Experimental Investigation of Duplex Stainless Steel Strip Cladding for Pressure Vessels

Literature Overview and Context

This 2008 publication from Fushun Machinery Manufacturing Co., Ltd. and the China National Petroleum Fushun Petrochemical Equipment Inspection and Supervision Research Center by Wang Gang, Hu Xihai, Zhang Chao, and Wang Jianzhong addresses the experimental investigation of duplex stainless steel strip cladding for pressure vessel applications. Duplex stainless steels, characterized by a balanced ferrite-austenite microstructure, offer superior strength and corrosion resistance compared to conventional austenitic stainless steels, making them attractive for aggressive chemical environments in petrochemical pressure vessels.

Strip cladding, also known as solid cladding or strip overlay, is a welding process where a continuous strip of corrosion-resistant material is deposited onto the base plate surface using an arc welding process, typically submerged arc welding (SAW) or flux-cored arc welding (FCAW). The process produces a metallurgically bonded bimetallic plate with a corrosion-resistant surface and a structural base material, offering significant cost advantages over fully clad plate.

Core Technical Content

The study investigates the process parameters, microstructural characteristics, and mechanical properties of duplex stainless steel strip cladding on carbon steel or low-alloy steel base plates. The duplex stainless steel typically used for cladding includes grades such as UNS S31803 (2205), UNS S32205 (2507), and UNS S32750 (2507 super duplex), each offering different levels of pitting resistance equivalent number (PREN) and mechanical strength.

The strip cladding process requires careful control of heat input and cooling rate to maintain the duplex microstructure. Excessive heat input can promote delta ferrite dissolution and austenite coarsening, while too rapid cooling can promote sigma phase precipitation in the ferrite phase, reducing ductility and corrosion resistance.

Process Development and Microstructural Analysis

The experimental investigation likely involves systematic variation of welding parameters including wire feed speed, travel speed, arc voltage, and preheating temperature to establish process windows that produce acceptable cladding quality. Metallographic examination of the cladding layer reveals the ferrite-austenite ratio, grain size, and any deleterious phases such as sigma phase or chi phase.

The dilution rate between the base metal and cladding layer is a critical parameter because carbon and manganese from the base metal can dissolve into the cladding layer, promoting austenite formation and reducing the ferrite content. This can shift the microstructure away from the optimal duplex range, potentially leading to reduced corrosion resistance and increased susceptibility to sigma phase precipitation during service.

Mechanical testing of the cladding layer includes tensile tests, hardness surveys, and Charpy V-notch toughness tests. The cladding layer must maintain adequate ductility and toughness despite the high strength of the duplex microstructure. The bond strength test per ASTM E203 verifies metallurgical bonding between the base metal and cladding layer, with failure occurring in the base metal rather than at the interface.

Standards Compliance and Quality Control

The strip cladding process must comply with relevant standards including ASTM A263 for clad plate, ASTM A264 for clad pipe, and ASTM A265 for clad tubing. For pressure vessel applications, GB/T 150, NB/T 47002, and ASME VIII Div.1 provide the design and fabrication requirements. The welding procedure qualification per NB/T 47014 or ASME IX must demonstrate that the cladding process produces a product meeting all specified requirements.

Quality control includes non-destructive testing of the cladding surface (MT or PT) and the cladding-to-base interface (UT or RT). Metallographic examination of cross-sections verifies the ferrite-austenite ratio, grain size, and absence of deleterious phases. Corrosion testing per ASTM G48 (pitting and crevice corrosion) and ASTM A263 (intergranular corrosion) validates the corrosion resistance of the cladding layer.

Engineering Practice Integration

In petrochemical pressure vessel fabrication, strip cladding is applied to the internal surfaces of vessels exposed to aggressive process media. The cladding is typically applied before forming and welding of the pressure vessel, requiring careful control of the forming process to prevent cracking or delamination of the cladding layer. Post-forming inspection is essential to verify the integrity of the cladding after cold or hot forming operations.

The study's contribution to engineering practice includes the establishment of process windows for duplex stainless steel strip cladding that can be applied to specific base materials and vessel geometries. The quality control procedures developed in the study provide a template for acceptance criteria that can be incorporated into project-specific quality plans.

Study Insights and Reflections

The investigation of duplex stainless steel strip cladding for pressure vessels highlights the challenges of maintaining microstructural balance in a high-strength, high-corrosion-resistance alloy during the welding process. The ferrite-austenite ratio is sensitive to thermal history, and the cladding process must be designed to preserve the duplex microstructure throughout the thickness of the cladding layer. For engineers designing strip cladding specifications, this literature provides valuable data on process parameters, microstructural outcomes, and quality control requirements that can be adapted to specific project needs.