CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding Process for Q345R Tube Sheets
Literature Overview
This paper, published in "Guangdong Chemical Industry" (2010) by Yan Shun, Tang Zhenxing, Wei Yuwei, and Lin Ying from Wenzhou Tianlong Light Equipment Co., Ltd., describes the development and implementation of a strip cladding (strip welding) process for Q345R tube sheets. Tube sheets are critical components in heat exchangers, and when used in corrosive service, they require a corrosion-resistant overlay layer on the tube-side surface. The paper provides detailed process parameters, quality control measures, and performance results for the strip cladding process applied to Q345R low-alloy steel tube sheets.
Technical Background
Q345R is a low-alloy steel widely used for pressure vessel and heat exchanger tube sheets due to its good combination of strength, toughness, and weldability. However, in corrosive service environments, Q345R lacks adequate corrosion resistance, necessitating the addition of a corrosion-resistant overlay layer.
Strip Cladding Process Overview
Strip cladding (also known as strip welding or continuous strip welding) is a specialized welding process that deposits a thin, continuous strip of corrosion-resistant material onto the surface of a base metal plate. The process is characterized by:
- High deposition rate: 5–15 kg/h compared to 1–3 kg/h for conventional GTAW
- Low dilution: 10–25% compared to 30–50% for conventional processes
- Good surface quality: Smooth surface with minimal rework required
- Uniform thickness: ±0.5 mm thickness tolerance over large areas
- Automation-friendly: Suitable for robotic welding
Process Configuration
The strip cladding process typically uses one of the following configurations:
| Configuration | Shielding | Consumable | Application |
|---|---|---|---|
| SAW strip cladding | Flux | Strip + flux | Large flat surfaces, thick overlay |
| GMAW strip cladding | CO₂ or Ar/CO₂ | Strip + wire electrode | Medium surfaces, moderate thickness |
| GTAW strip cladding | Ar | Strip + wire electrode | Small surfaces, thin overlay |
| Submerged flux strip | Flux | Strip + flux | Very large surfaces, heavy overlay |
For Q345R tube sheets, the GMAW or SAW strip cladding configuration is most commonly used, with a 304 or 316L stainless steel strip as the cladding material.
Process Parameters and Optimization
Key Process Parameters
| Parameter | Typical Range | Effect on Quality |
|---|---|---|
| Current (I) | 180–280 A | Higher current increases penetration and dilution |
| Voltage (U) | 22–30 V | Higher voltage increases strip melt rate |
| Travel speed (v) | 300–600 mm/min | Higher speed reduces dilution and overlay thickness |
| Wire feed speed (WFS) | 4–8 m/min | Controls wire electrode melting rate |
| Strip thickness | 0.8–1.5 mm | Thicker strip increases deposition rate |
| Shielding gas flow | 15–25 L/min | Prevents oxidation of molten pool |
| Torch angle | 5–15° from vertical | Affects weld profile and penetration |
Parameter Optimization Strategy
The optimization of strip cladding parameters follows a systematic approach:
- Dilution control: The dilution ratio must be kept below 25% to ensure adequate corrosion resistance of the overlay layer. This is achieved by using low heat input, high travel speed, and appropriate wire feed rate.
- Thickness uniformity: The overlay thickness must be uniform across the tube sheet surface to ensure consistent corrosion protection. This requires careful control of travel speed, wire feed rate, and strip feeding consistency.
- Surface quality: The surface of the overlay layer must be smooth enough for tube hole drilling and reaming. Surface roughness should be Ra < 6.3 μm after grinding.
- Bond strength: The bond strength between the overlay layer and the base metal must be verified by tensile or shear testing per ASTM A264 or GB/T 12965.
Quality Control and Inspection
Inspection Requirements
| Inspection Method | Standard | Acceptance Criteria |
|---|---|---|
| Visual inspection (VT) | JB/T 4730.1 | No surface defects, uniform coverage |
| Magnetic particle testing (MT) | JB/T 4730.4 | No linear indications > 1 mm |
| Ultrasonic testing (UT) | JB/T 4730.3 | Bond strength ≥ 90% of overlay thickness |
| Chemical analysis | GB/T 223 | Composition within specified range |
| Hardness testing | GB/T 231 | HV 150–350 (typical for 304/316L overlay) |
| Corrosion testing | GB/T 4334 | No intergranular corrosion after sensitization |
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Porosity | Contaminated strip, inadequate shielding | Clean strip, increase gas flow |
| Cracking | High residual stress, excessive dilution | Reduce heat input, preheat |
| Incomplete fusion | Low current, high travel speed | Increase current, reduce speed |
| Excessive dilution | High heat input, low travel speed | Reduce current, increase speed |
| Surface undercut | Improper torch angle, high speed | Adjust torch angle, reduce speed |
Engineering Practice and Case Study
Case Study: Heat Exchanger Tube Sheet Cladding
A typical application involves the cladding of a Q345R tube sheet for a heat exchanger operating in a chloride-containing environment:
- Tube sheet dimensions: 800 mm diameter × 80 mm thick
- Cladding area: Tube-side surface, 800 mm diameter × 30 mm depth
- Cladding material: 316L stainless steel strip, 1.2 mm thick
- Number of passes: 3 passes for 4 mm total overlay thickness
- Process: GMAW strip cladding with CO₂ shielding
- Parameters: I = 220 A, U = 26 V, v = 450 mm/min, WFS = 6 m/min
- Preheat: 150°C
- Post-weld treatment: Stress relief at 600°C × 2h
The resulting overlay layer had a dilution ratio of 18%, hardness of HV 220, and passed all inspection requirements including intergranular corrosion testing per ASTM A262 Practice E.
Study Insights and Implications
This paper provides practical guidance on the application of strip cladding to Q345R tube sheets, a common requirement in the heat exchanger industry. The emphasis on dilution control, thickness uniformity, and bond strength verification reflects the critical nature of tube sheet cladding in ensuring the long-term integrity of heat exchangers. The systematic approach to process parameter optimization and quality control is directly applicable to other tube sheet cladding applications and serves as a model for the implementation of strip cladding technology in pressure vessel fabrication. The paper also highlights the economic advantages of strip cladding over conventional overlay methods, with deposition rates 3–5 times higher and lower overall cost per unit area.