Regardless of the heat treatment process adopted, whether normalizing, annealing, tempering, quenching, or others, steel tubes undergo fundamental processes of heating, soaking, and cooling during heat treatment, all of which may lead to defects in the tubes. The heat treatment defects of steel tubes primarily encompass unsatisfactory microstructure and properties, oversized dimensions, surface cracks, scratches, severe oxidation, decarburization, overheating or overburning, as well as surface oxidation during protective gas heat treatment.
Unsatisfactory microstructure and properties of steel tubes: During heat treatment, incorrect heating temperatures, unreasonable soaking times, or excessively fast or slow cooling rates can cause the properties of steel tubes to fail to meet requirements. To address this, firstly, when formulating the heating process, it is essential to thoroughly consider the influence of alloy elements in steel, heating temperatures, and the original microstructure and dimensions on the austenitic transformation of steel. Secondly, establish the heating temperature for steel tube heat treatment based on the iron-carbon equilibrium diagram. Thirdly, clarify the heat treatment method, heating temperature, tempering temperature, and cooling rate. After formulating the process plan, it must be verified through small-batch production before mass production commences.
Unsatisfactory dimensions of steel tubes: After heat treatment, the dimensions of steel tubes may undergo significant changes in some cases, including changes in outer diameter, ovality, and bending. Outer diameter changes often occur during quenching, as the primary microstructure transforms into martensite and bainite, resulting in volumetric changes that increase the outer diameter. To reduce this change, a sizing process is often added after the tempering step. Ovality changes typically occur at the ends of steel tubes, primarily due to prolonged high-temperature heating of large-diameter thin-walled tubes. To prevent ovality changes, it is crucial to ensure a reasonable heating system. Even with a reasonable heating system, if the D/S ratio is too large, it can cause the tube to "collapse," resulting in an "unround" end. In such cases, ensuring that the steel tube rotates while being heated can prevent this issue.
Numerous factors influence bending, primarily uneven heating and cooling, especially inconsistent cooling rates along the longitudinal or transverse sections during quenching. Generally, bent steel tubes can be straightened using a straightening machine.
Surface cracks in steel tubes: Excessive thermal stresses during heat treatment can cause surface cracks in steel tubes, primarily due to excessively fast heating or cooling rates. During heating of alloy thick-walled steel tubes, if the furnace temperature is too high, the rapid heating of the tube upon entering the furnace can create a significant temperature difference between the surface and internal metals, generating thermal stresses. When these stresses reach the material's ultimate tensile strength, surface cracks appear.
Due to the nature of quenching, the likelihood of surface cracks is relatively high during metallographic quenching of steel tubes. The presence of non-metallic inclusions, compositional segregation, and microstructural segregation in steel tubes can increase the possibility of quenching cracks. To mitigate heat treatment cracks in steel tubes, on the one hand, it is necessary to formulate heating and cooling systems specific to the steel type, selecting suitable quenching media. On the other hand, tempered or annealed quenched steel tubes should be promptly treated to eliminate internal stresses.
Scratches and bruises on the surface of steel tubes: These defects primarily arise during heating in the furnace or after heating, within quenching equipment, or during roller conveyor transport, due to collisions or abrasions between the steel tube and contacting tools or workpieces. To prevent these defects, while ensuring the normal operation of heating equipment, minimize the relative sliding speed between steel tubes, workpieces, tools, and rollers, reducing opportunities for collisions.
In summary, whether it's the heating of billets before perforation for hot-rolled seamless steel tubes, reheating of rough tubes before sizing (reducing) after rolling, or intermediate annealing of cold-rolled (drawn) steel tubes, improper design and control of heating process parameters can lead to quality defects such as uneven heating, oxidation, decarburization, heating cracks, overheating, or overburning in billets (steel tubes), ultimately affecting the quality of the steel tubes. Therefore, it is imperative to strengthen quality control in all aspects of billet (steel tube) heating.
