Properties and Microstructure of T91 Steel TIG+MIG Welding Joints

Literature Overview

This study by Chang Tiejun, Gong Zhengchun, Li Zifeng, and Wang Changbai from Harbin Engineering University and Harbin Boiler Works Co., Ltd., published in the Transactions of the China Welding Institute in 2005, investigates the microstructure and mechanical properties of T91 steel welding joints fabricated using a TIG+MIG (pulse TIG root pass followed by MIG fill and cap passes) welding sequence.

Core Technical Points

T91 steel (9Cr-1Mo-V-Nb) is a martensitic heat-resistant alloy widely used in ultra-supercritical (USC) power plant boilers and steam generators. The welding of T91 steel is challenging due to its high hardenability, susceptibility to hydrogen-induced cracking, and the formation of brittle intermetallic phases in the HAZ.

Welding Sequence

The TIG+MIG sequence is a strategic approach that leverages the advantages of both processes:

1. TIG root pass: The pulse TIG process provides excellent penetration control and clean root formation. The low heat input minimizes the formation of coarse grain zones and reduces the risk of hydrogen-induced cracking.
2. MIG fill and cap passes: The MIG process provides high deposition rates for the bulk of the weld, while maintaining acceptable heat input.

Microstructural Evolution

Critical Defect: Laves Phase Formation

The most critical metallurgical concern in T91 welding is the formation of the Laves phase (Fe2MoW, M7C3) in the HAZ. This phase is extremely hard and brittle, significantly reducing the toughness of the joint.

• Formation temperature range: 1050-1150°C
• Critical cooling rate: Below 10°C/s, the Laves phase has sufficient time to nucleate and grow
• Prevention strategies:
• Preheat temperature: 200-250°C to reduce cooling rates
• Interpass temperature: Maintain 250-300°C to avoid re-heating into the Laves phase formation range
• Post-weld heat treatment (PWHT): 730-760°C for 1 hour per 25 mm thickness, followed by furnace cooling

Mechanical Properties

Engineering Practice Implications

T91 steel is increasingly used in USC boilers operating at pressures above 25 MPa and temperatures above 600°C. The welding integrity of T91 components is critical for the safe operation of these advanced power generation systems.

The TIG+MIG sequence offers several advantages for T91 fabrication:

1. Root quality: The TIG root pass produces a clean, defect-free root with minimal spatter and excellent fusion.
2. Productivity: The MIG fill and cap passes achieve 3-5 times the deposition rate of TIG alone, significantly reducing fabrication time.
3. Residual stress management: The lower heat input of the TIG root pass reduces the overall residual stress in the joint.

However, the following precautions are essential:

• Hydrogen control: Use low-hydrogen fluxes and thoroughly clean the weld area to prevent hydrogen-induced cracking.
• Preheat and interpass temperature: Maintain preheat at 200-250°C and interpass temperature between 250-300°C.
• PWHT: Mandatory post-weld heat treatment at 730-760°C to relieve residual stresses and temper the martensitic microstructure.
• PWHT cooling rate: Control the cooling rate below 100°C/hour in the 600-400°C range to prevent the formation of brittle phases.

Key Reflections

This research is particularly relevant to the fabrication of pressure vessels and heat exchangers for advanced power generation systems. The T91 steel is not only used in boilers but also in superheater tubes, reheater tubes, and high-temperature pressure vessels. The welding procedures developed for T91 must be carefully qualified according to ASME Section IX or equivalent standards.

The formation of Laves phase in the HAZ is a critical concern that must be addressed in the welding procedure specification. Engineers should consider the use of nickel-containing filler metals (e.g., ER911, ER912) that dilute the Laves phase formation tendency and improve the toughness of the HAZ.

The TIG+MIG sequence represents a practical compromise between weld quality and productivity. For large-scale fabrication of T91 components, this approach is particularly attractive because it reduces the fabrication time while maintaining acceptable weld quality. However, the welding procedure must be carefully optimized to ensure that the HAZ microstructure remains within acceptable limits.