Microstructure and Properties of Combined Cladding Layers on 2.25Cr-1Mo Steel

Literature Overview

The paper by Wang Jinliang, Zhang Shijian, Fu Junhao, Luo Weicong, Liu Yongfa, and Xi Xiaohui, published in Guangzhou Chemical Industry in 2022 (Vol. 50, No. 14, pp. 167-169), investigates the microstructure and properties of combined cladding layers produced by combining submerged arc welding with flux-cored gas shielded welding on 2.25Cr-1Mo steel for hydrogenation reactor internals. The research was conducted at Guangdong Ocean University and Shandong Steel Group Laiwu Branch. This study is directly relevant to the manufacturing of high-pressure hydrogenation reactors in the petrochemical industry, where the internal cladding layer must provide corrosion resistance while maintaining structural integrity.

Core Technical Approach

The authors employ a two-layer cladding strategy on 2.25Cr-1Mo steel, using submerged arc welding for the first layer and flux-cored gas shielded welding for the second layer to achieve a dual-layer stainless steel cladding. The study systematically examines chemical composition, ferrite content, microstructure, hardness, bend performance, and intergranular corrosion resistance of the combined cladding layer.

Cladding Process Configuration

Parameter	First Layer	Second Layer
Welding process	Submerged arc welding	Flux-cored gas shielded welding
Consumable type	Submerged arc wire	Flux-cored wire
Purpose	Provide base for stainless layer, reduce dilution	Achieve final stainless steel composition
Shielding	Flux	Inert/active gas mixture
Heat input	Higher	Lower

The combined approach is designed to address the challenge of achieving adequate stainless steel composition on a low-alloy base metal. The submerged arc layer acts as an intermediate layer that reduces dilution from the base metal into the final stainless steel layer.

Test Results Summary

Property	Result	Specification Requirement
Chemical composition	Meets stainless steel requirements	Per reactor design specification
Ferrite content	Within acceptable range	Typically 5-35% for austenitic-ferritic stainless
Microstructure	Austenite-ferrite dual phase	Balanced microstructure
Hardness	Within specification	Typically below 200 HV for cladding
Bend performance	Passes bend test	No cracking at specified bend angle
Intergranular corrosion	Passes ASTM A262 Practice E	No intergranular attack

Interpretation of Technical Points

The fundamental engineering challenge addressed by this study is the production of a corrosion-resistant stainless steel cladding layer on 2.25Cr-1Mo steel, a material commonly used for the shell of hydrogenation reactors operating at high temperatures and pressures with hydrogen-containing media. The cladding layer must resist corrosion from the process environment while maintaining good mechanical properties and resistance to cracking under thermal cycling.

Dilution Management

The key metallurgical challenge is dilution: when welding stainless steel onto 2.25Cr-1Mo steel, the base metal melts into the weld pool and dilutes the deposited metal, reducing the chromium and nickel content below the levels required for corrosion resistance. The combined cladding approach addresses this through:

Submerged arc first layer: The submerged arc process produces a wider, deeper weld bead with relatively high dilution, but this layer is not required to have full stainless steel composition. It serves as a transition layer.
Flux-cored gas shielded second layer: This layer is deposited on the submerged arc layer, where dilution from the base metal is significantly reduced because the intermediate layer already has some alloy content. The flux-cored wire provides additional alloying elements to achieve the target composition.

Microstructure Considerations

The austenite-ferrite dual phase microstructure of the combined cladding layer is critical for several reasons:

Corrosion resistance: The austenitic phase provides primary corrosion resistance, while the ferritic phase contributes to resistance against certain types of stress corrosion cracking.
Cracking resistance: A balanced austenite-ferrite ratio (typically in the range of 70:30 to 90:10) provides good resistance against solidification cracking and hot cracking.
Mechanical properties: The dual phase structure provides a favorable combination of strength, ductility, and toughness.

The ferrite content measurement is a critical quality control parameter. Too low a ferrite content increases susceptibility to hot cracking, while too high a ferrite content can reduce corrosion resistance and increase susceptibility to 475-degree embrittlement.

Integration with Engineering Practice

Hydrogenation reactors are critical assets in petrochemical and refining operations, and their reliability is essential for safe and continuous production. The internal cladding layer protects the reactor shell from corrosion by the process environment, which may include hydrogen, hydrocarbons, and various chemical species at elevated temperatures and pressures.

For engineers involved in the manufacturing or repair of such reactors, this study provides valuable practical information:

Process feasibility: The combined cladding approach has been demonstrated to produce cladding layers that meet all relevant quality requirements.
Quality assurance: The comprehensive testing program (composition, ferrite content, microstructure, hardness, bend, intergranular corrosion) provides a model for quality control procedures.
Standard compliance: The results demonstrate compliance with the manufacturing requirements for hydrogenation reactors, which are typically governed by codes such as ASME BPV Section VIII, API 510, and relevant national standards.

The study also highlights the importance of process selection: the combination of submerged arc and flux-cored gas shielded welding leverages the strengths of each process to achieve a result that would be difficult to obtain with a single process alone.

Study Insights and Reflections

This paper demonstrates a practical, engineering-driven approach to solving a real manufacturing challenge. The combined cladding method represents a pragmatic solution to the dilution problem that has long plagued stainless steel cladding on low-alloy steels. The authors' decision to conduct comprehensive testing across multiple quality dimensions reflects a thorough engineering approach that goes beyond simple composition analysis.

A notable aspect of the study is the use of flux-cored gas shielded welding for the second layer, which offers advantages over solid wire in terms of deposition rate, alloying efficiency, and process flexibility. The flux in the wire core provides deoxidation, alloying, and slag formation that contribute to weld quality.

The study's practical orientation is evident in the focus on meeting specific manufacturing requirements rather than pursuing fundamental metallurgical understanding. This is appropriate for the intended audience of manufacturing engineers and quality professionals who need actionable information for production decisions.

Reference Value and Outlook

This paper provides a valuable case study for engineers working on the manufacturing of hydrogenation reactors and similar pressure vessels requiring internal cladding. The combined cladding approach demonstrated here offers a practical solution that balances process efficiency, quality assurance, and cost considerations. The comprehensive testing program provides a template for quality control procedures that can be adapted to similar applications.

As the petrochemical industry continues to face challenges of maintaining and extending the service life of critical equipment, studies like this one contribute to the body of practical knowledge that supports reliable manufacturing. The approach of combining different welding processes to achieve optimal results is a strategy that can be extended to other challenging cladding applications where single-process approaches face limitations.