Hot rolling welding of mechanical inserts in steel
Hot rolling welding of mechanical inserts is a kind of solid phase welding method, which is the result of the common gravitational action between a large number of atoms on the surface of two metal bodies. High temperature heating makes the blade steel and the knife body low carbon steel have good plasticity, so that the two metal bodies are easy to get close under the action of external forces, the contact surface metal atoms are close to the interaction potential between atoms is low, can exchange electrons with each other, produce attractive distance; The surface before binding is in a separate state, and the electronic structure on the surface is localized, and the electronic structure after binding is de-localized to exchange with each other. Therefore, the energy applied in order to achieve the binding should consist of two parts, one is the interatomic binding energy to bring the surface atoms close to the gravitational action, and the other is the activation energy to restore the surface valence electrons to the delocalization. The oxide layer on the metal surface greatly hinders the solid phase bonding, so removing the oxide layer on the metal bonding surface and protecting the fresh clean surface is the key to solid bonding. In the welding flux, borax plays the role of anti-oxidation protection, while sodium fluoride mainly activates surface state iron atoms by reducing reaction. In the process of non-oxidation rolling without solder, the plastic deformation increases the amount of directional deformation at the contact surface bulge, breaks the residual oxide film, which is conducive to providing a clean contact surface, and the friction between the interfaces also provides the surface atomic thermal activation energy.
Most of the atoms on both sides of the contact surface are arranged in different crystalline directions, so the equilibrium distance between atoms cannot be reached when the gravitational force is formed. Theoretical calculation shows that in this case, it is difficult for metal atoms to diffuse each other in the rolling process, and the interstitial atoms such as carbon atoms are the main ones capable of thermal diffusion in this process [10]. It can be said that although a large number of atomic diffusion is conducive to improving the bonding strength, it is not a necessary condition for the early phase of solid bonding. However, during the annealing process after hot rolling welding, recrystallization can make a large number of interface atoms in the same grain, and also provide a way for the mutual diffusion of atoms, and a large number of atoms are strongest after adjusting to the equilibrium spacing Interatomic binding force.
The austenitic homogenization and plastic deformation ability of the blade steel and the cutter body are determined by the heating temperature of hot rolling. Undoubtedly, increasing the heating temperature and increasing the deformation of pressing and rolling are beneficial to the solid phase welding of the joint surface. However, too high temperature and too large reduction will not only cause damage to the microstructure (especially the high alloy steel insert steel machine blade), but also excessively increase the energy consumption and the loss of rolling mill power. When the hot rolling welding is only heated once, the power consumed by too high heating temperature and too large rolling deformation will not be proportional to the improvement of welding strength. Therefore, strict control of the upper temperature limit and reasonable selection of each roll reduction are not only the requirements for obtaining excellent microstructure, but also the needs of reducing energy consumption and rolling power. In addition, the iron powder in the solder is mainly used for filling in the concave of the contact surface, and its melting point is higher than Q235 steel, much higher than the edge steel, if the thickness is too large, pressing the iron powder also needs to consume more pressing work. Actual production shows that excessive flux increases slag inclusion and porosity, and the shear strength of the weld can only reach 150 ~ 200 MPa.
