There are usually two heating products for the pipe billet of the welded steel pipe, including pre-heating and rolling tube. When manufacturing welded steel pipe, the two heating products should be heated before in a given path, and it requires the use of intermediate annealing so that the residual stress can be removed. Sometimes, the welded steel pipe is manufactured to meet special requirements, for example, improving the mechanical properties and improving organizational and process performance and heat treatment. Therefore, the defects of the welded steel pipe manufacturing are uneven heating, oxidation, decarburization, heating cracks, overheating and over-burning.
The temperature on the steel pipe usually has a great influence on the quality of it, so the manufacturers must control the temperature precisely. However, the heating temperature is too low, too high or uneven heating. The deformation resistance of the metal may be increased and the plasticity of metals may be reduced because of too low heating temperature. Generally, if the complete conversion of the welded steel pipe microstructure of austenite grains cannot be guaranteed by the heating temperature tube, the cracks will be increased. What is more, the surface of welded steel pipe may be oxidation severely, decarburization overheated or burnt when the welded steel pipe is at a high temperature.
In the meanwhile, besides the too high temperature, if the steel pipe is heated for too long time, there are also defects such as the excessive austenite grain coarsening. When the heating temperature of the steel pipe reaches near the solidus temperature, the boundary may be oxidation or melted, and then a burnt will come out. If the fold is produced in the overheating perforated capillary welded steel pipe, then the burnt steel billets will be scrapped. When the welded steel pipe enters into the furnace where the temperature is too high, or the steel pipe is heated too fast in it, it is easy for the kind of steel pipe to crack. In general, the crack and the fold are easy to form both inner and outer surfaces of the pipe.
If the heat treatment is controlled improperly, the widmanstatten may be produced, which may have a bad impact on the working performance of the steel pipe, namely, reducing the strength of steel at room temperature and increasing brittleness.