1. Substandard Chemical Composition: Deviations in the alloy element ratio during billet smelting, with core elements such as carbon, manganese, and chromium exceeding the standard's allowable range, directly lead to substandard mechanical properties.
2. Excessive Non-metallic Inclusions: Insufficient deoxidation during smelting leaves residual oxides, sulfides, and other impurities, forming defects inside the steel pipe. This significantly reduces the material's strength and toughness, creating weak points for stress concentration.
3. Internal Structural Defects: Improper cooling during billet casting leaves unwelded shrinkage cavities, porosity, or component segregation, resulting in uneven internal structure and weakening the overall structural strength.
4. Abnormal Metallographic Structure: Uncontrolled heat treatment processes lead to coarse grains, severe banded structures, and may also leave excessive residual stress, reducing the material's fatigue resistance and corrosion resistance.
5. Substandard Mechanical Properties: Improper parameter settings during heat treatment processes such as quenching and tempering directly result in tensile strength, impact toughness, and other indicators of the steel pipe falling below standard requirements, making it unsuitable for the demands of corresponding operating conditions.
