Weld integrity under extreme conditions

For structures operating under low-temperature (below -46°C) and high-temperature (>500°C) creep conditions, the requirements for welding materials are drastically different, but both are extremely stringent.Weld integrity under extreme conditions

The following will elaborate on the most critical performance indicators and corresponding testing standards for these two areas.

These structures are commonly found in liquefied natural gas (LNG) storage tanks, cryogenic pipelines, polar vessels, and scientific research facilities. The core threat is brittle fracture.

Most Stringent Performance Indicator: Impact Toughness

●Requirement: The weld metal must possess extremely high Charpy V-notch impact absorption energy at the design minimum temperature (MDMT) to prevent the occurrence and propagation of brittle fracture.

▶ Toughness Transition: Steel and weld metal transition from a ductile to a brittle state as the temperature decreases. At low temperatures such as -46°C, it is essential to ensure that the weld remains within the ductile plateau region.

▶ Stress Concentration: Geometric discontinuities and microscopic defects in welded joints easily lead to stress concentration. High-toughness welds can blunt crack tips and prevent crack propagation through plastic deformation.

▶ Microstructure Sensitivity: The toughness of weld metal is extremely sensitive to its microstructure (such as grain size, inclusions, and second phases). Any minute metallurgical defect can lead to a sharp decrease in toughness.

● Standards: ASTM A370 / ASTM E23 (American Standard), ISO 148-1 (International Standard), GB/T 229 (Chinese National Standard).

⑴ Impact Absorbed Energy: At a specified temperature (e.g., -50°C, -60°C, or even -101°C), the minimum impact energy of a single specimen and the average impact energy of a group of three specimens must meet the requirements of the standard or design specification. For example, in the LNG field, a high impact energy is typically required even at -196°C.

(2) Lateral Expansion: The amount of plastic deformation on the back side of the specimen after fracture. This is another important indicator of toughness, typically requiring ≥ 0.38 mm.

(3) Percentage of Fiber Fracture: The percentage of area occupied by the shear lip, also a direct reflection of toughness.

● Applications: For particularly critical or thick-walled structures, the CTOD test is more accurate than the Charpy impact test in measuring a material’s resistance to crack propagation, providing fracture mechanics design parameters. It measures the critical crack tip opening displacement.

Key Points for Welding Consumable Selection and Control

● Use welding consumables from high-nickel (Ni) alloy systems (such as welding consumables for 3.5Ni, 5Ni, and 9Ni steels). Nickel significantly reduces the ductile-brittle transition temperature.

● Ensure the welding consumable produces a pure, fine weld microstructure (such as fine-grained ferrite and lower bainite).

● Strictly control welding heat input and interpass temperature to avoid grain coarsening and ensure toughness.

These structures are commonly found in power plant boilers, steam pipes, gas turbines, and chemical reactors. The core threat is progressive deformation and fracture under sustained stress and high temperature.

The most demanding performance indicators: Creep strength and creep fracture toughness

● Requirement: The weld metal must be able to resist fracture within its design life (e.g., 100,000 hours/200,000 hours) under the design temperature and working stress.

● Why is it so demanding: At high temperatures, the diffusion capacity of metal atoms increases. Even if the stress is below the yield strength, slow, continuous plastic deformation (creep) will occur, eventually leading to fracture under stress far below the tensile strength. The weld is the weakest link in creep failure.

● Requirement: The weld metal should possess sufficient elongation and reduction of area when creep fracture occurs.

● Why is it stringent: Low-plasticity welds are prone to Type IV cracking during creep, which occurs early failure in the fine-grained region of the heat-affected zone (HAZ) of the base metal. This type of cracking is difficult to detect and extremely dangerous. High creep fracture plasticity allows for a certain degree of deformation and stress redistribution, preventing brittle creep fracture.

● Requirements: The microstructure of the weld metal must remain stable under long-term high temperatures, without excessive phase transformation, coarsening of precipitates, or the formation of harmful phases (such as σ phase or Laves phase).

● Why is it stringent: These microstructure changes significantly accelerate the creep process, leading to premature loss of strength and plasticity.

● Standard: ASTM E139 (Standard Method for Creep, Creep Fracture, and Stress Fracture Tests).

▶ Creep Fracture Strength: The stress value that causes the material to fracture at a specified temperature after a specified time (e.g., 10^5 hours, 10^6 hours). These are the core data for welding consumable evaluation.

▶ Creep fracture life: Time to fracture under given stress and temperature.

▶ Creep fracture elongation/reduction of area: A key indicator of creep fracture plasticity.

● Application: Although it cannot replace long-term creep testing, it provides the yield strength and tensile strength of the material at high temperatures, forming the basis for design and screening.

● Methods: Metallurgical microscopy, scanning electron microscopy, energy dispersive spectroscopy.

● Application: To examine weld specimens after long-term aging or actual service, assessing the evolution of carbides, precipitation of harmful phases, etc., and verifying their microstructure stability.

Key points for welding consumable selection and control

● The composition of the welding consumable must match the base metal and have good resistance to temper softening and microstructure stability (usually containing elements that stabilize carbides such as Mo, V, and Nb).

● For welding dissimilar steels, special care must be taken in selecting welding materials (e.g., nickel-based alloys) to match the different coefficients of thermal expansion and creep behavior.

● The welding process must ensure that the weld and heat-affected zone are defect-free, and that a stable microstructure is obtained through appropriate heat treatment.

Ultimately, for these two extreme operating conditions, the selection of welding materials must be based on rigorous welding procedure qualification, and the test results must meet the mandatory requirements specified in the project design specifications (such as ASME BPVC Section I & VIII, EN 13445, API 620, etc.) for specific service conditions.

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