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Figure 7 shows a wall thick- ness pro le measured online on a hot seamless steel tube with Tecnar’s Lut at the output of a rotary process. The pro le was taken along a single line on the top section of the tube. As shown in the gure, several cycles are observed. On further study, one can identify three cycles in the wall thickness pro le: a slow cycle, and intermediate cycle and a fast cycle. It is possible to extract the information from each cycle and provide the oper- ator with a clearer picture of the source of eccentricities.
Fig. 7 Raw online wall thickness pro le of eccentric tube
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lar shape of the inner wall will results in third order eccentricity. Such inner triangular shape is often encountered with a three-roll processing mill, as shown in Figure 6. As the rolls applies pressure on the tube only along three speci c plane, the wall is slightly attened along these three planes, yield a slightly triangular inner shape.
More generally, it can be shown that a processing stand of a mill with N rolls will produced eccentricity of N order. In addition, as such pro- cessing mill are setup with alternating positioned roll sets, 2N order of eccentricity is also observed. Moreover, it can be shown that the con- tribution of each order of eccentricity essentially is added to provide the overall eccentricity of the tube.
In summary, an order of eccentricity is essentially the number of cycle of going from a minimum wall thickness value to a maximum wall thick- ness value along the circumference of the steel tube or hollow. The order of eccentricity may help identify the source of the eccentricity. For example, rst order eccentricity is generally caused by non-uniform heating of the billet or from a de-centered piercing mandrel. Second order eccentricity is generally caused by two-roll piercing process. More generally, N-order eccentricity and 2N-order eccentricity are generated by alternate stands of N-roll process mill.
Higher order of eccentricity may also be observed by the combination of processing mills, as the eccentricity formation is cumulative. For example, a two-roll piercing process followed by an alternate stand of three-roll process will results in two+six or eight order eccentricity. More generally, any wall thickness pro le can be decomposed as a sum of cycles, or eccentricity orders, allowing operator to get more informa- tion on the seamless tube manufacturing process.
WALL THICKNESS PROFILE DECOMPOSITION
As described previously, the wall thickness pro le contains a rich amount of information, which is not routinely used in seamless tube produc- tion. To better understand such capability, let us review an example from a seamless steel mill with a rotary manufacturing process.
Fig. 6 Third order eccentricity
ITAtube Journal No2/July 2018
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