Spiral steel pipes play a vital role in our daily lives. Below, based on the heat treatment processes of spiral steel pipes, we will introduce the common defects of each process.
Defects arising during the heating process. For the heating process, it is crucial to select the appropriate heat treatment equipment and heating medium. The issues that commonly arise or are prone to occur include the surface of the parts being affected by oxidizing heating media, and the heating temperature exceeding the process requirements. This can lead to excessively coarse austenitic grains, or even grain boundary melting, significantly impacting the appearance and internal quality of the parts. Therefore, during the actual process, feasible measures should be taken to address such defects through analysis.
Defects arising during quenching of small-diameter spiral pipes. After heating and austenitization, parts are cooled to obtain the desired structure and mechanical properties. Based on the material and specific hardness of the parts, an ideal cooling medium must be selected. An ideal cooling medium rapidly cools at high temperatures and slows down at lower temperatures (300°C). Common cooling media include air, water, oil (mineral oil, vegetable oil, etc.), 5%-10% saltwater, 5%-15% alkaline water, synthetic coolants, water-quenched oil cooling, water-quenched nitrate cooling, alkaline baths, nitrate baths, chloride salt baths, etc. These cooling media vary significantly in their cooling properties, especially for saltwater, alkaline water, oil, alkaline baths, nitrate baths, and chloride salt baths. If there are issues with the cooling medium, its performance may degrade (age), which, if not detected promptly, can become a significant source of defects. Common heat treatment defects include insufficient hardness, soft spots, quenching cracks, and poor deformation of quenched parts.
Defects arising during the tempering process. Parts are quenched to obtain high-hardness quenched martensite or lower-hardness bainite structures, but these structures are unstable and highly brittle. Before use in production, they must be tempered to achieve the desired microstructure and properties. Consequently, tempering process parameters significantly impact the heat treatment quality of parts, such as hardness, temper brittleness, temper cracks, and other defects. Effective measures must be taken during tempering to avoid these defects.
Defects in surface quenching. Full heat treatment of parts ensures that both the interior and exterior achieve the required hardness and specifications. In contrast, surface quenching only hardens the surface of the part, leaving the core in its original structural state. Therefore, factors such as surface quenching temperature, heating time, and hardened layer depth can affect the heat treatment deformation, cracking, hardness level, and service life of the part.
Defects in chemical heat treatment of small-diameter spiral pipes. Chemical heat treatment of spiral pipes involves infiltrating the surface with metallic or non-metallic atoms to achieve desired surface properties (e.g., high wear resistance). This process endows the part with dual functionality as a composite material. However, improper process formulations or altered process parameters can lead to part deformation, cracking, unqualified structure, and inadequate hardness. Thus, careful attention should be paid to the chemical heat treatment of parts, as any oversight could negate its significance. Heat treatment of parts should be safe, economical, and practical, while fostering a cool, clean, and quiet work environment.
Correct heat treatment processes are the prerequisite and foundation for ensuring the qualified heat treatment quality of parts. Once the above-mentioned quality issues are identified, they can be addressed from the perspectives of personnel, machinery, materials, methods, procedures, and inspections. Through analysis and judgment, the root causes of defects can be identified.
