Zhuojin Pipe Fitting Co., LtdSurfacing Layer Cracking Analysis and Repair in High-Pressure Heat Exchangers for Coal Tar Hydrogenation Refining
Literature Overview
This paper, authored by Yang Zhigang from the Henan Provincial Boiler and Pressure Vessel Inspection Technology Research Institute and published in Coal Chemical Industry (2023, Vol. 51, No. 4, pp. 72–75), presents a detailed case study of surfacing layer cracking in a high-pressure heat exchanger tube-side outlet nozzle within a coal tar hydrogenation refining unit. The case is particularly instructive because it documents not only the initial failure analysis but also the consequences of an inadequate repair attempt, ultimately leading to a successful factory repair. The paper integrates non-destructive testing (NDT), metallurgical examination, and process analysis to arrive at a root cause determination.
Failure Analysis Process
The investigation began with the detection of numerous surface cracks in the surfacing layer of the tube-side outlet nozzle. The following NDT methods were employed to characterize the cracking:
| NDT Method | Purpose |
|---|---|
| Penetrant Testing (PT) | Detection of surface-breaking cracks |
| Ultrasonic Testing (UT) | Assessment of crack depth and subsurface extent |
| Hardness Testing | Evaluation of local microstructure and residual stress state |
| Ferrite Content Testing | Verification of austenitic weld metal composition |
Based on the NDT results and equipment operating records, the analyst concluded that the primary cracking mechanism was chloride stress corrosion cracking (Cl-SCC). This determination is consistent with the known susceptibility of austenitic stainless steel surfacing layers to chloride-induced cracking under tensile stress conditions, particularly at elevated temperatures and pressures typical of hydrogenation refining operations.
The Repair Challenge
The most instructive aspect of this case is the sequence of repair attempts. Following the initial crack detection, an on-site repair was performed. However, post-repair inspection revealed new cracks forming around the repair weld area. This secondary failure prompted a more thorough investigation and ultimately led to the decision to return the heat exchanger for factory repair, which was subsequently successful.
Root Cause of Secondary Cracking
The paper identifies inadequate pre-weld dehydrogenation treatment as the fundamental cause of the new cracks that appeared after the on-site repair. This finding is critical and carries significant implications for field repair practices on pressure equipment.
| Repair Scenario | Key Process Step | Outcome |
|---|---|---|
| On-site repair | Inadequate pre-weld dehydrogenation | New cracks formed around repair weld |
| Factory repair | Proper dehydrogenation treatment | Repair qualified, no new cracking |
Technical Discussion
The issue of hydrogen-induced cracking in repaired austenitic stainless steel components is well documented in the literature, yet it remains a persistent challenge in field repairs. During welding, hydrogen is introduced into the weld metal from moisture in fluxes, contaminated surfaces, or the decomposition of organic compounds. In austenitic stainless steels, hydrogen can accumulate at grain boundaries and inclusion-rich regions, creating localized tensile stresses that, when combined with residual stresses from welding, can exceed the material's resistance to cracking.
The dehydrogenation treatment, typically involving heating the weld area to 150–250 °C for a sufficient duration, allows hydrogen to diffuse out of the metal before it can cause damage. The omission of this step during the on-site repair is a classic example of a process deviation that leads to equipment failure. In the context of coal tar hydrogenation, where the operating environment already contains hydrogen at high pressure, the risk of hydrogen-related damage is amplified, making dehydrogenation treatment even more critical.
Engineering Practice Implications
This case study underscores several important principles for field repair of pressure equipment:
- Complete root cause analysis before repair: Understanding the original failure mechanism is essential to designing an effective repair strategy.
- Adherence to prescribed welding procedures: Skipping steps such as dehydrogenation treatment, even when they seem minor, can lead to catastrophic secondary failures.
- Post-repair verification: New cracks around the repair area should be treated as a red flag indicating process problems rather than isolated defects.
- Factory vs. field repair decision-making: When field repair conditions are not fully controllable, returning the equipment to a factory with proper facilities may be the safer and ultimately more economical choice.
Summary
This paper provides a valuable real-world example of how a seemingly straightforward repair can lead to further damage when critical process steps are omitted. The identification of chloride stress corrosion cracking as the original failure mechanism, combined with the recognition that inadequate dehydrogenation caused the secondary cracking, demonstrates the importance of integrating metallurgical knowledge with welding practice. For engineers responsible for maintenance and repair of high-pressure equipment in the petrochemical and coal chemical industries, this case serves as a cautionary tale about the consequences of procedural shortcuts.