Evaluation of the effect of production and operating parameters on the durability and safety of assembled welded joints of valve fittings

Abstract

Questions are considered of the reliability of assembled welded joints of valve fittings. It is proposed to use impact strength as a criterion of welded metal durability for welded joints. The effect of welding heating cycle on the structure and phase composition, and properties of metal around the joint (MAJ) of the heataffected zone (HAZ) of welded joints made from steel of strength categories K42 and K52 is studied. The effect of strain ageing on impact strength characteristics of the MAJ of the HAZ is analyzed for steel 17GS with different structure and phase compositions. It is shown that metal of the MAJ of the HAZ with a predominantly bainitic structure is most resistant to strain ageing. Dependences are plotted for evaluating welded joint material endurance and determination of the duration of the operating period, during which there is no critical fall in impact strength, and rational welding regimes are also established that make it possible to prolong safe operation of valve equipment to 30-35 years.Valve fittings are one of the most important elements of a gas pipeline system, and therefore in assembly of the valve fittings high specifications are laid down both for the material used, and also the production process. In welding, the welded joint metal structure and properties change considerably compared with its original condition, and therefore in order to provide a service life of an assembly with welded joints, comparable with the standard service life of a gas pipeline (40 years), it is necessary to consider both the production factors in performing welding operations, and also the operating parameters.Most critical for the operating capacity of a welded joint is embrittlement of the heat affected zone (HAZ) metal, in particular metal around the joint (MAJ). In view of this impact strength may be adopted as a criterion for metal endurance of welded joints.In order to estimate the effect of welding technology on the loss of ductility properties for MAJ of the HAZ immediately after welding in the standard of Gazprom 2-4.1-212-2008, impact strength of welded metal is regulated with a test temperature of -40°C, that should not be less than 25 J/cm 2 . Alongside this in the standard of Gazprom 2-2.3-137-2007 specifications are regulated for metal hardness of a welded joint HAZ that should be not more than 300 HV 10 . A significant part of an assembled welded joint of valve fittings, used in main gas pipelines, is made in steels of the strength category K42 and K52, that have a ferrite-pearlite structure. The chemical composition of steels of these strength categories is given in Table 1.

Thermal Cycles and Peculiar Properties of Austenite Decomposition when Laser-Hybrid Welding Steel of K60 Strength Class

“…We studied kinetics of austenite decomposition in welding with the help of dilatometry method [20,21].…”

Comparison of procedures for evaluation of effect of welding thermal cycle on impact toughness of HAZ metal of welded joints from low-alloy steels

“…Effect of welding on base metal can be also evaluated using a method at which thermal action on metal is carried out without application of welding (GOST 23870-79) [3][4][5][6]. In this case, the billets (ingots of specific size) from studied steel are heated and cooled on mode, comparable with welding thermal cycle (WTC).…”