Stainless Steel Strip Cladding Process for 21/4Cr-1Mo Steel Study Notes

Overview and Technical Challenge

This study note addresses the electroslag welding (ESW) strip cladding process applied to 21/4Cr-1Mo steel substrates, a critical technology for manufacturing pressure vessels and heat exchangers in hydrogenation, hydrocracking, and high-temperature hydrogen service applications. The 21/4Cr-1Mo steel (ASTM A387 Gr.22 or equivalent) provides excellent high-temperature strength and resistance to hydrogen attack, while the stainless steel overlay layer (typically 309/310 or 347 grade) provides corrosion resistance against sulfidation, oxidation, and aqueous corrosion at elevated temperatures. The metallurgical compatibility between these dissimilar materials presents significant challenges related to dilution, residual stress, and phase transformation.

Material Compatibility Analysis

The metallurgical compatibility between 21/4Cr-1Mo steel and austenitic stainless steel overlay materials requires careful analysis of several factors including thermal expansion mismatch, dilution effects on phase stability, and the formation of brittle intermetallic phases at the interface.

Thermal Expansion Mismatch

The coefficient of thermal expansion (CTE) of 21/4Cr-1Mo steel is approximately 12.5 × 10⁻⁶ /K, while austenitic stainless steel (309L/347) has a CTE of approximately 17.5 × 10⁻⁶ /K. This 40% difference in thermal expansion creates significant residual stresses during cooling from the welding temperature. The mismatch-induced stress can be estimated as σ = Δα·ΔT·E/(1-ν), where Δα is the CTE difference, ΔT is the temperature change, E is Young's modulus, and ν is Poisson's ratio. For a ΔT of 1000°C, the induced stress can reach 300 to 500 MPa, which approaches or exceeds the yield strength of the base material at room temperature.

Dilution and Phase Stability

The dilution of 21/4Cr-1Mo base metal into the austenitic overlay layer is a critical concern. The 21/4Cr-1Mo steel contains approximately 2.25% Cr and 1.0% Mo, which when diluted into the overlay melt, affects the austenite stability. The dilution rate in ESW strip cladding typically ranges from 20% to 45% for the first pass, decreasing to 10% to 20% for subsequent passes. Excessive dilution can cause:

• Formation of delta ferrite in the overlay layer (target: <10% for corrosion resistance)
• Reduction in overlay layer hardness and corrosion resistance
• Potential formation of sigma phase during long-term service exposure

ESW Strip Cladding Process Parameters

Electroslag welding strip cladding offers the advantage of extremely low dilution rates compared to other arc welding processes, due to the large slag pool acting as a thermal buffer and the high deposition rates achieved. The following table presents typical process parameters for 21/4Cr-1Mo steel strip cladding with 309L/347 stainless steel strip.

Slag Composition and Properties

The slag composition is critical for ESW strip cladding success. A low-fluorine slag is preferred to minimize fluorine contamination of the overlay layer, which can cause hot cracking. Typical slag composition includes CaO (35-45%), SiO₂ (10-20%), Al₂O₃ (5-10%), CaF₂ (<5%), and Na₂O (10-15%). The slag must maintain a temperature range of 1400°C to 1600°C to ensure proper wetting and flow characteristics. The slag viscosity should be in the range of 50 to 150 mPa·s at operating temperature to allow proper strip feeding and slag removal.

Quality Control and Inspection Requirements

The quality of ESW strip cladding on 21/4Cr-1Mo steel is governed by standards including NB/T 47002 (Chinese standard), ASME IX, and API 934. The following inspections are mandatory for critical pressure vessel applications.

Visual and Dimensional Inspection

Visual inspection verifies surface quality, continuity of the overlay, and absence of surface defects such as craters, undercuts, and porosity. Dimensional inspection confirms that the overlay thickness meets the minimum specification (typically 3.0 mm for 309L, 4.0 mm for 347) and that the overlay width covers the full required area with adequate overlap (minimum 15 mm overlap between adjacent passes).

