Abstract
The effect of strain aging on the microstructure, including copper precipitation, and mechanical properties of an industrially produced Cu-containing microalloyed low carbon pipeline steel has been investigated. The precipitation of round bcc-Cu particles with diameters of ∼7 nm has been clearly observed in the microstructure after strain aging. The strength of pipeline steel significantly increased while the elongation and impact toughness did not apparently decrease. Unlike welded pipe, few carbon atoms in supersaturated solid solution diffuse to the mobile dislocations, forming Cottrell atmospheres and producing strain aging phenomenon in seamless pipe. This difference is attributed to the different pipe making technique: thermo mechanically controlled processed for welded pipe and traditional heat-treatment for seamless pipe.
Introduction
Ocean oil exploitation is becoming more and more important as oil requirement is rapidly increasing and reserves in land are reducing. It is well known that pipeline is one of the most important oil transportation tools in ocean oil exploitation. Cu-containing high strength low alloy pipeline steels provide a good combination of strength, toughness and weldability, especially excellent corrosion resistance in marine environment, making them suitable for applications in oil and gas pipelines exposed to marine environment.1,2 In other words, Cu-containing steels have been considered as promising candidates of pipeline materials for marine industry.
The vibration of pipeline is inevitable owing to the action of the wave and the current in sea, resulting in the occurrence of deformation of pipeline. Meanwhile, oil pipeline has often been heated by high temperature oil from heavy oil thermal recovery technologies in ocean oil exploitation. This simply means that the strain aging phenomenon will be caused during the oil transportation. In addition, pipeline steel is also subjected to a natural aging during a long time period at room temperature, resulting in the changes on the microstructure and mechanical properties after long-term service.3,4 The effects of strain aging on mechanical properties can include the increase of yield strength and yield ratio, and the decrease of toughness and ductility. In view of its engineering significance, strain aging has drawn more and more attention from scientists and steel industries, particularly for the strain-based design applications.5–9 Compared with welded pipe, seamless pipe has been increasingly applied for oil transportation in severe environment, for example the marine environment, due to the homogeneity of mechanical property without welded joints. At present, increasing demand for strain aging design of seamless pipe has been needed for Baoshan Iron & Steel Co. Ltd. However, such strain aging behaviour has been extensively investigated in welded pipes,3,4,10–12 but rarely in seamless pipes, especially for Cu-containing seamless pipes.
In the present study, a detailed investigation has been undertaken to characterise the changes on the microstructure and mechanical properties of Cu-containing steels before and after strain aging. In addition, the aim of this work is to evaluate the strain aging behaviour and understand the mechanism of strain aging by mechanical property measurements and microstructure analysis.
Experimental procedure
Chemical composition of Cu-containing microalloyed low carbon steel (wt-%)
Microstructural evaluations of test samples were carried out using optical microscopy (OM), EVO MA25 scanning electron microscopy (SEM) and JEM 2100F transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX). Nital etchant was used to reveal the microstructure under OM and SEM. The TEM thin foils of 3 mm diameter were prepared by the twin-jet polishing technique with a solution containing 4% perchloric and 96% ethanol at −30°C.
Results and discussion
The microstructure of Cu-LCS before strain aging is shown in Fig. 1. The microstructure consisted of polygonal ferrite, lath ferrite and many particles distributed within the grain and at the grain boundary (see Fig. 1a and b). Details of microstructure were further examined by TEM, as shown in Fig. 1c and d. Many particles were observed in Fig. 1c and d. EDX microanalysis showed that particles were all cementite. There were no copper precipitates detected in the microstructure of Cu-LCS before strain aging, as shown in Fig. 1c and d. It implies that copper is completely dissolved in iron at tempered temperature 700°C. It was reported13,14 that the solubility limit of copper in steel increases with the addition of Ni to some extent which could contribute to this phenomenon. In addition, Cu precipitation will not occur during the tempering process due to the rapid cooling rate in air.
The microstructure of Cu-LCS in quenched and tempered condition: a optical microscopic image; b SEM microscopic image; c and d TEM bright field images
SEM images did not exhibit any visible changes before and after strain aging. However, significant changes in microstructure were revealed in TEM images. Dislocation configurations of 7.5% pre-strain sample are shown in Fig. 2a and b. It can be seen that there were high density tangles of dislocations and subgrains from the rearrangement of dislocations in polygonal ferrite and lath ferrite (Fig. 2a). Bowing of dislocations resulting from pinning effect of nanoparticles (shown by arrows) was clearly seen in Fig. 2b. The density of dislocations in Cu-LCS after strain aging increased significantly in comparison with that of Cu-LCS, as shown in Figs. 1c and d and 2a and b. Further TEM observations (Fig. 2c) revealed the presence of fine, nearly round nanoparticles with diameters of about 7 nm. EDX analysis showed that these particles were Cu-rich precipitates, as shown in the inset of Fig. 2b.
