CLADDING TECHNOLOGY SHANXI CO., LTDProperties and Microstructure of T91 Steel TIG-MIG Weld 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 Institution in 2005, investigates the mechanical properties and microstructural characteristics of T91 steel weld joints produced by a TIG root pass combined with MIG fill and cap passes. T91 steel (9Cr-1Mo-V-Nb) is a martensitic ferritic steel widely used in ultra-supercritical power plant boiler tubes, headers, and pressure vessels operating at temperatures exceeding 600°C. The collaboration between academic researchers and a major boiler manufacturer provides direct industrial relevance to the findings.
Material Background and Welding Challenges
T91 steel is a precipitation-strengthened martensitic steel with the following nominal composition:
| Element | Content (wt%) |
|---|---|
| Cr | 8.5–9.5 |
| Mo | 0.85–1.05 |
| V | 0.18–0.22 |
| Nb | 0.06–0.10 |
| C | 0.08–0.12 |
| W | 0.25–0.50 |
The combination of martensitic matrix and fine L12-type MX carbonitride precipitates (M23C6 and MX phases) provides excellent creep strength and thermal fatigue resistance at elevated temperatures. However, this alloy presents significant welding challenges:
- High hardenability requiring strict preheat and interpass temperature control
- Susceptibility to hydrogen-induced cracking (HIC) in the as-welded and heat-affected zones
- Difficulties in achieving adequate weld joint toughness without extensive post-weld heat treatment
- Sensitivity to improper heat input that can lead to excessive grain coarsening or retained austenite formation
Welding Process Parameters
TIG Root Pass
| Parameter | Value |
|---|---|
| Current (A) | 80–120 |
| Voltage (V) | 12–16 |
| Travel speed (mm/min) | 100–200 |
| Shielding gas | Pure Ar or Ar + 2% H2 |
| Electrode | Pure tungsten (1.6 mm) |
| Filler wire | ER910 (matching composition) |
MIG Fill and Cap Passes
| Parameter | Value |
|---|---|
| Current (A) | 200–280 |
| Voltage (V) | 22–26 |
| Travel speed (mm/min) | 300–500 |
| Shielding gas | 80%Ar + 20%CO2 |
| Wire | ER910 (1.2 mm) |
| Preheat temperature | 200–250°C |
| Interpass temperature | 200–300°C |
Post-Weld Heat Treatment (PWHT)
| Parameter | Value |
|---|---|
| Temperature | 730–760°C |
| Soak time | 2–4 h (per 25 mm thickness) |
| Cooling rate | Furnace cooled to 400°C, then air cooled |
| Purpose | Tempering, HAZ softening, residual stress relief |
Microstructural Analysis
Weld Metal Microstructure
The weld metal solidifies as austenite and transforms to martensite during cooling. After PWHT, the microstructure consists of tempered martensite with fine carbide precipitates. The composition of the weld metal depends on dilution with base metal:
- Root pass: Higher dilution (30–50% base metal) due to TIG process characteristics
- Fill/cap passes: Lower dilution (10–20%) due to MIG process with wire feed
The weld metal hardness after PWHT typically ranges from 250–320 HV, which is within acceptable limits for T91 steel (generally <350 HV per code requirements to avoid HIC susceptibility).
HAZ Microstructural Evolution
The HAZ in T91 steel undergoes complex microstructural transformations:
| HAZ Sub-zone | Peak Temperature (°C) | As-Welded Structure | After PWHT | Hardness (HV) |
|---|---|---|---|---|
| Recrystallized HAZ | 700–900 | Fine martensite + retained austenite | Tempered martensite | 250–300 |
| Partially transformed HAZ | 900–1100 | Coarse martensite + retained austenite | Tempered martensite + coarse carbides | 280–340 |
| Fully transformed HAZ | 1100–1400 | Very coarse martensite + retained austenite | Tempered martensite + coarse grain | 300–380 |
| Base metal | — | Tempered martensite + MX carbides | Recreep-strengthened | 250–290 |
The fully transformed HAZ (FT-HAZ) is the most critical region, as it experiences the highest temperatures and most severe microstructural changes. The coarse grain size and high carbon equivalent in this zone can lead to elevated hardness and reduced toughness.
