How to solve and eliminate the problem of work hardening of 304 stainless steel pipes?

After cold processing, the grains of 304 stainless steel pipes are flattened and elongated, the lattice is distorted, and the grains are deformed, which reduces the plasticity of the metal and increases the strength and hardness. This phenomenon is called work hardening.

Work hardening is one of the methods to improve the strength of 304 stainless steel pipes, which is especially important for metals that cannot be strengthened by heat treatment methods. The high strength of cold-drawn pipes and precision-rolled pipes is the result of work hardening. In addition, work hardening also creates conditions for cold processing. Because the pipe cannot be further processed after hardening to a certain extent during the cold processing process, it will break due to deformation under certain conditions.

Work hardening increases the deformation resistance, but it also brings difficulties to the continued processing of stainless steel product pipes, because its hardness and strength are increasing, while its plasticity and toughness are decreasing. Among stainless steels, the work hardening phenomenon of austenitic and ferritic stainless steels is the most prominent.

For example, the tensile strength of austenitic stainless steel pipe after hardening reaches 1470-1960 MPa, and the yield limit increases with the increase of tensile strength; the yield limit of austenite in the annealed state does not exceed 30%-45% of the tensile strength, and reaches 85%-95% after work hardening. The depth of the work hardening layer can reach 1/3 or more of the cutting depth; the hardness of the hardened layer is 1.4-2.2 times higher than the original.

Because the plasticity of 304 stainless steel pipe is large, the lattice is distorted during plastic deformation, and the strengthening coefficient is large; and the austenite is not stable enough. Under the action of cutting stress, part of the austenite will transform into martensite; in addition, the compound impurities are easy to decompose and disperse under the action of cutting heat, so that a hardened layer is produced during cutting. The work hardening phenomenon generated by the previous process seriously affects the smooth progress of the subsequent process.

Carry out subsequent processing. After recrystallization, only the shape of the grains has changed, but the lattice type has not changed, and it is still the same as the original grains.

The recrystallized nuclei are generally formed at the grain boundaries or slip bands of the deformed grains and in places where the lattice distortion is serious. After the nuclei are formed, they rely on the diffusion movement of atoms to grow to the surrounding areas until the nuclei grow to contact each other and form new equiaxed grains.

Through recrystallization, the microstructure of 304 stainless steel pipes has undergone a thorough change, so its strength and hardness are significantly reduced, while the plasticity and toughness are greatly improved, the work hardening phenomenon is eliminated, and all the mechanical and physical properties of the deformed metal are restored to the state before cold deformation.