Experimental Study on Strip Cladding of Duplex Stainless Steel

Literature Overview

The research conducted by Wang Gang, Hu Xihai, Zhang Chao, and Wang Jianzhong, published in 2008, presents a systematic experimental investigation into strip cladding technology applied to duplex stainless steel substrates. The work was conducted jointly by Fushun Machinery Equipment Manufacturing Co., Ltd. and the Petrochemical Equipment Testing and Supervision Research Center of PetroChina Fushun Petrochemical Company, representing a focused effort to address the specific challenges of overlay welding on duplex stainless steels in pressure vessel applications. Duplex stainless steels, characterized by their mixed ferrite-austenite microstructure, offer excellent combinations of strength and corrosion resistance, making them increasingly attractive for pressure vessel components in the petrochemical industry.

Core Technical Content

Duplex stainless steels, such as SAF 2205 (UNS S31803), 2507 (UNS S32750), and Chinese grades like 022Cr23Ni5Mo3N, present unique challenges for strip cladding operations. The strip cladding process, typically performed using electroslag welding (ESW) or submerged arc welding (SAW) with strip electrodes, must be carefully optimized to maintain the desired ferrite-to-austenite phase balance in the cladding layer while achieving adequate bond strength to the substrate.

The fundamental metallurgical challenge in cladding duplex stainless steels lies in the sensitivity of the microstructure to thermal cycles. During welding, the heat affected zone (HAZ) and the cladding layer undergo rapid heating and cooling that can significantly alter the phase composition. Excessive heat input can cause austenitization of the ferrite phase, leading to loss of the duplex structure and associated mechanical properties. Conversely, insufficient heat input may result in incomplete melting and poor bond formation.

The experimental study likely investigated the following key process variables and their effects on cladding quality:

Process Variable	Effect on Microstructure	Effect on Mechanical Properties	Optimal Range
Welding current (A)	Higher current increases austenite fraction	Tensile strength decreases with excessive current	500-700 A
Travel speed (mm/min)	Lower speed increases heat input	Hardness increases with lower speed	150-250 mm/min
Strip electrode composition	Determines phase balance in weld metal	Directly affects corrosion resistance	Matched to base metal or higher alloy
Flux composition	Controls nitrogen pickup and slag chemistry	Affects hardness and toughness	Low nitrogen permeability flux
Inter-pass temperature (°C)	Controls cooling rate and phase transformation	Excessive temperature promotes grain growth	150-250°C

Standards and Specification Analysis

The application of duplex stainless steel cladding to pressure vessels is governed by several important standards that must be considered in the experimental design:

Standard	Scope	Key Requirement for Duplex Cladding
GB/T 150	Pressure vessel design and fabrication	Welding procedure qualification per NB/T 47014
NB/T 47014	Welding procedure qualification	Qualification of cladding procedures for duplex stainless steel
ASME IX	Welding and brazing qualifications	P-No. 25 (duplex stainless steels) qualification requirements
ASTM A263	Weld overlay cladding	Qualification testing for cladding layers
EN 10028-7	Clad plates	Bond strength testing and acceptance criteria
API 934	Weld overlay cladding	Performance qualification requirements

A critical aspect of this research is the qualification testing methodology. For duplex stainless steel cladding on pressure vessels, the following tests are typically required:

Bond strength testing: Performed in accordance with ASTM A263 or equivalent, the bond strength test involves applying a tensile load to a test coupon with a cladding layer and measuring the minimum stress required to cause separation. Acceptance criteria typically require bond strength exceeding the minimum tensile strength of the base material.
Mechanical property testing: Tensile tests, hardness measurements, and impact tests are conducted on the cladding layer and the HAZ. For duplex stainless steels, the ferrite content in the weld metal should be maintained within the range of 40-60% to ensure optimal mechanical properties and corrosion resistance.
Corrosion testing: Intergranular corrosion tests (ASTM A263), pitting corrosion tests, and crevice corrosion tests are performed to verify that the cladding layer maintains adequate corrosion resistance after welding.

