Research on mechanical properties of X100 pipeline steel after JCOE welded

Research on mechanical properties of X100 pipeline steel after

JCOE welded

Zhu Lixia

Tubular goods research center, CNPC

zhulx@ Qu Tingting Tubular goods research center, CNPC Qutt@

Li Na Tong Ke Zhuang Chuanjing

Tubular goods research center, CNPC Tubular goods research center, CNPCTubular goods research center, CNPC

Lin@ Tongk@ zhuangcj@

Han Xinli He Xiaodong Li Jinfeng

Tubular goods research center, CNPC Tubular goods research center, CNPCTubular goods research center, CNPC

Hanxl@ hexd@ Lijf@

Abstract: The Bauschinger effect, working-hardening and mechanical of X100 pipeline steel after JCOE welded were investigated by simple axial tension test. The effects of deformation microstructure and dislocation arrangement on tension properties were also studied by OM and TEM analysis. The results indicated that: working-hardening was occurred during JCOE welded processing, and long-range distributed dislocation glided along the sliding plane under the deformation stress to form high density dislocation cells on phase interface of acicular ferrite and matrix, resulting in high tensile strength and low ductility.

Key Word: X100 pipeline steel; JCOE; Work-hardening; Microstructure

0 Introduction

The pipeline transportation of petroleum and natural gas have been recognized as one of the most economic and safe way. The increasing length and discharge pressure of pipeline associated with their construction and operating costs has lead to the development of a new steel grades with higher performance. X100 is a new high strength steel for pipeline applications which can reduce overall cost about 5%~12% [1]. Recently, X100 pipeline steel has been developed and already tentatively used in some countries such as America, North Canada and Japan [2-3] one after the other. The research of X100 pipeline steel in our country is still under development, many steel corporations have trial-produced X100 pipeline steel successfully and rolled to LSAW line pipe by JCOE pipemaking process. During the pipemaking process, strength changes would occur from plate to pipe. During “ JCO” process, the outside of the pipe stand tension stress while the inner surface stand compression stress which engender a spot of plastic deformation; During the succeeding expanding process, the pipe stand expanding deformation which the whole pipe stand tension stress. Control the regularity of strength variation will provide according for pipemaking processing and pipeline safety design.

The object of this work presented was focus on microstructure and mechanical properties of X100 TMCP plate and LSAW pipe after JCOE pipemaking process, respectively. In addition, the effects of microstructure before after JCOE pipemaking process on tensile properties were also discussed, which may provide basic for pipemaking process.

1 Experimental

The pipeline steel used in this paper is X100 TMCP plate and LSAW pipe which rolled by TMCP plate. The specification of the X100 TMCP plate and pipe rolled by JCOE pipemaking process are 2500mm×15.0mm and Ф813mm×15.0 mm, respectively. Chemical composition of X100 pipeline steel is shown in table 1. Transverse direction tensile samples with diameter of

6.25mm and gauge of 25mm were cut from one-half wide of plate and at 180° from pipe weld seam, the samples were located in the center of thickness and machined to avoid work-hardening and overheating. Axial tension test was performed on MTS 810 material testing machine according to standard ASTM E 8/ E 8M.

Table 1 Chemical composition of X 100 pipeline steel (Wt×10-2)

Elements C Si Mn P S Cr Mo

X100 0.059 0.16 1.86 0.0088 0.00100.130.28Ni Nb0.38V Ti Cu B Al 0.0780.00560.012 0.22 ＜0.0001 0.0282 Results and discussion

2.1 Bauschinger effect and working hardening during JCOE processing

Table 2 shows the results of tensile tests of the plate and pipe. It is obvious that the pipe exhibited higher strength and lower ductility after JCOE processing, compare to that of plate. For example, the tensile strength of the pipe after JCOE pipemaking process increased 9.71% and the yield strength increased 13.84% compare with the plate, while the elongation decreased 9%. The result of tensile tests suggests that strength hardening occurs during the JCOE pipemaking process.

