Abramova Elena Anatolivna Physic -metallurgical faculty of Donetsk national technical university, master of group MT -06m

PERSPECTIVE OF QUALITY INCREASING OF WELDED JOIN OF PIPE LINE WITH USING DEFORMATIONO-THERMAL PROCESSING
 

Welded join on modern stage of the development of the technology irreplaceable at assembly of metallic pipe line; their reliability at usages is defined at other equal conditions by quality of the welded join, to which are presented the most high requirements on strength and particularly toughness.

The most dangerous point in welded pipe is a transition from welded seam to the main metal. Than more fluent is transition that appears of the stress concentrators is less. Besides, its parameters have negative influence on quality of exterior covering, because on seam the thickness of covering is got less, than on the main metal. Thereby, for obtainty of the even welded join under arc welding, which provides the minimum leash under the following usage, and for increasing quality of fixings corrosion resisting covering at factory condition it is necessary to obtain accuracy of the geometric sizes of the pipe and homogeneity of the structure on section. All this justifies application of local complex deformation for decision of this task - a thermal processing.

The purpose of this work is a study of the influence of local deformation - a thermal processing (LDTO) on structure and characteristic of the welded joining the pipes from low carbon steel for gas route.

For research the sample of the pipes have selected, welded square butt joint of arc welding, from low carbon steel 10, St.2sp., 20, St.3 sp with thickness of the wall is 4?5 mm. Chemical composition of filler metal of the pipes corresponds to on carbon equivalent of the main metal (Cekv = 0,184-0,255). The experimental mode of LDTO executed in condition of the "VAT DMPZ".
The sample was heated to the temperature 980-1000 0С with endurance during 15 -20 min., afterwards subjected to fractional deformation with total degree of deformation ? = 67, 57, 52, 75%. Then they have born at the temperature 650-670 0С during 25-30 min.; hereon cooling was realized on air. The mechanical characteristic of sample welded join were provided in table 1.

Table 1 - Mechanical characteristic of sample welded join

  From result of the studies is seen that at the average for welded joining of pipes from low carbon steel after LDTO the plastic characteristics (the elongation, cross section reduction) were increase in 1,15 times, impact toughness increases in 1,3 times, but strength characteristics remain is on the same level approximately; as before LDTO. Essential increasing of the plastic characteristics of the welded join tells about increasing of pipes quality, and so, reduction of the possibility of the forming the rifts, leash, warping.
Measurement of hardness on section of the welded join from centre of the zone of filler metal has shown that in initial condition hardness on section was distributed unevenly, most its value is reached Hv10, = 260 H/mm2 in centre - a zone of the welded seam, is least - 140 H/nm2 in zone of the main metal on distance 11-12 mm. After processing sample on technologies of LDTO the sharp swing of hardness on section is disappeared. The hardness of filler metal practically is equal with hardness of main metal and at the average - became to form 160-170 H/mm2. This is tells of disappearances of the stress, which can bring defect in welded joining of the pipes.

The got results are confirmed also on researched microstructure.

Thereby, processing of the welded join on technologists of LDTO promotes the obtaining of the even structure on section of the welded join, stabilizations of the complex characteristic and, as effect, increasing quality and long service life of welded product, and period to usages increases in 1,2-1,3 times.
 

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