CTOD Testing and Evaluation for Weld Joint of UOE Pipeline Steel

MIAO Ting,MIAO Zhang-mu,LIU Shuo,CAO Neng,MA Zhao-hui

(1 School of Transportation of Wuhan University of Technology,Wuhan 430063,China;

2 Baoshan Iron&Steel Co.,Ltd,Shanghai 201900,China)

CTOD Testing and Evaluation for Weld Joint of UOE Pipeline Steel

MIAO Ting1,MIAO Zhang-mu1,LIU Shuo2,CAO Neng2,MA Zhao-hui2

(1 School of Transportation of Wuhan University of Technology,Wuhan 430063,China;

2 Baoshan Iron&Steel Co.,Ltd,Shanghai 201900,China)

In accordance with the British Standard BS7448,Crack Tip Opening Displacement(CTOD)fracture toughness tests are conducted at different grades of UOE pipe steels.Combining with the standards CSA Z662-07 and NDV-OS-F101,this paper improved the fracture toughness evaluation in respects of the specimen types,notch positions and post-test metallography.The result shows that both X70 and X65 grades of LSAW(longitudinal submerged arc welded)pipeline steel have good toughness properties,and meet the requirement of the NDV-OS-F101 standard.X70 pipeline steel toughness of different parts distribute range from best to worst is:base metal,weld metal and HAZ(heat affected zone).The HAZ toughness of X65 pipeline steel is better than its base metal and weld metal.Because the base metal of X65 pipe steel contains a number of inclusions and pores and has delamination cracks on its fractograph.Thus lead CTOD test results to be highly disperse.Besides,as it contains a large amount of brittle structure M-A islands and mischcrystal structures which usually distribute unevenly,the weld metal toughness of X65 pipe steel declines.The results provide a basis for the improvement of offshore pipeline steels manufacturing and lay the foundation for the security assessment of offshore structures.

pipeline steel;CTOD;toughness;weld joint

1 Introduction

As the development of offshore engineering,the projects about the transportation of crude oil and natural gas have been increasing.Due to the transportation of higher capacity and longer distance of gases,the security of the pipeline has tended to be taken into consideration seriously.Recent trends in energy industry tend to use more efficient pipeline steels which have the properties of high-stress,high-toughness,big diameter and the capacity of working under a high pressure.UOE pipe production process,as the most advanced process with Longitudinal Submerged Arc Welding(LSAW),possesses the features such as high quality and high productivity.Based on meeting the current domestic and foreign demands for the high grade and high strength structural steel pipes,UOE pipeline steel is widely used in projects about transportation of crude oil and natural gas.

However,in the welding process,the defects and microstructural degradations tend to exist in the weld joint.As a result of being in a marine environment for a long time,the welding defects easily become to the weakest position of pressure pipelines.By the influence of the harsh marine environment such as cold seawater,earthquake and tsunami,these defects lead to the initiation,propagation and even unstable fracture of crack[1].Therefore,besides the study of fracture toughness on pipeline steel base metal,the weld joint fracture toughness research is also very important.

Conventionally,Charpy V-notch impact(CVN)test were adopted to assess the fracture toughness of pipeline steel.However,with the improvement of pipeline steel grades,and changes of service environment,CVN test value is no longer satisfied with the requests from the standard of quality acceptance and toughness evaluation.There is a great difference between the test environment and practical situation in pipeline operation.So that the measured test values cannot really reflect the resistance to initiation fracture of pipeline steels.As an important elastic-plastic fracture parameter,because of its intuition and testability,CTOD is gradually widely used by the researchers,and has become one of the important criteria to evaluate pipeline steel fracture toughness[2].

In accordance to the requirements of fracture mechanics toughness test standard BS7448[3-4],the CTOD criterion is adopted to carry out the test for UOE pipeline steel.And based on the oil and gas pipeline system standard CSA Z662-07[5]set by Canadian Standards Association and the submarine pipeline systems specification NDV-OS-F101[6]set by DET NORSKE VERITAS,the validity check of specimens and acceptance for test results have been conducted.Through the study of weld joint toughness in various regions of different grades of UOE pipe steels,this paper reveals the performance of the crack-resisting of UOE pipe steels.Thus lay the foundation for the security assessment to UOE pipelines.

2 Specimen preparation

2.1 Sampling

Test materials were taken from the UOE LSAW pipeline steels with different grades at X70 and X65.The specifications of weld pipes,basic mechanical properties and test temperatures are listed in Tab.1.

