Zhuojin Pipe Fitting Co., LtdStrip Electrode Overlay Welding of 304L onto 20MnMo Forged Tube Sheets
Literature Overview
This paper by Zhou Yinmei, Li Meijing, and Yang Wenming, published in Shanxi Chemical Industry (Vol. 29, No. 2, 2009, pp. 43–46), presents welding trial results and production experience for strip electrode overlay welding of austenitic stainless steel 304L onto 20MnMo forged tube sheets. The study addresses a common challenge in pressure vessel manufacturing: the need to create a corrosion-resistant or high-temperature resistant surface on a low-alloy steel base material through overlay welding. The authors emphasize that the selection of transition layer welding materials and the process parameters for each overlay layer are critical to controlling alloy dilution, ferrite content, and overall overlay quality.
Core Technical Content and Metallurgical Analysis
The metallurgical challenge in overlay welding 304L onto 20MnMo is fundamentally one of dissimilar material joining. 20MnMo is a low-alloy martensitic steel with Mo content of approximately 0.3–0.6% and Mn content of approximately 1.0–1.5%, while 304L is an austenitic stainless steel with approximately 18% Cr, 8% Ni, and low carbon (< 0.03%). The large difference in composition, thermal expansion coefficient, and microstructure between the two materials creates several challenges:
- Dilution: The base metal melts into the weld pool, diluting the austenitic composition and potentially forming a mixed microstructure with undesirable phases.
- Ferrite formation: Excessive dilution or improper composition can lead to ferrite formation in the overlay, which reduces corrosion resistance.
- Cracking sensitivity: The high carbon equivalent of the 20MnMo base metal increases susceptibility to cold cracking in the heat-affected zone.
- Residual stress: Differential thermal expansion between the base and overlay creates residual tensile stress at the interface, which can lead to cracking.
The strip electrode (带极) overlay welding process, also known as submerged arc strip welding (SAW with strip electrode), is particularly well-suited for this application because it provides:
- High deposition rate, which is essential for covering large tube sheet surfaces efficiently
- Excellent penetration control, allowing for precise layer-by-layer build-up
- Clean weld surface with minimal spatter
- Good metallurgical bonding with the base material
Process Parameters and Transition Layer Strategy
The authors highlight the importance of using a transition layer between the 20MnMo base and the 304L overlay. The transition layer serves to:
- Gradually adjust the composition from the base steel to the final overlay composition
- Reduce the dilution effect of the base metal on the final overlay layer
- Control the ferrite content in the overlay to within acceptable limits (typically < 10% ferrite per ASTM A262)
- Provide a metallurgical buffer that reduces residual stress concentration at the interface
A typical transition layer strategy might involve:
| Layer | Material | Purpose |
|---|---|---|
| First layer (on base) | E309L (310L) or equivalent | High dilution tolerance, bridges composition gap |
| Second layer | E309L or E308L | Further composition adjustment |
| Final layer(s) | E308L (304L) | Final corrosion-resistant surface |
The process parameters for strip electrode overlay welding typically include:
| Parameter | Typical Value |
|---|---|
| Strip electrode width | 25–50 mm |
| Strip electrode thickness | 2–3 mm |
| Flux type | Rutile or basic flux |
| Current | 500–800 A |
| Voltage | 25–35 V |
| Travel speed | 200–400 mm/min |
| Layer thickness | 3–5 mm per pass |
| Preheat | 150–200 °C |
| Interpass temperature | ≤ 250 °C |
Quality Control and Inspection
The quality of the overlay weld is critical for the long-term performance of the tube sheet. The following inspection methods should be employed:
- Ferrite number measurement: Using a ferrite scope per ASTM A262, the ferrite content in the overlay should be measured and verified to be within the specified range.
- Hardness testing: The hardness of the overlay should be uniform and consistent with the expected austenitic microstructure.
- Visual inspection: The overlay surface should be smooth, without porosity, undercut, or excessive reinforcement.
- Ultrasonic testing: To detect lack of fusion, cracks, or other internal defects at the base-overlay interface.
- Corrosion testing: Immersion tests or potentiodynamic polarization tests to verify the corrosion resistance of the overlay.
Reflections and Study Insights
This paper, though published in 2009, remains highly relevant to current engineering practice. The fundamental metallurgical challenges of dissimilar material overlay welding have not changed, and the principles of transition layer design, dilution control, and ferrite management remain the same. The strip electrode overlay welding process is particularly attractive for large-area applications such as tube sheets, where deposition rate and cost efficiency are important considerations.
One key insight from this work is that the transition layer is not merely a metallurgical necessity but also a process control tool. By carefully selecting the transition layer material and process parameters, the engineer can manage the dilution rate and microstructure evolution in a predictable manner. This is especially important when the final overlay must meet specific corrosion resistance requirements, such as resistance to chloride pitting or intergranular corrosion.
Another important consideration is the effect of the overlay on the mechanical properties of the tube sheet. The overlay layer, being austenitic, has a lower yield strength than the 20MnMo base. If the overlay is too thick or too soft, it may affect the overall structural integrity of the tube sheet. Therefore, the overlay thickness should be optimized to provide the required corrosion resistance without compromising the mechanical performance of the component.
The broader lesson is that overlay welding is not simply a surface treatment but a complex metallurgical process that requires careful consideration of the entire system, including base material, transition layers, final overlay, process parameters, and inspection methods. The engineer must balance competing requirements of corrosion resistance, mechanical strength, cost, and manufacturability to achieve a successful result.