Toughness improvement in coarse grain heat affected zone of X70 pipeline steel by accelerated cooling

Introduction

Significant economic advantages can be made by using in-service welding to repair oil and natural gas pipelines, since it avoids the costs of disrupting pipeline operation and secures continuity of supply.1^–3 However, a large amount of heat loss through the pipe wall is created by the flowing gas, resulting in accelerated cooling of the weld. The rapid cooling rates can promote the formation of intermediate transformation microstructures and improve the fracture toughness of the heat affected zone (HAZ). 4 4,5 In the present investigation, Gleeble 1500, optical microscope and transmission electron microscopy (TEM) were used to study the microstructure in the coarse grain HAZ (CGHAZ) of X70 pipeline steel by accelerated cooling. Charpy impact test was conducted in order to investigate the effects of accelerated cooling on fracture toughness.

Experiment

The specimens were machined from domestic X70 pipeline steel with a diameter of 1016 mm and thickness of 21 mm. The main chemical composition of the as received material was 0·05C–0·26Si–1·48Mn–0·027Cr–0·17Mo–0·22Cu–0·15Ni–0·003S–0·012P (wt-%).

The thermal cycle simulation was conducted in a Gleeble 1500 thermal/mechanical simulator. Square bar specimens (10·5×10·5×80 mm) were cut in the longitudinal direction with respect to the rolling direction. The thermal cycle was approximately equivalent to that of shielded metal arc welding 8 mm thick pipeline with heat inputs of 12 kJ cm⁻¹ during in-service repair. This involved heating to a peak temperature of 1350°C at a linear rate of 200°C s⁻¹ and holding at a peak temperature for 1·2 s and the cooling time from 800 to 500°C (Δt_8/5) in 8 s.

After the simulation test, the specimens were machined with standard V notch Charpy samples of 10×10×55 mm, and the toughness was evaluated at −20°C. Hardness tests were performed using a Vickers pyramidal indenter with an indenting force of 10 kg. Absorbed energy and hardness measurements are reported as an average of five results taken randomly in the heat treated specimens.

The specimens were prepared by conventional grinding and polishing techniques and etched with 4% nital solution, and then the microstructures were observed by an optical microscope. A more detailed microstructure examination was performed using TEM.

Results and discussion

Microstructures of CGHAZ

Figure 1 shows the optical micrographs of base metal (BM) and CGHAZ respectively. The following characteristics are indicated: in the BM (Fig. 1a), the microstructures are mainly acicular ferrite, polygonal ferrite and quasi-polygonal ferrite. It can be seen that the grain size of CGHAZ (Fig. 1b) is larger compared with that of BM for the effect of thermal cycle. The scanning electron micrograph in Fig. 2 reveals the morphology of the microstructure for the corresponding optical micrograph shown in Fig. 1b. It can be seen that the major microstructures of CGHAZ are granular and lath bainites with a large prior austenite grain size. Prior austenite grain boundaries are distinct in the CGHAZ.

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Figure 1

Optical micrographs of a BM and b CGHAZ

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Figure 2

Images (SEM) of CGHAZ: prior austenite grain boundary indicated by white arrow, granular bainite (A) and lath bainite (B)

For determining the fine structure of the CGHAZ, TEM microdiffraction was adopted. A few amount of martensite–austenite (M-A) constituents can be observed as a massive shape, which formed between two ferrite laths in CGHAZ, as shown in Fig. 3. In the TEM image, it can also be observed that there is a continuous film (Fig. 4a) with thickness of several tens of nanometres at the boundary between the ferrite laths.5 The thin film is retained austenite.

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Figure 3

a image (TEM) and b microdiffraction pattern of M-A constituents

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Figure 4

a film structure and b microdiffraction pattern of retained austenite

Properties of CGHAZ

Figure 5 shows the absorbed energy and hardness of CGHAZ and BM. It can be seen that the average absorbed energy of CGHAZ is 228 J at −20°C, with only a decrease of 9·53% compared with that of BM. The embrittlement phenomenon in the CGHAZ has not occurred for the X70 pipeline steel at −20°C with cooling time Δt_8/5 of 8 s. The hardness in the CGHAZ is 240 HV, which is higher than that of the base mental, but the susceptibility of hydrogen cracking may be small. Bruce et al.6 indicated that a significant risk of hydrogen cracking can be prevented when the hardness of HAZ is below 350 HV. Thus, accelerated cooling with appropriate cooling rate is a simple and efficient method to improve the CGHAZ toughness and obtain suitable hardness.

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Figure 5

Absorbed energy and hardness of BM and CGHAZ

Discussion

For the pipeline steel, on cooling from austenite state, bainite ferrite is formed, and the remaining austenite becomes stable, because of the carbon enrichment caused by the growth of bainitic ferrite. This enrichment is completed at about 400–350°C, and the carbon content of the remaining austenite can be reached at 0·5–0·8%. Cooled further, some parts of this austenite decompose into ferrite and carbide, in the temperature range of 300–350°C. If the cooling is rapid enough, this decomposition does not happen. Then, the undecomposed austenite transforms into lath and twin martensites at low temperature,7 and a little amount of austenite is retained. This is the reason for the formation of the M-A constituents.

Since carbon diffusion is insufficient due to the rapid cooling rate, the region of carbon enrichment is small, and the carbon content of the remaining austenite is low. In addition, the stress is generated for the volume expansion of austenite transforming into bainite, which will prevent the remaining austenite from transforming to martensite. Thus, some remaining austenite can be retained at room temperature and formed in the presence of thin film, as shown in Fig. 4.

The reasons for toughness improvement in CGHAZ by accelerated cooling rate during in service welding can be summarised as follows:

Conclusions

Accelerated cooling with cooling time Δt_8/5 of 8 s was introduced for in-service welding X70 pipeline to improve toughness and control microstructures. The average absorbed energy of CGHAZ is 228 J at −20°C, with only a decrease of 9·53% compared with that of BM. The main factor for toughness improvement is that the microstructure has a great amount of granular bainite. In addition, the M-A constituents in CGHAZ are reduced obviously in amount and replaced by good toughness austenite films. Accelerated cooling with cooling time Δt_8/5 of 8 s can be chosen in the field of in-service welding X70 pipeline to control the microstructures and improve the HAZ toughness.