Table 2 Tensile properties of X100 pipeline steel

Item Results Tensile strength, MPa Yield strength, Mpa Elongation, % Plate Pipe Plate Pipe Plate Pipe 776，769 880，880 657，660 722，724 21.0，23.5 19.5，20.0

Average results 773 880 659 723 22.0 20.0

The tensile stress-strain relationship of X100 TMCP plate and LSAW pipe after JCOE pipemaking process is shown in fig.1. It is evident that a continuum yield character was observed during the X100 TMCP plate tensile tests. The stress of the X100 TMCP plate increased with stain and reached to yield point with lower stain than that of JCOE processed LSAW pipe. Furthermore, the stress of the JCOE processed LSAW pipe increased with stain reaching a maximum at the beginning of tensile test, and then gradually decreased with increasing strain, the flow stress became smoothly and stably when the tensile test ended. It is worth to notice that a max stress was found in the stress-strain relationship of JCOE processed LSAW pipe, and this is not observed in that of TMCP plate, which confirmed that the work-hardening occurred during the JCOE pipemaking processing. 1000

800

Stress /MPa600400

200

plate

0.000.04pipe0.080.120.000.04

Strain0.080.12

Fig.1 The stress-strain curves of X100 plate and pipe

It is well accepted that the outside surface of the pipe resists tension stress while the inner surface stands compression stress during “JCO” process. The bauschinger effect occurred in the inner surface of pipe where the reversed stress were existed, which lead to low strength. While the deformation of the outer surface of pipe after “JCO” process caused work-hardening, and resulted in the increases of strength. In addition, an deformation strengthening was obtained during the succeeding expanding process, which is considered to be the other reason for the high strength. The above experimental results indicate that the tensile strength of pipe compare to the plate was the competition result by bauschinger effect and hardening.

Recently, it is reported that [4-5] no obvious varies in strength were observed after JCO process, which indicate that the bauschinger effect have similar influence on the strength of inner surface compare with that of work-hardening occurred in the outer surface of pipe during the “JCO” process of the plate, and the variety between plate and pipe mainly depend on work-hardening occurred in expanding process, which is the main reason for the high strength.

2.2 Influence of deformation microstructure on tensile properties

The characterizations of dislocation glides during the deformation were dependent on the microstructure of the metal material and straightly reflect the macro properties. In order to study the effect of deformation on mechanical properties, the metallurgical structure of the X100 pipeline steel has been performed in this work as shown in fig.2, where the microstructure of plate and pipe are mainly consisted of granular bainite and M/A constituent, and no obervious changes

were observed before and after pipemaking process.

Fig.2a Microstructure of X100 plate

Fig.2b Microstructure of X100 pipe

The dislocations arrangements before and after pipemaking process are investigated by high resolution TEM, and the results are shown in fig.3. Fig.3 (a) display the microstructure and dislocation arrangement obtained before pipemaking process, where the typically acicular ferrite was found in the X100 TMCP plate. Furthermore, it is observed that dislocations were long-range distributed in acicular ferrite (Fig.3 b), which is may produced by rolling process. While after pipemaking process, the TEM observations showed that the acicular ferrite were deformed by shear stress (Fig.3c), and higher density dislocation concentrated in the interface of acicular ferrite and matrix, which formed a dislocation cellular structure.

(a)

(b)

(c) (d)

Fig.3 Deformation microstructure of X100 pipeline steel ((a, b) Plate; (c, d) Pipe)

In general, the distribution of the dislocations during the JCEO process is shown schematically in fig.4. Dislocations moved and pile-up in the interface of acicular ferrite and matrix to form the cellular structure after pipemaking process, which result in work-hardening

effect.

Fig.4 the formation of the dislocations cell（a）anchoring of dislocation （b）dislocation cell

The influence of the dislocation on the hardening effect has been studied [6] and main results show that the strength is related to the arrangement, density and distribution of dislocations. In this study, it is found that the density and distribution of dislocations caused by the pipemaking process is the main reason for the different strength of both samples. In order to properly explain the effect of dislocation on YS, an equation based on classical dislocation theory was used as follow [7]:

σ0=σi+αGbρ

Where σ0 is yield strength, σi is yield strength of matrix, α is constant value, about 0.3~0.6, G is Young’s modulus, B is Burger’s vector, and P is dislocation density. It is evident that when temperature and stress during pipemaking process are given, the strength increased with increasing dislocation density, and high strength was obtained as the formation of dislocation cell occurred. It is reasonable to explain the relative variation in the strength.

Low ductility is attributed to the formation of dislocation cell in the interface of acicular ferrite and matrix, which is the principal mechanism for the nucleation of the microcracks. 4 Conclusions

(1) The X100 steel exhibited high strength and low ductility after pipemaking process. Both tensile strength and yield strength of pipe are higher than those of plate, but the elongation is fall down.

(2) Working-hardening was occurred during JCOE welded processing due to the formation of the dislocation cell, which is the main reason for high strength and low ductility.

5 References

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[6] Long Jianming, Yu Hao, Yin Yuqun, et al. Research on Bauschinger effect of X70 acicular

ferrite pipeline steel plate [J]. Journal of Materials Engineering, 2007, (10): 31-44.

[7] George E. Dieter. Mechanical metallurgy [M]. Beijing: Tinghua University publishing

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