Low-carbon Pipe Steel Research Articles

Main structural factors of strengthening of low-carbon low-alloy pipe steels after controlled rolling

The mechanical properties of a large group of commercial heats of three grades of pipe steel (17G1S-U, 10G2FB, and 12G2SB) are studied after controlled rolling. The interrelation of alloying, structure, and mechanical properties of the steels subjected to controlled rolling under the conditions of the Magnitogorsk Iron and Steel Works is studied, and the main structural factors responsible for strengthening of low-alloy steels are determined.

Chemical composition and structure of low-carbon low-alloy pipe steel following controlled rolling

We study the patterns of variations in chemical composition for a large number of commercial melts of three grades of pipe steel: 17G1S-U, 10G2FB, and 12G2Sb via statistical clustering of experimental data. A comparative structural study was performed; the effects of various steel alloy components on the amount, distribution, and morphology of the pearlite component; the size and shape of the ferrite grains; and the development of dynamic recrystallization processes.

Effects of Hydrogen on Fatigue Crack Growth and Stretch Zone of 0.08mass%C Low Carbon Steel Pipe

In order to investigate the influence of hydrogen and test frequency on fatigue crack growth rate, fatigue crack growth tests (R=0) were conducted on hydrogen charged and uncharged specimens of a 0.08 mass%C low carbon steel pipe at test frequency 0.001-10 Hz. Following the fatigue crack growth tests, the test for producing a stretch zone was carried out. Observing the morphologies of the striation and stretch zone on the fracture surface and slip bands on the specimen surface, it was revealed that uncharged specimens have no dependency of crack growth rate on test frequency and on the other hand, the crack growth rate of hydrogen charged specimens at the test frequency below 0.01 Hz is approximately 10 times faster than that of uncharged specimens. There exists an upper limit of the increase in crack growth rate of the hydrogen charged specimens at test frequency below 0.01 Hz. The degree of crack tip blunting is very different between uncharged and charged specimens. The crack tip is fully blunt and CTOD is large in uncharged specimens, while the crack tip is very sharp and CTOD is very small in charged specimens. Nevertheless, the stretch zone width of hydrogen charged specimens is approximately equal to that of uncharged specimens. Considering the increase in the crack growth rate and the morphology of the striation and stretch zone, it has been concluded that the acceleration of fatigue crack growth of hydrogen charged specimens is caused by the continuous hydrogen enhanced slip deformation ahead of crack tip (Hydrogen Enhanced Successive Fatigue Crack Growth, HESFCG).

Amorphous diffusion bonding of steel pipe and its impact toughness

An iron-based amorphous foil (FeNiCrSiB) was used as an interlayer for the amorphous diffusion bonding of low carbon steel pipes under argon flux. The microstructure and mechanical properties of the joint were analyzed using an electron probe micro-analyzer (EPMA), tensile test, bending test and impact test. The results show that the joint microstructure resembles that of the base metal and no precipitates form at the joint. Melting point depressants (B, Si) diffuse far away from the joint and the base metal element is homogenous across the joint. The joint impact toughness is greater than the base metal toughness and the mechanical properties of the joint are similar around the pipe.

Ratchetting and ratchetting boundary study of pressurized straight low carbon steel pipe under reversed bending

With a quasi-three point bending apparatus, ratchetting and the ratchetting boundary were studied experimentally for a pressurized low carbon steel pipe under reversed bending. The ratchetting strain occurs mainly in the hoop direction. Multi-step bending revealed that the ratchetting rate increases with increasing loading level but reduces or even vanishes at a lower bending level imposed after a higher bending level. Ratchetting simulation was performed by elasto–plastic finite element analysis with ANSYS in which the Ohno–Wang model and some modified Ohno–Wang models were applied. By comparison with the experimental data, it is found that the Jiang–Sheitoglu model with a minor modification gives reasonable simulation. The ratchetting boundary was determined by the KTA/ASME, RCC-MR and C-TDF with the lightly modified Jiang–Sehitoglu model. By comparison with the experimental data range in this study, it is shown that the ratchetting boundary determined by the C-TDF method, with the lightly modified Jiang–Sehitoglu model, divides the shakedown region well.

