Failure Analysis and Quality Control of Oil and Gas Pipe Fittings

A flange fracture and an elbow cracking incident occurred at an oilfield station. To determine the causes of these failures, physicochemical testing and microstructural analysis were conducted on the failed components. The results showed that the flange's chemical composition and hardness did not meet the requirements for Grade 304 stainless steel specified in JB 4728—2000. The flange also exhibited insufficient toughness, leading to brittle fracture at the thinnest section of the flange neck under low-temperature operating conditions. In contrast, the chemical composition and hardness of the elbow base material met the requirements for Grade 2205 duplex stainless steel specified in ASTM A790/A790M—2016. However, microstructural examination revealed decarburization in the regions surrounding cracks at both ends of the elbow. This decarburization had occurred before heat treatment and was exacerbated by the high temperatures during subsequent processing. Under operating pressure, the cracks continued to propagate, ultimately leading to failure.

To prevent similar incidents and improve pipe fitting quality, measures were proposed in the following areas:

• Procurement and supplier management
• Manufacturing process control
• Factory acceptance and delivery
• Incoming inspection and warehousing
• Personnel training

These measures aim to standardize safety management throughout the entire life cycle of pipe fittings and effectively reduce the risk of failure.

Oil and gas stations are key facilities in the oil and gas industry, performing critical operations such as dehydration, hydrocarbon removal, desulfurization, compression, and throttling. Their safe, reliable, and long-term operation is essential to national energy security, personnel safety, environmental protection, and economic performance. The complex piping systems and storage vessels at these stations handle oil and gas media under high-temperature and high-pressure conditions, with potential hazards associated with flammability, explosiveness, corrosivity, and toxicity. Pipe fittings, including flanges, elbows, tees, reducers, and caps, serve as essential connection points in these systems. Their quality and reliability are fundamental to the safe operation of oil and gas stations.

Even minor defects in the materials, manufacturing, inspection, or installation of pipe fittings can develop into serious safety hazards under actual operating conditions, potentially resulting in leaks, ruptures, or even explosions. As oil and gas exploration and production extend into increasingly challenging environments, including deep and ultra-deep formations, high-sulfur reservoirs, and deepwater fields, and operating facilities remain in service for longer periods, more stringent requirements are being placed on the performance, quality assurance, and quality control of pipe fittings used under harsh service conditions.

Furthermore, pipe fitting quality control faces several challenges, including:

• Discrepancies between design specifications and actual operating conditions
• Variations in supplier capabilities
• Inadequate monitoring of critical manufacturing processes, including material selection, heat treatment, welding, and nondestructive testing (NDT)
• Inconsistent enforcement of inspection standards
• Inadequate traceability of quality records
• Insufficient protection during storage and installation

Failures in any of these areas can introduce latent defects that remain undetected until operation, posing potential risks to facility safety. Accordingly, this paper investigates the causes of two typical pipe fitting failures and proposes quality control measures covering the entire life cycle of pipe fittings. The findings are intended to provide technical and managerial guidance for improving the inherent safety of oil and gas facilities and ensuring safe, stable, and reliable long-term operation.

A flange failure occurred in the cryogenic unit of a natural gas processing facility. The failure occurred at the flange downstream of the bypass valve on the overhead gas pipeline connecting the de-ethanizer tower to the cold box. The failed component was a raised-face (RF) weld-neck flange made of 304 stainless steel (06Cr19Ni10), with a nominal diameter of DN80 and a pressure rating of Class 300.

The flange had a design pressure of 2.1 MPa and a design temperature of −80°C. During operation, the pressure was 1.36 MPa, and the temperature ranged from −80°C to −20°C. The flange was manufactured in accordance with HG/T 20615—2009 (Steel Pipe Flanges) and JB 4728—2000 (Stainless Steel Forgings for Pressure Vessels).

Examination of the flange fracture surfaces revealed that the flange had fractured into three pieces along radial paths. The circumferential fracture surfaces were flat and smooth, with no obvious plastic deformation. However, they exhibited a stepped, uneven pattern, indicating multiple crack initiation sites, which is characteristic of macroscopic brittle fracture (Figure 1). Scanning electron microscopy using a TESCAN VEGA III revealed point-like features at the crack initiation sites and radial propagation marks. The fracture surfaces also exhibited a rock-candy-like intergranular morphology (Figure 2).