Mechanical Properties
Tensile Properties
| Location | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|
| Base metal | 620–680 | 440–500 | 12–16 |
| Weld metal | 580–650 | 420–480 | 14–18 |
| HAZ (worst) | 550–620 | 400–460 | 10–14 |
Hardness Profile
The hardness profile across the weld joint exhibits the characteristic "W" shape typical of martensitic steel weldments:
- Base metal: 250–290 HV
- HAZ peak: 300–380 HV
- Weld metal: 250–320 HV
- Maximum hardness location: Typically in the CGHAZ adjacent to the fusion boundary
Impact Toughness
Charpy V-notch impact energy at 20°C and 400°C:
| Location | CVN at 20°C (J) | CVN at 400°C (J) |
|---|---|---|
| Base metal | 40–60 | 80–120 |
| Weld metal | 30–50 | 70–100 |
| HAZ (worst) | 20–40 | 60–90 |
Engineering Practice Relevance
For pressure vessel and boiler engineers working with T91 steel, this study provides critical guidance on welding procedure development:
- Preheat requirements: Minimum preheat of 200°C is essential to slow cooling rates and prevent HIC. The interpass temperature must be maintained between 200–300°C to avoid excessive martensite formation and hydrogen accumulation.
- PWHT necessity: Post-weld heat treatment is mandatory for T91 weld joints to achieve acceptable toughness and hardness levels. The PWHT temperature of 730–760°C is specifically selected to temper the martensite without causing excessive grain growth or sensitization.
- Hydrogen control: The use of low-hydrogen consumables (ER910 with moisture-controlled flux) and strict gas shielding are essential to minimize diffusible hydrogen levels below 5 ml/100g in the weld metal.
- Code compliance: For ASME Section VIII Division 2 or GB/T 150 applications, the weld joint factor and stress relief factors must account for the reduced toughness in the HAZ. The maximum allowable hardness of 350 HV (or 300 HV for sour service) must be verified through hardness mapping.
Defect Analysis and Countermeasures
| Defect Type | Cause | Prevention/Countermeasure |
|---|---|---|
| Hydrogen-induced cracking | High diffusible hydrogen, slow cooling | Preheat, low-H consumables, post-weld bake-out |
| Excessive HAZ hardness | High carbon equivalent, fast cooling | Adequate preheat, controlled heat input |
| Reduced impact toughness | Coarse grain, retained austenite | PWHT, controlled interpass temperature |
| Incomplete fusion | Insufficient heat input, poor technique | Increased current, proper joint preparation |
| Cracking in weld metal | High dilution, composition mismatch | Proper filler selection, dilution control |
Key Reflections and Study Insights
The TIG-MIG combination for T91 steel welding represents a practical compromise between the precision of TIG (for root pass quality) and the productivity of MIG (for fill and cap passes). This hybrid approach is widely adopted in industrial practice for thick-section T91 weldments, including boiler headers, reactor internals, and pressure vessel components.
The study highlights the fundamental challenge of welding martensitic steels: achieving an acceptable balance between strength, toughness, and resistance to cracking while maintaining creep strength at elevated service temperatures. The PWHT is not merely a stress relief step but a critical metallurgical treatment that transforms the as-welded microstructure from a brittle, high-hardness martensite to a toughened, creep-resistant tempered martensite.
For pressure vessel engineers, the key takeaway is that T91 welding requires a comprehensive approach that integrates proper procedure design, consumable selection, process parameter control, and post-weld treatment. The hardness mapping and impact testing requirements in modern codes (ASME IX, NB/T 47014) must be rigorously satisfied to ensure long-term structural integrity in high-temperature service. The collaboration between Harbin Engineering University and Harbin Boiler Works exemplifies the essential partnership between academic research and industrial practice that drives the development of reliable welding technologies for critical infrastructure.