How to Eliminate the Problem of Work Hardening in 304 Stainless Steel Tube Processing

Work hardening is one of the methods to enhance the strength of 304 stainless steel tubes. It is particularly important for metals that cannot be strengthened through heat treatment. The high strength of cold-drawn tubes and precision-rolled tubes is the result of work hardening. Moreover, work hardening also creates conditions for cold processing. Because the tubes cannot be further processed once they have hardened to a certain extent during the cold processing, they may break due to deformation under certain conditions. 

Work hardening increases the resistance to deformation, but it also makes further processing of stainless steel products difficult because both its hardness and strength are increasing, while its plasticity and toughness are decreasing. In stainless steel, the work hardening phenomenon is most prominent in austenitic and ferritic stainless steels. 

The tensile strength of austenitic stainless steel tubes after hardening ranges from 1470 to 1960 MPa. Moreover, as the tensile strength increases, the yield limit also rises. The yield limit of the annealed austenitic steel does not exceed 30% to 45% of the tensile strength, while after processing hardening it reaches 85% to 95%. The depth of the processing hardening layer can reach one-third or more of the cutting depth; the hardness of the hardened layer is 1.4 to 2.2 times higher than that of the original. 

Because 304 stainless steel pipes have high plasticity, during plastic deformation, their shapes become distorted and the strengthening coefficient is very large; and the austenite is not stable enough. Under the action of cutting stress, some austenite will transform into martensite. Moreover, the compound impurities are prone to decompose and form a dispersed distribution under the action of cutting heat, resulting in a hardened layer during the cutting process. The work hardening phenomenon produced in the previous process seriously affects the smooth progress of the subsequent processes. 

Therefore, in order to eliminate the hardening effect of stainless steel welding pipes during processing, special heat treatment must be carried out during the processing. After recrystallization softening, the subsequent processing can be continued. After recrystallization, only the shape of the crystal grains has changed, but the crystal lattice type remains the same as the original crystal grains. 

The recrystallization nuclei are generally formed at the grain boundaries of deformed grains, in the slip bands, and in areas with severe lattice distortion. Once the nuclei are formed, they grow by the diffusion movement of atoms towards the surrounding areas until all the nuclei grow to contact each other, forming new equiaxed grains. 

Through recrystallization, the microstructure of the 304 stainless steel tube has undergone a complete transformation. As a result, its strength and hardness have significantly decreased, while its plasticity and toughness have greatly improved. The phenomenon of work hardening has been eliminated, and all mechanical and physical properties of the deformed metal have fully returned to the state before cold deformation.