Electroslag Band Hardfacing of Duplex Stainless Steel on Tube Sheets

Literature Overview

This paper by Xing Z. from Shenyang Institute of Instrumentation Science and Technology Co., Ltd., published in Pressure Vessel journal (2024, Vol. 41, No. 11, pp. 74-84), addresses the hardfacing of duplex stainless steel (DSS) onto tube sheets using electroslag band hardfacing (ESBW) technology. The work is classified under TG455 and focuses on achieving a balanced austenite-ferrite microstructure in the overlay layer while managing the unique thermal constraints of duplex stainless steels.

Core Technical Challenges

Duplex stainless steels derive their excellent mechanical properties and corrosion resistance from the presence of approximately equal amounts of austenite (γ) and ferrite (δ), typically targeting 40-60% ferrite content. The critical challenge in hardfacing DSS onto tube sheets lies in maintaining this phase balance during the welding process and subsequent thermal cycles.

Why Conventional Stress Relief is Prohibited

The sigma phase (σ, Cr25Fe2MoN) is a brittle intermetallic compound that precipitates in the temperature range of 600-900°C, particularly in the 700-850°C range where precipitation kinetics are most aggressive. This phase formation severely degrades ductility, toughness, and corrosion resistance of the duplex microstructure. Therefore, conventional post-weld stress relief heat treatment in the range of 600-900°C is strictly prohibited for DSS hardfacing layers.

Process Design Strategy

The paper presents two process routes based on the base tube sheet material characteristics:

Route 1: With Transition Layer (for High Hardenability Base Metals)

When the tube sheet material has high hardenability and requires post-weld heat treatment, the following sequence is employed:

1. Apply austenitic stainless steel transition layer first
2. Perform required PWHT (600-900°C) on the transition layer
3. Apply DSS hardfacing layer on top of the transition layer
4. No further high-temperature heat treatment is performed

This approach decouples the thermal requirements of the base metal from those of the DSS overlay. The austenitic stainless steel transition layer tolerates high-temperature PWHT without sigma phase formation because it contains no ferrite.

Route 2: Direct DSS Hardfacing (for Weldable Base Metals)

When the tube sheet material is carbon steel or low-alloy steel with good weldability:

1. Apply DSS hardfacing directly without transition layer
2. No conventional stress relief heat treatment is performed
3. Rely on the similar linear expansion coefficient between DSS and carbon/low-alloy steel to minimize residual stress

The linear expansion coefficient of duplex stainless steels (approximately 13-14 × 10⁻⁶/°C) is remarkably close to that of carbon steels (12-13 × 10⁻⁶/°C) and many low-alloy steels. This similarity reduces thermal mismatch stresses during welding and service, making direct hardfacing feasible without a transition layer.

Process Parameters and Results

Electroslag Band Hardfacing Parameters

Microstructure and Performance Results

The paper reports that both process routes (with and without transition layer) can achieve ferrite content exceeding 40% in the DSS hardfacing layer. This satisfies the minimum requirement for duplex stainless steel performance. The key findings include:

• Ferrite content of 40-60% achieved in both process configurations
• Good comprehensive mechanical properties maintained in the overlay
• Electroslag band hardfacing provides deep, uniform penetration suitable for thick overlays
• The process is suitable for tube sheet applications requiring thick corrosion-resistant overlays (typically 5-10 mm total thickness)

Engineering Practice Considerations

For pressure vessel tube sheet applications, the hardfacing process must be qualified according to relevant codes such as ASME Section VIII or GB/T 150. The key qualification requirements include:

1. WPS/PQR documentation: Each process route must be qualified with a welding procedure qualification record
2. Ferrite number testing: Post-weld ferrite content must be verified using a ferrite scope, targeting FN 40-60
3. Corrosion testing: Crevice corrosion and pitting corrosion resistance must be demonstrated, typically using ASTM G48 or ISO 9650
4. Mechanical testing: Tensile, hardness, and impact testing of the overlay metal

FMEA Analysis of Process Risks

Study Insights and Implications

This 2024 publication is particularly timely given the increasing use of duplex stainless steels in pressure vessel applications, particularly in the oil and gas, chemical processing, and marine industries. The dual-route approach presented in this paper provides engineers with flexibility in process selection based on the specific base material and service requirements. The key insight is that the prohibition of conventional PWHT for DSS does not necessarily preclude stress relief of the base metal; instead, it requires a strategic approach that separates the thermal management requirements of different layers.

The electroslag band hardfacing process is particularly well-suited for this application due to its ability to produce thick, uniform deposits with controlled cooling rates. The deep penetration and stable arc characteristics of ESBW provide consistent dilution control, which is critical for maintaining the target ferrite content in the overlay.