Thick-wall seamless steel tubes can be divided into three categories based on production methods: hot rolling (extrusion), welding, and cold working. Hot rolled seamless steel tubes are made by perforating solid steel billets or ingots and rolling them into hollow sections. The basic process includes perforation and pipe rolling. Cold worked seamless steel tubes are secondary processing of steel pipes. Cold working is an important method for obtaining high precision and high performance pipes, including cold rolling, cold drawing, cold reducing and cold rotary pressing. Metal seamless pipes for high-alloy steel types are mainly produced by extrusion.
The raw material of large-diameter thick-walled seamless steel tubes is round billets. Its production process is as follows: the billet is heated in a furnace with a temperature of about 1200 degrees Celsius. The fuel used is hydrogen gas or acetylene. The temperature control inside the furnace is a critical issue. After being heated, the round billet is pierced by a piercing machine. The most common piercing machine is a conical roller piercing machine. After piercing, the round billet is successively extruded, rolled or squeezed by three rollers. The extruded pipe needs to be sized and straightened. The sizing machine drills into the billet at high speed with a conical drill to form a thick-wall seamless steel tube. The inner diameter of the pipe is determined by the outer diameter length of the sizing machine drill.
Thick-wall seamless steel tubes enter the cooling tower and are cooled by water spray after sizing. After cooling, the steel pipes need to be straightened. Then they are sent to a metal flaw detector (or hydraulic test) for internal inspection. If there are cracks, bubbles or other issues inside the steel pipes, they will be detected. After quality inspection, thick-wall seamless steel tubes also need to be rigorously hand-selected.
Electric welding
High-frequency welding machines are generally used for large-diameter thick-walled seamless steel pipe fittings. High-frequency welding is a magnetic induction welding method that does not require welding fillers, electrodes, or fluxes. It has advantages such as no electric arc splash, narrow heat-affected zones, elegant appearance, and excellent physical properties. It is widely used in the manufacturing of seamless steel tubes. Based on the skin effect, proximity effect, and eddy current thermal effect of the current and magnetic induction in the conductive medium, the material at the welding edge is partially heated to a molten state. Then, the weld is completed by rolling the material using pressure rollers. After cooling, a solid straight-line weld is formed.
Welding clearance
The hot-rolled strip steel is sent to the welding pipe unit and is extruded into a circular pipe with a gap on one side. The gap of the welding seam is controlled at 1-3mm by adjusting the pressing amount of the squeeze roller, and the two sides of the welding seam are aligned. If the gap is too large, the proximity effect is reduced, and the eddy current heat generation is insufficient, resulting in poor intergranular joining or cracking. If the gap is too small, the proximity effect is enlarged, and the heat generation by electric welding is too high, resulting in burnt welding or the formation of large pits after compression and forming, which harm the surface quality of welding.
Electric welding temperature
The electric welding temperature for high carbon steel materials is 1250-1460 degrees Celsius, which can achieve full welding requirements for wall thicknesses of 3-5mm. The electric welding temperature is primarily controlled by adjusting the high-frequency eddy current heat power and the welding speed. When the heat input is insufficient, the welding edge that is heated cannot reach the electric welding temperature, and the metal structure still remains solid, resulting in incomplete or incomplete welding. When the heat input is excessive, the heated welding edge exceeds the electric welding temperature, resulting in coarse crystals or molten droplets, causing welding holes.
