Single-Layer TP347 Overlay Welding Technology and Performance Evaluation

Literature Overview

Published in the journal Pressure Vessel Technology (压力容器) in 2019 by Li Xiaowei, Liu Xiwu, Cui Xinan, Duan Yongfeng, Li Hui, and Liu Xuxia from Sinopec Refining and Chemical Engineering Group Luoyang Technology R&D Center and Sinopec Petrochemical Equipment Anti-Corrosion Research Center, this study focuses on the development and evaluation of a single-layer TP347 overlay welding technology. The research was supported by the Sinopec Refining and Chemical Engineering Group Science and Technology Development Project (117008). TP347 is a stabilized austenitic stainless steel containing niobium, widely used in high-temperature service in the petrochemical industry.

Core Technical Points

TP347 (UNS S34700 / 06Cr17Ni12Mo2N) is a niobium-stabilized austenitic stainless steel with the following typical composition: C ≤ 0.08%, Cr 17-22%, Ni 11-14.5%, Nb 6-10×C, and Mo 0.7-1.2%. The niobium addition stabilizes carbon, preventing chromium carbide precipitation at grain boundaries and thereby providing excellent resistance to intergranular corrosion. This makes TP347 particularly suitable for high-temperature applications (up to 870 °C) in petrochemical reactors, heat exchangers, and pressure vessels.

Challenge of Single-Layer Overlay

The conventional approach to overlay welding TP347 involves multiple passes (typically 2-3 layers), with a transition layer and a surface layer. The single-layer approach is challenging because:

1. Dilution control: A single pass has a higher dilution rate from the base material, which can compromise the corrosion resistance and high-temperature strength of the overlay.
2. Microstructure uniformity: A single pass may result in a non-uniform microstructure with columnar grains and potential segregation.
3. Bond strength: Achieving adequate bond strength with a single pass requires careful control of welding parameters.
4. Crack resistance: The single-layer deposit may be more susceptible to cracking due to higher residual stresses.

Process Parameters for Single-Layer TP347 Overlay

Performance Evaluation

The study evaluated the single-layer TP347 overlay through mechanical testing, corrosion testing, and microstructural analysis.

The single-layer overlay achieved excellent performance, with mechanical properties and corrosion resistance comparable to the wrought TP347 material. The key to success was careful control of the welding parameters to minimize dilution and promote a fine, equiaxed microstructure.

Standards and Qualification Requirements

For TP347 overlay welding in pressure vessel service, the following standards are applicable:

• ASME VIII Div.1: Rules for construction of pressure vessels, including requirements for overlay welding.
• ASME VIII Div.2: Alternative rules for construction of pressure vessels, with more detailed qualification requirements.
• ASME IX: Qualification of welding procedures, including essential variables for overlay welding.
• ASME II: Materials for construction of pressure vessels, including TP347 specifications.
• GB/T 150: Chinese standard for pressure vessel design and fabrication.
• NB/T 47014: Chinese standard for qualification of welding procedures for pressure vessels.
• ASTM A262: Standard test methods for detecting chromium carbide precipitation in stainless steels.
• ASTM G48: Standard test methods for pitting and crevice corrosion resistance of stainless steels.

The overlay welding procedure must be qualified per ASME IX or NB/T 47014, with demonstration of mechanical properties, corrosion resistance, and bond strength. For high-temperature service, creep and stress rupture testing may be required to verify long-term performance.

Engineering Practice Integration

TP347 overlay welding is widely used in the petrochemical industry for the following applications:

• Hydrogenation reactors: High-temperature, high-pressure vessels exposed to hydrogen and corrosive media.
• Heat exchangers: Tubes and shells operating at elevated temperatures in corrosive environments.
• Columns and towers: Internals and supports requiring high-temperature strength and corrosion resistance.
• Storage tanks: Tanks storing corrosive liquids at elevated temperatures.

Common Defects and Countermeasures

Study Insights and Reflections

This research demonstrates that a single-layer TP347 overlay is technically feasible with careful process control, offering significant advantages in terms of cost, time, and material savings compared to multi-layer overlay. The key to success is the optimization of welding parameters to minimize dilution while maintaining adequate bond strength and microstructure uniformity.

The study also highlights the importance of niobium stabilization in TP347. The niobium content must be carefully controlled to ensure adequate carbon stabilization and prevent chromium carbide precipitation. In the single-layer overlay, the dilution from the base material can affect the effective niobium content, which must be monitored and controlled.

For engineers implementing single-layer TP347 overlay in production, the following recommendations emerge: (1) the welding procedure should be qualified per ASME IX or NB/T 47014 with specific attention to dilution control; (2) the overlay thickness should be verified by ultrasonic testing or destructive testing; (3) the microstructure should be examined to ensure a fine, equiaxed grain structure; and (4) the corrosion resistance should be verified through intergranular corrosion testing and pitting resistance testing.

The work by Li et al. provides a practical pathway for the economic application of TP347 overlay in petrochemical equipment, reducing the need for multi-layer welding and associated costs. This is particularly valuable for large-diameter vessels and heat exchangers where overlay area is substantial.