Mechanical Properties and Microstructure of T91 Steel TIG-MIG Welded Joints

Literature Overview

This research, published in the Transactions of the China Welding Institution (2005, Vol. 26, No. 12, pp. 59-61) by Chang Tiejun, Gong Zhengchun, Li Zifeng, and Wang Changbai from Harbin Engineering University and Harbin Boiler Works Co., Ltd., investigates the welding metallurgy, mechanical properties, and high-temperature performance of T91 steel welded joints fabricated using a TIG+MIG combination welding process. T91 steel, a 9Cr-1Mo-V-Nb martensitic steel, is widely used in ultra-supercritical (USC) power boiler components and advanced nuclear reactor pressure vessels due to its excellent creep resistance and thermal stability at temperatures above 600°C. The study is of particular significance because the welding of T91 steel is notoriously challenging due to its high hardenability, susceptibility to hydrogen cracking, and the difficulty of achieving adequate toughness in the heat-affected zone.

Core Technical Findings

Mechanical Properties and Fracture Behavior

The study reports a ductile-to-brittle transition temperature (FATT) of -13°C for the T91 TIG+MIG welded joint. This value is acceptable for many industrial applications but requires careful consideration for cryogenic or low-temperature service. The FATT is determined by the Charpy V-notch impact test, where the transition temperature is defined as the temperature at which the absorbed energy equals 27 J (or 20 J in some standards). A FATT of -13°C indicates that the weld joint retains adequate toughness down to this temperature, below which the joint becomes susceptible to brittle fracture.

The high-temperature creep performance is equally important. The study reports a 625°C/17,958-hour creep rupture strength of 70 MPa, and an extrapolated 10^5-hour creep strength of 46.48 MPa. These values are critical for the design of USC boiler components, where the design stress at 625°C is typically limited to approximately 100-110 MPa for T91 steel, and the weld joint must retain at least 70-80% of the base metal strength to meet ASME and other pressure vessel codes.

The weld joint strength retention ratio of approximately 78-85% is generally acceptable for pressure vessel applications, which typically require a minimum of 70-75% strength retention. However, this value is sensitive to welding parameters, filler metal composition, and post-weld heat treatment.

Microstructure and Phase Evolution

The microstructure of the T91 welded joint is characterized by a martensitic matrix with dispersed carbides, consistent with the base metal microstructure. The key difference lies in the type, size, and distribution of carbides. The study identifies MC-type carbides (where M represents Cr, Mo, V, and Nb) as the primary contributors to creep strength. These carbides are thermodynamically stable at high temperatures and provide effective precipitation hardening through coherent or semi-coherent interfaces with the martensitic matrix.

The microstructure varies across the weld joint:

• Weld metal: A mixture of martensite and retained austenite, with a finer grain size than the base metal due to the rapid cooling from the liquid state. The carbide distribution is typically coarser and less uniform than in the base metal.
• Coarse-grained HAZ: Full martensitic transformation with the largest grain size in the joint. This zone is the most susceptible to cracking and has the lowest toughness.
• Fine-grained HAZ: Partial recrystallization with a mixed microstructure of martensite, bainite, and retained austenite.
• Base metal: Tempered martensite with fine, uniformly distributed MC and M23C6 carbides.

The high-temperature aging behavior is critical for long-term service. At 625°C, the martensitic matrix undergoes tempering, and the MC carbides can undergo coarsening (Ostwald ripening) or, in some cases, transformation to more stable but less effective M23C6 carbides. The study's finding that MC carbides play a dominant role in creep strength implies that the welding process must preserve or promote the formation of MC carbides in the weld metal and HAZ.

TIG+MIG Combination Process

The TIG+MIG combination welding process is specifically designed to address the challenges of welding thick-section T91 steel. The TIG root pass provides excellent weld quality, penetration control, and low dilution, while the subsequent MIG fill and cap passes provide high deposition rates and efficient multi-pass welding. This combination is particularly advantageous for thick-walled boiler tubes and headers where single-process welding would be impractical.