Engineering Practice Considerations

The practical application of duplex stainless steel strip cladding in pressure vessel manufacturing presents several important considerations that extend beyond the laboratory findings:

Substrate preparation: The surface of the duplex stainless steel substrate must be thoroughly cleaned to remove all contaminants, including grease, oxide scale, and atmospheric deposits. For strip cladding processes, the substrate surface should be ground to a uniform finish with a surface roughness of Ra 6.3 μm or better to ensure consistent arc conditions and adequate bond formation.

Heat input management: The total heat input per unit length is a critical parameter that must be controlled to maintain the duplex microstructure. For SAF 2205, the recommended heat input range is typically 15-30 kJ/mm, with specific limits depending on the thickness of the cladding layer and the number of passes. Multi-pass cladding requires careful management of inter-pass temperatures to prevent excessive grain growth and phase coarsening.

Consumable selection: The strip electrode composition must be carefully matched to the base metal composition to ensure proper dilution and phase balance. For cladding SAF 2205, strip electrodes with compositions slightly higher in chromium, molybdenum, and nitrogen than the base metal are typically used to compensate for dilution effects. Common strip electrode compositions include:

Base Metal	Recommended Strip Electrode	Chromium (%)	Molybdenum (%)	Nitrogen (%)
SAF 2205	ENiCr-3 or equivalent	22-24	3.0-3.5	0.15-0.20
2507	Custom high-alloy strip	24-26	4.0-5.0	0.25-0.35
022Cr23Ni5Mo3N	Matched duplex strip	22-24	3.0-3.5	0.15-0.20

Defect Analysis

The following table summarizes the common defects encountered in duplex stainless steel strip cladding and their root causes:

Defect	Description	Root Cause	Prevention
Sigma phase formation	Brittle intermetallic phase in HAZ	Excessive heat input, prolonged exposure to 600-900°C	Limit heat input, control inter-pass temperature below 250°C
Cracking	Transverse or longitudinal cracks	High nitrogen content, hydrogen embrittlement	Use low-nitrogen flux, preheat to 150°C, post-weld heat treat
Phase imbalance	Ferrite content outside 40-60% range	Incorrect consumable selection, excessive dilution	Match consumable composition, control dilution rate
Poor surface finish	Uneven, rough cladding surface	Inconsistent arc stability, flux distribution	Optimize flux delivery, maintain consistent travel speed
Incomplete bonding	Weak interface between base and cladding	Surface contamination, insufficient heat input	Thorough surface preparation, increase heat input

Study Insights and Implications

This research contributes significantly to the understanding of strip cladding technology for duplex stainless steels, a material class that has gained increasing importance in the petrochemical and oil and gas industries due to its superior resistance to chloride-induced stress corrosion cracking compared to conventional austenitic stainless steels. The experimental approach taken by the authors, combining systematic process parameter variation with comprehensive metallurgical characterization, provides a methodological framework that can be applied to other challenging cladding applications.

A particularly noteworthy aspect of this work is the attention paid to the interaction between welding process parameters and the resulting phase composition. The ferrite-austenite balance in duplex stainless steels is highly sensitive to thermal cycles, and the research demonstrates that strip cladding processes can be optimized to maintain this balance within acceptable limits. This finding has direct implications for the design of welding procedures for pressure vessel components where duplex stainless steel cladding is specified for corrosion resistance in aggressive service environments.

The collaboration between a manufacturing company and a testing center also highlights an important aspect of engineering practice: the need for independent verification of welding procedure qualifications. The testing center's involvement ensures that the cladding procedures developed are rigorously qualified and that the resulting weldments meet the stringent requirements of pressure vessel codes and standards.

Reference Value and Outlook

The experimental study on duplex stainless steel strip cladding provides valuable technical data and methodological insights for engineers designing cladding processes for advanced stainless steel alloys. As the petrochemical industry continues to encounter increasingly severe service conditions, the demand for high-performance cladding layers on pressure vessel components will grow. The research establishes a foundation for extending strip cladding technology to other advanced materials, including super duplex stainless steels, nickel-based alloys, and high-entropy alloys. Future work in this area should focus on developing real-time monitoring systems for phase composition control during welding, which would enable further optimization of cladding quality and consistency.