12Cr1MoVG Steel TIG Welded Joint Reheat Cracking Cause Analysis

Literature Overview

This case study published in Physicochemical Testing Physics Section (Volume 62, Issue 3, 2026, pages 61-65) by Pei Lixia and colleagues from Shanxi Vocational and Technical College and Taiyuan University of Technology investigates a field failure of a 12Cr1MoVG steel TIG welded joint in a power plant low-temperature reheater. The study employs a comprehensive failure analysis methodology combining macroscopic observation, metallographic examination, scanning electron microscopy with energy dispersive spectroscopy, and hardness testing, integrated with analysis of the welding process parameters and the structural characteristics of the reheater. The root cause was identified as reheat cracking, exacerbated by inadequate welding process execution and age segregation at the weld fusion line during prolonged high-temperature service.

Core Technical Content and Failure Analysis

12Cr1MoVG is a low-alloy heat-resistant steel widely used in power plant boiler components, particularly in high-temperature sections such as reheaters, superheaters, and steam pipes. The steel contains approximately 1.2 percent chromium, molybdenum, vanadium, and other alloying elements that provide creep strength and oxidation resistance at elevated temperatures. However, this alloy composition also makes the material susceptible to reheat cracking, a time-dependent cracking phenomenon that occurs in the thermally affected zone during post-weld heat treatment or during prolonged high-temperature service.

Failure Investigation Methodology

The failure analysis followed a systematic approach beginning with macroscopic examination of the cracked joint to identify crack location, orientation, and morphology. Metallographic examination revealed the microstructural features of the crack initiation and propagation zones. Scanning electron microscopy provided high-resolution images of the crack surface morphology, while energy dispersive spectroscopy identified elemental composition variations along the crack path. Hardness testing mapped the hardness distribution across the weld joint, identifying zones of susceptibility to reheat cracking.

Crack Classification and Mechanism

The analysis confirmed that the cracks were reheat cracks, characterized by intergranular cracking in the thermally affected zone, particularly in the prior austenite grain boundaries near the fusion line. Reheat cracking is a time-temperature-dependent phenomenon that occurs when the material is held at temperatures typically between 500 and 700 degrees Celsius for extended periods. The mechanism involves the precipitation of carbides and other secondary phases at grain boundaries, which depletes the surrounding matrix of alloying elements and creates a region of reduced cohesion. Under the combined action of thermal expansion stresses and internal pressure, cracks initiate at these weakened grain boundaries and propagate through the microstructure.

Contributing Factors

The investigation identified two primary contributing factors to the failure. First, the welding process was not executed in accordance with the specified procedure, leading to inadequate heat input control, improper preheating, or insufficient post-weld heat treatment. These process deviations resulted in an unfavorable microstructure in the heat-affected zone, particularly in the coarse grain region near the fusion line, which is most susceptible to reheat cracking. Second, during prolonged high-temperature service in the low-temperature reheater, age segregation occurred at the weld fusion line. This segregation involves the preferential precipitation of chromium and molybdenum carbides at grain boundaries, which depletes the boundary region of these strengthening elements and reduces its resistance to cracking.

Engineering Practice Implications

This failure case study is highly relevant to power plant maintenance and inspection practices. Reheat cracking is a well-known but frequently under-diagnosed failure mode in high-temperature boiler components. The low-temperature reheater, despite its name, operates at temperatures well above 500 degrees Celsius, placing it squarely within the temperature range where reheat cracking is most likely to occur. The combination of thermal cycling during start-up and shutdown, sustained high-temperature operation, and internal pressure creates a severe environment for weld integrity.

The case underscores the critical importance of strict adherence to welding procedures, including proper preheating temperatures, interpass temperature control, and post-weld heat treatment. Even minor deviations from the specified procedure can result in microstructural changes that significantly increase susceptibility to reheat cracking. Additionally, the age segregation phenomenon highlights the need for periodic inspection and assessment of weld joints in high-temperature service, as the cracking susceptibility increases with service time.

Failure Analysis Summary and Preventive Measures

Study Insights and Reflections

This failure analysis case study provides a valuable practical lesson for engineers working in power plant maintenance and welding quality assurance. The systematic approach to failure investigation, combining multiple characterization techniques with process history analysis, serves as a model for future failure investigations. The identification of both process-related and time-dependent factors demonstrates that weld integrity is a function of both initial manufacturing quality and subsequent service conditions.

For practitioners, the key takeaway is that welding procedure compliance is not merely a quality requirement but a safety imperative. The consequences of inadequate welding process execution in high-temperature applications can be catastrophic, leading to unexpected failures and potential safety incidents. Furthermore, the age segregation phenomenon reminds us that even properly manufactured welds can degrade over time, necessitating ongoing monitoring and periodic assessment. Future work should focus on developing more resistant materials or welding procedures that minimize the susceptibility of 12Cr1MoVG welds to reheat cracking, potentially through modified heat treatment cycles or alternative filler metal selection. This case study should be included in training programs for welding inspectors and maintenance engineers to reinforce the importance of process discipline and proactive failure prevention.