Zhuojin Pipe Fitting Co., LtdCrack Analysis of Reducing Cold Oil Tee in Hydrogenation Unit
Literature Overview
This paper by Liu Wenzhong from Sinopec Cangzhou Branch, published in "Petrochemical Corrosion and Protection" (Vol. 21, Issue 3, 2004, pp. 30-32), presents a detailed failure investigation of a reducing cold oil tee that developed cracks after only 10 months of service in the feed cold oil line of a gasoline and diesel hydrocracking/hydrogenation unit. The tee was fabricated from 0Cr18Ni9Ti (equivalent to austenitic stainless steel 321), and the analysis reveals that the cracking was caused by a combination of stress concentration from rough internal surface machining, residual stress from cold working, and the presence of a cast microstructure in the material. The paper concludes that fatigue fracture was the primary failure mechanism, exacerbated by environmental factors including moisture ingress.
Failure Mechanism and Root Cause Analysis
The investigation followed a systematic approach combining visual inspection, chemical analysis, mechanical property testing, metallographic examination, and scanning electron microscopy (SEM) of the fracture surface. The key findings are summarized below:
- Chemical composition: The material composition of 0Cr18Ni9Ti met the requirements of GB/T 12771 and GB/T 13296 for austenitic stainless steel pipe fittings, with adequate chromium (17-19%) and nickel (8-11%) content and sufficient titanium stabilization.
- Mechanical properties: Tensile strength, yield strength, and elongation values were within the specified ranges for the material grade, indicating that the base material properties were not deficient.
- Microstructural examination: Metallographic analysis revealed the presence of a cast microstructure in the tee body, indicating that the fitting had not undergone adequate hot working or forging to refine the grain structure. The grain boundaries were coarse and contained inclusions and segregation bands.
- Residual stress measurement: X-ray diffraction residual stress analysis showed significant tensile residual stresses on the internal surface of the tee, attributed to the cold working performed during internal machining.
- Surface roughness: The internal surface of the tee exhibited a surface roughness (Ra) significantly higher than the recommended value for fluid-carrying pipe fittings, creating localized stress concentration points.
- Fracture surface analysis: SEM examination of the crack initiation site revealed fatigue striations and a crack initiation zone consistent with fatigue failure. The crack propagated from the internal surface outward, with secondary crack branching indicating mixed-mode loading.
The following table presents the key test results and their implications:
| Test Method | Result | Implication |
|---|---|---|
| Chemical analysis (C, Si, Mn, P, S, Cr, Ni, Ti) | Conforms to GB/T 12771 | Material composition adequate |
| Tensile test | Rm = 520 MPa, Rp0.2 = 210 MPa, A = 38% | Meets specification requirements |
| Hardness test | HV = 185 | Within normal range for annealed austenitic SS |
| Metallography | Cast structure, coarse grain, segregation bands | Inadequate hot working, poor ductility |
| Residual stress (XRD) | +180 MPa (tensile) on internal surface | Cold working induced, promotes cracking |
| Surface roughness (Ra) | 3.2-6.3 um on internal surface | Excessive, causes stress concentration |
| SEM fracture analysis | Fatigue striations, internal crack initiation | Fatigue fracture mechanism confirmed |
| Corrosion examination | No pitting or intergranular corrosion | Corrosion not primary failure driver |
Contributing Factors and Failure Sequence
The failure sequence can be reconstructed as follows. The reducing tee was manufactured from a casting or extrusion billet that retained a coarse cast microstructure. During fabrication, the internal surface was machined to achieve the required geometry, but the machining process was classified as cold working, which introduced high tensile residual stresses on the internal surface. The surface roughness left by the machining process further amplified the local stress concentration.
During operation, the cold oil line was subjected to cyclic thermal and pressure loading. The combination of:
- Residual tensile stress from cold working
- Stress concentration from surface roughness
- Coarse cast microstructure with reduced fatigue resistance
- Cyclic thermal and pressure loading
- Potential moisture ingress from inadequate dehydration of the process fluid
...created conditions favorable for fatigue crack initiation and propagation. The crack initiated at a point of maximum stress concentration on the internal surface, propagated through the wall thickness, and eventually led to a through-thickness crack that was detected during routine inspection.
The paper also notes that the environmental conditions in the cold oil line, including the presence of hydrogen sulfide and moisture, may have contributed to hydrogen embrittlement or stress corrosion cracking, although these mechanisms were secondary to the fatigue failure.
Recommendations and Engineering Implications
Based on the investigation findings, the paper recommends the following corrective and preventive actions:
- Replace all tees from the same batch: Since the failure was attributed to material and manufacturing defects, all fittings from the same production batch should be replaced with fittings that have undergone proper heat treatment (solution annealing) and have verified microstructure and residual stress levels.
- Improve material quality control: Incoming material inspection should include metallographic examination to verify the absence of cast structure, and residual stress measurement should be performed on critical fittings after fabrication.
- Enhance internal surface finish: The internal surface roughness of pipe fittings should be limited to Ra <= 1.6 um for critical service applications, achieved through proper machining techniques and, where necessary, internal polishing or honing.
- Post-weld and post-machining heat treatment: For austenitic stainless steel fittings, solution annealing at 1050-1100 degrees C followed by rapid cooling should be performed after any cold working operation to relieve residual stresses and restore full ductility.
- Strengthen process fluid dehydration: The dehydration of the process fluid in the cold oil line should be enhanced to minimize moisture ingress, which can promote stress corrosion cracking and hydrogen embrittlement in austenitic stainless steel.
From a broader engineering perspective, this case study highlights the importance of considering the entire manufacturing chain, from raw material supply to final fabrication, when evaluating the reliability of pipe fittings in critical process applications. The failure of a single tee after only 10 months of service represents a significant reliability shortfall, and the root causes identified (cast structure, cold working residual stress, and surface roughness) are all controllable through proper quality management practices.
In the context of hydrocracking and hydrogenation units, where the process fluids contain hydrogen and sulfur compounds, the selection and qualification of pipe fittings is of paramount importance. Engineers should ensure that all fittings are manufactured in accordance with recognized standards such as ASME B16.9, ASTM A403, or ASTM A860, and that the manufacturing processes include adequate heat treatment, NDT, and dimensional inspection. The lessons from this failure should be incorporated into the procurement specifications and quality assurance plans for similar projects.
In conclusion, the cracking of the reducing cold oil tee was a multifactorial failure caused by inadequate material quality, excessive cold working residual stress, and poor surface finish, all of which contributed to fatigue crack initiation under cyclic loading. The case underscores the necessity of comprehensive material and manufacturing quality control for pipe fittings in critical process applications, and the recommendations provided in the paper offer practical guidance for preventing similar failures in the future.