T91 Steel TIG+MIG Welding Joint Properties and Microstructure

Literature Overview

This paper by Chang Tiejun, Gong Zhengchun, Li Zifeng, and Wang Changbai, published in the Transactions of the China Welding Institution (Volume 26, Issue 12, 2005, pp. 59-61), examines the mechanical properties, creep strength, high-temperature aging behavior, microstructure, and fracture characteristics of T91 steel welds produced by the TIG+MIG welding process. The research was conducted jointly by Harbin Engineering University and Harbin Boiler Works Co., Ltd., reflecting the close collaboration between academic research and industrial practice in power plant component fabrication.

Core Technical Findings

Mechanical and Creep Properties

The key performance data for the T91 steel TIG+MIG weld joint are summarized below:

The FATT of -13 °C indicates acceptable low-temperature toughness for the weld joint, although it is worth noting that T91 steel base metal typically exhibits a FATT in the range of -40 to -60 °C. The weld metal and HAZ therefore represent a relative weakness in terms of low-temperature toughness, which is a common characteristic of ferritic/martensitic steel welds.

Creep Strength and Microstructure

The creep strength data are particularly significant for power plant applications where T91 steel is used in high-temperature steam piping and boiler components. The 625 °C creep rupture strength of 70 MPa at 17958 hours, extrapolated to 46.48 MPa at 10^5 hours, demonstrates that the weld joint retains a substantial fraction of the base metal creep strength. This is largely attributed to the precipitation of MC-type alloy carbides (primarily (Fe, Cr)(C, N)) in the weld metal and HAZ, which provide effective strengthening through precipitation hardening and resistance to dislocation motion at elevated temperatures.

Microstructure and Fracture Analysis

The microstructural examination revealed that the high-temperature properties of the weld joint are governed by microstructural evolution, with MC carbides playing a critical role in creep resistance. The fracture analysis provides insights into the failure mechanisms under different loading conditions:

Engineering Practice Implications

Application in Power Plant Piping

T91 steel (equivalent to P91 per ASTM A213/A335) is widely used in supercritical and ultra-supercritical power plant steam piping systems, where operating temperatures reach 600-650 °C. The welding of T91 components is notoriously challenging due to:

1. High susceptibility to hydrogen-induced cracking (HIC) due to the high hardenability of the martensitic microstructure
2. Sensitivity to solidification cracking in the weld metal
3. Requirement for strict heat input control to minimize HAZ grain growth
4. Necessity for post-weld heat treatment (PWHT) to temper the as-welded martensite and relieve residual stresses

Welding Procedure Requirements

The TIG+MIG approach used in this study is well-suited for T91 steel welding, with the following procedure considerations:

Standards and Code Compliance

The welding of T91 steel components must comply with relevant codes and standards:

The creep strength data presented in this paper are directly relevant to the design of welded joints in high-temperature piping systems, where the joint strength reduction factor (F) specified in ASME B31.1 and B31.3 must be used in the allowable stress calculations. The weld joint strength ratio (WJSR) obtained from this study can inform the selection of appropriate joint strength reduction factors.

Key Questions and Reflections

A critical question arises regarding the long-term stability of the MC carbide precipitation observed in this study. While MC carbides provide excellent creep resistance, prolonged exposure to high temperatures may lead to carbide coarsening and phase transformation to more stable but less strengthening phases such as M23C6 or M6C3. The extrapolation from 17958 h to 10^5 h creep data relies on assumptions about the stability of the microstructure over extended service periods, which warrants further investigation through long-term aging studies.

Another important consideration is the effect of welding heat input on the HAZ microstructure. Excessive heat input can lead to over-tempering in the HAZ, reducing the strength and creep resistance of the base metal in the vicinity of the weld. Conversely, insufficient heat input may result in incomplete tempering of the as-welded martensite, leaving residual stresses and elevated hardness that increase cracking susceptibility. The TIG+MIG approach offers good control over heat input, but the transition from TIG root to MIG fill passes must be carefully managed to ensure consistent microstructural quality throughout the weld cross-section.

Summary

This study provides essential data on the mechanical properties, creep strength, and microstructural characteristics of T91 steel welds produced by the TIG+MIG process. The FATT of -13 °C and the 625 °C creep strength of 46.48 MPa at 10^5 hours demonstrate that the weld joint retains adequate performance for high-temperature power plant applications. The critical role of MC carbide precipitation in maintaining creep resistance underscores the importance of proper heat input control and PWHT in achieving the desired microstructure. Engineers working with T91 piping systems should use these findings to inform welding procedure design, heat input specifications, and post-weld treatment requirements, ensuring that welded joints meet the demanding performance criteria of modern supercritical power plant designs.