Failure Analysis of Right-Angle Elbow Cracking in Superheater Tubes

Literature Overview

The paper by Zhang Shuxin, Guo Kai, and Li Lifeng (2019), published in Heat Treatment of Metals (Vol. 44, Issue S1, pp. 348-352), presents a detailed failure analysis of a right-angle elbow that cracked during the sodium hydroxide boiler washing operation of a superheater tube system. The authors are affiliated with the Pipe Research Institute of PetroChina and the State Key Laboratory of Metal Materials for Oil and Gas Equipment. This work was supported by the National Key R&D Program of China (Grant No. 2017YFC0805804).

Failure Investigation Methodology

The investigation employed a comprehensive suite of analytical techniques to characterize the failure mechanism and identify the root cause. The methods used and their findings are summarized below:

Core Technical Analysis

Failure Context and Operating Conditions

The superheater tube right-angle elbow experienced cracking during the sodium hydroxide (NaOH) boiler washing operation, which is performed prior to commissioning to remove manufacturing residues and contaminants from the boiler system. The NaOH washing process involves circulating a hot alkaline solution through the boiler tubes to dissolve and remove iron oxides, scale, and other deposits. The operating conditions during this process include:

• Elevated temperature (typically 100°C to 150°C)
• High alkalinity (concentrated NaOH solution)
• Potential presence of dissolved oxygen
• Thermal cycling during the washing and subsequent drying phases

Stress Corrosion Cracking Mechanism

The investigation concluded that the failure mechanism was stress corrosion cracking (SCC), specifically alkaline SCC. The intergranular cracking morphology observed on the fracture surface, combined with the presence of secondary cracks, is characteristic of SCC in low-alloy steel materials exposed to concentrated alkaline solutions. The mechanism can be described as follows:

1. Initiation: The high alkalinity of the NaOH solution attacks the grain boundaries of the steel, preferentially dissolving the grain boundary carbides and creating localized anodic sites.
2. Propagation: The combination of the corrosive environment and the residual tensile stresses from the bending process drives the crack propagation along the grain boundaries.
3. Secondary cracking: As the primary crack propagates, branching and secondary cracking occur, creating a characteristic crack network on the fracture surface.

Microstructural Factors

The microstructure of the superheater tube elbow was identified as a pearlite-ferrite mixture, which is typical for carbon and low-alloy steels used in boiler applications. The microstructural features relevant to the SCC susceptibility include:

• Grain boundary carbides: The presence of carbides at grain boundaries can create preferential paths for crack propagation in alkaline environments.
• Ferrite-pearlite interface: The interface between ferrite and pearlite can act as a site for localized corrosion due to the electrochemical potential difference between the two phases.
• Grain size: Coarser grain sizes generally reduce SCC resistance, as they provide longer continuous paths for grain boundary attack.

Engineering Practice Integration

Prevention and Mitigation Strategies

Based on the failure analysis, several preventive measures should be considered for superheater tube elbows and similar components:

1. Material selection: Selection of materials with improved SCC resistance in alkaline environments, such as low-carbon steels with controlled grain size or steels with microalloy additions that refine the microstructure.
2. Manufacturing process control: Careful control of the bending process to minimize residual stresses, followed by appropriate stress relief annealing. The stress relief temperature and holding time should be optimized to ensure complete stress relief without adversely affecting the material properties.
3. Boiler washing procedure optimization: The NaOH washing procedure should be carefully controlled to minimize the risk of SCC. This includes:

• Limiting the NaOH concentration to the minimum effective level
• Controlling the washing temperature to avoid excessive thermal stresses
• Ensuring thorough rinsing after the washing process to remove residual alkaline solution
• Avoiding prolonged exposure of the tubes to concentrated alkaline solutions

1. Pre-commissioning inspection: NDT inspection of the superheater tubes and elbows after the boiler washing operation should be performed to detect any early-stage SCC cracks before the system is placed into service.

Quality Control Considerations

The investigation highlights the importance of quality control at multiple stages of the superheater tube manufacturing and installation process:

• Raw material verification: Chemical composition and microstructure verification of the raw material to ensure compliance with specifications.
• Bending process monitoring: Monitoring of the bending process parameters (bending radius, bending speed, temperature) to ensure consistent quality.
• Post-bending stress relief: Verification of the stress relief process to ensure complete elimination of residual stresses.
• Pre-commissioning inspection: NDT inspection of all critical components before commissioning to detect any manufacturing defects.

Key Questions and Reflections

The failure analysis raises several important questions for further investigation:

• What is the minimum NaOH concentration and temperature at which SCC initiation occurs in the specific steel grade used?
• How does the microstructure (pearlite fraction, grain size, carbide distribution) influence the SCC susceptibility of the elbow material?
• Can alternative washing procedures (e.g., acid washing, mechanical cleaning) be used to avoid the risk of alkaline SCC?

The intergranular cracking morphology with secondary cracks observed in this case is a clear indicator of SCC. The fact that the material composition met the standard requirements suggests that the failure was driven by the interaction between the material's inherent SCC susceptibility and the specific operating conditions during the boiler washing process. This underscores the importance of considering not only material specifications but also the specific service conditions when evaluating the integrity of piping components.

Study Insights and Implications

This failure analysis provides valuable insights into the SCC susceptibility of superheater tube elbows during the boiler washing process. The key finding is that even materials that meet chemical composition specifications can be susceptible to SCC under specific environmental conditions, particularly concentrated alkaline solutions. The presence of residual stresses from the bending process further exacerbates the SCC susceptibility.

For engineers involved in boiler design, manufacturing, and commissioning, this case study emphasizes the importance of a comprehensive approach to material selection and process control. The interaction between the material microstructure, manufacturing-induced residual stresses, and the specific service conditions during boiler washing must be carefully managed to prevent SCC failures. The implementation of robust quality assurance programs, including material verification, process control, and pre-commissioning inspection, is essential for ensuring the long-term integrity of superheater tube systems.