CLADDING TECHNOLOGY SHANXI CO., LTDManual Weld Overlay of Stainless Steel Inner Walls of Pipes
Literature Overview and Background
The internal cladding of pipes with stainless steel is a common requirement in chemical, petrochemical, pharmaceutical, and food processing industries where corrosion resistance of the flow path is critical. Unlike external cladding, which can be performed using automated processes such as strip cladding, electroslag welding, or submerged arc welding, internal cladding is constrained by accessibility, geometry, and the need to maintain the internal diameter of the pipe. This literature focuses on the manual welding overlay techniques—primarily gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW)—used for internal stainless steel cladding of pipes, and examines the process parameters, quality considerations, and practical challenges involved.
Internal pipe cladding presents unique challenges compared to flat plate cladding:
- Limited access for electrode positioning and filler wire feeding
- Gravity effects on the molten pool in vertical and overhead positions
- Difficulty in achieving uniform overlay thickness around the circumference
- Inability to use automated processes for many pipe sizes and configurations
- Higher labor costs due to manual operation
- Greater sensitivity to operator skill level
Process Selection and Equipment Requirements
Selection of Welding Process
The choice of welding process for internal pipe cladding depends on several factors:
| Process | Applicable Pipe Diameter | Overlay Thickness | Typical Application | Advantages | Limitations |
|---|---|---|---|---|---|
| GTAW (TIG) | DN25 - DN600 | 0.5 - 3.0 mm | Precision cladding, thin walls | Excellent penetration control, clean weld, low dilution | Low deposition rate, labor-intensive |
| GMAW (MIG) | DN100 - DN1200 | 1.0 - 5.0 mm | Medium to large diameter pipes | Higher deposition rate, good versatility | Higher dilution, more spatter |
| FCAW (Flux-Cored) | DN200 - DN1500 | 2.0 - 8.0 mm | Heavy-duty cladding | Very high deposition rate | Flux residue, slag removal difficulty |
| Hot-wire TIG | DN50 - DN800 | 0.5 - 2.0 mm | Precision cladding with higher deposition | Balanced deposition and control | Requires specialized equipment |
For most internal pipe cladding applications, GTAW is the preferred process due to its superior penetration control and the ability to achieve clean, consistent welds in confined spaces. The filler wire selection typically follows the 309L/316L or 309/316 series for carbon steel backing, with the first pass using a high-alloy consumable (309L) to minimize dilution-induced cracking, followed by 304L or 316L for subsequent passes.
Equipment Configuration
Manual internal pipe cladding requires specialized equipment configuration:
- Power source: Square wave or pulsed GTAW power source for improved arc stability and penetration control
- Filler wire feeding: Manual wire feeding for GTAW (typically ER309L or ER316L, 1.0-2.4 mm diameter) or semi-automatic wire feeding for GMAW
- Shielding gas: High-purity argon (99.99%) for GTAW; argon-helium mixture (80/20 or 75/25) for GMAW to improve arc characteristics in confined spaces
- Back purge: Essential for preventing internal oxidation; argon back purge with flow rate of 5-10 L/min
- Positioning: Pipe rotation fixture or manual pipe manipulation for multi-pass welding
- Welding position aids: Specialized torch holders and wire guide extensions for working inside pipes
Process Parameters and Welding Sequence
GTAW Internal Cladding Parameters
The welding parameters for GTAW internal cladding of stainless steel pipes are as follows:
| Parameter | First Pass (Dilution Pass) | Subsequent Passes | Final Pass (Capping) |
|---|---|---|---|
| Filler wire | ER309L (2.4 mm) | ER304L or ER316L (2.0 mm) | ER304L or ER316L (1.6 mm) |
| Current | 100-150 A | 120-180 A | 80-120 A |
| Voltage | 12-16 V | 14-18 V | 10-14 V |
| Travel speed | 30-50 mm/min | 40-70 mm/min | 50-80 mm/min |
| Heat input | 0.8-1.5 kJ/mm | 1.0-2.0 kJ/mm | 0.5-1.0 kJ/mm |
| Shielding gas | Ar 12-15 L/min | Ar 12-15 L/min | Ar 12-15 L/min |
| Back purge | Ar 5-10 L/min | Ar 5-10 L/min | Ar 5-10 L/min |
The welding sequence for internal pipe cladding typically follows a spiral or multi-pass approach:
- Surface preparation: Grind the internal surface to remove mill scale, rust, and contamination. The surface should be prepared to a minimum of Sa 2.5 cleanliness or equivalent mechanical preparation.
- First pass (dilution pass): A single bead along the pipe axis using high-alloy filler (309L) to create a transition zone with sufficient alloy content to prevent cracking in subsequent passes.
- Build-up passes: Multiple passes building up the overlay thickness, with each pass overlapping the previous one by 50-75% of the bead width.
- Capping pass: A final pass with lower heat input to achieve a smooth, dense surface with minimal porosity.
- Cooling and inspection: Allow controlled cooling, then perform visual inspection and NDT.
