Zhuojin Pipe Fitting Co., LtdStrip Electrode Submerged Arc Surfacing of 16MnR Tubesheet with Stainless Steel Overlay
Literature Overview
This technical paper by Hu Wei and Liu Rongjun from PetroChina Second Construction Company, published in China Chemical Equipment in 2005 (Vol. 7, No. 1, pp. 16–19), addresses the practical challenges of applying stainless steel overlay to 16MnR tubesheets using strip electrode submerged arc surfacing (SESAS). The paper focuses on two critical engineering problems: achieving the proper weld metal microstructure (austenite plus 3–12% ferrite duplex structure) for corrosion resistance and crack resistance, and controlling welding deformation on the large, thin-walled tubesheet geometry.
Core Technical Challenges
The 16MnR tubesheet is a critical component in heat exchangers and reactors used extensively in the petrochemical industry. The material designation 16MnR corresponds to a low-carbon manganese steel with good mechanical properties and weldability, but it is susceptible to corrosion in aggressive chemical environments. Applying a stainless steel overlay layer provides the necessary corrosion resistance while maintaining the structural integrity of the base material.
The two primary challenges identified in this paper are:
- Microstructural control: The overlay weld metal must achieve a specific austenite-ferrite balance. Too little ferrite leads to susceptibility to solidification cracking and intergranular corrosion, while too much ferrite can compromise corrosion resistance and mechanical properties. The target of 3–12% ferrite represents a carefully balanced composition that provides both crack resistance and corrosion performance.
- Deformation control: Tubesheets are typically large diameter, relatively thin-walled components. The significant thermal input from strip electrode submerged arc surfacing can cause substantial distortion, including bowing, warping, and local buckling, which can compromise the dimensional accuracy required for assembly with tubes and covers.
Welding Material and Process Parameter Selection
The selection of welding materials and process parameters is critical for achieving the desired overlay weld properties. The following table summarizes the key considerations:
| Parameter Category | Specific Consideration | Target/Requirement |
|---|---|---|
| Base material | 16MnR tubesheet | Low-carbon manganese steel, good weldability |
| Overlay material | Stainless steel strip electrode | Typically 304L, 316L, or equivalent |
| Weld metal microstructure | Austenite + ferrite | 3–12% ferrite (delta ferrite) |
| Ferrite number | Schaeffler diagram prediction | 5–20 FN |
| Dilution rate | Base metal into weld metal | Minimize while maintaining bond strength |
| Welding current | DC or AC, depending on equipment | Controlled to achieve desired penetration and dilution |
| Travel speed | Depends on strip width and thickness | Optimized for uniform bead profile |
| Shielding gas | Argon or Ar/CO₂ mixture | Protects molten weld pool |
| Preheating | Depends on base material thickness | Controls cooling rate and residual stress |
The strip electrode submerged arc process offers several advantages for this application:
- High deposition rates compared to wire electrode processes.
- Excellent weld quality with low porosity and uniform composition.
- Ability to achieve consistent multi-pass builds with controlled dilution.
- Lower welding fume and spatter compared to open-arc processes.
Microstructural Control Strategy
Achieving the target austenite-ferrite balance requires careful control of the weld metal composition, which is influenced by both the strip electrode composition and the dilution from the base metal. The Schaeffler diagram is the primary tool for predicting the weld metal microstructure based on the chromium and nickel equivalents of the base metal and filler metal.
The dilution rate in strip electrode submerged arc surfacing is typically lower than in conventional submerged arc welding due to the greater weld pool volume and the geometry of the strip electrode. However, the dilution rate must still be carefully managed, particularly for the first pass, which is most susceptible to base metal dilution.
| Pass Number | Expected Dilution Rate | Microstructure Concern |
|---|---|---|
| First pass | Highest (typically 15–30%) | Risk of excessive ferrite or martensite formation |
| Second pass | Moderate (typically 5–15%) | Better austenite-ferrite balance achievable |
| Subsequent passes | Lowest (typically <5%) | Composition approaches filler metal composition |
The first pass is the most critical from a microstructural standpoint. To minimize dilution and ensure adequate corrosion resistance, engineers may employ strategies such as:
- Using a higher-nickel strip electrode for the first pass to compensate for base metal dilution.
