Effect of Annealing Process on the Mechanical Properties of X70 Pipeline Steel

The influence of the mechanical properties of X70 pipeline steel under different annealing temperature was studied. The corresponding microstructure was investigated by the Field Emission Scanning Electron Microscopy. The results showed that the yield strength and the tensile strength both experienced from rise to decline with the increase of annealing temperature. The grain sizes were coarse and a large amount of cementite precipitated due to preserving temperature above 550 °C, which induced matrix fragmentation and deteriorate the -10 °C DWTT Toughness. There were little changes on the microstructure and mechanical properties when the annealing temperature was under 500 °C.


Introduction
With the increasing gas pressure and more harsh service conditions, long-distance pipeline is gradually required to possess thick wall, large caliber, high strength and toughness [1,2].Generally, X70 and X80 are applied during most of the major projects all over the world with a thickness from 15mm to 22mm.They are efficient, economic and can last for several decades and have been recognized as the mostly utilized steels for pipeline construction in the petroleum industry [3].It is reported that [4,5], heat treatment is usually adopted to achieve better comprehensive performance during the pipe making majorly for high-grade.Annealing is influential on the mechanical properties especially for low temperature toughness [6].Cleavage fracture splits in separations are usually detected on such material after tensile test, impact test and DWTT (Drop Weight Tear Test) [7].Cleavage in separations is a typical kind of fracture for controlled rolled steels, the mechanism of which however lacks of research.In the present work, the effect of annealing process on the mechanical properties was studied and the formation mechanism of cleavage in separations after DWTT was investigated.

Materials and sample preparation
The material studied in this work is X70 pipeline steel with thickness 20mm.The composition is shown in Table 1.The steel sample was taken from 1/4 position of the width and 45° to the rolling direction by flame cutting and was cut into rough samples with dimensions 420mm × 65mm × T used for tensile test, 45mm × 18mm × T for impact test and 300mm × 90mm × T for DWTT before heat treatment.

Experiment
Muffle furnace was applied for annealing treatment on prepared samples.The samples were heated in the furnace up to 400~650 ℃.Preserved for 4 hours respectively before furnace cooling down to 150 ℃ and then taken out for air cooling.The annealing process is shown in Table 2.After annealing, the rough samples were machined into standard tensile samples (Gauge Length 50mm), impact test samples (10 × 10 × 55mm) and DWTT samples (Pressed Notch).Tensile test was conducted on Zwick100 tensile test machine.The impact test was performed on PSW750 full automatic impact test machine from Zwick at a speed of 20 mm/min.The linear velocity at the point of impact was around 5.5m/s.
DWTT was performed on the JL40000 testing machine from China.

Using Metallography and Field Emission Scanning
Microscope to investigate the microstructure after the samples were polished and eroded by 4% nital.

Tensile test
The tensile properties with different annealing process are shown in Fig.

Microstructure observation
The microstructure was observed by optical microscope and the result is shown in Fig.

Scanning Electron Microscopy Analysis
The SEM microstructure morphologies are shown in Fig.In order to identify the precipitation particles, the energy spectrum analysis was conducted; the result is shown as Fig. 6 and Table 3.The particles are mainly cementite.The cementite precipitation particles can destroy the uniformity of the matrix [8,9].When the material is faced to impact, the stress will concentrate on the junction of the precipitation particles and the matrix and finally lead to fracture.

7 .
All results in weight [%] The cleavage in separations morphology of DWTT fracture of 5# sample which preserved 4h under 600℃ is shown in Fig. Massive initial cracks were detected on the direction which the crack travels.Inside these small initial cracks were the cementite precipitation particles.It is obvious that, the stress concentrated around the cementite precipitation particles and emerge initial cracks, more than several cracks began to grow and therefore caused the cleavage in separations and worsened the Low Temperature DWTT Toughness.