CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Workpiece Tilt Angle on Stainless Steel Strip Cladding Process
Literature Overview and Technical Background
Strip cladding (also referred to as strip surfacing or electroslag welding with strip filler) is a highly productive method for depositing corrosion-resistant overlay layers on carbon steel or low-alloy steel substrates. The process uses an electroslag welding (ESW) configuration in which a continuous strip of filler material (typically austenitic stainless steel such as 304, 316, or 321) is fed through an electric arc into a slag pool, creating a thick, uniform overlay layer with excellent metallurgical bonding. The workpiece tilt angle — the angle between the horizontal plane and the surface being clad — is a critical process parameter that significantly affects slag flow, arc stability, heat distribution, and ultimately the quality of the cladding layer.
This study investigates the systematic effect of workpiece tilt angle on the cladding quality, microstructure, and mechanical properties of stainless steel strip cladding. The research addresses a practical challenge encountered in the fabrication of large-diameter vessels, heat exchanger shells, and pressure vessel components where the cladding is applied to curved or inclined surfaces.
Process Principles and Tilt Angle Configuration
In strip cladding, the workpiece is typically positioned at an angle between 5° and 15° from horizontal to facilitate proper slag flow and pool control. The electroslag welding arc is established between the strip electrode and the workpiece, with a consumable flux covering the weld pool to create a protective slag layer. The tilt angle influences several interdependent process variables:
| Tilt Angle (°) | Slag Pool Stability | Arc Stability | Heat Distribution | Deposition Rate |
|---|---|---|---|---|
| 0° (Horizontal) | Poor — slag flows away | Unstable | Asymmetric | Low |
| 5° | Marginal | Moderate | Slightly asymmetric | Moderate |
| 10° | Good | Stable | Balanced | High |
| 15° | Good | Stable | Balanced | High |
| 20° | Excessive slag flow | Unstable | Asymmetric | Decreased |
| 25° | Poor — slag overflow | Unstable | Highly asymmetric | Very low |
The optimal tilt angle range of 10° to 15° provides the best balance between slag retention, arc stability, and heat input distribution. At angles below 5°, the slag pool cannot be adequately contained, leading to slag spatter, incomplete fusion, and porosity in the cladding layer. At angles above 20°, the excessive gravitational component causes the slag to flow ahead of the arc, resulting in incomplete slag coverage, oxidation of the weld pool, and potential undercut defects.
Effect of Tilt Angle on Microstructure and Mechanical Properties
The microstructure of the strip cladding layer is strongly influenced by the cooling rate, which is directly related to the tilt angle. At optimal tilt angles (10°–15°), the cooling rate is moderate, promoting the formation of a fully austenitic microstructure with minimal delta ferrite content. This is critical for maintaining the corrosion resistance of the cladding layer, as delta ferrite can act as a preferential site for intergranular corrosion and pitting.
| Tilt Angle (°) | Ferrite Content (%) | Grain Size (μm) | Hardness (HV) | Tensile Strength (MPa) |
|---|---|---|---|---|
| 5° | 15–25 | 30–50 | 180–220 | 450–520 |
| 10° | 5–10 | 25–40 | 160–190 | 500–580 |
| 15° | 3–8 | 20–35 | 155–185 | 520–600 |
| 20° | 10–20 | 35–60 | 170–210 | 470–540 |
| 25° | 20–35 | 40–70 | 185–230 | 430–500 |
The data clearly demonstrate that the 10°–15° tilt angle range produces the finest grain structure, lowest ferrite content, and highest tensile strength. The slightly elevated hardness at lower angles (5°) is attributed to the presence of higher ferrite content, which has a higher hardness than austenite but lower corrosion resistance.
Defect Analysis and Process Control
The study identifies several defect types that are directly related to tilt angle deviations:
- Slag inclusion: Occurs at tilt angles below 8° where the slag pool is not adequately contained. The slag inclusion appears as elongated voids or dark inclusions in the cladding layer, detectable by ultrasonic testing (UT) and magnetic particle testing (MT).
- Undercut: Develops at tilt angles above 18° where the slag flows ahead of the arc, leaving the weld toe unsupported. This creates a geometric stress concentrator that can initiate fatigue cracks.
- Porosity: Found at both low and high tilt angles due to incomplete gas exclusion from the weld pool. The porosity is typically distributed as isolated spherical voids and is detectable by radiographic testing (RT).
- Incomplete fusion: Most common at tilt angles below 5° where the heat input is insufficient to achieve complete fusion with the base metal. This is a critical defect that compromises the bond strength between the cladding layer and the substrate.
The recommended process control parameters for optimal strip cladding are summarized as follows:
| Parameter | Recommended Value | Tolerance |
|---|---|---|
| Tilt angle | 10°–15° | ±2° |
| Welding current | 800–1200 A | ±5% |
| Travel speed | 150–300 mm/min | ±10% |
| Strip thickness | 1.0–1.5 mm | ±0.1 mm |
| Strip width | 12–20 mm | ±0.5 mm |
| Flux coverage | 3–5 mm slag depth | ±1 mm |
| Preheat temperature | 100–150°C | ±20°C |
Engineering Practice and Quality Assurance
For pressure vessel fabrication involving strip cladding, the following quality assurance measures are recommended based on the study findings:
- Tilt angle verification: The workpiece tilt angle must be verified at the start of each cladding pass using a precision inclinometer or angle gauge. The angle should be checked at intervals of no more than 500 mm along the cladding length to ensure consistency.
- Slag pool monitoring: Visual monitoring of the slag pool during welding is essential. The slag pool should be convex and contained within the weld boundaries. Any signs of slag overflow or inadequate coverage should trigger immediate process adjustment.
- Post-weld inspection: The cladding layer should be inspected by magnetic particle testing (MT) for surface and near-surface defects, and by ultrasonic testing (UT) for subsurface defects such as slag inclusions and incomplete fusion. The acceptance criteria should comply with NB/T 47013 or equivalent standards.
- Corrosion testing: For critical applications, the cladding layer should be subjected to intergranular corrosion testing (ASTM A263 Method E or ASTM A264) to verify that the microstructure is fully austenitic with minimal ferrite.
- Bond strength testing: The bond strength between the cladding layer and the base metal should be verified by macrographic examination of a transverse section, ensuring that the fusion line is clean and free of inclusions or incomplete fusion.
Study Insights and Implications
The study provides a systematic understanding of how workpiece tilt angle affects strip cladding quality, offering engineers a clear process window for optimization. The finding that the optimal tilt angle range is relatively narrow (10°–15°) underscores the importance of precise workpiece positioning in strip cladding operations. For large-diameter vessels where the cladding is applied to cylindrical surfaces, the tilt angle varies continuously along the circumference, requiring careful management of the welding position and potentially the use of automated cladding equipment with tilt angle compensation.
The correlation between tilt angle, microstructure, and mechanical properties is particularly valuable for engineers who need to justify process parameters in quality assurance documentation. The data on ferrite content as a function of tilt angle provides a quantitative basis for specifying maximum ferrite limits in cladding specifications, linking process parameters directly to material performance.