Physical seamless weld process of ERW steel pipe

Carbon steel of carbon equivalent CE <0.2% is with good high-frequency butt welding ability, do not have to carry out heat treatment after welding. However, high strength low alloy steel pipe CE typically 0.2% -0.65% in the butt welding process of ERW steel pipe, due to the skin effect interaction, proximity effect and heat transfer, resulting in a gradient of temperature distribution near the edge of the tube formed a special zone melting zone, part of the melting zone, overheating tissue area, normalizing zone, incomplete normalizing zone and tempering area. As the temperature is above 1373K in the overheating tissue area, the austenite grain grew sharply, coarse grains after cooling, under certain chemistry and cooling rate conditions the hardened tissue may also occur. In addition, there is a temperature gradient which will produce welding stress, as the combined result, the joint mechanical properties will be lower than the base material, it must be carried out weld heat treatment, the so-called "physical seamless weld process."

There are two main methods:1, weld local treatment - after resection steel outer burr and before water cooling and sizing, weld heat affected zone is heated to about 1200K with medium frequency induction heating device, then cooled to 811K or less, this is a kind of online normalizing heat treatment, suitable for large diameter ERW steel pipe (OD 200mm or more);2, the overall treatment - the medium frequency induction heating method or flame is used to heat the tube to 1173K or more, then air cooled, or in a cooling chamber with a controlled atmosphere to cool down. When welding the tube which is easy to generate refractory oxides containing elements (such as Cr), in order to reduce the weld oxide inclusions, blown into the neutral gas flow (N2) at high-frequency welding equipment and gas inside the tube for protection, suitable for smaller diameter ERW steel pipe.