PSI - Issue 30

Mikhail M. Sidorov et al. / Procedia Structural Integrity 30 (2020) 149–153 Mikhail M. Sidorov, Nikolay N. Golikov and Yuriy N. Saraev / Structural Integrity Procedia 00 (2020) 000–000

150

1. Introduction The main method of connecting metal pipes is arc welding. Arc welding leads to formation of plastic deformations as a result of non-uniform heating of weld and heat-affected zone. This changes the original size and shape of the welded structure. The presence of localized yields contributes to the formation of residual welding stresses as stated by Dmitriev et al. (2013), by Leggatt (2018). Together with stresses from external loads, residual stresses can accelerate the process of pipeline failure, especially at low operating temperatures as stated by Golikov and Sidorov (2014), by Larionov et al. (1998). Analysis of actual cases of failure of main gas pipelines operated at low temperatures shows that brittle and fatigue cracks mainly originate in welded joints as stated by Kapitonova et al. (2019). Therefore, one of the main negative factors that determine the reliability and durability of pipelines are residual stresses in the zone of the girth weld. To reduce the level of residual stresses, additional technological processing of girth weld is used: heat treatment; vibration treatment; explosive treatment, etc. These treatments typically solve one of three main tasks: optimization of the stressed-deformed state of the weld; reduction of the stress concentration coefficient in the weld and heat affected zone; creation of favorable compressive stresses in the zone of the girth weld. One of the most effective post-welding hardening methods is ultrasonic impact treatment (UIT). The principle of the UIT operation is based on impact of low-frequency ultrasonic vibrations of a vibro-impact tool on the surface of a welded joint. As a result, a redistribution of high tensile residual stresses to compressive ones occurs on the treated surface as stated by Shestakov (2013). However, it is not always possible to know exactly how residual stress fields can be formed after treatment. This largely depends on the strength characteristics of steel, the geometric dimensions of the joint, the original stress state, and treatment modes. Therefore, research in this direction is a pressing scientific and technical task. Solving these tasks can help to correctly apply promising processing technologies in practice and increase the durability of welded structures. The objective is to ( исследование - слишком общее слово , вставьте конкретную задачу ) determination of the nature of the redistribution of residual stresses in girth weld of a pipe made of steel (0.10%C-2%Mn-1%V-1%Nb 1%Al) in depending on the speed of the ultrasonic impact treatment( может в зависимости от скорости UIT). 2. Materials and methods The girth butt weld of the pipe with a diameter of 530 mm, a wall thickness of 8 mm and a strength class of К 60 was investigated. Pipe steel made of (0.10%C-2%Mn-1%V-1%Nb-1%Al). The pipe connection is made by manual arc welding in 3 layers – facing, filling and root. The pipes were joined by arc welding. The number of passes was three (root, intermediate and cover). Pipes of strength class K56–K60 (respectively, with an ultimate tensile strength of 550-600 MPa) are used in the construction of gas pipelines with an operating pressure of 7.4 MPa and higher, for operation in the Far North. Mean values of mechanical properties of the base metal are given in Table 1.

Table 1. Mechanical properties of the base metal. Material

Mean values

Impact strength a (J/cm 2 ) KCV +20 KCV -40

Yield strength σ T , ( MPa )

Ultimate tensile strength σ B ( MPa )

Percent elongation δ 0,5 δ 0,5 (%)

Base metal of the pipe

550

631

26

247

241

The chemical composition of pipe metals - according to the results of spectral analysis as well as steel according to Russian State Standard GOST 19281-2014 is presented in Table 2. Comparison of the data of chemical analysis of the composition of pipe metals with the composition of steels according to Russian State Standard GOST showed