- Employing a reduced heat input for the first pass to limit penetration.
- Applying a thin transition layer before the main overlay buildup.
Welding Deformation Control
The deformation of the tubesheet during strip electrode submerged arc surfacing is a significant practical challenge. The large thermal input from the process, combined with the geometry of the tubesheet (large diameter, relatively thin), creates conditions conducive to significant distortion.
| Deformation Type | Cause | Mitigation Strategy |
|---|---|---|
| Bowing (overall curvature) | Asymmetric heat input | Balanced welding sequence, symmetric pass layout |
| Local warping | Uneven thermal gradients | Controlled travel speed, consistent parameters |
| Edge curling | Free edge heating and cooling | Backing plate support, fixture clamping |
| Tube hole distortion | Localized thermal stress | Hole plugging or protection during welding |
| Residual stress | Non-uniform cooling | Post-weld stress relief, controlled cooling rate |
Key measures for controlling deformation include:
- Balanced welding sequence: Welding should proceed in a balanced pattern that alternates between opposite sides of the tubesheet to minimize asymmetric thermal distortion.
- Fixture design: Rigid fixtures should be used to restrain the tubesheet during welding, though excessive restraint can lead to high residual stresses.
- Preheating: Moderate preheating reduces the thermal gradient between the weld zone and the base material, thereby reducing residual stresses and distortion.
- Back heat: Applying heat to the trailing edge of the weld during welding can help equalize the temperature distribution and reduce distortion.
- Post-weld treatment: Stress relief annealing can reduce residual stresses, though it must be carefully controlled to avoid affecting the overlay layer properties.
Engineering Practice Considerations
From a practical standpoint, the successful application of strip electrode submerged arc surfacing to 16MnR tubesheets requires a comprehensive approach that addresses both metallurgical and mechanical aspects:
- Pre-weld preparation: Surface cleaning, edge preparation, and fit-up are essential for achieving good weld quality. The tubesheet surface must be free of mill scale, rust, and contamination.
- Process monitoring: In-process monitoring of welding parameters, including current, voltage, travel speed, and wire feed speed, is essential for maintaining consistent weld quality.
- Post-weld inspection: Non-destructive testing methods such as ultrasonic testing, magnetic particle testing, and visual inspection should be employed to verify weld quality. Metallographic examination of the overlay weld metal should confirm the austenite-ferrite balance.
- Corrosion testing: The overlay weld should be subjected to corrosion testing appropriate for the intended service environment, such as chloride stress corrosion cracking testing, pitting resistance testing, or immersion testing.
Key Questions and Reflections
The paper raises several important practical questions:
- How does the strip electrode submerged arc process compare with alternative surfacing methods (such as wire electrode submerged arc, plasma arc, or laser cladding) for this specific application in terms of cost, quality, and productivity?
- What is the long-term performance of the stainless steel overlay in the specific chemical environment of the intended service? Does the overlay layer maintain its corrosion resistance over extended service periods?
- How does the residual stress state of the tubesheet affect the long-term structural integrity and fatigue performance of the component?
- Can the welding sequence and parameters be optimized further using numerical simulation to reduce distortion while maintaining productivity?
Study Insights and Implications
This paper provides valuable practical guidance for engineers involved in the overlay surfacing of large structural components. The emphasis on both microstructural control and deformation management reflects the holistic approach required for successful industrial welding applications.
The most significant insight is that achieving the target weld metal microstructure is only one aspect of the challenge; the deformation control is equally important for ensuring the dimensional accuracy and structural integrity of the final component. Engineers must consider the entire welding process—from material selection and process parameter optimization to sequence planning and post-weld treatment—to achieve a satisfactory result.
The paper also highlights the importance of process-specific considerations. Strip electrode submerged arc surfacing has unique characteristics that distinguish it from other surfacing processes, and these characteristics must be understood and managed to achieve optimal results. The dilution behavior, weld pool dynamics, and heat input characteristics of this process all influence the final weld quality and component performance.