Low carbon niobium alloyed high strength steel for automotive hot strip

Hot rolled strip from microalloyed high strength steel typically has a carbon content up to 0·12%. A carbon content close to the peritectic range enhances the susceptibility to surface defects such as edge and transverse cracks, which have to be removed by, for example, scarfing and grinding before hot rolling. These defects can be avoided when a low carbon content is applied. It has been demonstrated that by reducing the carbon content in linepipe steel to ∼0.03%, further improvements in toughness, ductility and weldability are achieved. Furthermore, a low carbon content promotes niobium carbide solution during reheating of the slab before hot rolling. This permits the application of higher niobium contents than are usually applied. Niobium contents up to 0.10% have recently been adopted in low carbon pipe steel. Besides refining the grain size during austenite processing, niobium in solid solution leads to an additional strength increase by delaying the γ→α transformation, thus promoting a higher volume fraction of bainite. In addition, NbC precipitation in ferrite gives a further strength increase. This concept is especially effective in producing high strength when accelerated cooling is applied after hot rolling. Based on this experience, a new hot strip steel concept has been developed. Besides its good surface properties, this steel shows high strength as well as excellent toughness and ductility. The properties achieved are homogeneous over the length and width of the hot strip. Changes in processing conditions have only a minor influence on the mechanical properties. In addition, this steel shows exceptional processing properties, for example in cold forming, welding and fine cutting as applied in the automotive industry.

Failure analysis of ruptured pipe connected to natural gas pre-heater

Failure Analysis was made on a ruptured low carbon steel pipe that was connected to a natural gas pre-heater in a chemical plant. The pipe has been in service for about 30 years. The analysis indicated that failure of the pipe was due to thin-lip rupture, resulting from high temperature sulphidation. The rate of sulphidation was much faster at the bend areas where turbulent flow was occurring. The pipe also suffered from graphitization due to decomposition of the pearlite phase as a result of long service time at moderately high temperature. It was recommended to replace the pipe with a new one and to regularly monitor the pipe thickness with special emphasis on bend areas. The plant was also advised to minimize the ingress of humidity and oxygen into the piping system during shutdowns.

Fatigue Crack Initiation on Low-Carbon Steel Pipes in a Near-Neutral-pH Environment under Potential Control Conditions

Abstract External transgranular cracking was observed on underground pipelines. The phenomenon took place under disbonded coating zones and was induced by the presence of diluted bicarbonate aqueous solution containing dissolved carbon dioxide (CO2). The observed environmental-assisted cracking is very often interpreted as a result of the inefficiency of the cathodic protection. However, some recent local electrochemical potential measurements on buried pipeline surfaces displayed important fluctuations with occasional overprotection conditions. The main objective of this work was to investigate the steel surface behavior under open-circuit potential conditions and over a wide range of cathodic potentials to identify the optimum electrochemical conditions to initiate cracking. Low-frequency cyclic loading tests were performed under various electrochemical conditions: underprotection, overprotection, and alternating stages of under- and overprotection. In low-pH simulated groundwater, the X-52 pipeline ste...

저합금 탄소강 배관재의 다층용접 열영향부의 미세조직 및 열이력 해석

저합금 탄소강 배관재의 다층용접 열영향부의 미세조직 및 열이력 해석

Some features of sulfide cracking of low-alloy and low-carbon steels

We analyze the susceptibility of low-carbon and low-alloy pipe steels and two types of rotor steel to corrosion cracking as a function of polarization in a NACE solution. It is shown that anodic and cathodic polarization inhibit the process of corrosion cracking only if the potential is shifted by 0.4–0.6 V. At the same time, full protection against cracking is never attained in testing at slow strain rates (10−5 sec−1). According to the results of the analysis of the diagrams of thermodynamic stability and measurements of the parameters ϕ and pH, sulfide cracking is initiated in the region of equilibrium between Fe2+ and FeS2 or between Ni2+ and NiS2 (NiS) if steel is alloyed with nickel.

