An Investigation into Microstructure Evolution and High-temperature Deformation Behavior during the Instability of In-service Welding

It is of great significance to reveal deformation behavior coupling with both the high-temperature effect and microstructure for the in-service welding instability zone. In the present paper, microstructure evolution and high-temperature deformation behavior at the representative temperatures 900 °C and 1100 °C inside the in-service welding instability zone were investigated based on the in-situ high-temperature laser scanning confocal microscope (LSCM). The results indicated that a sufficient condition of carbide precipitation is to hold it at 900 °C for a certain time. However, the phenomenon of carbide precipitation failed to be presented during continuous heating to 1150 °C. Although the slip and twining deformation occurred inside austenite grain, the fracture mechanism gave priority to intergranular cracking, which could be ascribed to the carbides with higher hardness and the grain boundary weakening caused by the temperature effect.


Introduction
Owing to the flowing high-pressure medium inside the in-service pipeline, the in-service welding instability is still the primary problem to be solved, which plays a crucial role in ensuring construction safety and controlling service quality [1].For the sake of the engineering application value, previous studies mainly focused on the in-service welding burn-through macro-criteria, which result in the lack of investigations into meso-and micro-scale high-temperature deformation behavior during the instability of in-service welding [2].
The essence of in-service welding belongs to the welding-heated process.And the in-service welding instability zone involves the transformation from room temperature to high temperature.Moreover, the Battelle criterion indicates that the welded pipe presents the risk of burn-through when the minimum inner-wall temperature is above 982 ℃ [3,4].And the radial deformation criterion points out that it is a typical burn-through sign when the pipe wall beneath the weld pool presents a radial deformation [5].However, the two macro-scale burn-through criteria ignore high-temperature meso-and microdeformation behavior during in-service welding instability before the burn-through, which not only affects the burn-through result but also particularly in-service welding repair quality and service life.Besides, during heating, the microstructure of the in-service welding instability zone transforms from the ferrite phase to the austenite phase.Hence, the fracture behavior coupling with both the hightemperature effect and the microstructure effect is different.Nevertheless, few types of research on meso-and micro-deformation behavior inside the in-service welding instability zone were reported [6][7].
Therefore, the present paper aims to investigate the microstructure evolution and high-temperature deformation behavior during the instability of in-service welding based on the in-situ high-temperature experiments.And the representative temperatures 900 ℃ and 1100 ℃ were selected as the object temperature.

Materials
In the present paper, X65 pipeline steel was selected as the based metal (BM).And the chemical composition and mechanical properties of BM at room temperature are shown in tables 1 and 2, respectively.

Experimental Procedures
A laser scanning confocal microscope (LSCM, VL2000DX-SVF17SP) equipped with a tensile furnace was used to perform the in-situ high-temperature tensile tests in order to investigate the crack initiation and propagation mechanism of the coarse grain zone in pipeline metal, as shown in figure 1.Firstly, the specimens were heated to 950℃ and 1150 ℃ with a heating rate of 100℃/min followed by a holding time of 10 min and 0.5 min, respectively.Subsequently, the specimens were cold to 900℃ and 1100 ℃ with a cooling rate of 50 ℃/min, respectively.The specimens were held at object temperature for a certain time and then loaded at a load rate of 1/s.Hence, the two specimens are referred to as Heat-Tension-900 (HT900) and Heat-Tension-1100 (HT1100), respectively.Figure 2 shows the schematic illustration of the processing steps utilized to heat the specimens used in the present study.And the microstructure evolution and deformation behavior were observed during this process.

Microstructure Evolution
Figure 3 shows the microstructure evolution of the specimen HT900 with increasing temperature.It could be found that the austenitization started to appear when the temperature was heated to around 752.0 ℃.And the grain boundary was relatively unambiguous with the temperature increase to around 780.6 ℃, which indicates that the specimen surface was completely austenized.Subsequently, the austenite grains swallowed each other and grew.And after holding the temperature at 950 ℃, the morphology and size of austenite grains were quasi-stable, as shown in figure 3(d-f).Figure 4 shows the microstructure evolution of the specimen HT900 during the holding temperature of 900 ℃.A small number of carbides first precipitated inside the austenite grain, subsequently, a large number of carbides precipitated along the austenite grain boundary.With the increase in holding time, the precipitated carbides failed to dissolve.Figure 5 shows the microstructure evolution of the specimen HT1100 during the heating.It could be noted that the phenomenon of carbide precipitation failed to be presented during continuous heating to 1150 ℃.Compared with result of figure 4, this displays the evidence that a sufficient condition of carbide precipitation is to hold it at 900 ℃ for a certain time.

High-temperature Deformation Behavior
Figure 6 shows the load-displacement curve of high-temperature tension at 900 ℃.The curve was divided into four stages: elastic stage, uniform plastic deformation stage, neck-shrinkage deformation stage as well as fracture stage.Figures 7 and 8 show the deformation behavior of uniform plastic deformation stage and neckshrinkage deformation stage from the specimen HT900, respectively.The plastic slip deformation first occurred inside the austenite grain.With the increasing load, local twining deformation emerged owing to local slip obstructed.Besides, the grain boundary around the carbides was easier to crack, as shown red arrow in figure 8.This could be attributed to two reasons: 1) the carbides with higher hardness, 2) the grain boundary weakening.Therefore, the temperature effect resulted in the grain boundary weakening and cracking during the neck-shrinkage deformation stage.Consequently, the fracture mechanism gave priority to intergranular cracking during the fracture stage, which is consistent with the results of in-service welding experiments [8].

Conclusions
Microstructure evolution and high-temperature deformation behavior at the representative temperatures 900 ℃ and 1100 ℃ inside the in-service welding instability zone were investigated based on the in-situ high-temperature LSCM.The main conclusions were drawn as follows: (1) Austenitization started to appear when the temperature was heated to around 752.0 ℃.And the austenite grains swallowed each other and grew.And after holding the temperature at 950 ℃, the morphology and size of austenite grains were quasi-stable.
(2) A sufficient condition of carbide precipitation is to hold it at 900 ℃ for a certain time.And the carbide existed inside the austenite grain and along the grain boundary.However, the phenomenon of carbide precipitation failed to be presented during continuous heating to 1150 ℃.
(3) Deformation at 900 ℃ was divided into four stages: elastic stage, uniform plastic deformation stage, neck-shrinkage deformation stage as well as fracture stage.Although the slip and twining deformation occurred inside austenite grain, the fracture mechanism gave priority to intergranular cracking, which could be ascribed to the carbides with higher hardness and the grain boundary weakening caused by temperature effect.

Figure 1 .
Figure 1.In-situ observation of high-temperature tensile test.(a) LSCM, (b) an overview of the load stage, (c) the environmental chamber, (d) sample size.

Figure 2 .
Figure 2. Schematic illustration of the processing steps utilized to heat the specimens used in the present study.

Figure 3 .
Figure 3. Microstructure evolution of the specimen HT900 with increasing temperature.

Figure 4 .
Figure 4. Microstructure evolution of the specimen HT900 during the holding temperature at 900 ℃.

Figure 5 .
Figure 5. Microstructure evolution of the specimen HT1100 during the heating.

Figure 7 .
Figure 7. Deformation behavior of uniform plastic deformation stage from specimen HT900.