Novomoskovsk Repair and Mechanical Works has launched a new multi-stand electrical tube welding mill 20–114
New solutions
Novomoskovsk Repair and Mechanical Works has launched a new multi-stand electrical tube welding mill 20–114
New technological solutions aimed at enhancement of the tube quality and reliability of high-frequency weld have been considered. A criteria for assessing reliability of the weld seam has been set forth.
Novomoskovsk Repair and Mechanical Works (hereinafter, NRMW) has launched a new multiple-stand electrical tube welding machine 20-114. The machine makes longitudinal electrical welded tubes of carbon steels, from 20 to 114 mm in diameter and up to 5 mm in wall thickness.
To improve quality of tubes and reliability of high-frequency weld, the electrical tube welding mill has the following features:
· Preliminary special strain placed on the strip before it is being shaped in order to alleviate deterioration of quality caused by dissymmetric macrogeometry of the strip;
· New sizing of tools that reduce the length of welded space and stabilize the welding of heated edges;
· Double reduction of the heated weld;
· A 1.5 time increase in the number of stands on the tube shaping mill;
· Enhanced kinematics of the electrical tube welding mill;
· Doubled number of stands of the calibrating mill;
· Reversal oval shaping of the tube prior to calibration.
Preliminary special strain placed on the strip before shaping makes it possible to reduce the difference between lengths of edges of an asymmetrical strip, e.g. of a crescent strip or a strip with one-sided ridges. Moreover, this strain favourably lowers the angle of edges’ flare and makes the edges’ ends more parallel to each other. All this stabilises the process of high frequency welding of hot edges and improves the quality of tubes.
New sizing of the tools features a new welding scheme which reduces the length of welding way and improves the welding cylinders’ ability to retain the passing billet, thus lowering the possibility of more ridges emerging from the welding process. Simultaneously, it speeds up immersion of edges and, correspondingly, reduces the quantity of oxides in the weld joint and boosts reliability of the weld.
The double reduction of heated weld means reduction in the welding gauge together with additional hot reduction in a stand located right next to the gauge, thus improving reliability of the welded seam. Because the number of shaping stands is 1.5 times as high as that in a regular electrical tube welding mills, the value of intermediate warps is lowered and, consequently, the resulting total residual stress on a billet approaching the point of edge welding is reduced. This way, the welding process is stabilised and tubes become more precise.
Enhanced kinematics of the tube welding mill means a right ratio of the number of revolutions of engines in the tube shaping and calibration mills, as well as the correct selection of rolling diameters of the driven rolls (choosing the diameter of driven rolls in relation to the gauge’s bottom). The key point in improving the kinematics is to prevent backing of billets between the two adjoining stands, as well as between the calibration and shaping mills. In other words, it is good to place a slight strain on the billet both between the reduction and calibration mills and between the two neighbouring stands of the mill. It is of special significance that the shaping mill should not propel a billet into welding area, but rather a billet should be drawn through the calibration mill. This stabilises the edge welding process.
Owing to the doubled number of stands on the calibration mill, precision of both the diameter and longitudinal curvature is improved. Furthermore, if needed, the reduction ratio in the mill can be raised and tube surface quality improved.
Additionally, greater number of stands on the reduction mill raises precision of tube shapes made of round infinite pipe on the electrical tube welding mill.
Reversal oval shaping of the tube prior to calibration enhances precision of the diameter and longitudinal curvature. Additionally, this reversal oval shaping runs sort of a permanent mechanical test of reliability of the weld seam on the infinite tube immediately at the site of electrical tube welding mill because it uncovers the possible welding faults, such as joining qualities of the weld etc.
In addition to electrical tube welding machine 20-114 we own a unit that length-cuts the strips, tube finishing unit (including the facing), hydraulic press, warehouses and other required equipment. NRMW has a strong machining department that can make cylinders and other required materials in-house.
NRMW keeps permanent contacts with the leading scientists and specialists of the tube industry in order to further improve technologies and equipment, expand product mix, enhance quality and reliability of tubes.
Currently, NRMW makes tubes in six diameter sizes, namely, 26.8 mm, 33.5 mm, 42 mm, 48 mm, 89 mm and 108 mm. In the nearest future, the company will start producing new tubes, 57mm and 60 mm in diameter, as well as a number of new sizes of tubular shapes.
Mechanical testing of tubes has shown that they withstand the strains that significantly exceed GOST 3262-75 and 10705-80 requirements. For instance, almost all the tubes tested endure complete flattening (to the point where opposite walls touch each other), while the standards only provide for maximum flattening at 2/3 of the outer diameter of the tube (2/3 Dh). At the cone expansion tests, tubes withstand the strain that exceeds the 4% standard requirements threefold and over. The bend tests revealed similar excellent results.
Reliability of the welded seam is among the most important features of quality of electrically welded longitudinal tubes. Unfortunately, currently there are no physical criteria for reliability of weld seams of small-diameter tubes, up to 114 mm. At the same time, we believe that availability of such a criterion would help consumers to make the right choice among the tubes offered and to make a more rational use of tubes. Additionally, should such criteria have been available, tube mills would have to constantly enhance reliability of their products.
Determination of the so-called ‘conditional reliability coefficient’ (CRC) of the weld is the backbone for the criteria for reliability of high-frequency weld seams for small-diameter tubes. As such, this coefficient is much alike the margin of safety ratio and depends on the stress of the weld and adjoining area at the time of mechanical tests.
For the toughest test of flattening when the welded seam is placed at 90° to the axis of the flattening force, the conditional reliability coefficient Кc is as follows:
where: Dh – outer diameter of the tube prior to flattening;
S – wall thickness of the tube;
n – hardening coefficient (0<n<1);
hr – the distance between flattening tools in compliance with GOST standards
(e.g. hr =2/3 Dh);
h – the distance between flattening tools at the moment when the first signs of cracks appears. Should there be no crack up to complete flattening, then h=2S.
The approach set forth as regards computation of the conditional reliability coefficient in small-diameter tubes is applicable to other kinds of mechanical tests as well. This way, an integrated coefficient of the welded seam reliability (Кo) can be determined subsequently.
For instance, let’s apply formulae (1) to determine Kc coefficient for tubes, 89 mm and 33.5 mm in diameter and 3.5 mm and 2.8 mm in wall thickness respectively, when such tubes are fully flattened, i.e. when h=2S and hr=2/3Dh
The analysis of tension curves for low carbon steels has revealed that the hardening coefficient ‘n’ lies between. For the purpose of computations, let’s accept n=0.35. Calculations give the values of Kc=3.8 for tubes, 89 mm in diameter and 3.5 mm in wall thickness, and Kc=2.8 for tubes, 33.5 mm in diameter and 2.8 mm in wall thickness. Therefore, when the tubes endure complete flattening, the conditional reliability coefficient of the weld seam in both cases exceeds the GOST standard requirements of 1.
I would like to point out that should the standards for mechanical tests of tubes consider all available factors, the output of the tests would depend not only on the tube diameter, but also on wall thickness and steel grades.
Novomoskovsk Repair and Mechanical Works guarantees the high quality of the tubes it makes and acceptable prices. We welcome cooperation.
Our address: Novomoskovsky RMZ, 2 Zavodskaya Street, Meliorativniy village, Novomoskovsky district, Dnepropetrovsk region.
Telephone/fax (+38-05693) 4-19-77
Figure 1. Determination of Kc coefficient for tubes.