Zhuojin Pipe Fitting Co., LtdMicrostructure Analysis of Austenitic Stainless Steel Strip Electrode Electroslag Surfacing on Low Carbon Steel
Literature Overview
This study by Yang Qin and colleagues from Hohai University, published in Electric Welding Machine in 2014 (Vol. 44, No. 1, pp. 77-80), examines the microstructure of austenitic stainless steel overlay welds produced using the strip electrode electroslag surfacing (SEES) process on Q235 low carbon steel substrates. The research is significant because it addresses the microstructural characteristics of austenitic stainless steel overlay welds, with particular attention to the δ-ferrite content and morphology, which are critical factors in determining the corrosion resistance and mechanical properties of the weld metal. The authors employed optical metallography and scanning electron microscopy (SEM) to characterize the overlay weld microstructure, and the study provides insights into the metallurgical behavior of austenitic stainless steel in the electroslag surfacing process.
Core Technical Findings
Process Characteristics and Overlay Weld Quality
The strip electrode electroslag surfacing process is characterized by several advantageous features that make it particularly suitable for thick overlay deposits:
- Good surface profile: The electroslag process produces smooth, well-formed overlay welds with minimal surface irregularities.
- Low dilution rate: The slag pool acts as a thermal barrier, reducing the dilution of the base metal into the overlay weld. This is crucial for maintaining the intended composition and properties of the austenitic stainless steel overlay.
- High deposition rate: The process is more productive than conventional arc welding methods, making it economical for thick overlay applications.
The low dilution rate is a direct consequence of the electroslag process geometry. The molten slag pool, which is significantly less dense than the molten metal, floats on top of the weld pool and acts as a thermal insulator. This insulation reduces the heat transfer from the weld pool to the base metal, limiting the amount of base metal that melts and mixes with the overlay. For austenitic stainless steel overlay applications, maintaining low dilution is essential because even small amounts of carbon from the base metal can significantly affect the weld metal composition and properties.
Microstructural Composition
The overlay weld metal microstructure consists primarily of austenite with a small amount of δ-ferrite. This austenitic-ferritic structure is typical for austenitic stainless steel welds and is governed by the Schaeffler diagram, which predicts the weld metal structure based on the chromium and nickel equivalent values of the weld metal composition.
The presence of δ-ferrite in austenitic stainless steel welds is not a defect but rather a desirable feature when present in controlled amounts. The δ-ferrite phase plays several important roles:
- Cracking resistance: δ-ferrite improves hot cracking resistance by providing a ductile phase that can accommodate strain during solidification and cooling.
- Corrosion resistance: A controlled amount of δ-ferrite (typically 3-10% by volume) can enhance resistance to intergranular corrosion by providing a pathway for chromium depletion to be distributed more uniformly.
- Mechanical properties: δ-ferrite contributes to the overall strength and toughness of the weld metal, providing a balance between ductility and strength.
δ-Ferrite Morphology Analysis
The study identifies three distinct morphologies of δ-ferrite coexisting in the overlay weld metal:
| Morphology | Description | Formation Mechanism | Significance |
|---|---|---|---|
| Skeletal | Network-like structure along grain boundaries | Primary solidification phase | High volume fraction indicates high ferrite content |
| Lamellar | Plate-like structures | Secondary transformation during cooling | Moderate volume fraction, good mechanical properties |
| Worm-like | Irregular, curvilinear shapes | Diffusion-controlled transformation | Low volume fraction, minimal impact on properties |
The coexistence of multiple δ-ferrite morphologies is a direct reflection of the complex solidification and cooling history of the electroslag surfacing process. The electroslag process involves a relatively slow cooling rate compared to conventional arc welding, which allows for more time-dependent transformations and the formation of multiple δ-ferrite morphologies. The skeletal δ-ferrite forms during the primary solidification stage when the weld pool is cooling rapidly at the surface. As the cooling rate decreases with increasing depth from the surface, the lamellar and worm-like morphologies become more prevalent.
Correlation with Intergranular Corrosion Resistance
The study emphasizes that a certain amount of δ-ferrite is beneficial for intergranular corrosion resistance. This finding is consistent with the well-established understanding that austenitic stainless steels with 3-8% δ-ferrite exhibit superior resistance to intergranular corrosion compared to fully austenitic welds. The δ-ferrite phase, being less susceptible to chromium depletion at grain boundaries, provides a more uniform distribution of chromium throughout the microstructure. This reduces the risk of chromium-depleted zones forming along austenite grain boundaries, which are the primary sites for intergranular corrosion attack.
Process and Standards Analysis
Electroslag Surfacing Process Parameters
The strip electrode electroslag surfacing process requires careful control of several parameters to achieve optimal overlay weld quality:
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Stripping current | 200-600 A | Higher current increases heat input and cooling rate variation |
| Travel speed | 50-200 mm/min | Affects cooling rate and grain structure |
| Slag composition | Custom formulation | Controls thermal properties and dilution rate |
| Electrode strip composition | 304/316 stainless steel | Determines weld metal chemistry |
| Substrate temperature | 100-200°C (preheat) | Reduces cracking risk and improves wetting |
The slag composition is a critical parameter in electroslag surfacing. The authors mention using a self-developed flux, which suggests that the slag formulation was optimized for the specific application. The slag must provide adequate thermal insulation to maintain low dilution while also ensuring proper fluidity for the electroslag process to function correctly.
