I. Material Defects: Identifying Hidden Dangers at the "Genetic" Level
Material is the foundation of steel pipe performance. If the chemical composition is substandard, such as low or fluctuating levels of key elements like carbon, chromium, and molybdenum, or excessive levels of harmful residual elements like arsenic, tin, and antimony, it can lead to risks like creep and brittle fracture during high-temperature operation.
Identification Method: Rapidly determine major elements using optical emission spectrometry (OES), combined with infrared carbon-sulfur analysis to detect gaseous element content, ensuring compliance with standard grade requirements.
Verification Method: During procurement, verify the Material Certificate (MTC) issued by the steel mill to confirm consistency in smelting process and composition, preventing the use of inferior materials (e.g., using ordinary 20# steel to impersonate 20G boiler tubes).
II. Surface Defects: Visible but Unignorable Risks
Surface problems are often caused by improper rolling, heating, or handling. Although not deeply hidden, they can easily become corrosion initiation points or stress concentration points.
Common types: Cracks (straight or spiral), folds (metal layers pressed in), scars (localized pits), scratches (transportation marks), residual oxide scale (affecting coating adhesion).
Identification methods: Visual inspection under sufficient light, using a 10x magnifying glass to observe minute defects; for ferromagnetic steel pipes, magnetic particle testing (MT) can be used to reveal surface cracks; for stainless steel, penetrant testing (PT) is used to assist in judgment.
III. Dimensional deviations: Key factors affecting assembly and sealing. Geometric accuracy directly affects installation compatibility and system sealing; any deviation can lead to leakage or connection failure.
Main problems: Outer diameter fluctuations, uneven wall thickness, excessive ellipticity, and excessive curvature (commonly known as "gooseneck bend"). Identification methods: Use micrometers and vernier calipers to measure the outer diameter and wall thickness at multiple points and record extreme values; use ultrasonic thickness gauges for non-destructive continuous thickness measurement; use laser diameter gauges to monitor the outer diameter and ellipticity online with an accuracy of 0.01mm; use a ruler and fine line method to measure the curvature per meter, with a standard requirement of ≤2mm/m.
IV. Substandard Mechanical Properties: Hidden Load-Bearing Capacity Crisis
Even if the appearance is intact, uncontrolled heat treatment processes can lead to insufficient strength and poor toughness, posing a risk of sudden fracture.
Core Indicators: Tensile strength, yield strength, elongation, hardness, impact toughness.
Identification Methods:
Tensile Test: Determines the ultimate load-bearing capacity of a material under tensile force;
Hardness Test (Bristol/Rockwell): Quickly assesses the effect of heat treatment;
Impact Test: Tests resistance to brittle fracture at low temperatures, especially suitable for pipes used in cold regions;
Hydraulic Pressure Test: Holds pressure for more than 30 minutes to check for leaks or plastic deformation, directly verifying pressure resistance.
V. Internal Structural Defects: Highly Concealed but Extremely Harmful
These problems cannot be determined by visual inspection and must be detected using non-destructive testing and metallographic analysis.
Common Types: Inclusions and porosity (smelting residues), delamination and internal cracks (unwelded steel billets), intergranular corrosion (caused by stainless steel sensitization). Identification methods:
Ultrasonic testing (UT): The primary method, capable of detecting area-type defects such as internal cracks, inclusions, and delamination. It has high sensitivity and is suitable for full-body scanning;
Radiographic testing (RT): Provides intuitive imaging of volumetric defects such as porosity and inclusions, but has lower sensitivity to crack-type defects;
Metallographic analysis: After sampling, observe grain size, non-metallic inclusions, decarburized layer, phase distribution, etc., to determine whether the heat treatment is appropriate, and rate according to GB/T 6394 and GB/T 10561.