According to the British Standard BS7448[3],three point bend specimens shall be used in the test.They shall be intercepted by Wire cut Electrical Discharge Machining(WEDM)from the longitudinal welded pipes.Based on the characteristic of UOE LSAW pipes,only the longitudinal weld specimen is considered.The thickness of the specimen should be made as close as possible to the original thickness,so that the CTOD characteristic value can reflect the integral toughness of the pipeline steel accurately.According to the standard BS7448[4]requirements,in order to get the single edge notched bend(SENB)specimen conform to the full section thickness requirements,a method for straightening specimen and minimizing the curvature of pipe specimens should be established.The specific method is to take partial three point bending on blank samples.Among them,for the requirements in BS7448 Part2[4]about welding joint specimens processing,the weld metal should be protected in the straightening process.The base metal specimens can be directly straightened.After blank specimens straightened,CTOD specimens shall be worked out as thick as possible.The specific sampling position and the sampling method after straightened are shown in Figs.1-2.

Fig.1 Sampling position of pipeline

Fig.2 Sketch of sampling process P-Rolling direction(parallel to the weld direction)

2.2 Notch placement

On the basis of the request for CTOD test in the Canadian standard CSA Z662-07[5]section K.4.4,this paper conducted the through-thickness notched specimen test on the base metal and weld metal to measure the fracture toughness in various positions.

In addition,the specification[5]also puts forward CTOD test requirements for the hardest area in the HAZ(the fracture toughness usually poor in this area),which is one of the key research in this test.Standard[5]-K.4.4.3-TableK.2 has following requirements of sampling in HAZ regions:(a)Where H(region of maximum hardness)is within 0.15 t(thickness of the specimen)of either specimen surface,a preferred geometry specimen with a through-thickness notch positioned through H should be used.(b)Where H is more than 0.15 t from both specimen surfaces,a subsidiary geometry specimen(a surface notched specimen)should be used with the notch cut from the nearer specimen surface and the notch tip located at H.The sketch of notch placement is shown in Fig.3[5].

In order to establish the right type of the specimen for CTOD test in weld joint HAZ,the hardness test should be taken in HAZ.Because of that the CGHAZ(coarse-grained heat affected zone)is the area with relatively high hardness.And according to the standard[5]-10.12.5,the hardness testing of welds demands:A minimum of five indentations shall be made in the CGHAZ at each weld toe.Therefore,the distribution of hardness in CGHAZ is the key research.This paper measured using Vickers methods based on indenter loads at 10 kg,and combined with NDV-OS-F101[6]to decide the test points.The distribution of hardness test points is shown in Fig.4,CGHAZ hardness distribution of X70 and X65 pipeline steels are depicted in Fig.5.

Fig.3 Specimen notch placement(a)Throughthickness notched specimen;(b)Surface notched specimen

Fig.4 Distribution of Vickers hardness test points

Fig.5 CGHAZ hardness distribution of X70 and X65 pipeline steels

From the hardness test result,the distribution form of CGHAZ hardness of X70 and X65 pipeline steels,presents as the values in the middle larger than those on both sides.Therefore the region of maximum hardness in HAZ is more than 0.15 t from both specimen surfaces.This meets the latter of the above requirements.That is to say,the surface notched specimen test should be taken for HAZ.

In accordance with sampling method in the standard[4],weld metal specimen take throughthickness notched specimen test,namely according to the NP direction sampling.HAZ speci-men adopt surface notched specimen test,namely according to the NQ direction sampling.As shown in Fig.6.

Fig.6 CTOD sampling direction,N-Normal to weld direction;P-Parallel to the weld direction;Q-Weld thickness direction

2.3 Dimensions of specimen

According to the sampling method and type selecting above,in accordance with the BS7448Part2[4],the dimensions of three point bending specimen in this test have been determined.After finishing the blank sample,the dimensions of the specimens are listed in Tab.2.Among them,the base metal and weld metal specimens adopt a rectangular section bend specimen （B=0.5 ）

W ;Surface notched specimen in HAZ use a square section bend specimen（B= ）

W .Both the spans are about S=4W.(B is the specimen thickness,W is the specimen width)

Tab.2 Dimensions of specimen

2.4 Fatigue precracking

Fatigue precracking shall be done at room temperature with high frequency fatigue testing machine.Based on the Ref.[3],in the process of fatigue precracking,the crack should not extend too fast.Especially for the NQHAZ surface notched specimen,the target area for the crack tip to locate is very small,if crack extends too fast,it may easily exceed the target area.So the specimen will be invalid.Therefore,before fatigue precracking,microstructure of weld joint should be analyzed after polishing and etching the surface of the specimen with nital.In this process,according to the crack propagation rule,mark the position where the surface crack tip should be extended,through that to control the propagation in the fatigue precracking.

