CLADDING TECHNOLOGY SHANXI CO., LTDExperimental Study on Strip Cladding of Duplex Stainless Steel
Literature Overview
The strip cladding of duplex stainless steel is a widely used manufacturing method for producing clad plates and pipes that combine the corrosion resistance of the duplex austenite-ferrite microstructure with the mechanical strength and weldability of a carbon or low-alloy steel base metal. This literature focuses on the experimental investigation of strip cladding processes, including explosive cladding, roll bonding, and electroslag welding (ESW) strip cladding, with emphasis on bond strength, microstructure evolution, and the effects of process parameters on the quality of the clad interface. The study is particularly relevant for engineers involved in the fabrication of pressure vessels and heat exchangers for the petrochemical and offshore industries.
Core Technical Points
Duplex Stainless Steel Microstructure and Phase Balance
The duplex stainless steel used in this study, typically grade UNS S31803 or S32750, consists of approximately equal volumes of austenite and ferrite phases. The phase balance is critical because an imbalance toward ferrite increases susceptibility to 475 °C embrittlement and sigma phase precipitation, while an excess of austenite reduces yield strength and increases susceptibility to pitting corrosion. The target phase balance for most applications is 40 to 60% ferrite, measured by ferrite number (FN) of 35 to 55.
Bond Strength Requirements
According to ASME Section VIII Division 1, Appendix 3 and API 934, the minimum bond strength for clad plates is specified in terms of the ultimate tensile strength of the cladding layer. For duplex stainless steel clad plates, the minimum bond strength must be at least 90% of the specified minimum tensile strength of the cladding layer. For S31803, this translates to a minimum bond strength of approximately 450 MPa. The literature demonstrates that properly executed strip cladding processes can achieve bond strengths of 500 to 550 MPa, well above the minimum requirement.
Process Parameter Analysis
Electroslag Welding Strip Cladding
ESW strip cladding is the most common method for producing large-format clad plates. The process involves passing a strip of duplex stainless steel through an electroslag bath formed between the strip and the substrate plate. The key process parameters include current, voltage, travel speed, slag composition, and strip feed rate.
| Parameter | Typical Range | Effect on Bond Quality |
|---|---|---|
| Current (A) | 3500-5000 | Higher current increases penetration and bond area |
| Voltage (V) | 28-35 | Higher voltage increases slag thickness and reduces penetration |
| Travel Speed (m/h) | 15-25 | Higher speed reduces heat input and can cause incomplete fusion |
| Slag Composition | CaO-SiO₂-MnO | Affects slag viscosity and wetting behavior |
| Strip Feed Rate | 15-25 m/h | Must match travel speed for stable slag pool |
The optimal parameter window for S31803 strip cladding on SA516 Gr.70 substrate is a current of 4000 to 4500 A, voltage of 30 to 32 V, and travel speed of 18 to 22 m/h. At these settings, the slag pool remains stable, the penetration into the base metal is sufficient to ensure full metallurgical bonding, and the dilution rate is controlled to 5 to 10%, preserving the duplex phase balance in the cladding layer.
Explosive Cladding
Explosive cladding uses the kinetic energy of a detonation-driven flyer plate to achieve cold welding at the interface. The process is characterized by the formation of a wavy or spiral interface pattern, which mechanically interlocks the two materials and provides excellent bond strength. For duplex stainless steel on carbon steel, the optimal velocity ratio (flyer plate velocity to detonation product velocity) is 0.20 to 0.25. At this ratio, the interface achieves intimate contact and cold welding without excessive interface roughness that would compromise fatigue strength.
| Velocity Ratio | Interface Morphology | Bond Strength (MPa) | Suitability |
|---|---|---|---|
| <0.15 | Smooth, no bonding | <200 | Unacceptable |
| 0.15-0.20 | Shallow waves | 300-400 | Marginal |
| 0.20-0.25 | Deep waves/spiral | 450-550 | Optimal |
| >0.25 | Overlap, delamination | <200 | Unacceptable |
Roll Bonding
Roll bonding is used primarily for clad pipes and smaller diameter products. The process involves passing a duplex stainless steel strip and a steel strip through a pair of rollers under controlled pressure and temperature. The key parameters are roll pressure, strip temperature, and reduction ratio. For S31803 on SA106B pipe, a roll pressure of 300 to 500 MPa and a strip temperature of 900 to 1100 °C achieves bond strengths of 480 to 520 MPa.
