Failure Analysis of Dissimilar Steel Tube Joints in Boiler Superheaters

Literature Overview

This failure analysis paper by Meng Qingsen, Luo Guangmin, Liang Jun, and Xue Jin (2000, Materials Science and Technology, Vol. 8, No. 2, pp. 101–104) investigates the cracking failure of dissimilar steel welded joints in the superheater tubes of the Shentou II Power Plant. Funded by the Shanxi Provincial Natural Science Foundation (Grant No. 9801034), this case study provides valuable insights into high-temperature failure mechanisms in power plant components—a critical concern for boiler integrity management.

Core Technical Content

Failure History

Failure Analysis Methodology

The investigation employed:

• Fracture surface examination: Visual and scanning electron microscopy analysis of crack morphology
• Metallographic analysis: Microstructural characterization of the HAZ, weld metal, and base metal
• Chemical analysis: Identification of carbon migration and carbide redistribution

Phase 1 Failure: Creep Failure

The initial failure was identified as creep failure in the heat-resistant steel heat-affected zone (HAZ). Evidence supporting this conclusion:

These features are classic indicators of creep damage in ferritic-martensitic steels at elevated temperatures. The HAZ is particularly vulnerable because the welding thermal cycle creates a microstructural gradient with reduced creep resistance.

Phase 2 Failure: Stress Corrosion Cracking (SCC)

The secondary failure following SMAW repair was attributed to stress corrosion cracking. Contributing factors:

1. Residual stress: Introduced by the repair welding process, creating tensile stress concentrations
2. Corrosive environment: Water vapor containing chloride ions (Cl⁻) in the superheater environment
3. Weld defects: Pre-existing or repair-induced defects creating stress concentration sites
4. Microstructural susceptibility: The HAZ microstructure providing preferential crack initiation sites

The combination of tensile stress, chloride-containing water vapor, and susceptible microstructure created the conditions for chloride-induced SCC.

Failure Mechanism Summary

Engineering Countermeasures

Welding Process Improvement

The recommended improvement involves:

Why Nickel-Based Filler Metal?

Nickel-based alloys (such as Inconel 625 or equivalent) offer several advantages for dissimilar steel joints in high-temperature service:

• Excellent creep resistance at elevated temperatures
• Superior resistance to stress corrosion cracking
• Lower thermal expansion coefficient reducing thermal stress
• Enhanced ductility accommodating thermal cycling
• Resistance to carburization in high-carbon-potential environments

Quality Control for Dissimilar Steel Welds

Based on this failure case, the following quality control measures should be implemented:

1. Pre-weld inspection: Verify base metal condition, ensure no pre-existing cracks or surface damage
2. Weld procedure qualification: Use WPS/PQR qualified for the specific dissimilar metal combination
3. Heat input control: Minimize heat input to reduce the extent of the vulnerable HAZ
4. Post-weld heat treatment: Stress relief to reduce residual stresses
5. Non-destructive testing: 100% UT or RT inspection of dissimilar joints
6. Periodic in-service inspection: Regular UT examination during operation to detect early-stage cracking

Study Insights and Reflections

This failure analysis case is a textbook example of how multiple failure mechanisms can interact in complex ways. The initial creep failure was not unexpected given the service conditions and material selection, but the secondary SCC failure following repair illustrates a critical lesson: repair welding must be designed with the same rigor as the original fabrication.

The use of SMAW for the repair was problematic for several reasons:

• Higher heat input than TIG welding, creating a larger and more vulnerable HAZ
• Greater risk of carbon dilution and microstructural degradation
• Higher residual stresses due to the less controlled nature of manual welding
• Difficulty in achieving consistent weld quality in the constrained geometry of a superheater tube joint

The recommendation to use TIG welding with nickel-based filler metal represents a best-practice approach for dissimilar steel joints in high-temperature service. This principle applies broadly to power plant maintenance and repair activities.

From a metallurgical perspective, the carbon migration phenomenon observed in this case is a fundamental concern for all ferritic-martensitic steel weldments in high-temperature service. The carbon-depleted zone in the HAZ and the carbon-enriched zone in the weld metal create regions of reduced strength and altered microstructure that are inherently more susceptible to creep and corrosion damage. Understanding and mitigating this phenomenon through proper material selection and welding procedure design is essential for reliable long-term service.