CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Pre-treatment on Microstructure and Properties of Yttrium-Containing Austenitic Weld Electrode Overlay Layer
Background and Research Motivation
Yttrium-containing austenitic weld electrodes are increasingly employed in weld overlay applications requiring exceptional resistance to thermal cycling fatigue, high-temperature creep, and intergranular corrosion. The addition of yttrium (Y) to austenitic stainless steel matrices such as 308L, 309L, and 316L compositions serves multiple metallurgical purposes: it acts as an oxygen scavenger to refine grain structure, modifies the morphology of inclusions to reduce stress concentration sites, and stabilizes the austenitic phase against transformation during cooling. However, the effectiveness of yttrium addition is critically dependent on the pre-treatment conditions applied to the base metal and electrode surface prior to welding, as these conditions directly influence the cleanliness of the weld pool and the subsequent microstructural evolution.
This study systematically investigates the influence of various pre-treatment methods on the microstructure, mechanical properties, and corrosion resistance of overlay layers deposited using yttrium-containing austenitic electrodes. The research is particularly relevant for overlay applications on power plant components, petrochemical reactors, and nuclear industry equipment where the overlay layer must withstand aggressive service environments for extended periods.
Pre-treatment Methods and Their Metallurgical Effects
The study evaluates four pre-treatment methods applied to both the base metal surface and the electrode flux coating prior to overlay welding. The following table summarizes the pre-treatment conditions and their effects on key overlay properties.
| Pre-treatment Method | Surface Cleanliness | Grain Size (μm) | Hardness (HV) | Intergranular Corrosion Resistance |
|---|---|---|---|---|
| No treatment (as-received) | Poor | 45–60 | 210–230 | Poor (failure) |
| Mechanical grinding (SiC P400) | Moderate | 30–40 | 220–240 | Marginal |
| Pickling and passivation | Good | 20–30 | 200–220 | Good |
| Ultrasonic cleaning + pickling | Excellent | 15–25 | 190–210 | Excellent |
| Plasma cleaning + passivation | Excellent | 12–20 | 185–205 | Excellent |
The results demonstrate that pre-treatment cleanliness has a direct and quantifiable impact on the overlay layer quality. Without any pre-treatment, the surface oxide layer acts as a source of inclusions in the weld pool, promoting the formation of coarse, elongated MnS and TiN inclusions that serve as crack initiation sites. The grain size reduction from 45–60 μm to 12–20 μm achieved through plasma cleaning and passivation represents a more than twofold improvement in grain refinement, directly attributable to the reduced oxygen content in the weld pool.
Microstructural Evolution and Phase Analysis
The microstructural analysis reveals that yttrium interacts with sulfur and oxygen in the weld pool to form Y2O3 and Y2S inclusions, which are significantly smaller and more uniformly distributed than the MnS inclusions formed in conventional austenitic welds without yttrium addition. The study reports that with proper pre-treatment, the Y2O3 inclusion size is reduced to below 2 μm with a number density of approximately 5×10^6 particles per cm², compared to MnS inclusions exceeding 10 μm in size in untreated welds.
The phase composition of the overlay layer is predominantly austenite (γ) with minor amounts of ferrite (δ) and carbide precipitates. The study quantifies the ferrite content using the magnetic permeability method and reports that pre-treatment reduces the ferrite content from 12–18% in untreated welds to 5–8% in properly pre-treated welds. This reduction in ferrite content is attributed to the cleaner weld pool chemistry, which allows for more precise control of the Creq–Meq balance through the Schaeffler diagram.
Effect of Yttrium on Inclusion Morphology
The following table presents the inclusion analysis results from scanning electron microscopy and energy-dispersive X-ray spectroscopy.
| Inclusion Type | Size (μm) | Shape | Distribution | Effect on Properties |
|---|---|---|---|---|
| Y2O3 | 0.5–2.0 | Spherical | Uniform | Grain refinement, improved ductility |
| Y2S | 1.0–3.0 | Spherical | Semi-uniform | Replaces MnS, reduces hot cracking |
| MnS (residual) | 5.0–15.0 | Elongated | Segregated | Stress concentration, crack initiation |
| TiN | 2.0–8.0 | Angular | Grain boundary | Moderate effect on toughness |
The near-elimination of elongated MnS inclusions through proper pre-treatment and yttrium addition is the most significant metallurgical finding of the study. MnS inclusions are well-documented as the primary cause of transverse cracking in austenitic welds, particularly during thermal cycling service. Their replacement with spherical Y2S inclusions fundamentally improves the crack resistance of the overlay layer.
Mechanical Properties and Corrosion Performance
The mechanical properties of the overlay layers show clear correlation with pre-treatment quality. Tensile strength values range from 480–520 MPa for untreated welds to 550–600 MPa for plasma-cleaned welds, with elongation improving from 35–40% to 45–52%. The hardness values, while showing only moderate variation, are important for applications requiring specific hardness ranges to prevent galling or wear against mating surfaces.
The intergranular corrosion testing conducted in accordance with ASTM A263 (Method A, 100% acid) reveals a critical threshold: overlay layers deposited without pre-treatment fail the test consistently, showing severe intergranular attack along grain boundaries. This failure is attributed to chromium carbide precipitation at grain boundaries promoted by the high oxygen content in untreated weld pools. With proper pre-treatment, the overlay layers pass the intergranular corrosion test with minimal grain boundary attack, confirming the effectiveness of yttrium addition combined with surface cleaning in preventing sensitization.
Engineering Implications for Bimetallic Pressure Vessels
For engineers specifying weld overlay procedures on bimetallic pressure vessels, the study provides critical guidance on the necessity of rigorous pre-treatment. The overlay qualification procedures under NB/T 47014 and ASME IX require demonstration of adequate mechanical properties and corrosion resistance, and the study's findings confirm that meeting these requirements with yttrium-containing electrodes is contingent upon proper surface preparation.
The study recommends that for critical applications involving hydrogen service, sour gas environments, or high-temperature oxidation, the plasma cleaning plus passivation pre-treatment should be specified as the minimum acceptable method. This recommendation is based on the consistent demonstration of superior microstructural quality, mechanical properties, and corrosion resistance across all tested conditions.
Conclusions and Study Insights
The study conclusively demonstrates that pre-treatment is not merely a quality assurance step but a fundamental process variable that determines the metallurgical outcome of yttrium-containing austenitic weld overlay deposits. The interplay between surface cleanliness, yttrium addition, and weld pool chemistry creates a synergistic effect that cannot be achieved by yttrium addition alone. Engineers must recognize that the specification of yttrium-containing electrodes without corresponding pre-treatment requirements represents an incomplete process specification that will result in unreliable overlay performance. The practical implication is that overlay procedure qualification must include pre-treatment methods as an essential variable, with the resulting overlay properties validated under the specific pre-treatment conditions that will be used in production. This holistic approach to process specification ensures that the full metallurgical benefits of yttrium addition are realized in service-critical overlay applications.