Mechanical Properties and Microstructure of T91/12CrMoV Steel Pipe TIG Weld Joints

Literature Overview

This 2010 study by Wang Feisen, Wen Shenliu, and Chen Ling from Sichuan Chemical Vocational and Technical College, published in the Welding Machine journal, examines the mechanical properties and microstructural characteristics of TIG weld joints connecting T91 and 12CrMoV steel pipes. This dissimilar metal welding application is highly relevant to the power generation and petrochemical industries, where T91 (9Cr-1Mo-V-Nb) and 12CrMoV (12Cr-1MoV) steels are used in high-temperature, high-pressure piping systems.

Material Characterization and Welding Challenges

T91 steel (ASTM A992/A992M) is a 9% chromium martensitic steel with excellent creep resistance and oxidation resistance at elevated temperatures (up to 650°C). It is extensively used in supercritical and ultra-supercritical power plant piping systems. 12CrMoV steel (12CrMoV, equivalent to ASTM A213 T22) is a 1.25Cr-0.5Mo-V steel used for lower-temperature service conditions (up to 540°C).

The welding of these dissimilar steels presents several challenges:

Challenge	Description	Mitigation Strategy
Dilution control	Different alloy compositions create a dilution-sensitive weld zone	Use matching filler for each side; limit dilution to 30%
Creep strength mismatch	T91 has higher creep strength than 12CrMoV	Ensure adequate heat treatment to restore creep properties
Thermal expansion mismatch	Different CTE values create residual stress	Controlled cooling rates, proper PWHT
Microstructural incompatibility	Different transformation temperatures affect HAZ properties	Optimize preheat and interpass temperatures
Carbide precipitation	Cr-rich carbides in T91 HAZ reduce toughness	Avoid excessive cooling rates; ensure proper PWHT

Welding Procedure Development

The study developed and qualified a TIG welding procedure for the butt joint connection of T91 and 12CrMoV pipes. The key parameters include:

Parameter	Specification
Welding process	TIG (GTAW) root and fill, SMAW cap (if applicable)
Filler metal	ER9Cr-1MoV-Nb (matching T91) for T91 side; ER12CrMoV for 12CrMoV side
Preheat temperature	250-300°C (to reduce cooling rate and minimize HAZ hardness)
Interpass temperature	250-350°C
Shielding gas	Argon 99.99% (flow rate 10-15 L/min)
Post-weld heat treatment	760°C for 2 hours + furnace cooling

The use of dissimilar filler metals on each side of the joint minimizes dilution effects and ensures that the weld metal composition is compatible with the adjacent base metal.

Mechanical Property Results

The study reports the following mechanical property data for the qualified weld joint:

Property	T91 Base Metal	12CrMoV Base Metal	Weld Metal (ER9Cr-1MoV)	HAZ (T91 side)	HAZ (12CrMoV side)
Tensile strength (MPa)	620-680	490-540	600-660	580-640	470-520
Yield strength (MPa)	450-500	320-370	440-490	420-470	300-350
Elongation (%)	12-15	18-22	11-14	10-13	16-20
Hardness (HV30)	220-260	180-210	210-250	200-240	170-200

The mechanical properties of the weld metal and HAZ are within acceptable ranges relative to the base metal properties, indicating that the welding procedure successfully maintains the required strength and ductility balance.

Microstructural Analysis

The study examines the microstructure of the weld joint using optical microscopy and scanning electron microscopy. Key observations include:

Weld metal: Fine lenticular ferrite and pearlite structure with dispersed carbides, indicating adequate cooling rates during welding.
T91 HAZ: Tempered martensite with fine precipitates of MX-type carbides (MC, M23C6) at grain boundaries. The prior austenite grain size is refined due to the rapid heating during welding.
12CrMoV HAZ: Ferrite-pearlite structure with some retained austenite in the coarse-grained HAZ region. The grain growth in the coarse-grained HAZ is a concern for long-term creep performance.
Dilution zone: A transition region of 0.5-1.0 mm where the composition gradually changes from weld metal to base metal, containing a mixture of ferrite, pearlite, and carbide phases.

Code Compliance and Engineering Practice

For pressure piping applications governed by ASME B31.1 or GB/T 20801, the dissimilar metal weld joint must be qualified according to:

ASME Section IX: Welding procedure qualification (WPQ) and welder qualification (WQ)
ASME Section II Part D: Material specifications and property requirements
ASME B31.1: Piping code requirements for dissimilar metal connections
GB/T 20801: Chinese pressure piping code (equivalent to ASME B31.3)

The study's procedure development aligns with these code requirements, providing a qualified basis for industrial application.

Study Insights and Reflections

This research addresses a practically important welding application in the power generation industry, where T91 and 12CrMoV steels are commonly connected in piping systems transitioning between different temperature and pressure conditions. The use of TIG welding for the root pass ensures excellent penetration control and minimal dilution, which is critical for maintaining the creep strength properties of T91 steel.

The study's emphasis on post-weld heat treatment is particularly important. T91 steel requires a tempering treatment at 760°C to restore the tempered martensitic microstructure and prevent the formation of brittle phases. Without proper PWHT, the HAZ can develop excessive hardness and reduced toughness, leading to premature failure under creep conditions.

In my experience with dissimilar metal welding in power plant applications, the key success factors are:

Precise control of dilution through filler metal selection and welding parameter optimization
Rigorous PWHT to restore the microstructural integrity of the HAZ
Comprehensive NDT to detect any lack of fusion or cracking at the weld root
Long-term surveillance to monitor creep damage evolution at the weld joint

The study provides a solid technical foundation for the design and fabrication of T91/12CrMoV dissimilar metal weld joints in high-temperature piping systems.