The process parameters typically include:

• TIG root pass: 100-150 A, 12-18 V, 2-4 m/min, with 2.0-2.4 mm filler wire
• MIG fill passes: 150-250 A, 18-25 V, 3-6 m/min, with 1.0-1.2 mm wire
• Preheat temperature: 200-300°C (critical for preventing hydrogen cracking)
• Interpass temperature: 250-350°C
• Post-weld heat treatment: 760°C for 2 hours followed by 620°C for 8 hours

The preheat and interpass temperature control are absolutely critical for T91 steel welding. Without adequate preheat, the high cooling rate in the CGHAZ can produce untempered martensite with hardness exceeding 400 HV, which is extremely susceptible to hydrogen-induced cracking. The recommended preheat of 200-300°C reduces the cooling rate to a level where the CGHAZ hardness remains below 350 HV.

Engineering Practice Integration

Filler Metal Selection

The selection of filler metal is one of the most critical factors in T91 welding. The filler metal must match or slightly exceed the base metal in creep strength while maintaining adequate toughness. Common filler metals for T91 welding include:

• ER9CrMoVBNb (AWS classification): A martensitic filler metal with composition matched to T91 base metal
• 9Cr-1Mo-V-Nb solid wire: Used in TIG and MIG processes, with compositions ranging from 8.5-9.5% Cr, 0.9-1.1% Mo, 0.2-0.3% V, and 0.04-0.06% Nb

The Nb addition in the filler metal is particularly important for promoting MC carbide formation. Without Nb, the weld metal tends to form M23C6 carbides, which are less effective for creep strengthening. The V and Nb elements act as carbide formers that stabilize MC carbides, while Cr and Mo contribute to solid solution strengthening and grain boundary strengthening.

Quality Control Measures

Given the critical nature of T91 welded joints in power generation applications, rigorous quality control is mandatory. The following measures are recommended:

1. Pre-weld inspection: Verify base metal chemistry, confirm PWHT status of the base material, and ensure proper fit-up and groove preparation.
2. In-process monitoring: Monitor preheat and interpass temperatures using thermocouples, control welding parameters within specified ranges, and perform visual inspection of each pass.
3. Post-weld testing: Include hardness survey across the weld joint, ultrasonic testing (UT) for internal defects, and radiographic testing (RT) for volumetric defects.
4. PWHT verification: Confirm that the PWHT cycle achieves the required temperature and duration, and verify that the post-PWHT hardness is below 300 HV.

Common Defects and Countermeasures

Key Questions and Reflections

The study does not address the long-term aging behavior of the welded joint beyond 10^5 hours, which is a significant limitation for power plant applications where design lives of 200,000-300,000 hours are common. The evolution of MC carbides during prolonged exposure at 625°C, including their size, number density, and distribution, directly affects the long-term creep resistance. Additionally, the study does not discuss the effect of welding sequence on the residual stress state in multi-pass welds, which is a practical concern for thick-section fabrication.

From an engineering perspective, the FATT of -13°C raises questions about the joint's performance in transient conditions. During power plant start-up and shutdown, the weld joint may experience thermal cycling between ambient temperature and 625°C. The combination of thermal stresses and the relatively high FATT could potentially lead to low-temperature brittle fracture during rapid cool-down events. Engineers should consider this when evaluating the joint's suitability for specific service conditions.

Study Insights and Outlook

This research provides valuable data on the welding performance of T91 steel using the TIG+MIG combination process. The reported FATT of -13°C and creep strength retention of approximately 78-85% are benchmark values that can be used for process qualification and weld design. The identification of MC carbides as the key phase for creep strength provides a clear metallurgical target for filler metal development and welding process optimization. Future research should focus on extending the creep data to longer durations, investigating the effect of welding sequence on residual stress and microstructure in multi-pass welds, and developing advanced filler metals with improved MC carbide stability. The practical significance of this study lies in its contribution to the qualification of T91 welding procedures for USC power boiler applications, where the reliability of welded joints is paramount for plant safety and availability.