GMAW Internal Cladding Parameters
For larger diameter pipes where GTAW deposition rates are insufficient, GMAW can be used with the following parameters:
| Parameter | Value |
|---|---|
| Filler wire | ER309L (first pass), ER316L (subsequent passes), 1.2 mm diameter |
| Current | 130-200 A |
| Voltage | 18-24 V |
| Travel speed | 50-100 mm/min |
| Shielding gas | Ar-He (75/25) 15-20 L/min |
| Wire feed speed | 4-6 m/min |
| Stick-out length | 10-15 mm |
Quality Control and Defect Prevention
Common Defects and Countermeasures
Internal pipe cladding is susceptible to several characteristic defects:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Porosity | Inadequate shielding, moisture in filler | Improve gas flow, ensure wire dryness, use back purge |
| Lack of fusion | Insufficient penetration, cold welding | Increase current, improve travel speed control, proper surface preparation |
| Cracking | Excessive dilution, residual stress | Use 309L for first pass, control heat input, apply preheat |
| Excessive dilution | High heat input, thin first pass | Reduce current, use high-alloy first pass filler |
| Uneven thickness | Operator inconsistency | Use welding guides, maintain consistent travel speed |
| Internal oxidation | Inadequate back purge | Ensure continuous back purge throughout welding |
Non-Destructive Testing
Quality assurance for internal pipe cladding requires a combination of NDT methods:
- Visual testing (VT): Using borescopes or flexible endoscopes to inspect the overlay surface for cracks, porosity, and unevenness
- Magnetic particle testing (MT): For surface and near-surface defects on the internal surface (applicable to ferritic base metal)
- Ultrasonic testing (UT): For bond strength verification and detection of internal defects
- Radiographic testing (RT): For through-thickness defect detection (limited by pipe geometry)
- Eddy current testing (ET): For surface defect detection on non-ferromagnetic overlay
Mechanical Property Verification
The cladded pipe must meet specified mechanical property requirements:
| Test | Requirement | Standard |
|---|---|---|
| Bond strength (peel test) | ≥ 2.0 MPa (typical) | NB/T 47014, ASME IX |
| Hardness (overlay) | 150-250 HV (for austenitic SS) | GB/T 150 |
| Charpy impact (if required) | ≥ 27 J at service temperature | GB/T 150 |
| Intergranular corrosion | Pass per ASTM A923 Method A | GB/T 150 |
| Hydrostatic test | 1.5 × design pressure | GB/T 150 |
Engineering Practice Cases
Case 1: Small Diameter Chemical Pipe Cladding
A series of DN80 chemical process pipes required internal 316L cladding for service in a 30% sulfuric acid environment at 80°C. The pipes were made of 20# carbon steel with 4 mm wall thickness.
Process approach:
- GTAW with ER309L for the first pass (0.5 mm thickness) and ER316L for the second and third passes (total overlay thickness 1.5 mm)
- Pipe rotation fixture with 12 rpm rotation speed for uniform cladding
- Preheat at 150°C to prevent cracking in the carbon steel base metal
- Back purge with argon at 8 L/min throughout welding
Results: All pipes passed UT inspection with no detectable defects. Bond strength testing showed an average of 2.8 MPa, exceeding the specification requirement. Intergranular corrosion testing confirmed the overlay met the required resistance level.
Case 2: Large Diameter Reactor Tube Cladding
A DN400 reactor tube required internal Hastelloy C276 cladding for service in a highly corrosive fluorine-containing environment at 200°C. The tube was made of 15CrMo with 12 mm wall thickness.
Process approach:
- GMAW with ERNiCrMo-16 (Hastelloy C276 equivalent) filler wire, 1.2 mm diameter
- Three passes with total overlay thickness of 3.0 mm
- Preheat at 200°C, interpass temperature controlled at 250°C
- Ar-He (75/25) shielding gas at 18 L/min with back purge
- Post-weld solution treatment at 1120°C for 1 hour followed by water quench
Results: The overlay showed excellent corrosion resistance in the target environment. However, the solution treatment caused slight distortion of the tube, requiring post-treatment straightening. This highlights the importance of considering the interaction between cladding and subsequent heat treatments.
Key Questions and Reflections
The manual internal pipe cladding process raises several important considerations for engineers:
- Deposition rate vs. quality trade-off: Manual processes inherently have lower deposition rates than automated methods, but they offer superior adaptability to complex geometries. How can this trade-off be optimized for specific applications?
- Operator skill dependency: The quality of manual cladding is heavily dependent on operator skill. What training and qualification programs are most effective in ensuring consistent quality?
- Cost-effectiveness analysis: For small batch production, manual cladding may be the most economical option. However, for large-scale production, the labor costs can become prohibitive. At what production volume does automated cladding become more cost-effective?
- Hybrid approaches: Can the combination of manual and automated processes (e.g., automated tack welding followed by manual build-up) provide an optimal balance of quality and productivity?
Study Insights and Implications
The study of manual internal pipe cladding reinforces the importance of process discipline and operator training in achieving consistent quality. Unlike automated processes, where quality is primarily determined by equipment calibration and parameter settings, manual cladding quality is fundamentally dependent on the skill and consistency of the welder.
The practical implication for engineers is that internal pipe cladding projects must include rigorous operator qualification programs, detailed welding procedure specifications (WPS), and comprehensive quality assurance protocols. The use of welding procedure qualification records (PQR) per NB/T 47014 or ASME IX is essential for establishing the process parameters and ensuring reproducibility.
Furthermore, the study highlights the importance of considering the entire manufacturing sequence, including post-weld treatments, when planning internal pipe cladding. The interaction between cladding residual stresses and subsequent heat treatments (such as solution treatment or stress relief) can significantly affect the final product quality and must be carefully managed through process planning.