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Self-reinforcing treatment is a process that applies excessive working pressure to generate residual stresses in the cylinder, thereby optimizing the distribution of working stresses and enhancing its yield strength. The principle involves applying high pressure to the inner wall of the cylinder, causing the inner wall to yield and develop radial residual deformation. Once the pressure is removed, the outer layer of material contracts elastically, which creates compressive stress in the plastically deformed inner layer, thereby resulting in residual compressive stress. Depending on the method used to generate radial force, the commonly employed self-reinforcement techniques in current production include mechanical extrusion, direct hydrostatic pressing, explosive expansion pressing, and solid self-reinforcement. The following describes the principles and characteristics of these methods. I. Principle of the mechanical extrusion method The core of the mechanical extrusion method lies in using a conical mandrel with an interference fit, which slides along the inner wall of the cylinder. During this process, the inner wall of the cylinder undergoes plastic deformation and residual stress due to compression, thereby achieving a self-reinforcing effect. There are mainly three ways to push the mandrel to slide inside the cylinder: First, the mandrel is pressed in using a punch and a hydraulic press; this is an indirect hydraulic method ； Secondly, transmit hydraulic pressure to the back of the mandrel for pushing, that is, the direct hydraulic method ； Thirdly, pulling the mandrel through a mechanical device is known as the mechanical traction method. The advantage of this method is that it is mainly suitable for open cylinders (such as tubes); it offers economic benefits, does not require external molding tools, and the sealing process is relatively simple. The pressure it applies is limited only by the compressive strength of the mandrel material, unaffected by the strength of the cylinder itself, allowing for very high residual stresses to be generated. Under the same radius ratio conditions, the pressure required for the mechanical extrusion method and the hydrostatic method both depends on the amount of inner diameter expansion; however, for materials with the same yield strength, the pressure required for the former is much lower than that for the latter, and greater circumferential residual stress is obtained. This is because the residual stresses generated by the hydrostatic method are prone to reduction due to the Bauschinger effect, whereas the extrusion method involves triaxial deformation, resulting in less reverse yielding of the inner wall and thus higher residual stresses. However, since this method differs from the hydrostatic method in terms of the stress state at high strains, during the self-reinforcement process using mechanical extrusion, the friction between the mandrel and the inner wall, along with the axial component of the normal stress on the contact surface, generates significant axial shear stress on the inner wall of the cylinder. As the axial stress increases, the circumferential stress decreases accordingly. This causes the open-cylinder treated by this method to exhibit a lower yield pressure and reduced elastic strength when subjected to another hydraulic yield test. However, for each radius ratio, there is an optimal superstrain at which the elastic strength limit of the cylinder can be comparable to that achieved by hydrostatic treatment at 100% superstrain. II. Principle of the direct hydrostatic method: The direct hydrostatic method involves applying liquid pressure directly to the inner wall of a cylinder, causing it to undergo plastic deformation and excess strain; once the pressure is removed, residual stresses are formed, thereby increasing the elastic strength limit and fatigue life of the container. As one of the earliest applied and most commonly used self-reinforcing methods, it is widely used in the self-tightening treatment of large and medium-caliber gun barrels, as well as high-pressure and ultra-high-pressure containers and pipelines. The advantage of this method is that its operation procedure is similar to that of hydraulic testing of containers; it features simplicity of operation and high flexibility. No special pressure components are required, and it enables uniform plastic deformation of the container walls, making it particularly suitable for self-reinforcement of closed containers. For open cylinders, they can be treated using end sealing devices. However, the direct hydrostatic method requires ultra-high pressure sources, ultra-high pressure pumps, as well as associated pipelines and accessories, and ultra-high pressure sealing issues become a key factor limiting its application scope. III. Principle of the explosive expansion method: The explosive expansion method utilizes high-energy explosives to generate extremely high pressures in an extremely short time, causing cylinders or tubes to undergo rapid plastic deformation under the action of these high-pressure gases and shock waves. The amount of plastic deformation of the inner wall is related to the strength of the explosive (the weight of explosives per unit length). By precisely controlling the amount of explosive used, the pressure generated by the explosion can be adjusted to meet the requirements for hyper-strain, thereby achieving the desired plastic deformation of the cylinder or tube. A feature of using this method is that end effects must be taken into full consideration; specifically, the amount of plastic deformation at the ends of the cylinder or tube gradually decreases. At the same time, the influence of the radial piston effect also needs to be addressed. The explosive expansion method can generate extremely high pressures in a short time, but it requires strict control over the amount of explosives used as well as high standards for environmental safety. IV. Principle of the solid self-reinforcement method: This method uses a solid medium with good plasticity and a low melting point (such as lead). The medium is melted and poured into a cylinder, after which a pressure rod is used to compress the solid medium, causing it to plastically deform and transmitting pressure to the cylinder walls, which in turn undergoes plastic deformation. This results in the formation of residual stresses. This method is particularly suitable for self-reinforcement in conditions of extremely high pressure. Advantages: This method has multiple benefits: first, it avoids the problem of the liquid medium solidifying under extremely high pressures ； Secondly, solid media are incompressible, allowing them to fully utilize self-reinforcing pressure, with relatively low requirements for such self-reinforcing pressure ； Thirdly, the solid medium has high viscosity after plasticization, which facilitates solving the sealing problem at the ends of the cylinder ； Fourth, it has high safety; even if a weak spot in the wall ruptures, it will not cause serious danger.