CLADDING TECHNOLOGY SHANXI CO., LTDAutomatic Overlay Welding of Inner Surface of Small Diameter Tubes
Literature Overview
This 2008 publication in China Chemical Equipment (中国化工装备) by an engineer from Lanzhou Lanchi Industrial Development Co., Ltd. addresses a challenging and specialized application: the automatic overlay welding of the inner surface of small-diameter tubes. Lanzhou Lanchi is a major manufacturer of petrochemical equipment, and the inner cladding of small tubes is a common requirement in heat exchangers, reactors, and transfer lines where the tube wall must resist internal corrosion or erosion.
Core Technical Content
The overlay welding of small-diameter tube interiors presents several unique challenges that distinguish it from flat plate or large-diameter pipe cladding:
| Challenge | Description | Consequence |
|---|---|---|
| Limited access | Small diameter restricts access of welding equipment | Requires specialized equipment and techniques |
| Poor visibility | Limited visibility inside the tube | Difficult to monitor weld quality |
| Restricted gas flow | Limited space for shielding gas circulation | Risk of oxidation and porosity |
| Heat dissipation | Small cross-section limits heat dissipation | High risk of overheating and distortion |
| Geometric constraints | Curved surface with limited straight length | Difficult to maintain consistent weld quality |
| Inspection difficulty | Internal surface is difficult to inspect | Requires specialized NDT methods |
The automatic overlay welding of small tube interiors typically employs one of the following approaches:
- Internal submerged arc welding (SAW): A wire electrode is fed through a guide tube into the interior of the work tube, with flux flowing around the electrode. This provides excellent shielding and high deposition rates but requires the tube to be positioned horizontally and the ends to be sealed or fitted with flux return systems.
- Internal gas metal arc welding (GMAW): A robotic arm or mechanized feed system delivers the electrode and shielding gas into the tube interior. This offers good flexibility but requires careful gas flow management.
- Internal plasma arc welding (PAW): A plasma torch is inserted into the tube, providing a concentrated, high-energy arc. This is suitable for small diameters but requires precise control to avoid overheating.
- Internal laser cladding: A laser beam is delivered through a fiber optic cable into the tube, with powder fed into the melt pool. This offers excellent precision and low heat input but is limited by powder delivery challenges in confined spaces.
Process Parameters for Small Tube Internal Cladding
| Parameter | Typical Range | Notes |
|---|---|---|
| Tube diameter | 25–100 mm | Below 25 mm, internal cladding becomes extremely difficult |
| Tube wall thickness | 3–10 mm | Thin walls are susceptible to distortion and burn-through |
| Overlay thickness | 1–3 mm | Limited by tube wall thickness and distortion tolerance |
| Base material | Carbon steel or low-alloy steel | Typically 20# steel, 15CrMo, or 09CrCuSb |
| Overlay material | 304, 316, 321, 347 stainless steel | Selected based on service environment |
| Welding current | 100–200 A | Dependent on tube diameter and wall thickness |
| Travel speed | 200–500 mm/min | Higher speed reduces heat input and distortion |
| Shielding gas | Ar or Ar + 2% O2 | Pure Ar for austenitic stainless; slight O2 addition improves wetting |
| Preheat | 50–150 °C | Minimal preheat for thin-walled tubes to avoid distortion |
Process Development and Optimization
The development of an automatic internal cladding process for small tubes requires systematic optimization of the welding parameters. The key process variables are:
- Welding current and voltage: These determine the heat input and penetration depth. Too high a current causes burn-through and excessive distortion; too low a current results in poor fusion and incomplete cladding.
- Travel speed: This controls the deposition rate and heat input per unit length. Higher travel speeds reduce heat input but may result in insufficient penetration.
- Wire feed rate: This must be synchronized with the travel speed to maintain a consistent weld bead. Mismatch between wire feed and travel speed causes porosity and uneven deposition.
- Shielding gas flow rate: In a confined space like a small tube interior, the shielding gas must be carefully managed. Insufficient gas flow allows oxidation and porosity; excessive gas flow can cause turbulence and gas entrapment.
- Electrode stick-out: The distance between the contact tip and the workpiece affects the arc length and heat input distribution. In internal welding, the stick-out is limited by the tube diameter.
The optimization process typically follows a Taguchi or response surface methodology approach, systematically varying the parameters and measuring the resulting weld quality. The key quality metrics are:
| Quality Metric | Measurement Method | Target |
|---|---|---|
| Penetration depth | Cross-section metallography | Full wall thickness fusion |
| Dilution rate | Spectrographic analysis of cross-section | < 30% for corrosion resistance |
| Porosity | Visual or radiographic inspection | No porosity |
| Cracking | Visual and MT inspection | No cracks |
| Surface quality | Visual and profile measurement | Smooth, uniform surface |
| Distortion | Dimensional measurement | Within tolerance |
Defect Analysis and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Burn-through | Excessive heat input, thin wall | Reduce current, increase travel speed, use backing rod |
| Porosity | Insufficient shielding gas, contamination | Increase gas flow, clean tube interior, use dry consumables |
| Incomplete fusion | Low current, high travel speed | Increase current, reduce travel speed, ensure proper fit-up |
| Cracking | High cooling rate, hydrogen | Preheat, use low-hydrogen consumables, post-weld bake |
| Distortion | Asymmetric heat input, thin wall | Symmetric welding sequence, reduce heat input per pass |
| Uneven thickness | Parameter instability, tube eccentricity | Monitor and control parameters, use seam tracking |
Engineering Practice Case
A typical application might involve the internal cladding of 32 mm OD × 4 mm wall tubes with 316L stainless steel for use in a heat exchanger handling corrosive process fluids:
| Specification | Value |
|---|---|
| Tube material | 20# carbon steel |
| Tube dimensions | 32 mm OD × 4 mm wall × 6000 mm length |
| Overlay material | 316L stainless steel wire |
| Overlay thickness | 1.5 mm |
| Welding process | Internal SAW with flux return |
| Welding current | 150 A |
| Travel speed | 300 mm/min |
| Preheat | 100 °C |
| Acceptance criteria | Full fusion, < 25% dilution, no defects |
The welding process would involve:
- Cleaning and preparation of the tube interior (grinding, degreasing).
- Positioning the tube horizontally in the welding fixture.
- Inserting the electrode guide and flux return system.
- Preheating the tube to 100 °C.
- Performing the overlay weld in a single or multi-pass operation, depending on the required thickness.
- Allowing the tube to cool to ambient temperature.
- Inspecting the overlay using borescope or endoscope examination.
- Performing any required NDT (UT, eddy current).
Study Reflection
The 2008 publication of this work reflects the increasing demand for high-performance, corrosion-resistant heat exchanger tubes in China's rapidly growing petrochemical industry. The internal cladding of small tubes is a challenging but essential technology for extending the service life of heat exchangers in corrosive service.
The key insight from this reference is that the automatic internal cladding of small tubes is a specialized technology that requires careful process development and optimization. It is not a simple adaptation of flat plate or large pipe cladding — the confined geometry, limited access, and restricted gas flow require fundamentally different process approaches.
In contemporary practice, the internal cladding of small tubes is increasingly performed using laser cladding or cold spray technologies, which offer lower heat input and reduced distortion. However, the fundamental principles of process control, defect prevention, and quality inspection described in this 2008 paper remain valid and are the foundation of modern internal cladding technology.
The practical challenge for engineers is to select the appropriate process for the specific application, considering factors such as tube diameter, wall thickness, required overlay thickness, production volume, and cost. There is no single best process — the optimal choice depends on the specific requirements of the application.