Ultrasonic Testing of Delamination in In-Service Hot-Wall Hydrogenation Reactor Overlay Layers

Literature Overview

This 2000 publication by Wang Qingmei, Qiang Tianpeng, Zhou Disheng, and Wei Zigang from Yangzi Petrochemical Company and the Jiangsu Provincial Boiler and Pressure Vessel Inspection and Research Center addresses a critical challenge in the petrochemical industry: detecting delamination between the weld overlay layer and the base material in in-service hot-wall hydrogenation reactors. These vessels operate under extreme conditions — high pressure (typically 15–30 MPa), high temperature (350–450°C), and a hydrogen-rich atmosphere containing H2S — making overlay integrity a matter of safety and asset protection. The study represents early Chinese engineering practice in applying ultrasonic techniques to a problem that would later become central to pressure vessel inspection codes.

Core Technical Challenges

Hot-wall hydrogenation reactors typically employ a carbon steel or low-alloy steel base shell (such as 16Mn or 15CrMo) with an overlay of austenitic stainless steel (commonly 304L or 321) or nickel-based alloy (Inconel 625) applied by electroslag welding (ESW) or submerged arc welding (SAW). The overlay thickness typically ranges from 3 to 10 mm. Over time, several mechanisms can cause delamination at the overlay-base interface:

• Hydrogen embrittlement and hydrogen blistering at the bond line, particularly where the base steel has poor hydrogen resistance.
• Thermal fatigue cracking due to cyclic heating and cooling during reactor start-up and shutdown.
• Stress corrosion cracking (SCC) in the overlay layer when exposed to H2S-containing hydrogen.
• Residual stress relaxation and interface weakening during prolonged service.

Ultrasonic Testing Methodology

The authors describe a phased-array or conventional contact ultrasonic technique adapted for detecting interface delamination. Key technical parameters and considerations include:

A significant challenge is the impedance mismatch between austenitic stainless steel and ferritic carbon steel. The large grain size of austenitic stainless steel causes severe beam scattering at the interface, reducing signal-to-noise ratio. The authors likely employed techniques such as:

1. Using lower frequency probes (2.5 MHz) to reduce scattering while maintaining sufficient resolution for detecting delamination.
2. Applying dual-probe transmission-reception configurations to improve sensitivity at the interface.
3. Utilizing water-coupled immersion testing where access permits, which provides better coupling and reduces operator variability.
4. Implementing phased-array scanning with focused beams to improve resolution in the near-surface region.

Engineering Practice Insights

From a practical standpoint, in-service inspection of hot-wall hydrogenation reactors presents unique challenges. The reactor shell is typically insulated and clad with refractory material, requiring careful preparation of the test surface. The inspection must be performed during planned shutdowns, often under time pressure, which demands efficient scanning procedures and rapid interpretation of results.

The detection of delamination is particularly critical because:

• A delaminated overlay effectively removes the corrosion protection from the base material, exposing it to the hydrogen environment.
• Hydrogen can then permeate into the base steel, causing hydrogen blistering and potential catastrophic failure.
• Repair of delaminated areas requires grinding back to sound material and reapplication of the overlay, which is costly and may require vessel requalification.

Key Technical Points and Reflections

The most important lesson from this study is that ultrasonic testing of overlay delamination requires process qualification specific to the material combination and geometry involved. Generic UT procedures developed for weld seam inspection are often inadequate for detecting interface defects. The following factors must be considered in procedure development:

• The orientation of the delamination relative to the probe beam direction.
• The effect of overlay grain structure on signal attenuation.
• The minimum detectable defect size, which should be based on the acceptable area of delamination per the applicable code (typically per ASME VIII Div. 1 or NB/T 47013).
• The influence of surface preparation quality on coupling and signal quality.

This early work laid important groundwork for the Chinese inspection community. Modern practice now increasingly employs phased-array ultrasonic testing (PAUT) and total-converted-mode ultrasonic testing (TOFD) for overlay inspection, offering improved defect characterization and quantitative assessment. However, the fundamental principles of signal interpretation and acceptance criteria remain rooted in the work described here.

Conclusion

The study by Wang and colleagues represents a significant contribution to the non-destructive examination of clad pressure vessels in China. The recognition that overlay delamination poses a serious threat to vessel integrity, combined with the development of practical ultrasonic techniques for its detection, demonstrates the maturity of Chinese pressure vessel inspection practice at the turn of the millennium. Engineers involved in the inspection and maintenance of hydrogenation reactors should be aware that overlay integrity is not guaranteed for the life of the vessel and must be periodically verified through appropriate non-destructive testing methods.