CLADDING TECHNOLOGY SHANXI CO., LTDStrip Submerged Arc Surfacing Process for 16MnR Tube Sheets
Background and Technical Context
Tube sheets are critical components in heat exchangers, reactors, and pressure vessels. They serve as the structural interface between the shell and the tube bundle, and they must withstand internal pressure, thermal cycling, and often corrosive media. The 16MnR steel (equivalent to ASME SA-516 Gr.70) is widely used for tube sheet fabrication due to its good combination of strength, weldability, and cost-effectiveness. However, when the tube sheet is exposed to corrosive media on one side, a protective surfacing layer is required. Strip submerged arc welding (strip SAW) provides an efficient, high-deposition-rate method for applying corrosion-resistant cladding to large tube sheet surfaces.
This study examines the process development, metallurgical considerations, quality control, and code compliance for strip SAW surfacing of 16MnR tube sheets with austenitic stainless steel overlay. The findings are directly applicable to engineers involved in pressure vessel fabrication and repair.
Process Design and Parameter Selection
Strip submerged arc welding uses a wide flat strip of electrode (typically 50–100 mm wide) that is fed through a flux blanket, creating a broad, shallow weld bead with minimal penetration into the base metal. This geometry is ideal for surfacing applications because it minimizes dilution while maximizing deposition rate.
Electrode and Flux Selection
| Component | Specification | Rationale |
|---|---|---|
| Strip electrode | 308L or 316L, 60 mm wide | Low carbon to prevent sensitization |
| Flux | Low-alloy, basic type | Provides shielding, deoxidization, and slag protection |
| Backing strip | 308L stainless steel | Prevents burn-through and ensures full penetration of first pass |
| Base metal | 16MnR, minimum 25 mm thickness | Adequate thermal mass for stable process |
The selection of 308L or 316L strip electrode depends on the corrosion environment. 308L provides adequate resistance to most oxidizing environments, while 316L is selected when chloride resistance is required. The low carbon specification (C < 0.03%) is essential to prevent intergranular corrosion sensitization during welding heat cycles.
Process Parameters
| Parameter | Value | Notes |
|---|---|---|
| Welding current | 800–1000 A | Higher current for wider bead |
| Arc voltage | 28–32 V | Controls bead width and profile |
| Travel speed | 200–300 mm/min | Balances deposition rate and bead quality |
| Strip width | 60 mm | Matches bead width to tube sheet geometry |
| Flux coverage | 10–15 mm depth | Ensures complete arc shielding |
| Preheat temperature | 100–150°C | Prevents cold cracking at fusion line |
| Interpass temperature | < 200°C | Controls heat input and dilution |
| Number of passes | 2–3 | Builds overlay thickness to 3–5 mm |
The deposition rate for strip SAW is approximately 8–12 kg/h, which is 3–5 times higher than conventional SAW with round electrodes. This makes strip SAW economically attractive for large tube sheet surfaces where total overlay area exceeds 2 m².
Metallurgical Considerations
Dilution Control
Dilution is the primary metallurgical concern in strip SAW surfacing. The wide, shallow bead geometry of strip SAW inherently produces low dilution (typically 5–15%), which is favorable for maintaining the corrosion resistance of the overlay. However, dilution can increase if process parameters are not carefully controlled.
| Condition | Dilution (%) | Overlay Hardness (HV) | Corrosion Resistance |
|---|---|---|---|
| Optimal parameters | 8–12 | 180–200 | Excellent |
| High current (1100 A) | 15–20 | 160–180 | Good |
| Low travel speed (150 mm/min) | 18–25 | 150–170 | Moderate |
| Excessive heat input | 25–35 | 140–160 | Poor |
The literature confirms that maintaining dilution below 15% is critical for achieving acceptable corrosion resistance in the overlay. When dilution exceeds 20%, the overlay composition shifts toward the base metal, and the corrosion resistance degrades significantly. The interpass temperature control is particularly important—maintaining interpass temperature below 200°C limits heat input and prevents excessive dilution in subsequent passes.
