CLADDING TECHNOLOGY SHANXI CO., LTDBack-Side Cladding Strip MIG Welding of Stainless Steel Silo
Literature Overview
This technical document by Zhang Baoyan, Gao Yan, and Liang Daipu from Daqing Petrochemical General Plant Machine Repair Factory and Daqing Labor Bureau presents a practical engineering case study on back-side cladding strip MIG welding for stainless steel silos. Published in 1997 in the journal Welding, this work represents valuable industrial experience in the fabrication of corrosion-resistant storage vessels for the petrochemical industry.
Technical Context and Application
Stainless steel silos used in petrochemical applications require corrosion-resistant linings to withstand aggressive chemical environments. The back-side cladding strip MIG welding technique provides an effective method for applying stainless steel overlay layers to carbon steel base plates, creating bimetallic composite structures that combine the strength of carbon steel with the corrosion resistance of stainless steel.
The back-side welding approach is particularly advantageous for silo fabrication because it allows the cladding layer to be deposited on the interior surface of the vessel, which is the surface exposed to the corrosive medium. This technique avoids the need for post-fabrication cladding application and ensures intimate bonding between the cladding and base metal.
Process Description and Parameters
The back-side cladding strip MIG welding process involves depositing a layer of stainless steel filler metal (typically 308L, 309L, or 316L wire) onto the back side of a carbon steel plate or joint, creating a cladding layer that becomes the interior surface of the finished vessel. The process parameters are carefully controlled to ensure adequate bonding strength and minimal dilution of the stainless steel cladding with the carbon steel base metal.
| Parameter | Typical Value | Rationale |
|---|---|---|
| Base metal | Q235 or 16Mn carbon steel | Structural strength requirement |
| Cladding wire | ER308L or ER316L | Corrosion resistance and low carbon |
| Current | 180–260 A | Adequate penetration and bonding |
| Voltage | 22–28 V | Stable arc and proper bead profile |
| Travel speed | 0.2–0.5 m/min | Controlled dilution and deposition rate |
| Wire diameter | 1.0–1.2 mm | Balance of deposition and penetration |
| Shielding gas | 98% Ar + 2% CO2 | Stable arc and good wetting |
| Preheat temperature | 100–150°C | Reduce hydrogen cracking risk |
| Interpass temperature | <250°C | Prevent excessive grain growth |
The back-side welding technique requires careful joint design to ensure that the cladding layer achieves adequate thickness and uniform coverage. The typical joint configuration involves a T-butt or square butt joint with a slight root gap to allow the cladding filler metal to flow through and form a continuous layer on the back side.
Quality Control and Inspection
Quality control for back-side cladding strip MIG welding involves several critical inspection steps:
- Visual inspection – Check for uniform cladding coverage, absence of cracks, and proper bead profile on the back side.
- Magnetic particle testing (MT) – Detect surface and near-surface defects in the cladding layer and at the cladding-base metal interface.
- Ultrasonic testing (UT) – Assess bond strength and detect lack of fusion between the cladding and base metal.
- Chemical analysis – Verify the composition of the cladding layer, particularly carbon content and dilution level.
- Mechanical testing – Perform tensile, hardness, and bend tests to verify the mechanical properties of the cladding and the bond interface.
- Corrosion testing – Conduct intergranular corrosion tests and salt spray tests to verify the corrosion resistance of the cladding layer.
The dilution rate—the percentage of carbon steel base metal mixed into the stainless steel cladding—is a critical quality parameter. For back-side welding, dilution rates of 20–35% are typical, and the resulting cladding composition must still meet the minimum requirements for corrosion resistance. If dilution exceeds acceptable limits, additional cladding passes may be required.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Lack of fusion | Insufficient heat input or poor joint fit-up | Increase current, improve joint preparation |
| Cracking | High carbon dilution or hydrogen embrittlement | Use low-carbon wire, control preheat and interpass temperature |
| Porosity | Contamination or gas shielding issues | Clean base metal, ensure proper gas coverage |
| Excessive dilution | Excessive penetration into base metal | Reduce current, increase travel speed |
| Undercut | Improper travel speed or angle | Adjust torch angle and travel speed |
| Incomplete cladding | Inadequate filler metal deposition | Add additional cladding passes |
Engineering Practice Considerations
The back-side cladding strip MIG welding technique offers several advantages for silo fabrication:
- Cost efficiency – Eliminates the need for separate cladding plate application and subsequent welding, reducing material and labor costs.
- Quality assurance – The cladding layer is deposited in a controlled process with immediate quality verification possible.
- Structural integrity – The cladding layer is integrally bonded to the base metal, providing superior bond strength compared to mechanical attachment methods.
- Design flexibility – Allows complex silo geometries to be fabricated with corrosion-resistant interiors without additional cladding steps.
However, the technique also presents challenges that must be addressed in engineering practice:
- Back-side accessibility – The back side of the silo must be accessible for welding, which may require special fixtures or sequential welding strategies.
- Distortion control – The asymmetric heat input from back-side welding can cause plate distortion, requiring careful welding sequence planning.
- Procedure qualification – Welding procedure qualification in accordance with applicable standards (such as NB/T 47014 or ASME IX) is essential for production implementation.
- Inspection access – Post-weld inspection of the back-side cladding may require special access provisions.
Study Insights and Reflections
This engineering case study provides valuable practical insights into the application of back-side cladding techniques for petrochemical equipment fabrication. The technique represents a practical solution to the challenge of creating corrosion-resistant vessels with cost-effective fabrication methods.
The key lesson from this work is that back-side cladding strip welding is not merely a welding operation but a comprehensive process that requires careful integration of joint design, welding procedure, quality control, and inspection. The success of the technique depends on the coordinated control of all these factors, and any compromise in one area can compromise the overall quality of the finished product.
For engineers involved in bimetal pressure vessel fabrication, this study reinforces the importance of process understanding and quality control in achieving reliable cladding performance. The technique remains relevant today and continues to be used in various industrial applications where corrosion-resistant linings are required on carbon steel structures.