CLADDING TECHNOLOGY SHANXI CO., LTDSubmerged Arc Strip Cladding of Stainless Steel on Low-Alloy Steel Surfaces
Literature Overview
This 1993 paper from Guangzhou Heavy Machinery Works, authored by Lin Qingguo, Wang Yunzhong, and Chen Zhixin, and published in Petrochemical Equipment, presents experimental work on submerged arc strip cladding (also known as submerged arc strip welding or multi-wire SAW cladding) of stainless steel onto low-alloy steel substrates. This work addresses a critical technology in the fabrication of bimetal clad plates and pressure vessels for the petrochemical industry, where corrosion resistance of the inner surface combined with mechanical strength of the base metal is required.
The submerged arc strip cladding process, which uses a flat strip electrode (typically 25–100 mm wide) fed continuously into the arc, represents a high-productivity alternative to conventional submerged arc welding for cladding applications. The strip electrode provides a larger cross-sectional area of molten metal per unit time, resulting in deposition rates that can be 3–5 times higher than those achieved with solid wire SAW. This makes the process particularly attractive for large-scale cladding operations such as the fabrication of pressure vessel heads, heat exchanger shells, and large diameter pipe cladding.
Process Description and Technical Parameters
The submerged arc strip cladding process involves feeding a continuous strip of the cladding material (in this case, austenitic stainless steel such as 304, 316, or 321) into the arc formed between the strip tip and the base metal. The strip is typically 0.5–1.5 mm thick and 25–100 mm wide, with a length of 2–10 meters per coil. The process parameters are critical to achieving a sound bond and a uniform overlay composition.
| Parameter | Typical Range | Notes |
|---|---|---|
| Strip width | 25–100 mm | Wider strips increase productivity but reduce flexibility |
| Strip thickness | 0.5–1.5 mm | Thicker strips require higher currents |
| Welding current | 1500–3000 A | Depends on strip width and thickness |
| Welding voltage | 22–30 V | Maintains arc stability with strip electrode |
| Travel speed | 200–600 mm/min | Inversely proportional to current for uniform deposition |
| Flux type | Rutile or basic flux | Basic flux provides lower hydrogen levels |
| Preheat temperature | 100–200°C | Depends on base metal carbon equivalent |
| Interpass temperature | 150–250°C | Prevents excessive cooling and cracking |
| Number of passes | 1–3 | First pass for bond, subsequent passes for thickness |
The first pass is critical for achieving metallurgical bonding between the stainless steel overlay and the low-alloy steel base. The bond line composition is typically a dilution of approximately 30–50% base metal into the first pass, resulting in a ferritic or martensitic microstructure at the bond line. This is acceptable from a mechanical standpoint but may compromise corrosion resistance if the overlay is thin. Subsequent passes dilute the bond layer with more austenitic stainless steel, progressively improving the corrosion resistance of the near-surface layers.
Bond Line Metallurgy and Dilution Control
One of the central challenges in submerged arc strip cladding is the control of dilution at the bond line. When the austenitic stainless steel strip melts and mixes with the underlying low-alloy steel, the resulting first-pass weld metal has a composition intermediate between the two materials. For example, if a 304 stainless steel strip (approximately 18% Cr, 8% Ni) is welded onto a low-carbon steel (approximately 0.1% C, 0.3% Mn, 0.2% Si), the first-pass dilution of 40% base metal would result in a weld metal with approximately 11% Cr and 5% Ni, which is in the ferritic or duplex range.
The paper discusses methods to control and minimize dilution:
- Using a wider strip electrode: A wider strip deposits more overlay metal per unit length, reducing the relative dilution from the base metal.
- Multiple pass welding: Each subsequent pass dilutes the previous pass, progressively increasing the Cr and Ni content toward the surface. After 2–3 passes, the top layer typically achieves over 90% stainless steel composition.
- Flux composition optimization: The flux acts as a thermal barrier and can be formulated to reduce heat input into the base metal, thereby limiting dilution.
- Travel speed adjustment: Higher travel speeds reduce the heat input per unit length, which decreases the volume of base metal melted and thus reduces dilution.