Figure 1 Macroscopic morphology of the flange fracture

(a) 100x

(b) 200x

(c) 500x

Figure 2 Microscopic morphology of the flange fracture surface

The chemical compositions of the flange neck and flange disc were analyzed using an ARL 4460 direct-reading spectrometer. The results showed that the C, Mn, Cr, and Ni contents did not meet the requirements for 304 stainless steel specified in JB 4728—2000 (Table 1). In addition, the hardness of the flange neck and flange disc was measured using a BH3000 Brinell hardness tester. The measured hardness values exceeded the limits specified in JB 4728—2000 for 304 stainless steel (Table 2).

Table 1. Chemical Composition Test Results (Mass Fraction, %)

Table 2. Hardness Test Results

The test results indicate that the flange did not meet the applicable quality requirements. The elevated C and Mn contents increased the material's hardness and brittleness, while the reduced Cr and Ni contents raised its ductile-to-brittle transition temperature, thereby reducing its toughness at low temperatures.

Regarding the service environment, the flange was installed in the cryogenic section of an ethane recovery unit, where the operating temperature ranged from −80°C to −20°C. Under these low-temperature conditions, the material was more susceptible to brittle fracture, while exposure to the cryogenic medium imposed additional thermal loads on the flange. From a structural perspective, the flange neck represents a geometric discontinuity where stress concentrations can develop under internal pressure. The thinnest section of the neck wall therefore constituted a critical load-bearing region. Under the combined effects of the cryogenic environment, operating loads, and additional external loads, the flange ultimately experienced brittle fracture under relatively low stress.

A bypass elbow connected to a check valve at a pigging station developed a crack and began leaking. The failed component was a 90° seamless elbow made of 2205 duplex stainless steel, with an outer diameter of 60 mm and a wall thickness of 4 mm (Φ60 mm × 4 mm).

The elbow had a design pressure of 16 MPa and a design temperature of 40°C. Its operating pressure was 11.79 MPa, and its operating temperature was 28°C. The elbow was manufactured in accordance with ASTM A790/A790M—2016 (Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe) and ASME B16.9—2012 (Factory-Made Wrought Buttwelding Fittings).

Inspection of the cracked elbow revealed longitudinal cracks on the inner surface at the upstream and downstream ends, measuring 30 mm and 5 mm in length, respectively. The remaining inner and outer surfaces were smooth and uniform, with no obvious abnormalities (Figure 3).

Cross-sectional metallographic examination of the cracks was conducted using an OLS 4100 laser confocal microscope. The results showed that the base metal consisted of ferrite and austenite, with an appropriate ferrite fraction and no harmful precipitated phases. Both cracks originated at the inner surface, and numerous secondary cracks were observed around the main cracks. Decarburization was evident in the microstructure around the crack roots and middle sections of both cracks, whereas no obvious decarburization was observed near the crack tips (Figure 4).

Figure 3 Macroscopic morphology of the cracked elbow

Figure 4 Metallographic morphology of the cracks

The chemical composition of the elbow was analyzed using an ARL 4460 direct-reading spectrometer and a LECO TC600 oxygen/nitrogen analyzer. The results met the requirements for Grade 2205 duplex stainless steel specified in ASTM A790/A790M—2016 (Table 3). Hardness testing was performed using an RB2002T Rockwell hardness tester, and the measured values also complied with the requirements of ASTM A790/A790M—2016 for Grade 2205 (Table 4).

Table 3. Chemical Composition Test Results (Mass Fraction, wt.%)

Table 4. Hardness Test Results

Note: I standardized the specimen location names for clarity and retained the original hardness values and specification limit.

The test results revealed no abnormalities in the chemical composition, hardness, or microstructure of the elbow base metal. However, decarburization was observed in the microstructure surrounding the cracks at both ends of the elbow. Decarburization occurs when carbon in steel reacts with oxygen or hydrogen at elevated temperatures to form gaseous products, such as carbon monoxide or methane, thereby reducing the carbon content in the surface layer.

