High Strength Linepipe Steels Research Articles

The Present and the Future of Line Pipe Steels for Petroleum Industry

The growth of petroleum distribution systems has always been closely associated with the development of microalloyed steel grades having high strength, toughness, and weldability. In many ways, the research in microalloyed steel drew impetus from the huge demands of such steels from the petroleum pipeline projects. The article reviews the evolution and trends in steel for petroleum pipelines, the manufacturing processes, metallurgy and quality management aspects, research, and frontiers, followed by the corresponding Indian scenario. The first specification for steel for pipelines introduced by the American Petroleum Institute (API) in 1948 was X 42 with yield strength of 42 ksi or 290 MPa. The strength levels underwent large scale changes over the decades, and at present X 80 is commonplace, while X 100 and X 120 are in sight. The advantage of the relatively recent high temperature process (HTP) for producing higher grades of line pipe steel under the Indian context is discussed. The necessity of incorporating facilities for producing high-strength line pipe steels in up-coming steel plants has been briefly dealt with. With increasing strength levels in line pipe steels and higher operating pressures coming into vogue, line pipe steel emerges as a subject of fresh research, innovation, and area of specialization.

Relationship between hydrogen-induced cracking and type I sulfide stress cracking of high-strength linepipe steel

Hydrogen-induced cracking (HIC) resistance of high-strength linepipe steel was investigated using the cathodic hydrogen charging method. Sulfide stress cracking (SSC) resistance was also evaluated using a dead weight device. This study clearly proves that HIC occurs as an initial crack of type I SSC, which is the same as a hydrogen-induced blister crack (HIBC), and thus the initial crack behavior of HIBC as a precursor to type I SSC can be evaluated by the cathodic hydrogen charging method.

Assessment of Properties and Microstructure of X100 Pipeline Girth Welds

Over the last few years, interest in the development and construction of large diameter, high pressure pipelines from the Mackenzie Delta in northern Canada and Alaska’s North Slope has steadily increased as a result of the projected decline in recoverable natural gas from existing reserves in western Canada. The success of such enormous pipeline projects, which will require utilisation of a strain-based design approach, is largely based on successful application of high strength line pipe steels, such as X80 and X100. To fully realise the economic benefits of high strength steels, welding processes and procedures must be developed to ensure that the requirements for high strength and good low temperature toughness are confidently obtainable in field girth welds used in the construction of such pipelines. This study aims to provide a better understanding of the factors that control weld metal strength and toughness for a series of mechanised X100 girth welds produced using a range of narrow-gap gas-metal arc welding procedures. The attainment of high yield strength (≥ 810 MPa) is considered to be an important target to ensure overmatching strength relative to the longitudinal property distribution of X100 line pipe steel. Some factors that have been considered in this evaluation include influences of gas-metal arc welding process variants (single and multi-wire) and changes in welding procedure specifications, including joint preparation. Key components of this work relate to the reliable measurement of weld metal strength variation through-thickness and as a function of position around the circumference of a given girth weld.

Characterisation and Quantification of Complex Bainitic Microstructures in High and Ultra-High Strength Linepipe Steels

This paper provides a detailed description of complex bainitic microstructures obtained during the recent development of low carbon linepipe steels with strengths in the range of X100 to X120. New experimental techniques based on a high resolution FEG-SEM and EBSD have been used to characterise and quantify the mixture of ultrafine bainitic ferrite and nanosize second phases in these steels. It was found that the occurrence of incomplete transformation generates new, previously unexplored bainitic microstructures with a wealth of microstructural features that is beyond classification based on conventional concepts. Clear differences in distributions of boundary misorientations and effective grain size were noted between upper, lower and granular bainites. Based on these results a new classification scheme and definition of bainite is proposed.

FE-SEM Study of Fine Nb Precipitates in Carbon Extraction Replicas

Precipitation strengthening is an important parameter controlling the mechanical properties of low carbon steels. These precipitates are very fine and are normally analyzed using either thin foils or carbon extraction replicas under a transmission electron microscopy (TEM). In this work, field emission gun scanning electron microscope (FE-SEM) was applied successfully in the characterization of niobium (Nb) carbo-nitride (C,N) precipitates using carbon extraction replicas. FE-SEM observation of high strength linepipe steel replicas before and after aging at 400°C for 1 hr confirmed the presence of Nb(C,N) precipitates in ferrite. The FE-SEM could analyze small particles (below 50 nm) embedded in the steel but the analysis had to be carried out at low voltages to maximize spatial resolution resulting in a poor signal. However, carbon extraction replicas in the FE-SEM can be analyzed using high voltages, since the interaction volume effect is no longer a problem.

Effect of Microstructure on the Yield Ratio and Low Temperature Toughness of Linepipe Steels.

The present study aims at elucidating the effects of microstructural features on the yield ratio and toughness of high strength linepipe steels. The main emphasis has been placed on understanding the effects of constituents on the properties. Several alloy systems with different constituents, i.e. ferrite-pearlite steels, ferrite steels with acicular ferrite as second phase, acicular ferrite steels with ferrite as second phase, and bainite steels, have been investigated. Experimental results show that while the refinement of ferrite grain size improves both yield strength and low temperature toughness of ferrite-base steels, it increases the yield ratio. Modification of matrix from ferrite to acicular ferrite or bainite results in improvements in both yield strength and yield ratio. However, bainite steels have worse low temperature toughness (i.e., higher DBTT) than the other types of steels. It has been shown that the low temperature toughness of acicular ferrite steels can be improved by the introduction of polygonal ferrite as a second phase. This is mainly due to the refinement of effective grain size by the introduction of second phases. The relationship between the yield ratio and work hardening exponent has also been established using the Swift equation. Based on the results, the optimum microstructure for a better combination of strength, toughness and yield ratio is suggested to be the one having second phase of polygonal ferrite in an acicular ferrite or bainite matrix.