Development of a database of pipe fracture experiments

Over the last 35 years, researchers worldwide have conducted hundreds—if not thousands—of pipe fracture experiments. In the early years, researchers focused their attention on studying the failure pressure and crack propagation behavior of axially cracked pipe loaded by internal pressure. The earliest work was sponsored by the oil and gas industry and, as such, involved relatively thin-walled, low toughness carbon steel pipes. This work was eventually followed up by efforts in the USA and Germany on nuclear piping with axial cracks. In recent years, attention has turned to understanding the behavior of circumferentially cracked nuclear piping subjected to both pressure and bending loads. The loading histories for these experiments range from the relatively simple case of quasi-static, monotonic displacement control to the more complex cases of dynamic cyclic loading, and pipe system experiments. In this paper, two of the leaders in this research, i.e. Battelle in the USA and MPA Stuttgart in Germany, have collaborated to develop a database of pipe fracture experiments. The database includes data from other organizations as well as the data from Battelle and MPA. In addition, as part of this paper, an example of how the database was used to assess the failure pressure of axially cracked pipe is given.

Study of the effect of defects on the fatigue behaviour and the fracture toughness for low carbon steel (API 5L grade B) gas transmission pipelines

The use of low carbon steel pipes in gas transmission pipelines and network gas pipes has increased. API 5L grade B (American Petroleum Institute) pipes are welded and inspected according to the BGC/PS code (British Gas Standard). Since root undercut and lack of penetration are common defects, the acceptance and nonacceptance of these defects based on the BGC/PS/P2 code of butt welded pipes are evaluated. Pipes are welded with different degrees of root undercut and lack of penetration defects and are X-rayed. The defects are categorized based on both X-ray films and final crack area. Specimens are cut and fatigue precracked at different periods depending on the initial defect size. An equivalent crack length is calculated based on Paris' law for each defect size in the fatigue crack propagation. The crack opening displacement technique, J-integral method based on Begley and Landes and the residual strength method have been used to determine the fracture toughness. The results have been compared to the parent material and the initial defect size to evaluate the restrictions of the BGC/PS/P2 code. The equivalent crack length results showed that the depth of lack of penetration welding defect should be taken into consideration for both API and BGC codes since this depth has a significant effect on the equivalent crack length. The application of the Begley and Landes approach is shown to be suitable since it permits the use of specimens of different initial crack lengths. The technique is applied here for cracks of different shapes. The results are close and can be used as comparative results between different categories and the parent material since the conditions of specimen dimensions are not valid as restricted by Begley and Landes. Critical crack opening displacement is calculated which gives reliable results for different categories. Crack opening displacement technique based on the British Standard requires different specimens with the same initial crack length, which is not satisfied. The residual strength method based on ASTM gives reliable results for the different categories of defects. The depth of lack of penetration welding defect should be taken into consideration for both API and BGC codes. The BGC code should specify an allowable depth for the root undercut welding defects.

Mathematical Modeling of Cathodic Protection Using the Boundary Element Method with a Nonlinear Polarization Curve

The distributions of potential and current density around a cathodically protected pipeline in seawater were determined using the boundary element technique. A nonlinear polarization curve for a low carbon steel in artificial sea water was obtained from dc‐potentiodynamic measurements and was fitted for use as the boundary condition on the pipe. The program was used to evaluate cases in which one or two aluminum sacrificial anodes are used to protect a low carbon steel pipe in seawater. The results show that the number of anodes, the sizes of the anodes, and the distance between the anodes and the cathode are of importance for cathodic protection.

Pit Holes in Thin Low Carbon Steel Pipes Cast by Metal Mold Centrifugal Casting

Pit Holes in Thin Low Carbon Steel Pipes Cast by Metal Mold Centrifugal Casting

An Experimental Study on the Torsional Fatigue Strength of Hollow Specimens with Drill Holes

A series of fatigue tests have been made to determine experimentally the size and notch effects of hollow specimens on the fatigue strength in reversed torsion. The hollow specimens were carefully machined out from one stock, composed of hot drawn low carbon steel pipe with 0.14%C, nearly satisfying the condition of geometrical similarity, that is, with their external and internal diameters of 41 and 31mm respectively for the large testpieces, and 8 and 6mm for the small. For the sake of comparison some solid cylindrical specimens were also prepared each with diameter of 8mm. Holes of several sizes were drilled reaching transversally to the central axis of the specimens. The material has been experimentally examined regarding the homogeneity of its quality with its static mechanical properties. Its agreement with the specimens annealed in vacuum has been established. In this experiments, size effect has been hardly observed in the testpieces of geometrically similarisize, and the notch effect of the drill holes has been determined as 1.6 and 2.7, for the diametral ratio of the drill holes and the specimens of 0.06 and 0.25, respectively.