Given that the elbow operated at 28°C, decarburization could not have occurred under normal service conditions, suggesting that the cracks originated during manufacturing. The solution treatment temperature for the 2205 duplex stainless steel elbow was (1075 ± 15)°C, with a soaking time of at least 30 min. Therefore, the cracks were likely present before heat treatment, during which exposure to elevated temperatures caused decarburization in the surrounding material.

During subsequent operation at a pressure of 11.79 MPa, stress concentrations developed at the original crack tips, causing the cracks to propagate toward the outer surface. The absence of decarburization at the crack tips suggests that crack propagation continued after heat treatment, ultimately reducing the elbow's load-bearing capacity to the point of failure.

Both failures were quality-related incidents caused by nonconforming products, with their root causes originating during manufacturing.

Flange manufacturing typically involves the following stages:

50. Raw material preparation (steel ingot)
50. Forging
50. Machining
50. Surface treatment
50. Final inspection

In this case, the failure originated at the raw material preparation stage, where nonconforming materials were used. Specifically, the Cr and Ni contents were approximately 20% and 70% below the respective minimum levels specified by the applicable standards. Given the relatively high cost of these alloying elements, the use of substandard materials may have been intended to reduce manufacturing costs. The material nonconformity was not detected during incoming inspection or acceptance.

Elbow manufacturing generally involves the following stages:

50. Preparation of straight-pipe raw materials
50. Cutting
50. Push forming
50. Sizing
50. Heat treatment
50. Surface treatment
50. Final inspection

In this case, the cracks likely formed before heat treatment, potentially during raw material preparation, cutting, push forming, or sizing. They remained undetected during subsequent factory inspection, incoming inspection, and installation.

Lessons learned from these failure cases highlight several deficiencies in current pipe fitting quality control:

Unclear technical requirements during procurement: Critical parameters are not adequately specified, resulting in products that fail to meet performance requirements.

Inadequate supplier qualification: Supplier selection lacks standardized procedures, allowing suppliers with insufficient technical capabilities to enter the supply chain and deliver nonconforming products.

Insufficient incoming inspection and inventory management: Inadequate inspection and warehousing controls make it difficult to identify and isolate nonconforming products effectively.

Limited personnel competence and quality awareness: Personnel lack the necessary technical expertise and quality awareness, quality oversight is inadequate at multiple stages, and systematic mechanisms for reviewing past failures and driving continuous improvement are lacking.

Therefore, pipe fitting quality control should prioritize strict source control and extend throughout the entire life cycle, from design and procurement to manufacturing, construction, and installation.

Supplier Qualification and Approval: Establish a rigorous supplier evaluation system covering qualifications, quality management systems, production capacity, technical capabilities, supply history, reputation, and after-sales service. Maintain an approved supplier list and restrict procurement to qualified suppliers.

Technical Specifications: Develop clear and comprehensive procurement specifications that define applicable standards, chemical composition, dimensional tolerances, inspection and testing requirements, product marking, packaging and transportation, and quality documentation.

Tendering and Bid Evaluation: Clearly specify quality requirements and technical criteria in tender documents. During bid evaluation, prioritize suppliers' quality assurance capabilities, past performance, and compliance with technical requirements rather than selecting suppliers based on price alone.

Contractual Quality Requirements: Clearly define quality requirements, acceptance criteria, quality responsibilities, nonconformity handling procedures, rights to conduct manufacturing surveillance and inspections, quality documentation submission requirements, and liabilities for breach of contract in procurement contracts.

1) Raw Material Control: Suppliers must provide complete material quality certificates for all raw materials, including plates, bars, forgings, and castings. The chemical composition and mechanical properties of materials must be verified through testing as required, and full traceability must be maintained for heat numbers, batch numbers, and heat treatment lot numbers.

2) Manufacturing Process Control: Suppliers shall establish documented control procedures for key manufacturing processes and provide process qualification records. Critical processes, including heat treatment and welding, shall be closely monitored to ensure that all process parameters meet specified requirements.

3) Manufacturing Surveillance and Third-Party Inspection: On-site surveillance shall be conducted for critical pipe fittings. Inspectors shall witness key operations and review documentation covering raw materials, manufacturing processes, critical production stages, and final inspections and tests in accordance with applicable specifications and inspection and test plans. Alternatively, qualified third-party inspection agencies may be engaged to conduct in-process spot checks and final inspections.