Microstructural Analysis and Performance
The paper examines the microstructure of the cladding weld using metallographic examination and reports on mechanical properties including hardness, tensile strength, and impact toughness. The typical microstructural sequence from the base metal to the overlay surface is as follows:
- Base metal: Ferritic-pearlitic microstructure of the low-alloy steel.
- Bond line (first pass): Ferritic or martensitic structure with some austenite, depending on the dilution level. Hardness typically 200–300 HV.
- Intermediate passes: Transition from ferritic to duplex (ferrite + austenite) to predominantly austenitic with some ferrite.
- Surface layers (final passes): Predominantly austenitic with 5–15% delta ferrite, consistent with the expected weld metal composition of austenitic stainless steels. Hardness typically 150–200 HV.
The presence of delta ferrite in the overlay layers is generally acceptable and is in fact beneficial for preventing hot cracking. However, excessive delta ferrite (above 20%) can reduce corrosion resistance, particularly in chloride-containing environments. The paper recommends controlling the ferrite content through appropriate filler metal selection and welding parameters.
Common Defects and Countermeasures
Submerged arc strip cladding is susceptible to several characteristic defects:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Bond line cracking | Excessive residual stress, high carbon equivalent of base metal | Increase preheat, reduce travel speed, use lower carbon filler |
| Porosity | Moisture in flux, surface contamination | Dry flux at 250–300°C, clean base metal surface |
| Lack of fusion | Insufficient heat input, excessive travel speed | Increase current, reduce travel speed |
| Crater cracking | Inadequate crater filling, rapid cooling | Use crater fill with proper technique, apply post-weld heat treatment |
| Excessive dilution | Low travel speed, high current, narrow strip | Increase travel speed, use wider strip, reduce current |
| Strip misalignment | Mechanical feed instability | Ensure proper strip guide and feed mechanism alignment |
Engineering Practice Integration
For engineers involved in the fabrication of bimetal clad pressure vessels, the submerged arc strip cladding process offers significant advantages in terms of productivity and cost-effectiveness, particularly for large flat surfaces and large diameter cylindrical shells. The process is well-suited for the cladding of heat exchanger tubesheets, reactor shells, and distillation column shells where large areas of corrosion-resistant overlay are required.
However, the process has limitations that must be considered:
- Geometric flexibility: The strip electrode is less flexible than solid wire, making it difficult to clad complex geometries, small diameter pipes, or areas with tight radii.
- Bond line quality: The bond line is inherently a dilution zone and may not meet the same corrosion resistance requirements as the surface layers. For applications requiring full-thickness corrosion resistance, additional passes or a different cladding process may be necessary.
- Equipment requirements: Submerged arc strip cladding requires specialized equipment including a strip feed mechanism, a flux distributor, and a welding head capable of handling the high currents involved.
The paper's experimental results provide valuable data for process qualification under NB/T 47014 and ASME IX, which require demonstration of weldability for specific base metal, filler metal, and welding process combinations. The mechanical property data, including tensile strength and impact toughness values, can be directly used in the qualification of welding procedures for clad plate fabrication.
Study Insights and Implications
This paper represents an important contribution to the understanding of submerged arc strip cladding technology in the early 1990s, a period when China's petrochemical industry was rapidly expanding and the demand for corrosion-resistant equipment was growing. The experimental approach, combining process parameter optimization with microstructural and mechanical property evaluation, provides a methodology that remains applicable to modern cladding operations.
One of the key insights from this work is the importance of dilution control in determining the overall performance of the clad structure. The bond line, while mechanically sound, may not provide the same level of corrosion resistance as the surface layers, and engineers must be aware of this when specifying cladding thicknesses and pass sequences. For applications where the entire cladding thickness must be corrosion resistant (such as in highly aggressive chemical environments), a minimum of 3–4 passes may be required to achieve adequate dilution reduction throughout the overlay thickness.
The paper also highlights the practical challenges of strip electrode feeding and alignment, which can be critical in achieving consistent weld quality over long cladding runs. Modern automated systems have addressed many of these challenges, but the fundamental principles of process control described in this paper remain essential for achieving high-quality submerged arc strip cladding welds.