How to make butt welded joints?

Butt welding refers to the method of placing butt welding gate valves between two clamping devices, aligning their end faces, and applying electric heating to the contact to perform welding. Butt welding requires that the cross-section size and shape of the contact point of the gate valve is the same or similar to ensure uniform heating of the valve contact surface. Butt welding technology is widely used because of its high productivity and easy automation.

Flashing Technology

The main function of the flashing technology is to heat the workpiece. In this stage, first turn on the power and make the two end faces of the workpiece slightly contact to form many contact points. When an electric current passes, the contact points melt and become a liquid metal lintel that connects the two end faces. Due to the extremely high current density in the liquid lintel, the liquid metal in the lintel is evaporated and the lintel is burst. With the slow advancement of the moving clamp, the lintel is constantly produced and blasted. Under the action of vapor pressure and electromagnetic force, liquid metal particles are continuously ejected from the interface to form a rapid stream of sparks-flashes.

During the flashing process, the workpiece is gradually shortened, and the tip temperature is gradually increased. As the temperature of the tip rises, the speed of blasting of the lintel will increase, and the advancement speed of the movable clamp must also be gradually increased. Before the end of the flashing process, a liquid metal layer must be formed on the entire end surface of the workpiece, and the metal must reach plastic deformation temperature at a certain depth.

Due to the strong oxidation of the metal vapor and metal particles generated during the blasting of the lintel, the oxygen content of the gas medium in the interface gap is reduced, so its oxidation ability can be reduced, thereby improving the quality of the joint. But the sparks-flashes must be stable and intense. The stability means that no broken circuit or short circuit occurs during the flashing process. The broken circuit will weaken the self-protection of the welding place, and the joint is susceptible to oxidation. The short circuit will over-burn the workpiece, resulting in the workpiece scrapping. The intensity means that there are quite a few blaster blasts in a unit time. The stronger the sparks-flashes, the better the self-protection of the weld, which is especially important in the later stage of the flashing technology.

Upset Technology

At the end of the flashing process, sufficient top pressure is immediately applied to the workpiece, the gap between the joints is quickly reduced and the blasting of lintel is stopped. That is to say, it enters the upset stage. The function of upset technology  is to seal the gap between the end face of the workpiece and the fire port left by the explosion of the liquid metal lintel. At the same time, the liquid metal and oxide inclusions on the end face are extruded to make the clean plastic metal contact closely, and to produce certain plastic deformation in the joint area, so as to promote the recrystallization, form common grains and obtain a firm joint.

When butt welding ball valve or gate valve, in addition to the general rules of butt welding process, attention should be paid to the influence of shunt and ring deformation elastic force. When the valve is butt welded, the influence of deformation and rebound force must be considered in the selection of upset pressure, but the increase of upsetting pressure is not large because of the effect of shunt on the heating of ring.