4) Inspection and Test Plan (ITP): Suppliers shall submit a detailed inspection and test plan before manufacturing begins, clearly identifying inspection points, inspection methods, acceptance criteria, and applicable standards.

5) Non-Destructive Testing (NDT): NDT shall be performed in strict accordance with applicable standards and technical specifications. NDT personnel shall hold valid qualifications and certifications. Test results shall be clearly legible, and all NDT reports shall be complete, accurate, and valid.

1) Final Inspection: Compile and review all manufacturing records, inspection reports, test reports, raw material certificates, NDT reports, heat treatment records, and quality certificates. Conduct final visual inspections, dimensional spot checks, marking and identification checks, and packaging and protection inspections.

2) Quality Documentation: Suppliers shall provide a complete, authentic, and traceable product quality certificate containing all essential product data and test results. All documents shall be clear and complete, bear the required signatures and stamps, and comply with the applicable technical specifications.

3) Marking and Traceability: Pipe fittings shall bear clear, durable markings, such as stamped markings or labels, that include, at a minimum, the manufacturer's name or trademark, material grade, size and dimensions, pressure rating, heat number, batch number or serial number, and applicable standard number. All markings shall correspond to the quality documentation to ensure full traceability from raw materials to finished products.

4) Packaging and Transportation: Appropriate packaging shall be used to prevent damage during handling and transportation, with particular attention to protecting flange sealing surfaces, threads, and protective coatings.

1) Receiving and Unpacking Inspection: Verify the delivered goods against the packing list and purchase order, and check the outer packaging for integrity and signs of transport damage. Conduct a preliminary visual inspection for rust, deformation, physical damage, and unclear or illegible markings. Verify that all accompanying documents, including quality certificates, certificates of conformity, and operating manuals, are complete and valid.

2) Incoming Inspection and Acceptance: Conduct a detailed inspection before accepting the goods into inventory, in accordance with applicable technical specifications and standards. Verify product markings against the quality documentation, check critical dimensions and other specified parameters, and assess visual quality. Complete a detailed incoming inspection record. Any nonconformities shall be promptly addressed in accordance with established nonconformance control procedures.

3) Warehouse Management: Establish designated storage areas for accepted goods and implement classified storage and zoning to prevent material mix-ups. Maintain appropriate warehouse temperature and humidity conditions to prevent corrosion and other damage to pipe fittings.

1) Personnel Training: Provide appropriate training to all personnel involved in pipe fitting quality control, including design, procurement, manufacturing surveillance, inspection, warehousing, and construction. Training shall cover applicable standards, technical specifications, quality requirements, inspection methods, and safety practices. Strengthen personnel's quality awareness, sense of responsibility, and ability to identify and manage risks.

2) Continuous Improvement: Regularly collect and analyze quality issues identified during the manufacturing, installation, and operation of pipe fittings. Summarize lessons learned and use the findings to update and improve quality control standards, procedures, and supplier evaluation systems. Keep abreast of developments in industry technologies, standards, and materials, and continuously improve quality control practices.

1. The chemical composition and hardness of the flange did not meet the requirements for Type 304 stainless steel specified in JB 4728-2000, resulting in inadequate material toughness. Under the combined effects of low-temperature service conditions, operating loads, and additional loads, low-stress brittle fracture occurred at the thinnest section of the flange neck wall.
2. The chemical composition and hardness of the elbow base material met the requirements for UNS S32205 duplex stainless steel specified in ASTM A790/A790M-2016. However, decarburization was observed in the microstructure surrounding the cracks at both ends of the elbow. This indicates that the cracks originated as manufacturing defects and that decarburization occurred during high-temperature heat treatment. During subsequent service, the cracks propagated under operating pressure, ultimately leading to failure.
3. Both the flange and elbow failures were attributed to product quality defects associated with inadequate controls in procurement, supplier selection, and acceptance inspection. To address these issues, targeted measures were proposed to strengthen procurement and supplier management, manufacturing process control, factory acceptance and shipment, receiving inspection and warehouse management, and personnel training. These measures aim to improve pipe fitting quality and ensure the safe, stable, and reliable operation of oilfield production systems.