ESSURING OF THE WORKING STAND FRAME RELIABILITY OF THE PIERCING MILL 350 FOR ROLLING PIPES AFTER ITS LONG TERM OPERATION
Purpose. Investigation of the stress-strain state of the frame of the working stand of the piercing mill of the pipe rolling unit 350. Development of a method for determining the reliability of the frame of the working stand of the piercing state pipe rolling unit 350 after its long-term operation. Development of a strategy for further operation of the pipe rolling unit 350 piercing mill.
Methods. The tensometric measurements of the stresses arising in the frame of pipe rolling unit 350 piercing mills were carried out; studies of the 3D model of the frame were carried out. Mathematical models have been developed to determine the function of the distribution of the durability of the frame of the piercing mill and the safety index, as well as to determine the durability of the frame in the presence of existing defects.
Results. The results of the 3D model research of the piercing mill of pipe rolling mill 350 are presented. The stress-strain state of the working stand of the piercing millof pipe rolling mill350 is determined. It is established that maximum stresses occur in zones of the working stand of the piercing mill in the zones of previously detected cracks, places of concentration of maximum stresses "migrate" when changing the conditions of fixing of the frame.Defects (cracks) occurring in the stressed areas of the working standare found to be dangerous. Initial defect sizes were found to be small for reliable diagnosis, but their critical dimensions were large enough to not be detected in the examinations. It has been established that the safety of the piercing millunder the tougher operating conditions actually observed in reality has been exhausted after 75 years of operation. Studies of stress-strain state found that the gaps at the junction of the cover and the frame of the workingstand contribute to the growth of local stresses, the consequences of which were analyzed in this work.
Originality. For the first time the research of reliability of the frame of a piercing condition of pipe rolling mill 350 after 80 years of operation is carried out. In developing a mathematical model of resistance to fatigue failure, I and II modes of destruction were used.
Practical implications. The calculations show that at the moment the workingstand of the piercing millis operating in the zone of complete risk, which is accompanied by the formation of cracks. The survivability period of a spherical defect is about 5 years, after which the defect is transformed into a crack with badly defined geometry and develops according to regularities that require separate study. It is shown that the most effective in this regard is to control it with non-destructive diagnostic methods. To continue the operation of the piercing mill it is proposed to securely fasten the upper cover of the piercing millwith a screw fixing mechanism. It is suggested to minimize (exclude) the number of tubes being manufactured from deformed alloys, to diagnose stands and to regularly check the size of cracks.
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