Spiral steel pipes play a vital role in our daily lives. Below, we will introduce the common defects associated with each heat treatment process for spiral steel pipes based on their respective processes.
(1) Defects arising during the heating process. For the heating process, it is essential to select appropriate heat treatment equipment and heating media. Common issues that arise or are prone to occur include the surface of the parts being affected by oxidative heating media, and the heating temperature exceeding 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, feasible measures should be taken to address such defects during the actual process.
(2) Defects occurring during quenching of small-diameter spiral pipes. After heating and austenitizing, parts are cooled to obtain the desired structure and mechanical properties. The selection of an ideal cooling medium is crucial, based on the material and desired hardness of the parts. An ideal cooling medium should provide rapid cooling at high temperatures and slow cooling at lower temperatures (around 300°C). Common cooling media include air, water, oil (mineral, vegetable, etc.), 5%-10% saline water, 5%-15% alkaline water, synthetic coolants, water-quenched oil cooling, water-quenched nitrate cooling, alkaline baths, nitrate baths, chloride salt baths, etc. These media vary significantly in their cooling performance, especially for saline water, alkaline water, oil, alkaline baths, nitrate baths, and chloride salt baths. If the cooling medium deteriorates (ages), its performance will decline, which, if undetected, can become a significant source of defects. Common heat treatment defects include insufficient hardness, soft spots, quenching cracks, and deformation of quenched parts.
(3) Defects arising during the tempering process. Parts are quenched to obtain high-hardness martensitic structures or slightly lower-hardness bainitic structures, but these structures are unstable and highly brittle. To achieve the desired structure and properties for use in production, tempering is necessary. Therefore, tempering process parameters significantly impact the heat treatment quality of parts, including hardness, temper brittleness, temper cracks, and other defects. Effective measures must be taken during tempering to avoid these defects.
(4) Surface quenching defects. While bulk heat treatment ensures that both the interior and exterior of parts achieve the desired hardness and requirements, surface quenching is used solely to harden the surface of parts, leaving the core in its original structural state. Thus, factors such as surface quenching temperature, heating time, and hardened layer depth can affect heat treatment-induced deformation, cracking, hardness levels, and service life.
(5) Defects in the chemical heat treatment of small-diameter spiral pipes. Chemical heat treatment of spiral pipes involves infiltrating metal or non-metal atoms onto the surface of parts to achieve desired surface properties (e.g., high wear resistance). This process imparts dual functionality to the composite material. However, improper process formulations or changes in process parameters can lead to part deformation, cracking, unsatisfactory structure, and inadequate hardness. Therefore, utmost attention should be paid to the chemical heat treatment of parts, as failure to do so negates the purpose of the treatment. Heat treatment of parts should be safe, economical, and practical, with a focus on creating 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 through a comprehensive analysis involving personnel, machinery, materials, methods, procedures, and inspections. By analyzing and judging, the root cause of the defects can be pinpointed.
