Abstract
The microstructure and precipitate characterizes in the API X90 hot bend related to mechanical properties was investigated by OM, TEM, EBSD and mechanical tests. X90 pipeline steel is consist of quasi-polygonal ferrite (QPF), acicular ferrite (AF), lath bainite (LB) and a small amount of M/A constituents. The width of bainite lath is about 0.2 ~ 0.3 μm. After hot induction bending, hardly observe AF in the bend zone. In the outer arc side, the width of LB was coarsened to 0.53 ~ 1.34 μm, and sharp M/A constituents formed along the prior austenite grain boundaries. Compared to the parent pipe, the strength of X90 bend decreased 30 ~ 80 MPa, and the Charpy impact energy increased 20 J. The outer arc side with the weakest low temperature impact toughness, is 153 J. The main component of the precipitate is NbC with a small amount of TiC, possibly (Ti, Nb) C, and the size is about 15nm. The fraction of the high angle grain boundaries (HAGBs) and the kernel average misorientation (KAM) value of the outer arc side is 21% and 0.62 ° respectively, which is the higher than the other positions.
Keywords:X90; hot induction bending; microstructure; TEM; EBSD
Introduction
To increase transportation efficiency, high strength thick-walled pipeline steels are used to transport the crude oil and natural gas under high press. However, high strength is generally achieved at the expense of reduced toughness and ductility. Low carbon and rich microalloying design is preferred because of excellent low-temperature impact toughness and high strength combination [1-3]. To achieve an improvement of strength and toughness combination, thermos-mechanical control processing (TMCP) is the common way and grain refinement plays an important role [4-7]. API (American Petroleum Institute) X70 and X80 grade steels are achieved by TMCP development rapidly, and employed in the field of high pressure, large diameter, long distance transportation. However, API X90 ~ X120 pipeline steel as the next generation of pipelines has been researched to enhance the strength in the future. The bends with large wall thickness and high strength grade, not only change the direction of pipeline transportation in accordance with the terrain requirements, but also buffer the tensile, compressive stress and torsion attach to the pipeline because of ground movement, earthquakes and environmental temperature changes [8-9].
For the pipeline steel in service, microalloying design and TMCP parameters changing promoted the excellent mechanical properties. Addition of microalloyed elements Ti, V, Nb form fine carbide and carbonitride precipitates, which dispersed uniform sufficiently to achieve precipitation strengthening and refine the grains effectively [10]. Literatures [11-13] reported that the precipitation behavior and the relationship to mechanical properties of low carbon microalloyed steels during TMCP schedule. However, the manufacture of thick-walled large-diameter bends is mostly carried out by hot induction bending, which with the double effect of re-heating and deformation affecting the original microstructure and mechanical properties, especially the thick-walled pipeline steels were obtained by the TMCP process. In general, bending parameters would seriously affect the performance of the bend, such as the re-heating temperature, push speed, cooling water flow, tempering temperature. The re-heating temperature is order to austenitizing the microstructure that can be plastic deformed under the pushing force. Therefore, the heating temperature is higher than the austenite phase transition temperature, in the range of 950~1050 °C [14-18]. Microstructure transformation, fine carbide (or carbonitride) precipitates dissolving and redistribution causing an unstable factor during the hot induction bending process [19].
The high-strength pipeline steel is easily obtained the complex microstructure of acicular ferrite(AF) (or bainite ferrite(BF)), granular bainite(GB) and martensite/austentite (M/A) constituents after hot induction bending [15-18]. Only a part of carbide (or carbonitride) precipitations formed by microalloying elements Ti, V, Nb dissolved. The precipitations are benefit for prevent grain boundary moving to prevent grain growth significantly. In addition, the microstructure almost distributed with different orientation, and a large number of dislocations existed in fine grains. The composite microstructure of AF and GB ensures that the bend with a good toughness, but the plasticity and large strain resistance is difficult to reach the level of the parent pipeline steel [17, 18]. Many literatures [18, 20, 21] investigated the hot induction bend were focus on the effects of hot bending parameters on microstructure and mechanical properties, and the relationship of microstructure and mechanical properties through thermal simulation tests. However, the deformation in the bending process was not take into account. The research about the microstructure and mechanical properties changes in the bend zone still has a lot work waiting to be completed.
The hot induction bending process for API X70, X80 pipeline steel has been studied maturely, especially the effect on the microstructure and mechanical properties, but the research on API X90 hot bending process has not been completed. It is therefore the main objective of this paper to address the microstructure and precipitate in the bend zone of thick-walled API X90 pipeline steel related to mechanical properties. The evolution of microstructure, precipitation distribution, grain boundaries distribution, local strain distribution and mechanical properties in API X90 bend are established.
Experimental procedure
The material in the study for investigation is submerged arc welded API X90 grade pipeline steel with Φ1219 mm × 26.4 mm. The chemical components are listed in Table 1. The experimental steel is low in carbon content and rich microalloying elements with contains 0.065% C, 1.82% Mn, 0.43% Mo, and Nb+V+Ti less than 0.1%. The relatively low carbon content ensures good weldablity, toughness and ductility. The role of small amount of Ti was to tie-up nitrogen and limit grain growth, while microalloying element Nb facilitates grain refinement and provides strengthening via precipitation as NbC in the ferrite matrix and on the dislocations [22]. Element boron addition reduced Ar3 and Ar1 temperature about 100 ℃ that can produce bainitic ferrite and martensite, and effectively suppress pearlite and ferrite formation that revealed the increase of hardness related to tensile strength [23]. Microalloying elements Cu, Ni, Cr are added to promote the corrosion resistance.
Table 1 Chemical components of X90 pipeline steel (wt.%).