Non-Destructive Testing

• Magnetic Particle Testing (MT): Applied to the overlay surface and base metal surface to detect surface and near-surface cracks, particularly at the overlay-base metal interface. Acceptance criteria per NB/T 47013.4 typically allow no linear indications exceeding 3 mm in length.
• Ultrasonic Testing (UT): Used to verify bond integrity at the overlay-base metal interface. The shear wave technique with a 45° or 60° probe is standard. Acceptance requires no unbonded area exceeding 3 mm in any dimension.
• Radiographic Testing (RT): Applied to the overlay welds to detect internal porosity, slag inclusion, and incomplete fusion. Type II film radiography or digital radiography per NB/T 47013.2.

Mechanical and Metallurgical Testing

Common Defects and Process Optimization

The following defects are commonly encountered in ESW strip cladding of 21/4Cr-1Mo steel and require specific process adjustments.

Cracking

Cracking in ESW strip cladding can occur in the overlay layer, at the interface, or in the base metal heat-affected zone (HAZ). Overlay cracking is primarily caused by excessive sulfur and phosphorus content in the strip material or by hot cracking due to columnar grain growth. Countermeasures include using strips with S < 0.015% and P < 0.025%, maintaining adequate preheat, and controlling the travel speed to avoid excessive cooling rates. Interface cracking is caused by insufficient heat input or excessive residual stress. Increasing the current by 10% to 20% or reducing the travel speed can mitigate this defect.

Excessive Dilution

If dilution exceeds 45% in the first pass, the overlay layer may develop excessive delta ferrite (>30%), reducing corrosion resistance. The dilution rate in ESW can be controlled by adjusting the current-to-speed ratio: higher current increases the weld pool size and dilution, while higher travel speed reduces heat input per unit length and dilution. A practical approach is to use the first pass with a 309L strip at slightly higher current to ensure good bonding, then reduce current for subsequent passes to minimize dilution.

Engineering Practice Cases

In a recent project involving the cladding of a hydrogenation reactor shell made of 21/4Cr-1Mo steel (ASTM A387 Gr.22) with 347 stainless steel, the following approach was successfully implemented. The shell inner diameter was 3200 mm with a wall thickness of 85 mm, requiring a full internal cladding of 4.0 mm minimum thickness. The ESW strip cladding was performed with 347 strips (25 mm × 4 mm) using a current of 5500 A, voltage of 50 V, and travel speed of 200 mm/min. A total of 12 passes were required to achieve the specified thickness. Preheat was maintained at 250°C using induction heating, and interpass temperature was controlled at 300°C. Post-weld stress relief was performed at 620°C for 4 hours. The final dilution in the first pass was measured at 32% and in the final pass at 14%, both within acceptable limits. UT inspection confirmed 100% bond integrity across the entire cladded surface area of approximately 80 m².

Study Insights and Conclusions

The study of ESW strip cladding for 21/4Cr-1Mo steel highlights that this process represents the optimal solution for achieving thick, high-quality stainless steel overlays on heavy-section low-alloy steel substrates used in high-pressure hydrogen service. The key success factors are: maintaining adequate preheat to prevent HAZ cracking in the 21/4Cr-1Mo steel, controlling dilution through multi-pass strategies and appropriate current-speed ratios, and ensuring thorough post-weld stress relief to mitigate the residual stresses arising from thermal expansion mismatch. The ESW process dilution rate of 20% to 45% in the first pass, while higher than laser cladding or PTA, is acceptable for austenitic stainless steel overlays because the austenite stability is maintained even with 30% to 40% dilution from 21/4Cr-1Mo steel, provided the overlay alloy contains sufficient Ni and Mn to stabilize the austenite phase. Engineers should always verify dilution through metallographic analysis of cross-sections and adjust process parameters accordingly to ensure the overlay microstructure meets the required delta ferrite content and corrosion resistance specifications.