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.%).