Method for heat treatment of thick-walled steel pipes

The method serves for the tempering of pipes with wall thicknesses over 16 mm. An optimum formation of intermediate stucture will be obtained with minimum energy expenditure and high speed of production. The pipe, which is transported in axial direction by means of driven bearing rollers, is successively passing sections for heating up, quenching, tempering. The sections for heating up and tempering each contain an induction coil to be fed with a.c.; in the quenching section two annular tubes fed with cooling water enclose, in axial distance, the pipe passing through. The first annular tube, by means of its nozzles, aims inclined water jets at the pipe. The second annular tube limits the cooling section by means of water jets radially directed at the pipe. The distance between this annular tube and the first one is adjusted by means of an engine-driven spindle. Behind each of the three sections, the pipe surface temperature is measured and fed into a controller each. By means of the differential temperature, as compared with a set nominal value, the heating power of the induction coil on the heat-up and tempering section and the distance of the annular tubes on the quenching section will be controlled. In the heat-up section, the pipe surface is heated up till austenitizing. In the quenching section, martensite is formed in the surface zone and finally, in the tempering section, intermediate structure in the non-quenched layer. The tempering heat necessary is provided by the non-quenched inner layer if the thicknesses of the two layers are dimensioned correspondingly. Only little auxiliary heat to prevent the pipe wall from cooling down too fast has to be added by the second induction coil. The overall heat balance and the structure formation can be improved by proper insulation along the pipe. Especially thick pipes (over 30 mm) may additionally be quenched from the inside, thus 3 concentrical structural zones being formed. (RW)