CLADDING TECHNOLOGY SHANXI CO., LTDStainless Steel Strip Cladding Process for A508-III Steel
Literature Overview
This 2009 paper by Hui Yuanyuan from Xi'an Aeronautical Polytechnic Institute examines the strip cladding (strip overlay) welding process applied to ASTM A508-III steel, a low-alloy high-strength steel widely used in pressure vessel fabrication. The study addresses the metallurgical challenges and process optimization required to achieve a sound metallurgical bond between austenitic stainless steel overlay and ferritic low-alloy steel substrate, a combination commonly employed in pressure vessels requiring corrosion resistance on the internal surface.
Technical Background
ASTM A508-III is a Cr-Mo-V low-alloy steel with a typical composition of 0.9-1.05% Cr, 0.45-0.65% Mo, and 0.15-0.30% V. It is used in high-temperature and high-pressure applications such as power generation boilers, pressure vessels, and heat exchangers. When exposed to corrosive media, an overlay layer of austenitic stainless steel (typically 309L, 310, or 347) is applied to provide corrosion resistance while retaining the mechanical strength of the base steel.
Material Compatibility Analysis
| Property | A508-III (Base) | 309L / 310 (Overlay) | Compatibility Issue |
|---|---|---|---|
| Thermal expansion (μm/m·K) | 12.0-12.5 | 17.0-17.5 | High residual stress |
| Thermal conductivity (W/m·K) | 28-32 | 14-16 | Asymmetric cooling |
| Dilatation tendency | Low | Low | Acceptable |
| Carbon equivalent | 0.45-0.55 | 0.25-0.35 | Cracking risk in HAZ |
| Weldability | Good (with pre-heat) | Excellent | Good overall |
Strip Cladding Process Description
The strip cladding process involves the sequential deposition of stainless steel strips onto the prepared base plate surface through electroslag welding (ESW) or submerged arc welding (SAW) techniques. The process is characterized by:
- Surface preparation: Machining or grinding of the base plate surface to remove scale and ensure a clean, flat surface. The surface roughness should be controlled to Ra 12.5-25 μm.
- Pre-heat application: Uniform pre-heating of the entire plate to 200-300°C, with verification using temperature measurement points at multiple locations.
- Stripping sequence: A systematic pattern of strip placement, typically in a zigzag or serpentine pattern, to minimize cumulative distortion.
- Slag removal and inspection: After each strip is deposited, the slag is removed and the surface is inspected for defects before proceeding.
Process Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Pre-heat temperature | 200-300°C | For plates > 25 mm |
| Interpass temperature | 250-400°C | Maximum limit |
| Stripping current | 180-250 A | Electroslag |
| Stripping voltage | 25-35 V | Electroslag |
| Travel speed | 100-200 mm/min | Electroslag |
| Strip thickness | 3-5 mm | Per pass |
| Overlap between strips | 10-15 mm | For bonding |
| Total overlay thickness | 3-6 mm | Per side |
Metallurgical Considerations
The fusion between austenitic stainless steel and ferritic-martensitic low-alloy steel creates a complex metallurgical interface. Key concerns include:
- Dilution control: The carbon content and alloy elements from the base steel dilute into the overlay layer, potentially forming brittle phases and reducing corrosion resistance. The dilution ratio should be kept below 30% for satisfactory overlay performance.
- HAZ microstructure: The HAZ in A508-III may develop coarse-grained martensite or bainite structures, particularly in the critical cooling rate zone. This can reduce toughness and increase susceptibility to cracking.
- Residual stress: The significant difference in thermal expansion coefficients between the overlay and base creates substantial residual stresses at the interface, which can reach 200-350 MPa without stress relief.
- Carbon migration: At elevated service temperatures, carbon can migrate from the ferritic base into the austenitic overlay, creating a carbon-depleted zone adjacent to the fusion line that is susceptible to intergranular corrosion.
Quality Control Requirements
| Inspection Method | Acceptance Criteria | Reference Standard |
|---|---|---|
| Visual inspection | No surface defects, uniform coverage | NB/T 47014 |
| Magnetic particle testing (MT) | No linear indications | JB/T 4730 |
| Ultrasonic testing (UT) | No bond defects | NB/T 47013 |
| Hardness test | Base: 160-260 HBW; Overlay: 180-250 HBW | ASTM E182 |
| Bond strength | > 200 MPa (tensile) | ASTM E8 |
| Corrosion test | No intergranular corrosion | ASTM A262 Practice E |
Study Reflections
The strip cladding process for A508-III steel represents a well-established technology in pressure vessel fabrication, but it continues to present challenges that require careful process control. The authors' emphasis on process parameter optimization is well-founded, as the wide range of acceptable parameters means that minor deviations can lead to significant quality variations. In practice, the success of strip cladding depends not only on the welding parameters but also on operator skill, equipment calibration, and environmental conditions. The study provides valuable guidance for process development, but engineers should note that qualification testing according to NB/T 47014 or ASME IX is mandatory before production application. The interplay between dilution control, residual stress management, and corrosion resistance requires a holistic approach to process design that considers the entire service life of the component.