Crack Analysis and Prevention Research of Boiler Pipe Tee Welds

Literature Overview

This paper by Zhao Jiaxing, Pan Qingchuan, and Liu Xiang from the China Special Equipment Inspection Research Institute (2018) presents a comprehensive failure analysis and preventive engineering study of weld cracks in tee fittings of a 600 MW ultra-supercritical subcritical变压 (variable pressure) direct-steam boiler. The specific component analyzed is the HG-1900/25.4-YM7 type main steam pipe tee, where cracks were detected at both sides of the tee welds during operational inspection. The study combines metallurgical examination, hardness testing, stress analysis, and creep-fatigue life estimation to identify root causes and propose preventive measures.

Core Technical Viewpoints

The primary finding of this investigation is that the weld cracks are caused by a combination of improper weld design (inadequate reinforcement and geometry at the tee junction) and unfavorable stress conditions imposed by the main steam pipe system layout and support arrangement. The ultra-supercritical operating conditions—temperatures exceeding 593°C and pressures above 25.4 MPa—subject the tee welds to severe creep-fatigue loading during boiler start-stop cycles. The study proposes a multi-faceted prevention strategy encompassing weld design improvement, stress management through support optimization, and enhanced inspection protocols.

Interpretation of Key Technical Points

Defect Analysis and Root Cause Identification

The metallurgical examination of the cracked welds revealed:

Stress Analysis and Life Estimation

The stress analysis of the tee welds during boiler start-stop transients reveals:

• Peak thermal stress: Approximately 180–220 MPa during rapid temperature changes (heating rate of 150–200°C/h during startup)
• Creep stress: Sustained stress of 35–45 MPa at operating temperature (593°C)
• Combined damage: The interaction of cyclic thermal stress and sustained creep stress accelerates damage accumulation according to the Miner's rule for cumulative damage

The creep-fatigue life estimation indicates that under the original design conditions, the expected weld life is approximately 15,000–20,000 start-stop cycles, which is below the design life requirement of 30,000 cycles for ultra-supercritical boilers.

Weld Design Deficiencies

The analysis identifies several design deficiencies in the original tee weld configuration:

• Insufficient weld reinforcement: The weld leg length is at the minimum allowable value per ASME Section VIII, providing no margin for stress concentration reduction.
• Poor fillet geometry: The weld toe radius is small, creating a high stress concentration factor (Kt > 2.5).
• Inadequate weld preparation: The groove angle and root gap are not optimized for the specific stress state at the tee junction.
• Lack of post-weld heat treatment (PWHT): The residual stress from welding is not relieved, compounding with operational stresses.

Process and Standards Analysis

The relevant standards for boiler tee welds include:

The material specification for the HG-1900/25.4-YM7 boiler tee is typically 12Cr1MoV or 1.25Cr-0.5Mo (ASTM A335 P91), which is a creep-strength enhanced ferritic alloy suitable for ultra-supercritical service. The welding procedure must be qualified per ASME Section IX or NB/T 47014, with specific attention to:

• Preheat temperature: 250–350°C to prevent cold cracking
• Interpass temperature: 250–350°C to control HAZ hardness
• Heat input: 0.8–1.5 kJ/mm to balance cooling rate and HAZ microstructure
• PWHT: 730–760°C for 2 hours to relieve residual stress and soften HAZ

Integration with Engineering Practice

This failure analysis provides critical lessons for boiler designers, welding engineers, and inspection personnel. The following engineering recommendations are derived from the study:

Design Improvements

1. Weld geometry optimization: Increase weld reinforcement to provide a larger transition radius at the weld toe, reducing the stress concentration factor to below 2.0.
2. Support arrangement modification: Redesign the pipe support layout to minimize thermal expansion restraint at the tee junction, reducing cyclic thermal stress by 20–30%.
3. Stress relief features: Incorporate stress-relieving fillets or notches at critical locations to redistribute stress away from the weld HAZ.

Welding Process Improvements

1. Multi-layer multi-pass welding: Use a combination of GTAW for the root pass and SMAW/FCAW for fill and cap passes, with careful control of heat input per pass.
2. Post-weld heat treatment: Mandatory PWHT at 730–760°C for a duration proportional to the wall thickness (minimum 1 hour per 25 mm of thickness).
3. Weld repair procedures: Establish strict procedures for weld repair, including full penetration grinding of defective areas and re-welding with qualified procedures.

Inspection and Monitoring

1. Initial inspection: 100% UT (TOFD or PAUT) of all tee welds after fabrication and before installation.
2. In-service monitoring: Periodic UT inspection at intervals of 2 years during the first 10 years of service, then annually thereafter.
3. Thermographic monitoring: Infrared thermography during startup to identify thermal stress hotspots at tee junctions.
4. Fitness-for-service assessment: Regular FFS evaluation of any detected indications using ASME API 579 methodology.

Key Questions and Reflections

A fundamental question that emerges from this analysis is the adequacy of current welding procedure qualification standards for ultra-supercritical boiler applications. The standard qualification tests (tensile, bend, impact) may not fully capture the creep-fatigue behavior of the weld metal and HAZ under actual operating conditions. Supplemental qualification tests—including long-term creep rupture testing of weld metal and HAZ samples at service temperatures—should be considered for critical applications.

Another important reflection is the role of digital simulation in preventing such failures. Finite element analysis of the tee weld during start-stop transients, coupled with fracture mechanics analysis of potential crack initiation and propagation, could have predicted the failure mode during the design phase. The integration of simulation-based design with traditional engineering judgment is essential for preventing failures in high-consequence applications.

Study Insights and Implications

This failure analysis exemplifies the importance of a systematic, multi-disciplinary approach to addressing weld defects in critical pressure-containing components. The combination of metallurgical examination, stress analysis, and life estimation provides a comprehensive understanding of the failure mechanism and enables targeted preventive measures. For welding engineers and boiler inspectors, the key lessons are:

• Weld geometry and design significantly influence the long-term performance of pressure-containing components, and minimum-code-compliant designs may be inadequate for severe service conditions.
• The interaction between welding residual stress, operational thermal stress, and creep stress must be considered holistically rather than treating each factor in isolation.
• Enhanced inspection protocols and fitness-for-service assessment provide essential safety margins during the operational life of critical components.
• Preventive engineering—through design optimization, process control, and monitoring—is always more cost-effective than corrective action after failure.

This case study reinforces the principle that weld quality in critical applications extends far beyond meeting code requirements; it demands a comprehensive understanding of the operating environment, material behavior, and damage mechanisms that will be encountered during the component's service life.