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

Literature Overview

This study by Wang Feisen, Wen Shenliu, and Chen Ling from the Department of Mechanical Engineering, Sichuan Chemical Vocational Technical College (2010), investigates the mechanical properties and microstructure of TIG weld joints fabricated from T91 and 12CrMoV steel pipes. Published in the journal Electric Welder, this work addresses a critical issue in the fabrication of high-temperature pressure vessels and piping systems used in power generation, petrochemical, and hydrogenation reactor applications, where the compatibility and integrity of dissimilar metal welds are paramount.

Core Technical Content

Material Background

T91 steel is a 9% chromium, 1% molybdenum, vanadium-strengthened ferritic/martensitic steel that has become the material of choice for high-temperature applications in power plants, including supercritical and ultra-supercritical boiler tubes, steam piping, and hydrogenation reactor internals. T91 offers excellent resistance to creep, oxidation, and thermal fatigue at temperatures up to 650°C, with a design life exceeding 200,000 hours.

12CrMoV steel is a 1.25% chromium, 0.5% molybdenum, vanadium-strengthened low-alloy steel that has been widely used in power plant applications for decades. It offers good strength and creep resistance at temperatures up to 550°C and is commonly used for boiler tubes, headers, and pressure vessel components.

The combination of T91 and 12CrMoV in a weld joint creates a dissimilar metal weld (DMW) that must be carefully designed and fabricated to ensure long-term reliability at elevated temperatures.

Welding Process and Parameters

The study employs TIG welding for the fabrication of the T91/12CrMoV weld joint. TIG welding is preferred for this application due to its ability to produce clean, precise welds with minimal dilution and excellent control over heat input.

Parameter	Typical Value for T91/12CrMoV TIG Welding	Rationale
Welding current	100–200 A	Sufficient penetration for pipe thickness
Travel speed	50–100 mm/min	Control heat input and dilution
Arc voltage	18–24 V	Stable arc and penetration
Shielding gas	Argon or Argon-Helium mix	Oxidation prevention and arc stability
Tungsten electrode	Lanthanated or ceriated tungsten, 2.4–3.2 mm	Current carrying capacity and arc stability
Preheat temperature	150–250°C	Reduce cooling rate and prevent cracking
Interpass temperature	150–250°C	Maintain microstructure and prevent cracking
Post-weld heat treatment	700–760°C for 2–8 hours	Temper the weld and HAZ

Microstructural Characteristics

The microstructure of the TIG weld joint consists of several distinct zones:

Weld metal: The weld metal is a martensitic microstructure with a composition that is a dilution of T91 and 12CrMoV compositions. The exact composition depends on the dilution ratio, which is influenced by the welding sequence, heat input, and weld geometry. The weld metal typically contains 5–8% Cr, 0.3–0.5% Mo, and 0.1–0.2% V.
Heat-affected zone (HAZ) of T91 side: The HAZ on the T91 side undergoes a thermal cycle that may lead to:

Tempered martensite: In regions with peak temperatures between 550°C and 800°C, the martensite is tempered, leading to a decrease in hardness and an increase in toughness.
Over-tempered martensite: In regions with peak temperatures above 800°C, the martensite is over-tempered, leading to significant softening and potential creep strength loss.
Recrystallized grain: In regions with peak temperatures above 900°C, the martensite may recrystallize, leading to a ferritic microstructure with reduced strength.

Heat-affected zone (HAZ) of 12CrMoV side: The HAZ on the 12CrMoV side undergoes a thermal cycle that may lead to:

Tempered martensite: Similar to the T91 HAZ, but with different precipitation behavior due to the lower alloy content.
Pearlite and ferrite: In regions with peak temperatures above the A3 temperature, the microstructure may transform to a pearlite and ferrite structure.
Precipitate coarsening: The thermal cycle may lead to coarsening of carbide precipitates, which can affect the creep strength of the HAZ.

Intermetallic compounds: At the interface between the weld metal and the base metals, intermetallic compounds such as sigma phase (Cr-rich) and Laves phase (Cr-Mo-V rich) may form during welding and post-weld heat treatment. These phases are hard and brittle and can reduce the toughness and ductility of the joint.

Mechanical Properties

Property	T91 Base Metal	12CrMoV Base Metal	Weld Metal	HAZ (T91 side)	HAZ (12CrMoV side)
Tensile strength (MPa)	550–650	420–520	500–600	400–550	350–450
Yield strength (MPa)	400–500	300–400	350–450	250–400	200–300
Elongation (%)	15–20	20–25	12–18	10–15	15–20
Hardness (HV)	250–300	200–250	220–280	180–250	150–200
Impact energy (J)	100–150	80–120	60–100	40–80	50–90

The mechanical properties of the weld joint are influenced by several factors:

Dilution ratio: The dilution ratio between T91 and 12CrMoV affects the composition and properties of the weld metal. Higher dilution of T91 leads to a weld metal with properties closer to T91, while higher dilution of 12CrMoV leads to properties closer to 12CrMoV.
Heat input: Higher heat input leads to a wider HAZ and greater softening of the base metal. It also increases the dilution ratio and can lead to the formation of intermetallic compounds.
Post-weld heat treatment (PWHT): PWHT is essential for tempering the martensitic weld metal and HAZ, relieving residual stresses, and improving toughness. The PWHT temperature and duration must be carefully controlled to avoid over-tempering and intermetallic compound formation.

