Overlay Layer Cracking Inspection Analysis and Repair Process for High-Pressure Heat Exchangers in Coal Tar Hydrogenation Refining Units

Literature Overview and Background

Coal tar hydrogenation refining units operate under severe conditions — high pressure (10–20 MPa), high temperature (350–450 °C), and a highly corrosive environment containing hydrogen sulfide, ammonia, and organic sulfur compounds. The high-pressure heat exchangers in these units are typically clad with stainless steel or nickel-based alloy overlay layers to resist corrosion. However, cracking of these overlay layers is a well-documented failure mode that can lead to catastrophic leaks and production shutdowns. This study presents a comprehensive analysis of overlay layer cracking in such heat exchangers, including the root cause investigation, non-destructive testing (NDT) methodology, and repair process development.

The study is particularly relevant for engineers working in the petrochemical and coal chemical industries, where coal tar hydrogenation is an increasingly important process for producing clean fuels and chemicals.

Core Technical Points and Failure Analysis

The overlay layer cracking observed in these heat exchangers typically manifests as:

• Intergranular cracks: Cracks propagating along the grain boundaries of the overlay layer, often associated with sensitization and intergranular corrosion (IGC).
• Transgranular cracks: Cracks propagating through the grains of the overlay layer, often associated with hydrogen-induced cracking (HIC) or stress corrosion cracking (SCC).
• Bond line cracks: Cracks at the interface between the overlay layer and the base metal, often associated with poor fusion or thermal fatigue.

The root cause analysis typically involves a combination of visual inspection, non-destructive testing, and metallurgical examination. The following NDT methods are commonly employed:

• Magnetic particle testing (MT): For detecting surface and near-surface cracks in ferromagnetic overlay layers.
• Penetrant testing (PT): For detecting surface cracks in non-ferromagnetic overlay layers (e.g., austenitic stainless steel).
• Ultrasonic testing (UT): For detecting subsurface cracks and bond line defects.
• Phase array ultrasonic testing (PAUT): For advanced imaging of complex crack geometries.
• Eddy current testing (ET): For detecting cracks in conductive overlay layers.

Metallurgical Analysis and Root Cause Determination

The metallurgical examination of cracked overlay layers typically reveals the following findings:

1. Sensitization and intergranular corrosion: If the overlay layer is an austenitic stainless steel (e.g., 304 or 316), sensitization can occur during welding or post-weld heat treatment if the material is exposed to the sensitization temperature range (450–850 °C) for an extended period. This leads to chromium carbide precipitation at grain boundaries, depleting the boundary region of chromium and rendering it susceptible to intergranular corrosion.
2. Hydrogen-induced cracking (HIC): In hydrogen-rich environments, atomic hydrogen can diffuse into the overlay layer and accumulate at microstructural defects (inclusions, voids, grain boundaries). This can lead to blister formation and eventual cracking. The 316L and 321 grades are more resistant to HIC than 304 due to their lower carbon content and stabilizing elements (Ti or Nb).
3. Stress corrosion cracking (SCC): In chloride-containing environments, austenitic stainless steel overlay layers can be susceptible to chloride stress corrosion cracking (Cl-SCC). This is particularly problematic in coal tar hydrogenation units where chloride contamination can occur from upstream processes.
4. Thermal fatigue cracking: Cyclic thermal loading during start-up and shutdown can induce thermal fatigue cracks, particularly at the bond line where thermal expansion mismatch between the overlay and base metal creates cyclic stresses.

Repair Process Development and Implementation

The repair process for cracked overlay layers depends on the crack type, severity, and location. The following repair strategies are recommended:

The repair procedure typically involves:

1. Crack mapping: Using MT and PT to map the extent of cracking. The cracks are marked on the surface for reference during repair.
2. Material removal: The cracked overlay layer is removed by grinding or machining. For intergranular cracks, the entire sensitized zone (typically 1–3 mm depth) must be removed. For transgranular cracks, the entire overlay layer may need to be removed.
3. Surface preparation: The exposed base metal is cleaned and prepared for re-cladding. Any defects in the base metal (e.g., corrosion pits, machining marks) must be repaired.
4. Re-cladding: The overlay layer is rebuilt using the appropriate welding process and filler material. For stainless steel overlay layers, GTAW or SAW with low-carbon or stabilized filler wires (e.g., ER316L, ER321) is recommended.
5. Post-weld treatment: The repaired area undergoes stress relief annealing (1050–1100 °C for austenitic stainless steel) to reduce residual stresses and restore corrosion resistance.
6. Post-repair inspection: The repaired area is inspected using MT, PT, and UT to verify the absence of defects. A hydrostatic test at 1.25 times the design pressure is performed to confirm the integrity of the repair.

Engineering Practice and Quality Control

The quality control protocol for overlay layer repair in high-pressure heat exchangers is critical. The following measures are recommended:

• Welding procedure qualification: The repair welding procedure must be qualified in accordance with ASME IX or NB/T 47014, with specific attention to the re-cladding of previously clad surfaces.
• Filler material selection: The filler material must be compatible with the base metal and the original overlay layer. For 316L overlay layers, ER316L or ER316LJ filler wire is recommended.
• Heat input control: The heat input during re-cladding must be carefully controlled to avoid sensitization of the new overlay layer. Low heat input (below 15 kJ/mm) is preferred for austenitic stainless steel overlay layers.
• Post-weld inspection: 100% MT and PT of the repaired area, plus UT of the bond line to verify fusion quality.
• Documentation: All repair activities must be thoroughly documented, including crack mapping, material removal, welding parameters, and inspection results.

Key Questions and Reflections

The study raises an important question about the long-term reliability of repaired overlay layers. While the repair process can restore the overlay layer to a sound condition, the repaired area may be susceptible to future cracking if the root cause is not addressed. For example, if the cracking is caused by sensitization during the original fabrication, the repair must include a sensitization-resistant overlay material (e.g., 316L instead of 304) and a modified welding procedure that minimizes heat input.

Another reflection concerns the role of operational factors in overlay layer cracking. Fluctuations in operating pressure and temperature, as well as contamination of the process stream with chlorides or other corrosive species, can accelerate cracking. Engineers must work closely with operations personnel to identify and mitigate these factors.

Study Insights and Implications

This literature provides a comprehensive framework for the inspection, analysis, and repair of cracked overlay layers in high-pressure heat exchangers. The key takeaway is that a systematic approach — combining NDT, metallurgical analysis, and careful repair planning — is essential for successful repair and long-term reliability.

In conclusion, the overlay layer cracking problem in coal tar hydrogenation heat exchangers is a complex challenge that requires a multidisciplinary approach. Engineers must understand the metallurgical mechanisms of cracking, select appropriate repair strategies, and implement rigorous quality control protocols. By doing so, they can extend the service life of critical equipment and minimize production losses.