Overlay Welding Repair of Material Inclusion Defects in 16MnReR Low Temperature Pressure Vessels

Literature Overview

This paper by Liu Xiaoxian and Sun Liancheng from Shenyang Construction Machinery Factory Installation Company, published in the journal Welding in 1980, documents a practical case study of overlay welding repair for material inclusion defects discovered in 16MnReR low-temperature pressure vessel spherical tanks. The case involves four 1000 m³ spherical tanks with a diameter of 12.3 m and a working pressure of 1.57 MPa, constructed in 1984. During the non-destructive testing phase, one of the tanks was found to have super-standard material inclusions in the heat-affected zone, necessitating a repair strategy that would maintain the structural integrity and low-temperature performance of the vessel.

Defect Characterization and Assessment

The defect in question was identified during radiographic and ultrasonic testing of the weld seams. Material inclusions, also known as laminations, are planar defects that originate in the steelmaking and rolling process. They appear as elongated voids or oxide inclusions aligned in the rolling direction. In low-temperature pressure vessels, such defects are particularly critical because they can act as stress concentrators and crack initiation sites, especially under cyclic loading or thermal cycling conditions. The 16MnReR steel, being a low-temperature pressure vessel steel, must meet strict requirements for Charpy V-notch impact energy at the minimum design temperature, typically -40°C or lower.

The assessment of the defect severity is governed by the relevant standards, which in this case would be GB 150 (Pressure Vessel Code) and SY/T 0410 (Ultrasonic Testing of Welds in Steel Pipelines). The key parameters for defect acceptance are:

Parameter	Acceptance Criteria	Defect Condition
Defect length	Less than 10% of weld length	Exceeded acceptance limit
Defect height	Less than 10% of plate thickness	Exceeded acceptance limit
Defect depth from surface	Less than 20% of plate thickness	Near-surface inclusion
Location	Not in the weld fusion line	Located in HAZ

Given the severity of the defect and the criticality of the application, the decision was made to perform an overlay welding repair rather than a full replacement of the affected plate section. This approach is economically advantageous and avoids the disruption of the vessel assembly schedule.

Repair Strategy and Welding Procedure

The repair procedure involved several critical steps. First, the defective area was machined to remove the inclusion and the surrounding affected material. The machining was performed to a radius of at least 3 times the plate thickness to minimize stress concentration at the repair boundary. The machined surface was then inspected using magnetic particle testing to ensure that no residual defects remained.

The welding electrode selected for the repair was 1507FcNi, which is a low-hydrogen, high-toughness electrode designed for low-temperature applications. The electrode composition provides a weld metal with excellent low-temperature impact properties, typically exceeding 27 J at -46°C, which is well above the requirements for 16MnReR steel at -40°C. The welding procedure was designed to minimize the risk of hydrogen-induced cracking and to ensure adequate penetration and fusion.

Welding Parameter	Value
Electrode type	1507FcNi
Electrode diameter	3.2 mm and 4.0 mm
Welding current	100-130 A for 3.2 mm; 160-200 A for 4.0 mm
Arc voltage	22-26 V
Preheating temperature	100-150°C
Interpass temperature	Below 150°C
Number of passes	2-3 layers
Post-weld heat treatment	Stress relief at 550-600°C

The preheating temperature of 100-150°C is critical for preventing hydrogen-induced cracking in the heat-affected zone of the 16MnReR steel. This steel has a carbon equivalent (CE) of approximately 0.45-0.50, which places it in the category of steels susceptible to cold cracking. The interpass temperature limit of 150°C ensures that the previous pass cools sufficiently before the next pass is deposited, preventing excessive grain growth and maintaining the toughness of the weld metal.

Quality Control and Verification

After the repair welding was completed, the repaired area was subjected to a comprehensive non-destructive testing regime. The testing sequence included:

Visual inspection: To verify the surface appearance, weld profile, and absence of visible defects such as undercut, porosity, or incomplete fusion.
Magnetic particle testing (MT): To detect surface and near-surface cracks that may have formed during welding due to residual stresses or hydrogen diffusion.
Ultrasonic testing (UT): To verify the internal quality of the repair weld and to confirm that no new defects were introduced during the repair process.
Radiographic testing (RT): To provide a permanent record of the internal quality of the repair weld and to verify the penetration and fusion characteristics.
Dye penetrant testing (PT): As a supplementary method for detecting surface-breaking defects in the machined transition area.

The repair weld was also subjected to mechanical property testing, including tensile testing and Charpy V-notch impact testing at the design temperature. The tensile strength of the weld metal was required to be within the range of 520-640 MPa, and the impact energy at -40°C was required to be no less than 47 J. These requirements are consistent with the specifications for 16MnReR steel in GB 150.

Engineering Practice Integration

This case study highlights several important lessons for engineers involved in pressure vessel repair. First, the decision to repair rather than replace must be based on a thorough assessment of the defect severity and the structural implications. The repair must be designed to restore the original load-bearing capacity and fracture toughness of the component. Second, the welding procedure for the repair must be qualified and documented in accordance with the applicable code requirements. Third, the post-repair inspection must be as rigorous as the original inspection, with no compromise on acceptance criteria.

In modern practice, we would also consider the use of advanced welding technologies such as pulsed GTAW or FCAW with flux-cored wire to achieve better control over the heat input and the dilution ratio. These processes can produce welds with finer microstructures and lower residual stresses, which are advantageous for low-temperature applications. Additionally, the use of finite element analysis to simulate the stress distribution in the repaired area can provide additional confidence in the structural integrity of the repair.

Key Questions and Reflections

A question that arises from this case study is the long-term reliability of the repair weld under cyclic loading. The repair weld introduces a region of different microstructure and residual stress state into the vessel wall, which can act as a preferential site for fatigue crack initiation. In the context of a spherical tank, the stresses are predominantly membrane stresses, which are relatively uniform, but local stress concentrations at the repair boundary can still initiate cracks under cyclic pressure loading. To mitigate this risk, the repair weld should be designed with a smooth transition to the base metal, and the residual stresses should be reduced through post-weld stress relief.

Another reflection is that the use of a nickel-containing electrode (1507FcNi) for the repair weld is a conservative choice that provides excellent toughness but may introduce a slight galvanic coupling issue if the weld metal is significantly more corrosion-resistant than the base metal. In this case, since the vessel is used for storage of liquefied gases rather than aggressive chemicals, this concern is minimal. However, in applications involving corrosive environments, the compatibility of the repair weld metal with the base metal must be carefully evaluated.

Study Insights and Implications

This paper is a valuable contribution to the body of knowledge on pressure vessel repair, particularly for low-temperature applications. The case study demonstrates that material inclusion defects, when properly assessed and repaired, need not result in the scrapping of an entire component. The systematic approach to repair, from defect characterization through welding procedure design to post-repair verification, provides a model for similar repair operations in other industries. The emphasis on the use of appropriate electrode materials and welding parameters is a reminder that repair welding is not simply a matter of filling a defect but requires a careful consideration of the metallurgical and mechanical requirements of the repaired component. The paper reinforces the principle that repair welding must be treated with the same rigor as original fabrication welding, with full documentation and inspection at every stage.