Microstructure of the Overlay
The overlay microstructure consists of a fully austenitic matrix with fine delta ferrite (2–5%) distributed in interdendritic regions. This microstructure is characteristic of low-dilution 308L/316L deposits and provides excellent resistance to both general corrosion and intergranular corrosion. The delta ferrite content is beneficial because it prevents hot cracking during solidification and provides additional resistance to stress corrosion cracking.
The fusion line microstructure shows a narrow transition zone (0.1–0.3 mm) between the austenitic overlay and the ferritic 16MnR base metal. This transition zone contains a mixture of austenite and martensite, with hardness values of 250–300 HV. The narrow width of this zone is a direct result of the low dilution achieved by strip SAW, and it is critical for maintaining the corrosion resistance of the overlay.
Quality Control and Non-Destructive Testing
Quality control for strip SAW surfacing on tube sheets is governed by applicable codes and standards, including GB/T 150, NB/T 47002, and ASME Section VIII. The following NDT methods are required:
| NDT Method | Purpose | Acceptance Criteria | Standard |
|---|---|---|---|
| Visual inspection (VT) | Surface defects, bead profile | No undercut, no excessive convexity | NB/T 47002 |
| Magnetic particle testing (MT) | Surface and near-surface cracks | No linear indications > 3 mm | JB/T 4730 |
| Ultrasonic testing (UT) | Bond strength, lack of fusion | Full bond, no lack of fusion | JB/T 4730 |
| Penetrant testing (PT) | Surface cracks in overlay | No linear indications > 2 mm | JB/T 4730 |
| Radiographic testing (RT) | Internal defects (selective) | No porosity > 3 mm, no slag inclusions | NB/T 47013 |
The ultrasonic testing for bond strength is particularly important. The overlay must be fully bonded to the base metal across the entire surface area. UT examination using a contact method with a 5 MHz probe at a 0° incidence angle can detect lack of fusion at the fusion line. The acceptance criterion is typically 100% bond with no indications exceeding the reference block signal level.
Mechanical Property Verification
The overlay must meet minimum mechanical property requirements specified in the applicable code. For 308L/316L overlay on 16MnR:
| Property | Minimum Requirement | Typical Achieved Value |
|---|---|---|
| Hardness (HV) | 150–250 | 180–210 |
| Tensile strength (MPa) | ≥ 450 | 520–580 |
| Elongation (%) | ≥ 30 | 35–45 |
| Impact energy (J, RT) | ≥ 27 | 60–120 |
The hardness range of 150–250 HV is specified to ensure that the overlay is not too hard (which would compromise toughness) or too soft (which would compromise wear resistance). The impact energy requirement ensures adequate toughness for the operating temperature range.
Common Defects and Countermeasures
| Defect | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Lack of fusion at fusion line | Insufficient heat input, poor flux coverage | UT, RT | Increase current, ensure full flux coverage |
| Cracking at fusion line | High carbon in base metal, rapid cooling | MT, PT | Preheat to 150°C, use low-hydrogen flux |
| Excessive porosity | Moist flux, insufficient arc shielding | RT, UT | Dry flux at 300°C, ensure complete flux blanket |
| Hardness below specification | Excessive dilution (>20%) | Hardness test | Reduce heat input, increase travel speed |
| Surface undercut | Excessive current, improper electrode angle | VT, MT | Reduce current, adjust electrode angle |
| Overlay spalling | Poor bond, residual stress | UT, hammer test | Improve fusion, reduce residual stress |
Code Compliance and Documentation
Strip SAW surfacing of pressure vessel tube sheets requires compliance with applicable fabrication codes. The welding procedure specification (WPS) must be qualified in accordance with NB/T 47014 or ASME Section IX. The qualified WPS must include:
- Base metal specification (16MnR, SA-516 Gr.70, etc.)
- Overlay material specification (308L, 316L, etc.)
- Process parameters (current, voltage, travel speed, flux type)
- Preheat and interpass temperature requirements
- Post-weld heat treatment requirements (if applicable)
- Acceptance criteria for NDT and mechanical testing
The welder qualification must demonstrate the ability to produce welds meeting the acceptance criteria. For strip SAW, the qualification test typically involves welding a coupon with the same geometry and parameters as the production tube sheet, followed by full NDT and mechanical testing.