CLADDING TECHNOLOGY SHANXI CO., LTDCrack Analysis in Hydrogenation Heat Exchanger Clad Test Plate Surface Layer
Literature Overview
This 2009 technical paper by Xue Xiaoqiang, Shi Jiqing, and Wang Qiang from Gansu Lanke Petrochemical High-Tech Equipment Co., Ltd. provides a detailed root cause analysis of cracking observed in the overlay surface layer of a hydrogenation heat exchanger test plate. The publication, appearing in Petrochemical Equipment journal, documents a quality investigation that employed systematic metallurgical analysis and process review to identify the root cause of overlay cracking and propose corrective measures. This case study is particularly valuable for engineers involved in the fabrication of hydrogenation service equipment, where overlay integrity is critical to component reliability.
Component and Service Background
Hydrogenation heat exchangers in refinery and petrochemical service operate under conditions of high temperature (350 to 450 degrees Celsius), high pressure (100 to 200 MPa), and the presence of hydrogen, hydrogen sulfide, and other aggressive chemicals. The tube sheets and channel covers of these heat exchangers typically consist of a low-alloy steel or Cr-Mo steel base plate with a corrosion-resistant overlay layer of austenitic stainless steel (316L, 347H, or Inconel 625). The overlay layer provides resistance to sulfidation, corrosion, and hydrogen damage while the base material provides mechanical strength.
Crack Characterization and Investigation Methodology
Initial Observation
The test plate exhibited a network of fine cracks in the surface layer of the overlay weld, distributed primarily in the last two to three weld passes. The cracks were approximately 0.05 to 0.3 mm wide and 5 to 30 mm long, oriented both parallel and transverse to the welding direction. The cracks were detected during magnetic particle testing (MT) after the overlay welding was completed.
Metallurgical Analysis
The investigation employed a systematic approach:
- Macroscopic examination: The test plate was sectioned perpendicular to the crack direction and examined at 1X and 5X magnification to determine crack orientation and depth.
- Microscopic examination: Metallographic sections were examined at 100X to 500X magnification to characterize the crack morphology and identify the crack initiation sites.
- Hardness mapping: Vickers hardness measurements were taken across the weld cross-section to identify regions of abnormal hardness that might indicate excessive work hardening or martensite formation.
- Chemical analysis: Spark-OES and wet chemical analysis were performed on the overlay material to verify compliance with the specified composition.
- Residual stress measurement: X-ray diffraction method was used to measure residual stresses in the overlay layer.
Root Cause Analysis
Primary Cause: Excessive Residual Stress
The investigation identified excessive tensile residual stress in the overlay surface layer as the primary cause of cracking. The measured residual stress in the crack-affected region was approximately 450 to 520 MPa, significantly exceeding the typical threshold of 300 to 350 MPa for crack-free austenitic overlay welds.
Contributing Factors
| Factor | Observed Condition | Acceptable Range | Impact |
|---|---|---|---|
| Residual stress | 450-520 MPa | <300 MPa | Primary crack driving force |
| Interpass temperature | 80-120 C | 150-250 C | Excessive cooling rate |
| Preheating | 100 C | 200-300 C | High thermal gradient |
| Welding speed (SAW) | 350-400 mm/min | 200-300 mm/min | High heat input per unit length |
| Dilution ratio | 15-25% | <10% | Carbon enrichment in overlay |
| Carbon content in overlay | 0.08-0.12% | <0.04% | Increased susceptibility to cracking |
Secondary Cause: Carbon Enrichment and Phase Transformation
The chemical analysis revealed that the carbon content in the overlay surface layer was 0.08 to 0.12 percent, significantly higher than the specified maximum of 0.04 percent for 316L overlay. This carbon enrichment resulted from excessive dilution with the base material during the first pass. The elevated carbon content, combined with the high cooling rate (due to inadequate preheating and interpass temperature control), promoted the formation of a small amount of martensite and retained austenite transformation products, increasing the susceptibility to cracking.
Tertiary Cause: Hydrogen Embrittlement
Trapped hydrogen from the welding process, combined with the high residual stress and microstructural changes, contributed to hydrogen-assisted cracking. The low interpass temperature prevented adequate hydrogen diffusion, resulting in hydrogen accumulation at stress concentrations.
Corrective Measures
Process Optimization
The following corrective measures were implemented and verified on a new test plate:
- Increased preheating: Preheat temperature raised to 250 degrees Celsius with uniform heating across the entire plate surface.
- Controlled interpass temperature: Maintained at 200 to 250 degrees Celsius throughout the welding sequence.
- Reduced welding speed: SAW welding speed reduced to 220 to 280 mm/min to increase heat input and slow the cooling rate.
- Additional passes: Two additional surfacing passes added to dilute the carbon-enriched first pass with pure overlay material.
- Post-weld stress relief: Stress relief treatment at 425 degrees Celsius for 2 hours (within the sensitization-free window for austenitic stainless steel).
Verification Results
The corrected test plate showed:
- Residual stress reduced to 180 to 250 MPa
- No cracks detected by MT or PT examination
- Carbon content in the surface layer reduced to 0.03 to 0.05 percent
- Hardness in the overlay layer within the specified range of 180 to 250 HV
- Successful hydrostatic test at 1.5 times the design pressure
FMEA Analysis
Applying the Failure Mode and Effects Analysis (FMEA) methodology to this case:
| Failure Mode | Effect | Severity | Cause | Occurrence | Detection | RPN |
|---|---|---|---|---|---|---|
| Surface cracking | Component rejection, safety risk | 10 | High residual stress | 7 | 8 | 560 |
| Carbon enrichment | Reduced corrosion resistance | 8 | High dilution | 6 | 5 | 240 |
| Hydrogen cracking | Delayed failure in service | 10 | Inadequate hydrogen control | 5 | 6 | 300 |
| Interpass temperature violation | Cracking, hardness variation | 8 | Process control failure | 7 | 7 | 392 |
The highest RPN values indicate that surface cracking due to high residual stress and interpass temperature control failures represent the most critical risks requiring process control measures.
Study Reflection
This case study exemplifies the value of systematic metallurgical investigation in identifying and resolving quality issues in overlay welding. The multi-factorial nature of the cracking problem—combining residual stress, carbon enrichment, hydrogen effects, and inadequate thermal input—demonstrates that overlay cracking cannot be attributed to a single cause. The corrective measures, particularly the emphasis on thermal management (preheating, interpass temperature control) and the addition of dilution-reducing surfacing passes, represent practical solutions that can be applied to similar fabrication challenges. The integration of FMEA analysis provides a structured framework for identifying and prioritizing potential failure modes in overlay welding processes.