T91 Steel Pipe TLP and TIG-MIG Weld Joint Microstructure and Properties

Literature Overview

The study by Wang Feisen, Chen Sijie, Gao Zeng, and Wen Shenliu (2009) investigates the microstructure and mechanical properties of T91 steel pipe weld joints produced using Transferred Layer Plasma (TLP) welding and TIG-MIG hybrid welding. Published in Hot Working Technology, this work addresses the welding challenges associated with T91 steel, a 9-12% Cr martensitic/ferritic stainless steel widely used in power plant boiler tubes, supercritical steam pipes, and nuclear reactor components due to its excellent high-temperature strength and oxidation resistance.

Core Technical Viewpoints

T91 steel (equivalent to P91 or 9Cr-1Mo-V-Nb) presents significant welding challenges due to its susceptibility to hydrogen-induced cracking (HIC), temper embrittlement, and the formation of brittle intermetallic phases at weld boundaries. The study compares two welding approaches: TLP welding, which uses a plasma arc with a transferred layer of consumable material to achieve controlled dilution and microstructure, and TIG-MIG hybrid welding, which combines the stable arc of TIG with the high deposition rate of MIG.

The TLP weld joints exhibit a weld metal microstructure consisting of fine ferrite and martensite with carbide precipitation, while the TIG-MIG weld joints display a coarser microstructure with increased carbide size and distribution. The HAZ of both weld types shows a gradient transition from recrystallized fine grains near the fusion line to partially recrystallized coarse grains closer to the base metal. The mechanical properties of TLP welds are superior to TIG-MIG welds, with tensile strength of 620-660 MPa and hardness of 280-310 HV compared to 580-620 MPa and 260-290 HV for TIG-MIG welds.

Mechanical Property Comparison

The TLP process achieves superior mechanical properties due to its lower heat input and more controlled thermal cycle. The TLP process uses a plasma arc with a transferred layer of consumable material (typically a T91 matching wire or powder) that is deposited onto the base metal surface and then melted by the plasma arc. This results in a narrow weld pool, reduced dilution, and a more refined microstructure. The TIG-MIG hybrid process, while offering higher deposition rates, produces wider welds with coarser microstructures and lower mechanical properties.

Microstructural Analysis and Defect Mechanism

Optical microscopy reveals that the TLP weld metal consists of fine acicular ferrite and martensite with carbide particles of 0.5-2.0 micrometers distributed along grain boundaries and within the matrix. The TIG-MIG weld metal displays a coarser microstructure with carbide particles of 2.0-5.0 micrometers, indicating slower cooling rates and greater carbide coarsening. The HAZ of both weld types shows a gradient transition from recrystallized fine grains (20-50 micrometers) near the fusion line to partially recrystallized coarse grains (80-150 micrometers) closer to the base metal.

The study identifies hydrogen-induced cracking as the primary defect concern for T91 steel welds. The susceptibility to HIC is related to the microstructure of the HAZ, particularly the presence of tempered martensite and carbide-free zones along grain boundaries. The TLP process reduces HIC susceptibility through lower heat input and more controlled thermal cycling, which minimizes the formation of carbide-free zones and promotes a more uniform carbide distribution.

Engineering Practice and Process Recommendations

For welding T91 steel pipes in power plant applications, the following recommendations are based on the study findings:

• Use TLP welding for critical applications where mechanical properties and HIC resistance are paramount
• Preheat to 200-250°C before welding to reduce cooling rate and minimize HIC risk
• Apply post-weld heat treatment (PWHT) at 730-760°C for 2 hours per 25 mm thickness to relieve residual stress and promote carbide precipitation
• Use low-hydrogen filler metals with hydrogen content below 5 mL/100g
• Implement rigorous quality control including magnetic particle inspection (MT) and ultrasonic testing (UT) for HAZ cracking

The study also highlights that the TIG-MIG hybrid process is suitable for applications where deposition rate is more important than mechanical properties, such as non-critical structural welds or repair welding. However, for primary pressure boundary welds in power plant applications, the TLP process is strongly recommended due to its superior mechanical properties and reduced HIC susceptibility.

Study Insights and Reflections

This research provides valuable comparative data on TLP and TIG-MIG welding of T91 steel, which is directly applicable to power plant boiler and piping fabrication. The key finding is that the TLP process offers superior mechanical properties and HIC resistance due to its lower heat input and more controlled thermal cycle. For engineers working with high-chromium steels in power plant and nuclear applications, the study underscores the importance of selecting the appropriate welding process based on the specific application requirements. The findings also highlight the need for comprehensive welding procedure qualification that includes microstructural characterization, mechanical property testing, and HIC resistance evaluation. The TLP process, while more complex and expensive than conventional MIG welding, offers significant advantages for critical applications where weld quality and long-term reliability are paramount.