using LePera's reagent, ranges from 5% to 15% depending on pre-treatment severity.
| Pre-Treatment Condition | Ferrite Content (%) | Average Grain Size (μm) | Hardness (HV) | Corrosion Potential (mV vs. SCE) |
|---|---|---|---|---|
| No preheating, rough surface | 3–5 | 180–220 | 185–200 | -720 |
| 100 °C preheat, solvent clean | 7–10 | 120–150 | 195–210 | -680 |
| 200 °C preheat, ground + clean | 12–15 | 85–110 | 205–220 | -640 |
| 250 °C preheat, ground + clean | 14–16 | 70–95 | 210–225 | -620 |
The presence of yttrium inclusions at grain boundaries acts as preferential nucleation sites for austenite during solidification, effectively suppressing the growth of columnar dendrites. The rare-earth oxide particles (Y₂O₃) formed during welding serve as heterogeneous nucleation agents, particularly evident in the heat-affected zone (HAZ) of the overlay layer.
Engineering Practice Integration
From a practical standpoint, this research has direct implications for the cladding of pressure vessel internals and heat exchanger tubes where austenitic stainless steel overlay is specified for corrosion resistance in aggressive environments. The findings suggest that:
- For overlay work on thick-section carbon steel vessels (e.g., hydrogenation reactor shells exceeding 80 mm wall thickness), preheating to 200–250 °C is essential to prevent cold cracking at the base metal/overlay interface while simultaneously optimizing the overlay microstructure.
- Surface preparation should be documented and controlled as part of the welding procedure specification (WPS) in accordance with NB/T 47014 and ASME IX requirements, with surface roughness and cleanliness verified before welding commences.
- The interpass temperature should be maintained between 150–250 °C to prevent excessive grain coarsening in subsequent overlay passes while avoiding hydrogen-induced cracking.
Key Questions and Reflections
Several important questions arise from this study. First, while yttrium addition improves grain refinement, excessive amounts (above 0.08 wt%) may lead to the formation of brittle intermetallic phases at grain boundaries, potentially reducing toughness. Second, the long-term stability of yttrium-modified overlay layers under cyclic thermal loading (as encountered in hydrogenation reactors operating between 250–450 °C) requires further investigation through accelerated thermal cycling tests. Third, the interaction between yttrium and sulfur/phosphorus impurities in the base metal warrants attention, as these elements may compete with yttrium for grain boundary segregation sites.
Study Insights and Implications
This research demonstrates that the quality of weld overlay is not solely determined by the consumable selection but is profoundly influenced by the preparation state of the substrate. The concept of "pre-treatment as a process variable" should be elevated to the same level of importance as welding parameters themselves. In engineering practice, this means that quality control procedures for cladding operations must include rigorous verification of surface preparation and thermal pre-conditioning, not merely inspection of the finished overlay layer. The incorporation of rare-earth elements such as yttrium into welding consumables represents a promising avenue for enhancing overlay performance, but their effectiveness is contingent upon proper process control from the very beginning of the operation. Future work should focus on developing standardized pre-treatment protocols that can be universally applied across different industries while accounting for the specific requirements of each application environment.