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Fig. 4 First order eccentricity
eters and directly see the impact on the useable length of tube. Figure 3 shows a typical Lut wall thickness pro le obtained from a tube with heavy ends. When modifying a parameter, such as the insertion or extraction speeds, the operator may immediately view the impact of his action on the wall thickness of the tube.
Such pro le analysis can be further expended. However, in order to better understand the analysis, it is important to understand the source and type of eccentricities produced in seamless tube manufacturing, and its impact on the wall thickness pro les.
ECCENTRICITY IN SEAMLESS TUBE MANUFACTURING
Low eccentricity is of prime importance for most seamless steel tube application. Eccentricity, for seamless tube manufacturing, is de ned as the offset and/or deformation of the inner wall of the tube with respect to the outer wall. It is generally the results of misfunction or of wear in the manufacturing process. It is an unavoidable result of the process that can be, however, managed and controlled. Eccentricity is generally produced at the beginning of the seamless tube manufac- turing process. Given its nature and shape, it is usually not possible to correct eccentricity using downstream processes. Therefore, it is of prime importance to detect eccentricity formation as soon as possible, to quickly identify the source of the eccentricity, and to take the proper remedial actions.
The eccentricity of a tube may be expressed in the term of “order”. The order corresponds to the cycle or frequency at which the wall thickness varies from a minimum value to a maximum value over a full circumfer- ence of the tube. For example, if the inner wall of the tube is round but has a slight centering offset with respect to the outer wall, as shown in Figure 4, the measured wall thickness will go from a minimum wall to a maximum value, then back to the initial minimum wall. Hence, for wall thickness measurements over the full circumference, one will see one cycle or rst order eccentricity. Such rst order eccentricity is often encountered in a piercing press mill. If the piercing mandrel is slightly of off-centered, the inner pierced hole will be off-centered, resulting in an eccentric hollow.
Other manufacturing process may yield different orders of eccentricity. As shown in Figure 5, a two-roll piercing process applies pressure on the hollow on two-sides, resulting in some bulging. The pierced tube will generally have an elliptic inner shape. Measuring the wall thickness over the full circumference will show two locations with a minimum wall thickness and two locations with a maximum wall thickness. Hence, we will have two cycles or second order eccentricity.
It should be noted that for a rotary piercing process, the eccentricity is “rotating” as a function of the length of the tube. Measuring the wall thickness along a single line will provide a direct observation of the cycle resulting from the inner elliptical shape. For rotary processing mills, a single axis wall thickness pro le is suf cient to provides good information of the eccentricities of all orders.
More complex shapes of the inner of seamless tube leads to the forma- tion of eccentricity of higher order. For example, the case of a triangu-
Fig. 5 Second order eccentricity
ITAtube Journal No2/July 2018
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