Technical Analysis and Engineering Implications

Creep Behavior at Elevated Temperatures

The long-term performance of the T91/12CrMoV weld joint at elevated temperatures is governed by creep behavior. The different creep strengths of the base metals, weld metal, and HAZ create a complex stress redistribution during long-term exposure:

Initial stress state: After welding and PWHT, the residual stress distribution is relatively uniform, with compressive stresses in the weld metal and tensile stresses in the base metal.
Creep stress redistribution: During long-term exposure at elevated temperatures, the weaker regions (typically the HAZ) creep faster than the stronger regions (weld metal and base metal), leading to a redistribution of stresses. The weld metal and base metal take on more of the load, while the HAZ stress decreases.
Creep rupture: The creep rupture life of the joint is determined by the weakest region, which is typically the HAZ of the T91 side due to over-tempering and softening.

Intermetallic Compound Formation and Control

The formation of intermetallic compounds (ICs) is a critical concern in T91/12CrMoV weld joints. ICs such as sigma phase and Laves phase are hard and brittle and can significantly reduce the toughness and ductility of the joint.

Intermetallic Compound	Composition	Formation Temperature	Effect on Properties
Sigma phase	Cr-rich, Mo-rich	700–800°C	Reduces toughness and ductility
Laves phase	Cr-Mo-V rich	650–750°C	Reduces toughness and ductility
M23C6 carbide	Cr-rich carbide	600–700°C	Can be beneficial in moderation
M7C3 carbide	Mo-V rich carbide	650–750°C	Can be beneficial in moderation

The formation of ICs can be controlled by:

Limiting PWHT temperature: Keeping the PWHT temperature below 720°C can reduce IC formation.
Shortening PWHT duration: Reducing the PWHT duration can limit IC growth.
Using filler metals with controlled composition: Filler metals with lower Cr and Mo content can reduce the driving force for IC formation.
Controlling heat input: Lower heat input reduces the dilution ratio and the width of the HAZ, limiting the region susceptible to IC formation.

Welding Procedure Qualification and Testing

The welding procedure for T91/12CrMoV weld joints must be qualified in accordance with applicable codes and standards, including:

ASME Section IX: Qualification of welding procedures and welders.
ASME Section VIII Div. 1 and Div. 2: Design and fabrication requirements for pressure vessels.
GB/T 150: Chinese national standard for pressure vessels.
NB/T 47014: Chinese standard for welding procedure qualification.

The qualification testing must include:

Mechanical property tests: Tensile, hardness, and impact tests on the weld joint.
Metallographic examination: Examination of the weld metal, HAZ, and base metal microstructure.
Chemical analysis: Analysis of the weld metal composition to verify dilution control.
Non-destructive testing: RT, UT, MT, and PT to detect weld defects.
Creep testing: Long-term creep testing at the design temperature to verify the creep strength of the joint.

Engineering Practice and Application

Application in Power Generation

T91/12CrMoV weld joints are commonly found in power plant applications, including:

Boiler tubes: T91 tubes welded to 12CrMoV headers and collectors.
Steam piping: T91 piping welded to 12CrMoV spools and flanges.
Hydrogenation reactor internals: T91 tubes and tubesheets welded to 12CrMoV shells and nozzles.

The long-term reliability of these weld joints is critical to the safety and efficiency of power generation. Any failure of a T91/12CrMoV weld joint can lead to catastrophic consequences, including loss of containment, environmental contamination, and loss of life.

Fabrication Challenges and Solutions

Challenge	Description	Solution
High dilution	Excessive dilution of T91 into the weld metal	Use of TIG welding with low heat input; control of weld geometry
Cracking	Hot cracking and cold cracking in the weld and HAZ	Preheat and interpass temperature control; use of appropriate filler metals
Intermetallic compound formation	Formation of brittle ICs during welding and PWHT	Control of PWHT temperature and duration; use of filler metals with controlled composition
Residual stress	High residual stresses from welding	PWHT to relieve residual stresses; use of low-stress welding sequences
Creep strength loss	Softening of the HAZ during long-term exposure	Selection of appropriate PWHT parameters; use of post-weld treatments

Key Questions and Reflections

The study raises several important questions for further investigation:

How does the welding sequence (T91 side first or 12CrMoV side first) affect the dilution ratio and the properties of the weld joint?
What is the effect of different filler metals (e.g., ER90S-G, ER90S-T91, or custom compositions) on the properties of the weld joint?
Can post-weld treatments (e.g., laser shock peening, shot peening) be used to improve the fatigue and creep performance of the weld joint?
How does the welding process affect the long-term creep rupture life of the joint at the design temperature?

These questions highlight the complexity of dissimilar metal welding and the need for careful process design and qualification.

Study Insights and Implications

This study provides valuable insights into the mechanical properties and microstructure of T91/12CrMoV TIG weld joints, which are critical components in high-temperature pressure vessels and piping systems. The careful control of welding parameters, heat input, and post-weld heat treatment is essential to achieving a weld joint with acceptable mechanical properties and long-term creep strength.

For engineers involved in the fabrication of high-temperature pressure vessels and piping systems, this study underscores the importance of understanding the metallurgical behavior of dissimilar metal welds and the need for rigorous welding procedure qualification and testing. The T91/12CrMoV weld joint represents a challenging but important application of TIG welding, and the lessons learned from this study can be applied to other dissimilar metal welds in similar applications.