In the welding process of high-frequency steel pipe, due to the combination of skin effect, proximity effect and heat conduction, the temperature distribution gradient near the edge of the tube blank is formed, forming a melting zone, a partial melting zone, an overheated tissue zone, a normalizing zone, Incomplete normalized area, tempered area, and other characteristic areas. Among them, the microstructure of the superheated zone is higher than 1100 °C, the austenite grains grow sharply, and the grains are coarse after cooling. A hard and brittle phase is formed under certain chemical composition and cooling rate. In addition, welding stress is also generated due to the presence of a temperature gradient. As a comprehensive result, the overall mechanical properties of the weld zone are lower than that of the parent metal. The physical seamlessness of the welded pipe is through the heat treatment of the weld to achieve the purpose of eliminating stress, homogenizing and refining the structure, and improving the comprehensive mechanical properties of the welded heat affected zone.
There are two main methods for physical seamless processing of seamless steel pipe welds: localized weld treatment and integral heat treatment.
1) Localized treatment of welds
Since the heat affected zone of the weld is mainly hardened and the mechanical properties are low, we should first consider the local treatment of the weld heat affected zone.
The localized treatment of the weld is to use a medium frequency induction heating device to heat the weld heat affected zone to 927 degrees Celsius, then air cool to below 538 degrees Celsius, and then add water cooling if necessary.
The localized treatment of welds is more suitable for steel pipes with larger diameters. There are two reasons: one is that the heating weld can be partially energy-saving; the other is that the weld seam is convenient when the diameter of the welded pipe is large.
2) Overall normalization
For smaller diameter steel tubes, the tube blank can be heated to above 900 degrees Celsius by medium frequency induction or flame heating and then cooled or cooled in a cooling chamber with a controlled atmosphere.