Strip Cladding Process Research and Application for 16MnR Low-Alloy Steel

Overview of the Study

This literature investigates the strip cladding process and its application to 16MnR low-alloy steel, which is one of the most widely used pressure vessel steels in China per GB/T 150. Strip cladding, also known as electroslag welding (ESW) strip cladding or submerged arc welding (SAW) strip cladding, is a highly productive method for depositing corrosion-resistant overlay layers on large steel plates and structural components. The study examines the process parameters, metallurgical behavior, mechanical properties, and quality control aspects of strip cladding 16MnR steel with stainless steel or nickel-based alloy strips.

As a specialist in bimetal product manufacturing, I consider this topic of paramount importance because 16MnR is the backbone material for pressure vessels in petrochemical, power generation, and chemical processing industries. The ability to apply reliable corrosion-resistant cladding to 16MnR plates directly determines the feasibility and cost-effectiveness of bimetal pressure vessel fabrication.

Core Technical Content

Process Description and Parameters

Strip cladding involves the welding of a pre-formed strip of the cladding material onto the surface of the base plate using electroslag welding or submerged arc welding. The strip is typically 1–3 mm thick and is fed continuously into the weld pool while the welding head traverses the plate surface. The process produces a uniform, continuous overlay layer with excellent bond strength and minimal dilution.

Metallurgical Behavior of the 16MnR/Cladding Interface

The metallurgical behavior at the 16MnR/cladding interface is critical to the bond strength and corrosion resistance of the cladding. The study examines the dilution ratio, which is defined as the percentage of base metal in the cladding layer. For 16MnR steel clad with 304L stainless steel strips, the dilution ratio typically ranges from 5–15 percent depending on process parameters. Higher dilution ratios result in lower corrosion resistance of the cladding layer due to the increased carbon and manganese content from the base metal.

The microstructure of the cladding layer near the interface typically consists of a fine-grained martensitic or bainitic structure in the transition zone, transitioning to the austenitic or austenitic-ferritic structure of the cladding material in the upper portion. The transition zone may exhibit hardness values of 250–350 HV, which is higher than both the base metal (150–200 HV) and the cladding material (150–200 HV). This hardness gradient is generally acceptable for most applications but may require attention in highly stressed regions.

Mechanical Properties and Performance

The study presents comprehensive mechanical property data for strip-clad 16MnR plates, including tensile strength, yield strength, elongation, hardness, and impact toughness. The results demonstrate that the strip cladding process produces overlay layers with mechanical properties that closely match the specifications of the cladding material, provided that the dilution ratio is controlled within acceptable limits.

Application in Bimetal Pressure Vessel Fabrication

Design Considerations

When using strip-clad 16MnR plates for bimetal pressure vessel fabrication, several design considerations must be addressed in accordance with GB/T 150 and NB/T 47002. The cladding layer thickness must be sufficient to provide the required corrosion allowance, typically 1.5–3.0 mm depending on the service environment. The minimum cladding layer thickness after machining is specified by the design code and must be verified by dimensional inspection.

The design must also account for the thermal expansion mismatch between the carbon steel base and the stainless steel cladding. During thermal cycling in service, differential thermal expansion can induce interfacial stresses that may lead to delamination or cracking if not properly managed. The study provides guidance on maximum operating temperature differentials and thermal cycling limits for strip-clad 16MnR pressure vessels.

Fabrication Sequence and Quality Control

The fabrication of strip-clad 16MnR pressure vessels follows a specific sequence to ensure quality and dimensional accuracy:

1. Base plate inspection and certification verification, including chemical analysis, mechanical properties, and non-destructive testing.
2. Surface preparation of the base plate, including grinding to remove scale, rust, and contamination to a minimum grade of Sa 2.5 per ISO 8501-1.
3. Strip cladding welding with controlled parameters and in-process monitoring.
4. Post-weld stress relief annealing at 580–620 °C for 2 hours per 25 mm of plate thickness.
5. Non-destructive testing of the cladding layer, including magnetic particle testing (MT) for surface defects and ultrasonic testing (UT) for subsurface defects and bond quality.
6. Bond strength testing per ASTM E23 or equivalent, with acceptance criteria typically requiring a minimum bond strength of 150 MPa or fracture occurring in the base metal.
7. Mechanical forming and welding of the vessel components, with appropriate welding procedures to prevent cracking of the cladding layer.
8. Final inspection including dimensional verification, hydrostatic testing, and surface finish verification.

Common Defects and Quality Assurance

The study identifies several common defects in strip-clad 16MnR plates and provides countermeasures:

Engineering Practice Cases

Case Study: Petrochemical Reactor Shell

A practical application of strip-clad 16MnR plates was demonstrated in the fabrication of a hydrogenation reactor shell for a petrochemical plant. The reactor required 316L stainless steel cladding on the interior surface to resist sulfuric acid corrosion at operating temperatures up to 350 °C. The strip cladding process was selected for its high productivity and uniform coverage over the large plate dimensions (12,000 mm × 2,500 mm × 45 mm).

The cladding was performed with 2.5 mm thick 316L strips using electroslag welding with a current of 1200 A, voltage of 38 V, and speed of 250 mm/min. The dilution ratio was controlled at 8–12 percent through careful parameter optimization. Post-weld stress relief was performed at 600 °C for 4 hours. Non-destructive testing revealed excellent bond quality with no lack of fusion or cracking detected. The final hydrostatic test at 1.5 times the design pressure was passed without leakage.

Lessons Learned

The case study highlights several important lessons for strip cladding applications:

1. Surface preparation is critical; inadequate cleaning of the base plate surface was identified as the primary cause of lack of fusion defects in earlier production batches.
2. Parameter consistency is essential for uniform dilution control; real-time monitoring of welding parameters is recommended to detect and correct drift during long production runs.
3. Post-weld stress relief is not optional but mandatory for strip-clad pressure vessels to prevent delayed cracking and to relieve residual stresses that could compromise the bond strength.
4. The transition from cladding to forming requires careful planning; mechanical forming operations must be performed with appropriate procedures to prevent cracking of the cladding layer.

Key Questions and Reflections

The study raises an important question about the long-term performance of strip-clad 16MnR plates in highly aggressive environments. While the initial corrosion resistance is satisfactory, the dilution zone near the interface may be susceptible to intergranular corrosion or crevice corrosion under certain conditions. Long-term exposure testing in simulated service environments would provide valuable data to establish service life predictions and maintenance intervals.

Another area requiring further investigation is the effect of multiple thermal cycles on the cladding integrity. In pressure vessels that undergo frequent start-up and shutdown cycles, the repeated thermal expansion and contraction at the cladding/base interface could lead to fatigue cracking over time. The study suggests incorporating fatigue life assessment into the design process for cyclically loaded strip-clad vessels.

Study Insights and Implications

This literature provides comprehensive technical guidance for the strip cladding of 16MnR low-alloy steel, covering process parameters, metallurgical behavior, mechanical properties, and quality control. The findings are directly applicable to the fabrication of bimetal pressure vessels in accordance with Chinese and international standards. The strip cladding process offers excellent productivity and cost-effectiveness for large-scale cladding applications, but requires careful attention to process parameters, surface preparation, and quality assurance. Engineers should use the data and recommendations presented in this study as a foundation for developing their own procedures, while recognizing that specific applications may require additional investigation and qualification testing to ensure reliable performance in service.