Impact Strength of Main gas Pipeline Steel After Prolonged Operation (original) (raw)
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Low temperature impact toughness of the main gas pipeline steel after long-term degradation
Central European Journal of Engineering, 2014
The correlation of microstructure, temperature and Charpy V-notch impact properties of a steel 17G1S pipeline steel was investigated in this study. Within the concept of physical mesomechanics, the dynamic failure of specimens is represented as a successive process of the loss of shear stability, which takes place at different structural/scale levels of the material. Characteristic stages are analyzed for various modes of failure, moreover, typical levels of loading and oscillation periods, etc. are determined. Relations between low temperature derived through this test, microstructures and Charpy (V-notch) toughness test results are also discussed in this paper.
Analysis of the causes of fracture of the main gas pipeline
Zastita materijala
A complex of research has been conducted to establish the causes of the accident of the main gas pipeline made of 1420x15.7 mm X70 steel after 20 years of operation. It is found that pipe destruction was initiated by the longitudinal crack with a length of 470 mm and a maximum depth of 6.8 mm, oriented in the direction of the pipe and perpendicular to the outer surface of the pipe identified as stress-corrosion cracking. The crack initiation is due to the disboundment of the tape protection coating, the high corrosion activity of the soil, and the complex stress-strain state caused by deviations from the design objectives during construction. The chemical composition the base metal of all investigated pipes corresponds to the requirements of the technical conditions for steel pipes in category X70; pipes welding are performed with the typical materials used in pipe welding plants. Despite the differences, the structural and mechanical characteristics (yield strength, ultimate streng...
Materials
Regularities of steel structure degradation of the “Novopskov-Aksay-Mozdok” gas main pipelines (Nevinnomysskaya CS) as well as the “Gorky-Center” pipelines (Gavrilovskaya CS) were studied. The revealed peculiarities of their degradation after long-term operation are suggested to be treated as a particular case of the damage accumulation classification (scheme) proposed by prof. H.M. Nykyforchyn. It is shown that the fracture surface consists of sections of ductile separation and localized zones of micro-spalling. The presence of the latter testifies to the hydrogen-induced embrittlement effect. However, the steels under investigation possess sufficiently high levels of the mechanical properties required for their further safe exploitation, both in terms of durability and cracking resistance.
A Damage Mechanics based Evaluation of Dynamic Fracture Resistance in Gas Pipelines
Procedia Materials Science, 2014
Investigation of running ductile fracture in gas transmission pipelines and the derivation of reliable crack arrest prediction methods belong to major topics in pipeline research. The yet available crack arrest criterion, known as the Battelle Two-Curve Method (BTCM), leads to reliable predictions up to grade X70 line pipe steels for which it has been validated. This includes specific limits in terms of mechanical properties, pressure and geometry. The application of this criterion to modern pipeline steels, i.e. especially grades X80 and beyond in combination with larger diameters and high pressure, has led to mispredictions of the BTCM. Hence, in order to ensure safe design of pipelines, new methods are required based on in depth knowledge and appropriate characterization of material resistance. This paper presents a procedure for the assessment of dynamic ductile fracture resistance based on combined experimental and numerical investigations. The procedure involves quasi-static and dynamic dropweight tear testing (DWTT) on modified specimens with pre-fatigued crack for grades X65, X80 and X100 materials, and the application of cohesive zone (CZ) and Gurson-Tveergard-Needleman (GTN) models to describe ductile material damage. The damage model parameters are calibrated on basis of DWTT results and subsequently used to simulate dynamic crack propagation in a pipeline. The influence of material properties (strain hardening, toughness), pipe geometry, usage factor and decompression behaviour on ductile fracture propagation behaviour is studied and evaluated. The results will contribute to an enhanced understanding of major parameters controlling ductile fracture propagation and will help to establish a reliable procedure for safe design of new high-capacity pipelines with regard to crack arrest.
Effect of long-term operation on steels of main gas pipeline: Structural and mechanical degradation
Journal of King Saud University - Engineering Sciences, 2016
Based on the results of experimental studies of 17MnSi steel the regularities of the inservice degradation influence into its deformation and strength properties were established with the use of full strain diagrams. The important role of the hydrogen absorption that takes place under operation and its negative influence onto the mechanical properties of 17MnSi steel are shown. The latter is manifested through the microdefect growth in the gas pipeline material wall (in the form of dispersed damages) and reduction of its resistance to the brittle fracture.
Dynamic fracture behavior of a high strength pipeline steel
The occurrence of a crack propagating along a pipeline is a catastrophic event, which involves both economic losses and environmental damage. Therefore, the study of the fracture initiation and propagation properties of a pipeline is an essential part of its integrity assessment. Fracture prediction, however, is a challenging task, since it requires knowledge of the interaction between the dynamic forces driving crack growth, and the resistance forces opposing fracture propagation. Moreover, plenty of material properties should be taken into account. Aiming at a better understanding of the plastic hardening, damage and fracture properties of an API 5L X70 pipeline steel, and how these are affected by the strain rate, in present contribution, a comprehensive set of test results is presented. The program includes static and dynamic tensile tests on smooth and notched samples, and compression tests on cylindrical samples. Test result analysis is supported by finite element (FE) modelling. As such, the study aims at providing data needed for both fundamental material research and constitutive material modelling.
A Damage Evolution Approach in Fracture Mechanics of Pipelines
The paper concentrates on perspectives of the damage evolution approach in fracture mechanics of oil and gas pipelines. This approach is based on the generalised concept of damage. It is postulated that deformation and fracture processes in solids are determined by some general functional law related to the accumulation of damage. Fracture mechanics parameters are accepted as the controlling parameters for the failure processes. The approach leads to a description of fatigue crack growth, stress corrosion cracking, a correlation between hydrogen redistribution in the vicinity of a crack tip and the stress intensity factor during crack propagation under cyclic loads. The damage evolution approach has been also employed to quantify the shift of the ductile-to-brittle transition temperature of gas pipelines due to physical-mechanical damage of the steel during long-term operation of pipelines. The ductile–brittle transition curve of the steel pipeline shifts to higher temperature which...
The Mechanical and Brittle Properties of Gas Pipeline in Service
Acta Physica Polonica A, 2017
The paper deals with the determination of mechanical and brittle properties of gas pipeline after more than 40 years in operation. The heat affected zone in weld of pipeline is the risk area in terms of embrittlement structure, which resulted in an increase of yield strength and shift of transit temperature to more positive values. The specimens were cut from heat affected zone for this reason. The tensile test according to ISO 6892-1 showed the increase in yield strength: it had a value of 295 MPa at the beginning operation and 400 MPa after more than 40 years in pipeline operation was recorded. The transition temperature at the value of 20 • C was determined by using the Charpy impact test according to EN 10045-1. The operators of gas pipelines report that the mean temperature on the outer surface of pipe is about 4 • C during the winter. The risk of brittle fragile is very likely if the transition temperature is higher than operating temperature.