Influence of Softened Heat-affected Zone on Stress Oriented Hydrogen Induced Cracking of a High Strength Line Pipe Steel.

The susceptibility to hydrogen induced cracking under applied stress was examined for a large diameter high strength sour service line pipe steel produced by employing the thermo-mechanical control process, i.e. controlled rolling followed by accelerated cooling. The low carbon content aimed at high toughness and weldability resulted in softening in the grain-refined and inter-critical heat-affected zone by thermal cycles of longitudinal sub-merged arc welding. The cracking susceptibility of weldment was transversely evaluated by TM0177-90 method A and compared with the parent metal.The parent metal indicated high resistance to cracking. Nonethelss, the weldment indicated the decrease in threshold stress, since the weldment ruptured preferably along the softened heat-affected zone. The cracking morphology in the softened heat-affected zone was recognized as stress oriented hydrogen induced cracking. Also performed was a large scale test to reveal the internal incipient cracks in the softened heat-affected zone. The observation confirmed that the initiation site was a small cluster of non-metallic inclusions which had probably no adverse effect on the resistance of the parent metal. He stress analysis of the weldment proved that the tension through thickness was generated at mid-thickness under the deformation constraint from surrounding harder parts. The enhanced tri-axiality of stress in company with the through thickness tension facilitates the onset of cracking caused by the hydrogen pressure mechanism. The crack morphology was consistently associated with the stress distribution.

High strength line pipe steel plates based on TM-processing

Thermomechanical control process (TMCP) is more and more exploited to produce steel plates for manufacturing pipes used under Arctic conditions in sour gas service. A high level of yield strength and toughness is obtained without an exaggerated addition of alloying. It avoids costs and impairment of weldability. Good steel cleanliness and homogeneity is to be achieved together with well defined rolling and cooling procedures.

Development of High Strength Line Pipe Steels in Bhilai Steel Plant

Development of High Strength Line Pipe Steels in Bhilai Steel Plant

Hydrogen Induced Cracking Susceptibility of High-Strength Line Pipe Steels

Abstract The effects of chemical compositions, rolling conditions, and homogenization treatment on hydrogen induced cracking (HIC) have been studied metallurgically using laboratory heats of chemical compositions corresponding to those of the center segregation zones in continuous cast slabs. The hardness of the center segregation zones in steel plates is one of the predominant factors affecting the HIC susceptibility of high-strength line pipe steels. An increase of the carbon equivalent (Ceq) or phosphorus content hardens the center segregation zones in conventional carbon steels produced by the controlled rolling (CR) process. Lowering the carbon content to the range of 0.01 to 0.05% markedly decreases the hardness of center segregation zone and reduces the HIC susceptibility, regardless of the increases in other alloy elements. Hot rolling at relatively lower temperature increases the hardness of the center segregation zone and results in a slightly high HIC susceptibility. The design of the chemical ...

The Physical Metallurgy of HSLA Linepipe Steels—A Review

The discovery of oil and natural gas in severe climate conditions such as arctic regions has resulted in more-demanding mechanical property requirements for high-strength linepipe steels. These steels should have higher strength and toughness as well as excellent weldability. New high-strength, low-alloy (HSLA) steels for linepipe have been developed from the laboratory to full-scale production with astonishing speed in the last decade. These steels can be divided into three categories based on the microstructure, namely, ferrite-pearlite steels, acicular ferrite or bainitic steels, and multiphase steels. The physical metallurgy of these steels is reviewed and discussed, with emphasis on processing, structure-property relationship, and response to the U-O-E pipe forming process.

Cold cracking susceptibility of girth welding for arctic grade line pipe

High strength line pipe steels require not only good low temperature toughness but also excellent weldability. The use of cellulose welding electrodes has introduced the possibility of cold cracking due to the introduction of hydrogen. This report deals with the cracking susceptibility of line pipe steels. Test results make it possible to establish a diagram to predict and determine the welding conditions for crackfree girth welds. This diagram contains an emperical parameter, Pw, which includes the alloying composition of the steel, the effective hydrogen concentration at the root of the weld, and the restraining stress. When Pw is plotted against the cooling time (the time to cool from the weld condition to 100 °C), a distinct separation is seen between cracked and crack-free weld joints. Thus, the Pw parameter has utility.

Influence of vanadium on transformation characteristics of high-strength line-pipe steels

The mechanical properties ofa new line-pipe steel containing 0·45% V are strongly dependent upon the ability of the V to influence transformation characteristics in addition to providing strengthening from precipitation of V(CN). The V addition depresses the bainite start temperature and in this respect is similar to Mo. It is also shown that, although V increases the ferrite start temperature, it stabilizes the high-C austenite remaining at the end of the ferrite reaction. This stabilized austenite subsequently transforms to a martensite–bainite (M–B) constituent which is responsible for the continuous yield behaviour in a tensile test. The volume fraction of the M–B phase is strongly affected by the extent of interphase precipitation of V(CN) concurrent with ferrite formation and factors which increase the amount of interphase precipitation, such as N content, cause a reduction in the amount of M–B phase.