Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-11-10
2003-12-23
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S599000, C073S602000, C073S622000
Reexamination Certificate
active
06666094
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method and a device for contactless online measuring of the wall thickness of hot pipes for detecting undesirable inner wall structures such as inner polygons etc. of hot-rolled, in particular, stretch-reduced pipes.
2. Description of the Related Art
In the manufacture of seamless and welded steel pipes it is conventional to employ the so-called stretch reduction method in order to produce in a very flexible way, based on a few semi-finished product dimensions, a plurality of diameters and wall thicknesses of finished pipe sizes. The advantage of this method, which does not require inner tools, resides in the quick and inexpensive variation of the wall thickness and the diameter.
The deformation of the pipe blank is carried out in a plurality of sequentially arranged roll stands wherein by speed variations in the individual stands a defined tension between the roll stands is produced and thus the wall thickness of the finished pipe can be adjusted in a directed way. The shaping within the stretch reduction rolling mill is nowadays carried out generally in three-roll or four-roll stands whose pass is not circular but oval on three or four sides. This form of the pass is generally unavoidable, and only the last pass of such roll stands is generally circular because the finish-rolled pipe should be substantially of a round and circular shape.
As a result of the oval pass there are often distinct irregularities in the cross-sectional wall thickness of the stretch-reduced pipe. These irregularities of the wall thickness have different shapes. For example, for a three-roll stand they have a hexagonal shape and are referred to as inner polygon. In a four roll stand the shape is octagonal. Like all other deviations of the wall thickness, the inner polygon formation also means a quality loss.
Since the inner polygon formation is the function of the wall thickness or, in more precise terms, of the ratio wall thickness to pipe diameter, it is actually necessary to provide different passes for the rolls, i.e., different oval appearances of the roll pass, for producing a large wall thickness range. However, since the making available of roll stands requires a considerable expenditure, in general, only two different passes are used, one round pass with minimal oval appearance of the pass opening for thick-walled pipes as well as:one oval pass with large oval appearance of the pass opening for thin-walled pipes. Otherwise, it is attempted to keep the occurring inner hexagon formation as small as possible by adjusting the average tension stress or the “tension” in the rolling stock during deformation optimally. This is so because it was found by experimentation that the degree of polygon formation changes as a function of tension. Once this tension optimization has been carried out laboriously, it is still not possible to obtain at all times pipes with minimal inner polygon because momentary unavoidable changes of the influencing parameters occur, i.e., an inner polygon formation as a result of momentarily changing deformation conditions as well as a considerable expenditure had to be accepted in order to perform an optimization prior to production.
Seamless steel pipes are conventionally produced in three deformation steps including hole punching in a cross-rolling mill, stretching in an “Assel” rolling mill, a continuum rolling mill or other rolling mills, and finish-rolling in a stretch-reduction rolling mill. All three deformation steps cause the pipe wall to have characteristic, undesirable deviations from the nominal dimensions which are overlaid by each successive deformation step and in this overlaid form are found in the wall of the stretch-reduced pipe. For example, in a two-roll cross-rolling mill, two thickened wall portions are formed which extend spirally about the pipe which, in a cross-section of the pipe, are expressed as a circumferential eccentric shape. When the second deformation step is carried out on an “Assel” rolling mill, it is also possible that spirally extending thickened wall portions result which extend either in the same direction but with different pitch about the pipe or have an oppositely arranged rotational direction and may cross the spirals of the cross-rolling mill.
On the other hand, in the case of a stretch-reduced pipe which has been pre-rolled in a continuum rolling mill, a quadrangle formation can occur in addition to the inner polygon of SRR (stretch reduction rolling) and the circumferential eccentric shape of the cross-rolling process. This quadrangle formation can be detected with respect to its phase position at the SRR exit so that a precondition is provided for counteracting these inner disturbances.
The problem of the undesirable wall structures occurring in pipes could be solved if it were possible during the production process to perform a correction of the inner flaw formation by means of the control circuit, for example, by variation of the tension parameters (change of the speed series). Since, as is known in the art, between the parameter of tension distribution and the inner polygon formation a definite correlation exists, the inner polygon formation could be automatically reduced without affecting the wall thickness of the semi-finished pipe. However, this requires that the course of the inner polygon formation and of the overlaying errors is known, for example, by contactless measuring of the wall thickness of the hot-rolled pipes directly after rolling when they exit the rolling mill with a constant center of the pipe. However, this requires an economical measuring method and a cost-effective measuring device which, in addition to measuring the wall thickness course across the length of the pipe or via the time for passing through, provides important information in regard to the inner polygon formations occurring during stretch reduction.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and a device for contactless online wall thickness measurement of the hot-rolled pipe with which the undesirable wall structures such as inner polygons, eccentric shapes, or quadrangles can be detected with minimal measuring-technological expenditure in order to be able to carry out measures for quality improvement at an early point in time.of the manufacturing process.
In accordance with the present invention, this is achieved in that, with the aid of the laser ultrasound method and by using at least one measuring head, a segment of the wall of the pipe to be measured, during or directly after the rolling process, is scanned in the circumferential direction and, optionally by mathematical analyses and symmetry considerations, the course of the wall of the pipe cross-section is reconstructed in a computer wherein, when using several measuring heads pivotable in the circumferential direction, each measuring head sweeps across a different correlated portion of the pipe wall.
With the laser ultrasound wall thickness measuring method the classic principle of ultrasound propagation time measurement is used. Based on the time of the ultrasound pulse (twice) passing through the pipe wall, the desired wall thickness will result based on the known speed of sound. Since the coupling of the ultrasound in the thickness measurement of hot walls with temperatures in the range of 1000° C. must be carried out in a contactless way at the excitation as well as the detection side, this is realized by optical methods in which the measuring head itself can remain at a thermally safe spacing from the rolling stock to be measured. High energy light pulses in the infrared range, generated by a flashlamp-pumped laser which is directed onto the rolling stock to be measured, are absorbed in the pipe surface and this results partially in an evaporation of extremely thin surface layers. As a result of the evaporation pulse, based on pulse conservation, an ultrasound pulse results in the pipe which enters the pipe wall perpendicularly to the pipe surface. The th
Kueffner Friedrich
Saint-Surin Jacques
SMS Demag AG
Williams Hezron
LandOfFree
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