Standards and Specification Considerations
For austenitic stainless steel overlay welding, several standards provide guidance on acceptable weld metal properties:
- ASME B31.3: Specifies requirements for stainless steel weld overlays in process piping, including minimum hardness limits and corrosion testing requirements.
- ASTM A403: Covers wrought and cast austenitic chromium and chromium-nickel castings for pressure-containing parts.
- ISO 3506: Specifies requirements for stainless steel fasteners and weld overlays.
- EN 10217-7: Covers heat-resistant austenitic stainless steel tubes and fittings.
The δ-ferrite content in austenitic stainless steel welds is typically specified by standards such as ASTM A388 and AWS D8.1, which recommend a maximum of 10% δ-ferrite by volume for general applications and up to 20% for applications requiring improved cracking resistance.
Engineering Practice Integration
Application in Pipe and Piping Systems
Austenitic stainless steel overlay welding is widely used in the following piping applications:
- Corrosion-resistant cladding: Overlaying austenitic stainless steel on carbon steel pipes to provide corrosion resistance in aggressive environments such as chemical processing, pulp and paper, and marine applications.
- Repair of corroded sections: Restoring the wall thickness of corroded stainless steel piping by overlay welding with matching or compatible materials.
- Transition joints: Creating transition sections between carbon steel and stainless steel piping, where the overlay provides a metallurgical and corrosion-resistant interface.
Quality Control for Electroslag Surfacing
When implementing electroslag surfacing for austenitic stainless steel overlays, the following quality control measures are essential:
- Dilution rate measurement: Determine the dilution rate by chemical analysis of the overlay weld metal. For austenitic stainless steel overlays on carbon steel, dilution should typically be below 5-10% to maintain acceptable weld metal properties.
- δ-ferrite content determination: Use magnetic permeability methods or metallographic analysis to determine the δ-ferrite content. The target range is typically 3-10% by volume.
- Corrosion testing: Conduct intergranular corrosion testing (e.g., ASTM A262 Practice E) on the overlay weld metal to verify corrosion resistance.
- Hardness testing: Measure hardness across the overlay weld to ensure uniformity and compliance with specification limits.
Defect Analysis and Countermeasures
Common defects in electroslag surfacing overlay welds include:
| Defect | Cause | Countermeasure |
|---|---|---|
| Excessive dilution | High heat input, poor slag composition | Optimize slag formulation, reduce current |
| Cracking | High carbon dilution, excessive cooling rate | Preheat substrate, use low-carbon consumables |
| Porosity | Inadequate flux coverage, moisture contamination | Ensure proper flux coverage, use dry consumables |
| Uneven surface profile | Inconsistent travel speed, electrode misalignment | Maintain consistent travel speed, align electrode properly |
Key Questions and Reflections
The study provides valuable insights into the microstructural behavior of austenitic stainless steel in electroslag surfacing, but several questions remain for further investigation:
- How does the δ-ferrite content vary with depth from the overlay surface? The electroslag process creates a thermal gradient that may result in different δ-ferrite contents at different depths.
- What is the effect of cooling rate on the relative proportions of the three δ-ferrite morphologies? Understanding this relationship would enable better control of the microstructure through process parameter optimization.
- How does the slag composition influence the δ-ferrite content and morphology? The authors mention using a self-developed flux, but the specific slag composition and its effect on the weld metal microstructure are not detailed.
- What are the long-term effects of δ-ferrite on the corrosion resistance of the overlay weld? While the study confirms that δ-ferrite is beneficial, the long-term behavior under sustained corrosion exposure should be investigated.
The coexistence of three δ-ferrite morphologies is an interesting finding that has implications for the mechanical properties of the overlay weld. The skeletal δ-ferrite, being located along grain boundaries, may act as a crack initiation site under certain loading conditions. The lamellar and worm-like morphologies, being more dispersed, are less likely to cause such issues. Understanding the distribution and volume fraction of each morphology would provide a more complete picture of the overlay weld's mechanical behavior.
Study Insights and Implications
This research contributes to the understanding of austenitic stainless steel overlay welding using the electroslag surfacing process. The key insight is that the electroslag process, with its low dilution rate and controlled thermal conditions, produces overlay welds with a favorable austenitic-ferritic microstructure. The presence of δ-ferrite in controlled amounts is beneficial for both cracking resistance and intergranular corrosion resistance, making the electroslag surfacing process a viable option for corrosion-resistant overlay applications.
For engineering practice, the study reinforces the importance of understanding the microstructural evolution in overlay welds. The δ-ferrite content and morphology are not merely academic concerns but directly affect the performance and reliability of the overlay system. Engineers should consider δ-ferrite content as a critical quality control parameter when specifying and inspecting austenitic stainless steel overlay welds.
The study also highlights the potential of the electroslag surfacing process for producing high-quality overlay welds on large components. The combination of low dilution, good surface profile, and favorable microstructure makes this process particularly suitable for applications where both corrosion resistance and mechanical integrity are required. Future work should explore the effects of different slag compositions and process parameters on the δ-ferrite content and morphology, to develop comprehensive guidelines for electroslag surfacing of austenitic stainless steel overlays.