Microstructure at the Clad Interface
The microstructure at the interface between the duplex stainless steel cladding and the carbon steel base metal is a critical quality indicator. A proper interface should show a gradual transition from the ferrite-austenite duplex structure in the cladding to the ferrite-pearlite structure in the base metal, with no unmelted zones or intermetallic compounds. The presence of intermetallic phases such as sigma (Cr₂N) or chi (Cr₂₃C₆) at the interface indicates excessive heat input and is a cause for rejection under most codes.
The literature reports that the interface width, defined as the zone of mixed microstructure, typically ranges from 0.1 to 0.3 mm for ESW strip cladding and from 0.05 to 0.15 mm for explosive cladding. The narrower interface in explosive cladding is due to the absence of melting and the purely mechanical bonding mechanism.
Common Defects and Countermeasures
Incomplete Fusion
Incomplete fusion at the interface is the most critical defect in strip cladding, as it directly compromises bond strength and can lead to delamination during service. Causes include insufficient heat input, excessive travel speed, and inadequate slag pool stability. Countermeasures include increasing the current by 5 to 10%, reducing the travel speed by 1 to 2 m/h, and ensuring the slag composition provides adequate wetting and penetration.
Dilution and Phase Imbalance
Excessive dilution from the base metal into the cladding layer can shift the phase balance toward austenite, reducing the corrosion resistance of the duplex steel. The literature recommends monitoring the dilution rate through chemical analysis of the interface zone and adjusting process parameters to maintain dilution below 10%. Post-weld heat treatment at 1050 to 1100 °C can restore the phase balance if minor imbalance occurs.
Cracking in the Cladding Layer
Hot cracking in the duplex stainless steel cladding layer is associated with the presence of low-melting-point sulfide and phosphide phases at grain boundaries. The literature recommends using duplex stainless steel strips with sulfur content below 0.015% and phosphorus content below 0.020% to minimize cracking susceptibility. Additionally, preheating the substrate to 100 to 150 °C and controlling the interpass temperature below 200 °C reduces residual stress and cracking risk.
Non-Destructive Testing and Quality Assurance
The quality of strip clad plates is verified through a combination of non-destructive testing methods. Visual inspection checks for surface defects, undercut, and slag inclusions. Dye penetrant testing (PT) is applied to the exposed cladding surface to detect surface-breaking cracks and incomplete fusion. Ultrasonic testing (UT) is used to assess bond strength and detect subsurface delamination. For ESW strip cladding, UT is performed according to ASME Section V Article 23, with a minimum acceptance of no indications exceeding 3 mm in length at the interface.
Engineering Practice Integration
In the fabrication of hydrogenation reactors for the petrochemical industry, duplex stainless steel clad plates are widely used for the vessel shell and heads. The combination of high yield strength (minimum 550 MPa for S31803) and excellent resistance to chloride stress corrosion cracking makes duplex clad plates ideal for high-pressure, high-temperature service. A case study from a refinery project demonstrated that ESW strip cladding with the optimized parameters described in this literature produced clad plates with consistent bond strengths above 500 MPa, passing 100% UT inspection with no rework required.
Key Questions and Reflections
The experimental study raises important considerations for the scaling up of strip cladding from laboratory conditions to production-scale manufacturing. The stability of the slag pool during long continuous runs, the effect of substrate plate thickness on interface quality, and the long-term performance of duplex clad plates under cyclic thermal and pressure loading are areas that require further investigation. Additionally, the cost-effectiveness of explosive cladding versus ESW strip cladding for large-format plates remains a practical concern for manufacturers, as explosive cladding requires specialized facilities and safety controls that increase capital expenditure.
Study Insights and Implications
The experimental study on strip cladding of duplex stainless steel provides a comprehensive understanding of the process parameters, microstructure, and quality requirements for producing high-quality clad plates. The key insight is that the optimal process window is relatively narrow, and deviations in current, voltage, or travel speed can lead to significant quality degradation. Engineers must adopt a rigorous quality assurance program that includes parameter monitoring, in-process inspection, and post-weld testing to ensure that every clad plate meets the stringent bond strength and phase balance requirements. The study also highlights the importance of material selection for the cladding strip, as the sulfur and phosphorus content of the duplex stainless steel directly affects cracking susceptibility and long-term service performance.