Why is a return process necessary during the production of cold-drawn steel pipes

In the production process of cold-drawn steel pipes, the "annealing" step—commonly known as "heating"—is not a superfluous process, but a crucial operation that determines the final performance and subsequent reliability of the cold-drawn steel pipe. To understand why this "heating" is necessary, we must first understand the impact of the cold-drawing process itself on the steel pipe.

Cold drawing is a process in which steel pipes are forcibly stretched and shaped using a die at room temperature. The core purpose is to achieve more precise dimensions, a smoother surface, and increased strength and hardness. However, this "forced shaping" process can lead to two "side effects" for cold-drawn steel pipes: firstly, it generates a large amount of residual stress internally; secondly, it causes "work hardening," making the cold-drawn steel pipe brittle and losing its toughness. Just as a person's body becomes stiff and sore from prolonged tension, the cold-drawn steel pipe is in a state of "over-fatigue," and annealing is the key step that "relaxes" and "rejuvenates" it.

First, annealing effectively eliminates residual stress, preventing deformation and cracking of cold-drawn steel pipes. During cold drawing, the metal flow rate differs between the inner and outer surfaces and the core layer of the steel pipe, resulting in uneven plastic deformation. This uneven stress generates continuous internal "internal friction"—that is, residual stress. These hidden stresses act like time bombs: during subsequent processing (such as cutting and welding), the stress balance is disrupted, and the cold-drawn steel pipe may suddenly deform, exhibit ellipticity, or even crack directly. Even after processing is complete, in long-term use, especially under corrosive environments or alternating loads, residual stress can accelerate fatigue failure and shorten service life. Annealing, by heating the cold-drawn steel pipe to a specific temperature (typically 500-650℃ for steel) and holding it at that temperature for a period of time, allows the metal atoms to gain energy. Through localized plastic deformation or stress relaxation, most of the residual stress is released (eliminating 80%-95%), allowing the internal stress of the cold-drawn steel pipe to tend towards equilibrium, fundamentally ensuring dimensional stability and safe use.

Second, annealing alleviates work hardening and restores the plasticity and toughness of the cold-drawn steel pipe. During cold drawing, the grains of the steel pipe are forcibly elongated and broken, resulting in a distorted crystal structure. This is akin to forcibly shuffling and compressing a neatly stacked pile of building blocks, leading to increased hardness and strength, but a significant decrease in plasticity and ductility—making it both hard and brittle, unsuitable for subsequent bending, stamping, and other forming processes. Even minor collisions during transportation and installation can cause damage. During annealing, the combined effects of temperature and holding time allow these "disordered" broken grains to rearrange and grow, forming uniform equiaxed crystals. Like rearranging disordered building blocks, this process retains some of the strength advantages of cold drawing while restoring plasticity and toughness to appropriate levels. For example, after annealing, the yield strength of Q345B cold-drawn steel pipe decreases by 15%-20%, while the elongation increases by more than 30%, perfectly meeting the "strength-toughness balance" requirements of industries such as machinery manufacturing and the automotive industry.

Furthermore, annealing optimizes the internal structure of cold-drawn steel pipes, improving subsequent processing and performance. Cold-drawn steel pipes often exhibit uneven microstructure, potentially including inconsistent grain size and structural defects, which negatively impact subsequent heat treatment and corrosion resistance. Annealing not only improves microstructure uniformity but also refines the grain size, enhancing the fatigue resistance and stress corrosion resistance of cold-drawn steel pipes. Refined grains effectively inhibit crack propagation, making cold-drawn steel pipes more durable under long-term alternating loads and less prone to intergranular corrosion cracking in corrosive environments. For precision steel pipes requiring multi-pass cold drawing, annealing is crucial: after each cold drawing, annealing restores plasticity before the next drawing round; otherwise, over-hardening can prevent further deformation and even breakage during the drawing process.

In summary, cold drawing is a process of "shaping and hardening," making the cold-drawn steel pipe "stronger" but also "brittle" and containing "internal stress." Annealing, on the other hand, is a process of "relieving pressure and restoring blood," aiming to eliminate internal stress, restore toughness, and optimize the microstructure. This ensures that the cold-drawn steel pipe meets both precision and strength requirements and can be used stably and reliably. This step is not a waste of energy but a "must-do" to guarantee the quality and practicality of cold-drawn steel pipes.