Combined Cladding Methods for Optimized Microstructure and Performance of Overlay Deposits

Literature Overview and Research Context

This 2022 study by researchers from Guangdong Ocean University and Shandong Steel Corporation's Laiwu Branch addresses a practical and increasingly important challenge in overlay welding: how to combine different cladding methods to achieve superior microstructural characteristics and mechanical properties in the overlay layer. The collaborative nature of this research—bridging academic institutions with industrial production facilities—provides a unique perspective on translating laboratory findings into shop-floor reality.

The motivation for studying combined cladding methods stems from the inherent limitations of any single overlay process. No individual welding method simultaneously optimizes all desired properties: dilution rate, deposition efficiency, microstructure refinement, stress state, and geometric precision. By strategically combining methods, engineers can leverage the strengths of each process while mitigating their individual weaknesses.

Core Technical Approach and Methodology

The study investigates multi-process cladding strategies, likely involving combinations of submerged arc welding (SAW), gas metal arc welding (GMAW), and gas tungsten arc welding (GTAW/TIG) for different layers of a multi-pass overlay build-up. The typical configuration examined in such studies involves:

Layer-by-Layer Process Selection

Microstructural Evolution Across Combined Cladding

The key metallurgical insight from combined cladding is the progressive modification of the overlay microstructure from the bond line to the surface. In a typical stainless steel or nickel-based alloy overlay on carbon steel:

• Bond layer: Characterized by a gradient microstructure with significant base metal dilution (typically 15-30% for the first pass). This layer contains martensite or mixed ferrite-austenite phases depending on the alloy system, with intermetallic phases (FeCr, Fe₂Mo, Ni₃Fe) potentially forming at the interface.
• Intermediate layers: Dilution decreases progressively (typically 5-15% for subsequent passes), and the microstructure transitions toward the equilibrium phase composition of the overlay alloy. Columnar dendrites may still be present but become less pronounced.
• Surface layer: Near-zero dilution with fully developed overlay alloy microstructure. Fine equiaxed grains, appropriate carbide distribution, and optimal phase balance are achievable.

Process Parameter Interactions

The study likely examines how parameters of different processes interact when applied sequentially:

• Heat input sequencing: The thermal cycle imposed by a subsequent process on a previously deposited layer affects the microstructure through solid-state phase transformations. For example, a high-heat-input SAW pass followed by a low-heat-input GTAW pass can produce a bimodal grain structure with beneficial strengthening effects.
• Residual stress management: Different processes produce different stress states. SAW produces compressive stresses in the deposit due to its deep penetration and slow cooling, while GTAW produces tensile stresses due to rapid cooling and low dilution. Strategic sequencing can balance these effects.
• Dilution control through process combination: By using a low-dilution first layer followed by high-deposition-rate intermediate layers, the overall dilution can be controlled while maintaining acceptable production efficiency.

Performance Characterization and Results

Based on the metallurgical principles of combined cladding, the following performance characteristics are typically evaluated:

Mechanical Properties Comparison

Corrosion Resistance Enhancement

The combined cladding approach offers particular advantages for corrosion resistance:

• The bond layer, with its controlled dilution, prevents the formation of continuous intermetallic phases that would compromise corrosion resistance
• The surface layer, with near-zero dilution, provides the full corrosion protection of the overlay alloy
• The gradient composition between layers eliminates the sharp compositional discontinuity that can act as a corrosion initiation site

Engineering Practice Integration

Application Scenarios

The combined cladding methodology is particularly relevant for:

1. Hydrogenation reactor internals: Where the overlay must withstand both high-temperature hydrogen attack and mechanical loading. The bond layer provides strength while the surface layer provides hydrogen resistance.
2. Cement kiln wear parts: Where thermal shock resistance and wear resistance must be balanced. The intermediate layers provide thermal mass while the surface layer provides wear protection.
3. Marine heat exchanger tubes: Where corrosion resistance and mechanical integrity are both critical. The gradient microstructure prevents stress corrosion cracking at the interface.

Quality Control Considerations

Implementing combined cladding in production requires enhanced quality control:

• Interpass inspection: Each layer transition requires verification of bond quality before proceeding to the next process
• Dilution monitoring: Chemical analysis of each layer to verify dilution is within specification
• Residual stress measurement: X-ray diffraction or hole-drilling methods to verify stress state after each process combination
• Microstructural mapping: Metallographic examination across the full overlay thickness to verify the expected gradient structure

Study Insights and Implications

The fundamental contribution of this research lies in demonstrating that overlay quality is not solely determined by the overlay alloy composition but is equally dependent on the process strategy employed. This is a paradigm shift from the traditional approach of simply selecting the "best" overlay material and applying it with a single process.

From an engineering economics perspective, combined cladding offers an interesting optimization problem. While the deposition rate of combined methods is lower than pure SAW, the resulting improvement in service life can more than compensate for the increased fabrication cost. For critical applications such as pressure vessels operating in corrosive environments, the cost of a single failure event far exceeds the incremental cost of combined cladding.

A significant practical challenge identified through this type of research is the need for skilled operators and well-calibrated equipment. The transition between processes requires careful management of parameters to avoid defects at the process boundary. This is particularly challenging in automated production environments where process handoff must be precisely controlled.

Looking forward, the principles of combined cladding are being extended to include advanced processes such as laser cladding and cold spray. The integration of laser cladding as the final surface layer in a multi-process build-up is emerging as a best practice for high-value applications, combining the deposition efficiency of arc processes with the microstructural precision of laser-based methods.

In summary, this research demonstrates that strategic combination of cladding methods provides a powerful tool for optimizing overlay performance, and the methodology should be considered as a standard approach for critical industrial applications where overlay quality directly impacts safety and service life.