TEM micrographs of Cu-LCS with 7.5% pre-strain after strain aging a dislocation substructure; b nanoparticles showing dislocations interactions; c TEM bright field image showing nanoparticles and rod-shaped cementite particles; d diffraction pattern of Cu-rich phase particle; e HRTEM image of Cu precipitate
Precipitation of copper in ferritic steels has been studied extensively,2,15–18 particularly in reference to pressure vessel steels19–21 which are used for nuclear reactors. Based on these studies, it is now generally accepted that the following sequence is characteristic of precipitation in this system: bcc → 9R → 3R → fcc.22,23 Initially, the bcc precipitates which are fully coherent with the bcc ferrite matrix are observed in Cu-containing steel. When the precipitates are larger than the critical size for transformation and loss of coherency, and thus likely have a martensitic transformation, first to a twinned 9R structure (fcc with stacking faults) and then to 3R structure (a distorted fcc). Finally, at larger precipitate sizes, the precipitates attain the equilibrium phase with the fcc structure. When the bcc-Cu precipitates reach a critical size, the bcc → 9R transformation occurs in an early stage of precipitation of copper in aged ferritic steels. The critical size depends on temperature and possibly some other factors, being about 12 nm diameter at 550°C and 4 nm diameter at room temperature in model Fe–Cu alloys. 23
In this case, micro diffraction of the Cu-rich precipitates only showed the same diffraction pattern as ferrite matrix with bcc structure (see Fig. 2d). There were also no stacking fault contrast and no twin contrast which are characteristics of twinned 9R (3R) structure, observed in the high resolution image (see Fig. 2e). It is therefore concluded that the structure of precipitates of ∼7 nm was coherent bcc-Cu at aging temperature of 250°C under pre-strain conditions. If no pre-strain had been applied before aging, the precipitation would not have occurred at 250°C. Because copper precipitation in both iron and steels occurs above 350°C according to the previous literatures.2,17 In this case, the copper precipitation in pre-strain condition can be attributed to an increase in the dislocation density due to the deformation. The copper precipitation in pre-strain condition can be attributed to an increase in the dislocation density due to the deformation. It has been reported that the presence of dislocations in the matrix favours copper precipitation during aging process.2,18 The principle relies on a decrease in the activation energy for nucleation of the copper precipitates, most probably by a relief of the coherency strains. 16 With high dislocation density, nucleation of Cu precipitates is promoted due to their higher energy and copper diffusion is improved due to fast diffusion paths along dislocations. This simply means that an acceleration of the precipitation kinetics will be occurred in pre-strain condition.
The mechanical properties of all the samples are shown in Fig. 3 and Table 2. As seen in Fig. 3a and d, with the increase of pre-strain, the strength increases remarkably while the elongation decreases slightly. The yield ratios of all samples were significantly increased and exceeded 0.90 in comparison with that of the unaged Cu-LCS sample (0.77). If the error bars were taken into account, impact toughness did not experience obvious change for pre-strains of 2.5, 5.0 and 7.5% after strain aging (see Fig. 3c). The dispersion of the nanoparticles provides stronger interactions and obstacles to the dislocation motion, as seen from bowing of dislocation, thus resulting in an increase in the strength of the steel. Compared with the change of the strength, the elongation and impact toughness of the steel did not experience an apparent decrease due to the particle size remaining extremely small (∼7 nm). The dislocation density will increase with the increase in strain from 2.5, 5.0 to 7.5%. It is therefore concluded that the strength and yield ratio gradually increase with the increase of the pre-strain.
Effect of strain aging on mechanical properties: a stress–strain curves; b yield ratio; c impact toughness; d the change value Effect of strain aging on mechanical properties of samples
During controlled rolling process of thermo mechanically controlled processed for welded pipe, carbon and nitrogen go into solution supersaturatedly in the ferrite phase because of rapid cooling after controlled rolling. Aging of pre-strain material allows the interstitial solute atoms to diffuse to the existing dislocations, forming Cottrell atmospheres and pinning them, which causes strain aging phenomenon. 10 In this case, during the heat-treatment process of Cu-LCS (quenched/tempered), all carbon has been precipitated with the form of cementite according to Fe–C binary phase diagram. This simply indicates that few free carbon atoms exists in ferrite for Cu-LCS. Therefore, conventional seamless pipe, which have a ferrite and carbide precipitate structure, has few strain aging phenomenon, as initially they contain no pinned dislocations. It is worth pointing out that the presence of the plateau at the elastic-plastic transition clearly measured in Cu-LCS before strain aging. The origin of this phenomenon is not clear at this stage and need further study.
Conclusions
Static strain aging tests with different amounts of pre-strain were performed on Cu-containing microalloyed low carbon steels. According to the changes of the microstructure and mechanical properties of pipeline steel, the main results can be summarised as follows:
After strain aging, the strength of pipeline steel significantly increased while the elongation and impact toughness did not increase apparently. The structure of precipitates of ∼7 nm is coherent bcc-Cu and the precipitation is attributed to an increase in the dislocation density due to pre-strain used in this study. Unlike welded pipe, few strain aging phenomenon could be observed in seamless pipe due to few carbon atoms in supersaturated solid solution diffuse to the mobile dislocations, forming Cottrell atmospheres.