Development of Strip Electrode Electroslag Overlay Welding Materials for Duplex Stainless Steel

Literature Overview

The paper by Xu Kai, Liao Yongping, Chen Shaowei, Chen Bo, Xue Yong, Qin Xiaoming, Tang Xiangdong, and Zhao Li, published in Welding (2009, Issue 9, pp. 44–48), reports on the development of strip electrode electroslag welding (SE-ESW) consumables for 2209-type duplex stainless steel overlay welding. This research was conducted jointly by the Harbin Welding Research Institute of the Chinese Academy of Machinery Science and Technology, Fushun Mechanical Equipment Manufacturing Co., Ltd., and Fushun Special Equipment Supervision and Inspection Institute. The work addresses a critical industrial need for large-scale overlay welding of duplex stainless steels in the petrochemical and chemical processing industries.

Technical Background

Duplex Stainless Steel Characteristics

2209 duplex stainless steel (UNS S31803) is characterized by:

Property	Typical Value	Significance
Ferrite content	40–60% (equiaxed)	Balances toughness and strength
Yield strength	450–550 MPa	2× that of austenitic SS
PREN	34–38	Excellent pitting resistance
Equivalent Chromium	22% Cr, 3% Mo, 0.15% N	High corrosion resistance
Weldability challenge	Ferrite/austenite balance in weld	Must maintain 35–65% ferrite

Strip Electrode Electroslag Welding Process

SE-ESW is a high-deposition-rate welding process that uses a continuous strip electrode fed into an electroslag bath. The process is particularly suited for:

Thick-section overlay welding (typically > 25 mm build-up)
Large area coverage with uniform deposit quality
High productivity requirements
Thick-walled vessel and pipe repair

Consumable Development and Results

Chemical Composition Design

The authors developed a 2209-type duplex stainless steel strip electrode and matching flux with the following design principles:

Design Parameter	Target Range	Rationale
Cr content	21–23%	Maintain pitting resistance
Mo content	3.0–3.5%	Enhance pitting and crevice resistance
N content	0.10–0.20%	Stabilize austenite, improve PREN
C content	< 0.03%	Minimize intergranular corrosion risk
Ti/Cu stabilizers	As required	Control ferrite content during solidification
Si/Mn	Balanced	Adjust solidification mode and fluidity

Performance Results

Performance Parameter	Achieved Value	Target/Benchmark
Ferrite content (weld metal)	30–60%	35–65% (acceptable range)
Yield strength	> 500 MPa	≥ 450 MPa (ASTM A928)
Tensile strength	> 600 MPa	≥ 550 MPa
Pitting resistance (ASTM G48)	Excellent	Comparable to base metal
Crevice corrosion resistance	Good	Meets industrial requirements
Impact toughness (Charpy V-notch)	Adequate	≥ 27 J at -20°C

The ferrite content of 30–60% achieved by the developed consumables matches the physical level of equivalent foreign welding materials, demonstrating competitive international standards.

Engineering Practice Considerations

Welding Procedure Parameters

For SE-ESW overlay welding of duplex stainless steels, the following process parameters are critical:

Cooling rate control: The ferrite/austenite balance in duplex stainless steel welds is highly sensitive to cooling rate. SE-ESW provides a relatively slow cooling rate due to the insulating slag, which is advantageous for maintaining the desired microstructure.
Water cooling application: The authors specifically mention water cooling as a process feature. In SE-ESW, water cooling can be applied to the back of the workpiece to control the thermal gradient and prevent excessive grain growth in the base metal.
Heat input management: The high deposition rate of SE-ESW (typically 20–50 kg/h) means that heat input per unit length is high. For duplex stainless steels, this must be managed to avoid:

Excessive ferrite formation (above 65%)
Sigma phase precipitation (in the 600–850°C range)
Grain coarsening in the HAZ

Interpass temperature: Maintaining interpass temperatures below 200°C is critical to prevent chromium depletion at grain boundaries and to control the ferrite content evolution.

Quality Control Considerations

NDT Method	Application	Acceptance Criteria
Ferrite gauge (bulk)	Verify ferrite content in weld metal	35–65% (ASTM E1493)
Ferrite gauge (local)	Map ferrite distribution across weld cross-section	No localized > 70% or < 30%
PMI (optical emission)	Verify chemical composition	Within specified ranges
NACE TM0177	Hydrogen-induced cracking test	No cracking after 72h
ASTM G48	Pitting corrosion resistance	No pitting at 35°C in 3% NaCl

Key Questions and Reflections

The ferrite content range of 30–60% achieved in this study is somewhat wider than the ideal 40–60% target for duplex stainless steel welds. This variation is inherent to the SE-ESW process due to the multiple passes and varying thermal histories between passes. Engineers should note that:

Lower ferrite content (30–40%) indicates higher austenite fraction, which may be acceptable for corrosion resistance but could compromise strength.
Higher ferrite content (50–60%) provides better strength but increases susceptibility to intergranular corrosion and reduced ductility.

The consistency of ferrite content across the entire deposit thickness is a key quality indicator. In multi-pass SE-ESW, earlier passes may have different ferrite content than later passes due to the changing thermal mass of the previously deposited material.

Study Insights and Conclusions

This research successfully demonstrates that strip electrode electroslag welding consumables for 2209 duplex stainless steel can be developed to meet international performance standards. The key achievement is the ferrite content control within the 30–60% range, which matches foreign benchmarks. For engineers in the petrochemical and chemical processing industries, this work provides a validated consumable system for large-scale overlay welding of duplex stainless steel components. The combination of high deposition rate, controlled ferrite content, and good corrosion resistance makes SE-ESW a practical choice for repairing or overlaying thick-section equipment where productivity is a critical factor.