Failure Analysis of 12Cr1MoVG Steel Pipe in High-Temperature Superheater of Steam Boiler

Literature Overview

This 2025 paper by Zhang Congmin, Xu Luojun, Huang Luofei, Rao Xiaolin, and Xu Chao from the Yichang Branch of Hubei Special Equipment Inspection and Testing Research Institute presents a forensic failure analysis of a 12Cr1MoVG steel pipe rupture in the high-temperature superheater of a steam boiler. The study employs a comprehensive methodology combining macroscopic observation, chemical composition analysis, tensile testing, hardness testing, metallographic examination, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) to determine the root cause of the failure.

Failure Background and Incident Description

12Cr1MoVG is a chromium-molybdenum-vanadium alloy steel specifically designed for high-temperature applications in power generation equipment, including boiler tubes, superheaters, and reheaters. The steel is normalized to provide a fine-grained ferrite-pearlite microstructure with excellent creep resistance and thermal stability at operating temperatures of 550-620°C.

The incident involved a burst (rupture) of a high-temperature superheater tube in a steam boiler, resulting in a loss of containment and potential safety hazards. The investigation aimed to determine the root cause to prevent recurrence and to evaluate the adequacy of existing maintenance and inspection procedures.

Investigative Methodology

The failure analysis followed a systematic approach consistent with industry standards for forensic engineering investigations:

Analytical Techniques Applied

Root Cause Analysis

Primary Failure Mechanism

The investigation identified a multi-factorial failure mechanism:

1. Internal scale formation: Deposits accumulated on the inner wall of the superheater tube, creating a thermal barrier that impeded heat transfer from the hot gases to the working fluid. This caused the tube wall temperature to exceed the design limit.
2. Sustained over-temperature operation: The thermal barrier from scale deposits caused the tube wall temperature to rise significantly above the design temperature, leading to accelerated creep and microstructural degradation.
3. Severe spheroidization: Prolonged exposure to elevated temperatures caused the originally dispersed carbide particles in the 12Cr1MoVG steel to spheroidize—transforming from fine, uniformly distributed particles into large, rounded globules. This microstructural change drastically reduced the steel's strength and creep resistance.
4. Scale detachment and redistribution: During operation, portions of the internal scale layer detached and migrated downstream, accumulating at a downstream elbow. This redistribution created a new thermal barrier at the elbow location.
5. Secondary over-temperature at straight section: With the scale now concentrated at the elbow, the upstream straight tube section experienced severe over-temperature due to the loss of the original thermal barrier at that location combined with continued high-temperature gas flow.
6. Bulging and rupture: Under the combined action of working pressure and thermal stresses, the over-temperature straight tube section underwent creep deformation (bulging), eventually leading to rupture.

Microstructural Evidence

The metallographic examination revealed:

• Original microstructure: Fine ferrite-pearlite with dispersed M23C6 and M6C carbides (as-delivered condition)
• Degraded microstructure: Coarse spheroidized carbides (5-15 μm diameter) in a ferrite matrix, with evidence of prior austenite grain coarsening

The hardness test results confirmed the extent of degradation:

• Original hardness (as-delivered): 180-210 HB
• Measured hardness at failure location: 120-140 HB
• Hardness reduction: 30-40%

Material Degradation Mechanisms

Spheroidization in 12Cr1MoVG Steel

Spheroidization is a time-temperature-dependent microstructural degradation mechanism that occurs in low-alloy steels containing carbide-forming elements (Cr, Mo, V). The process involves:

1. Dissolution of fine carbides at elevated temperatures
2. Re-precipitation as larger, thermodynamically stable spherical carbides
3. Coarsening of the spheroidized carbides over time (Ostwald ripening)

The kinetics of spheroidization in 12Cr1MoVG steel follow an Arrhenius-type relationship:

t = A × exp(Q/RT)

Where:

• t = time to significant spheroidization
• A = material constant
• Q = activation energy (~200-250 kJ/mol for 12Cr1MoVG)
• R = gas constant
• T = absolute temperature

At design temperatures (550-600°C), significant spheroidization typically occurs after 20,000-50,000 hours of operation. However, over-temperature conditions accelerate this process dramatically—spheroidization that would normally take decades can occur in months under sustained over-temperature conditions.

Creep Behavior Under Degraded Conditions

The spheroidized microstructure exhibits significantly reduced creep resistance:

• Creep strength at 600°C: Reduced by 40-60% compared to as-delivered condition
• Creep rupture life: Reduced by 50-80% compared to as-delivered condition
• Creep ductility: May increase initially but then decrease as voids nucleate and grow

Preventive Measures and Recommendations

Based on the failure analysis findings, the following recommendations were formulated:

Operational Measures

Maintenance and Inspection Recommendations

1. Scale control: Implement rigorous water and steam chemistry programs to minimize internal scale formation. Target dissolved oxygen < 10 ppb, pH 9.0-9.5 for boiler feedwater, and conductivity < 0.2 μS/cm for steam.
2. Temperature monitoring: Install additional thermocouples at critical locations (elbows, bends, and sections with historical scale accumulation) to detect over-temperature conditions early.
3. Periodic inspection: Conduct comprehensive inspections including:

• Visual inspection of accessible areas
• Ultrasonic thickness measurement at critical locations
• Eddy current testing for internal defect detection
• Metallographic sampling from representative locations

1. Material qualification: For replacement tubes, verify the as-delivered microstructure meets specification requirements (fine, dispersed carbides without spheroidization).

Engineering Practice Implications

This failure case study provides valuable lessons for engineers involved in the design, fabrication, and maintenance of high-temperature pressure piping systems:

Fabrication and Welding Considerations

For 12Cr1MoVG steel pipe fabrication and welding:

• Preheat temperature: 200-300°C (to prevent hydrogen-induced cracking in the susceptible HAZ)
• Interpass temperature: ≤ 350°C
• Heat input: 1.5-4.0 kJ/mm (controlled to prevent excessive grain growth)
• Post-weld heat treatment: 780-820°C for 2-4 hours (to relieve welding residual stresses and restore microstructure)
• Welding electrode: Low-hydrogen type, pre-dried at 300-350°C for 2 hours

Welding Quality Requirements

Study Reflections

This failure analysis exemplifies the importance of systematic forensic engineering methodology in determining the root cause of equipment failures. The multi-factorial nature of the failure—combining scale formation, over-temperature, microstructural degradation, and mechanical loading—highlights the complexity of high-temperature equipment failures and the need for comprehensive investigation approaches.

The case underscores the critical importance of water chemistry control in preventing internal scale formation, which can have cascading effects on equipment integrity. A seemingly minor operational issue (scale formation) led to a catastrophic failure through a chain of degradation mechanisms that would not have been apparent from any single inspection technique.

For engineers in the steel pipe and welding industry, this case reinforces the importance of material quality verification at the point of manufacture and the need for proper welding procedures that maintain the material's high-temperature performance characteristics. The post-weld heat treatment requirement for 12Cr1MoVG welds is not merely a code compliance issue but a critical requirement for maintaining the long-term creep resistance of the welded joint.

The systematic approach to failure analysis presented in this study—combining multiple complementary techniques from macroscopic to microscopic scales—provides a model for investigating similar failures in other high-temperature pressure equipment. The findings emphasize that prevention of such failures requires a holistic approach encompassing material selection, fabrication quality, operational monitoring, and periodic inspection, with each element contributing to the overall integrity of the system.