Investigation of Microstructure and Properties in Combined Cladding Methods

Literature Overview and Research Significance

This study investigates the metallurgical behavior and mechanical performance of cladding layers produced through the sequential combination of different welding overlay methods. The research addresses a critical practical challenge in industrial cladding operations: the need to deposit thick cladding layers with specific microstructural and property gradients that cannot be achieved by any single welding process alone. By combining processes such as submerged arc welding (SAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), plasma transferred arc (PTA), and laser cladding in a carefully designed sequence, engineers can achieve optimized dilution profiles, microstructural transitions, and property gradients that satisfy demanding service requirements.

The study is particularly relevant to the fabrication of bimetal pressure vessels and heat exchanger components governed by standards such as GB/T 150, ASME VIII Div.1 and Div.2, and NB/T 47002. In these applications, the cladding layer must provide corrosion or wear resistance while maintaining adequate bond strength and mechanical compatibility with the base material. The combined method approach offers a practical solution to the dilution problem that plagues single-process thick cladding operations, where excessive base metal dilution in the first few passes compromises the corrosion resistance of the final cladding surface.

Combined Method Sequences and Process Parameters

The literature examines several combined method sequences, each targeting specific property profiles and dilution control objectives. The most commonly studied sequences include:

Each process in the sequence serves a specific metallurgical function. SAW provides high deposition rates and deep penetration, making it ideal for building up the base layer with controlled dilution. GMAW offers moderate deposition rates with good arc force and bead shaping, suitable for intermediate layers where dilution control becomes critical. GTAW provides precise heat input control and minimal dilution, ideal for the final surface layer where composition accuracy is paramount. PTA offers excellent composition control with minimal dilution (typically 5 to 15 percent) and uniform powder feed, making it suitable for both intermediate and surface layers. Laser cladding provides extremely low dilution (10 to 25 percent), rapid solidification, and fine microstructural features, making it ideal for the final surface layer in high-performance applications.

Microstructural Analysis Across Cladding Layers

The microstructural evolution across the combined cladding layers was characterized using optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The analysis reveals distinct microstructural zones corresponding to each welding process:

The dilution profile across the combined cladding layers demonstrates the effectiveness of the sequential approach. In a typical Sequence A configuration using 316L overlay material on a carbon steel substrate, the dilution decreases from approximately 35 to 40 percent in the first SAW passes to 20 to 25 percent in the GMAW intermediate passes and 10 to 15 percent in the final GTAW surface pass. This progressive dilution reduction ensures that the final surface layer achieves the target composition with adequate chromium and nickel content for corrosion resistance.

Mechanical Property Evaluation

Mechanical property testing included hardness profiling, tensile testing of overlay coupons, bend testing, and intergranular corrosion testing per ASTM A262 Practice A or GB/T 4334. The results demonstrate that combined method cladding achieves superior property gradients compared to single-process cladding of equivalent thickness.

The combined method approach demonstrates a significant advantage in achieving low dilution in the surface layer while maintaining adequate bond strength at the base metal interface. This is achieved through the strategic use of high-deposition-rate processes for the base layer (where dilution is less critical) and low-dilution processes for the surface layer (where composition accuracy is paramount).

Defect Analysis and Quality Control

Defect analysis across the combined cladding layers reveals that the primary quality concerns are located at the process transition interfaces. The SAW-to-GMAW transition zone is susceptible to lack of fusion due to differences in penetration characteristics and bead geometry. The GMAW-to-GTAW transition zone may exhibit porosity due to changes in shielding gas coverage and arc stability. The PTA-to-laser transition zone can develop microcracks due to the rapid cooling rate change and thermal stress concentration.

Quality control procedures for combined method cladding must include dedicated inspection at each process transition. Ultrasonic testing (UT) per JB/T 4730 or ASME V should be performed after each process transition to detect lack of fusion and subsurface defects. Radiographic testing (RT) may be required for critical applications, particularly at the base metal-cladding interface. Visual inspection (VT) and magnetic particle testing (MT) should be performed on each completed layer to detect surface and near-surface defects.

Engineering Application and Case Study

A representative engineering application involves the fabrication of a hydrogenation reactor pressure vessel with a 316L stainless steel cladding layer on a 16MnR carbon steel base. The vessel operates at 25 MPa and 350°C in a hydrogen-rich environment containing trace amounts of hydrogen sulfide, requiring a cladding layer with excellent resistance to hydrogen embrittlement and sulfide stress cracking.

The combined method sequence employed was SAW (3 passes, 309L electrode) → GMAW (2 passes, ER316L wire) → GTAW (2 passes, ER316L wire). The total cladding thickness achieved was 8 mm, with the dilution profile decreasing from 38 percent in the first SAW pass to 12 percent in the final GTAW pass. The resulting cladding layer exhibited a chromium content of 17.5 percent, nickel content of 11.2 percent, and carbon content of 0.035 percent, all within the 316L specification limits.

Post-fabrication testing included hydrostatic testing at 30 MPa (1.2 times design pressure), air-tightness testing at 1.1 times design pressure, and full-penetration weld inspection by RT and UT. All tests were passed, demonstrating the effectiveness of the combined method approach for critical pressure vessel applications.

Key Reflections and Technical Insights

The study provides several important insights for engineering practice. First, the combined method approach offers a flexible and effective strategy for achieving thick cladding layers with controlled dilution profiles, which is particularly valuable for applications requiring thick corrosion-resistant overlays on thick-section base materials. Second, the microstructural transition between different welding processes creates natural property gradients that can enhance the overall cladding performance by combining the high deposition rate of SAW with the low dilution and fine microstructure of GTAW or laser cladding.

Third, the quality control implications of combined method cladding are significant. Each process transition represents a potential defect initiation site, requiring dedicated inspection procedures and acceptance criteria. The literature recommends implementing a systematic approach based on the PDCA (Plan-Do-Check-Act) cycle, where each process transition is planned with specific quality objectives, executed with controlled parameters, checked through comprehensive NDT, and acted upon through documented corrective actions.

Fourth, the economic analysis of combined method cladding must consider both direct costs (equipment, consumables, labor) and indirect costs (rework, inspection, downtime). While the combined method approach requires multiple welding equipment types and more complex procedure qualification, it can reduce overall costs by minimizing rework and ensuring first-time quality acceptance. For critical applications such as pressure vessels, the investment in combined method cladding is justified by the enhanced reliability and reduced risk of in-service failure.

The broader implication of this research is that cladding technology should evolve beyond single-process approaches toward integrated, multi-process solutions that leverage the unique strengths of each welding method. This philosophy aligns with modern manufacturing trends toward process integration and optimization, and it represents a significant advancement in the field of bimetal product manufacturing and pressure vessel fabrication.