Meanwhile,the specimen should be guaranteed that the notch comprising machined notch and fatigue crack shall have a depth of 45%-55%of the specimen thickness for base metal;and 45%-70%of the specimen thickness for weld joint.

3 CTOD test

After the preparation of specimens containing the precrack,the main test shall be conducted.In accordance with BS7448,the main process is as follows:specimen locating and adjusting on the electro-hydraulic servo universal testing machine;loading and recording F-V curve by X-Y function recorder;unloading;additional fatigue cracking and break down specimens;measure the crack length a0and Δa.According to the measured data and related information of F-V curves,combined with standard[3]formula(6),the data calculation will be conducted to get the CTOD values.

4 Test results and analysis

4.1 Fracture morphology

The typical macroscopic fracture morphology of CTOD specimens is shown in Fig.7,7(a)is for BM specimen,7(b)is for NPWM specimen,and 7(c)is for NQHAZ specimen.

Fig.7 Typical fracture morphology in CTOD test

The figure indicates that the typical fracture can be divided into two kinds:brittle crack extension fracture and stable crack extension fracture.Brittle crack extension fracture consists of five parts,from down to up as:machined notch,precrack,stable extension area,instable extension area and broken zone.Compared with the brittle crack extension fracture,the stable crack extension fracture does not have the instable extension area.And the size of stable crack extension is relatively obvious.It has been observed with the shape of crescent,owning a lot of corrugated patterns on it.That is related to the much dislocation slip of the crack tip in plastic deformation process[7].To protect the stable extension area of stable crack extension fracture,additional fatigue crack should be made in front of the extended area.

As shown in Fig.7,X702 is a typical brittle crack extension fracture,and has obvious instable spreading area before the broken area.While for other specimens,all belonged to stable crack extension fracture,present the fracture morphology which has been protected by the additional fatigue crack.

4.2 Post-test metallography and validity check

According to the related standards,the validity check for NQHAZ specimens should be carried out to confirm whether the crack tip falls in the target area.Both the British standard BS7448Part2[4]and the specification NDV-OS-F101[6]set by DET NORSKE VERITAS have put forward the corresponding provisions of post-test metallography and specific test method.Standard[6]-APPENDIX B-A900 provides that post-test metallography shall be applied to the specimens designated for FL/HAZ testing in order to establish if the crack tip has been successfully located in the target microstructure.If the specimen meet the following conditions,it will be regarded as valid:(1)The precrack tip is not more than 0.5 mm from fusion line;(2)CGHAZ microstructure is present within a region confined by a plane perpendicular to the crack plane through the crack tip and a parallel plane 0.5 mm ahead of the crack tip.

According to the post-test metallography requirements in BS7448Part2,the prepared metallographic section shall be taken from the HAZ samples within the central 75%of specimen thickness.Through the observation under the metalloscope for its microstructure,whether the specimen is valid can be confirmed.The typical microstructures(e.g.for NO.658 and No.659)are shown in Fig.8.The microstructure of No.659 specimen section can be seen in Fig.8(a).In the figure,the crack tip crosses the fusion line and just locates in the coarse grain area.The distance form fusion line is 0.1 mm to the parallel plane.Belonging to the limit within 0.5 mm of CGHAZ,thus the specimen can be regarded as valid.On the contrary,for No.658 specimen(see in Fig.8(b)),the crack tip does not pass the fusion line but falls in the weld metal.And the distance from CGHAZ to the plane of the crack tip is 0.3 mm,which being far from the target microstructure in the CGHAZ,is judged as invalid.

Fig.8 Metallographic analysis for specimen section

4.3 CTOD test results

According to the test method and the validity check results above,CTOD test values of valid specimens have been concluded,listed in Tabs.3-4.

Tab.3 X70 grade pipeline steel CTOD values

4.4 Results analysis and discussion

Analyzing the CTOD values of valid specimens,the corresponding distribution of δ(crack tip opening displacement)is shown in Fig.9.

From the above test results,it can be seen that all the CTOD values of X70 pipeline steel in various regions are quite high.The maximum can reach 0.898 mm(base metal),and the minimum is 0.224 mm(HAZ).The data is uniformly distributed on the base metal and weld metal in chart.In general,the longitudinal weld joint of X70 steel pipe has great resistance to initiation fracture,and the property of longitudinal weld performs uniformly on vertical rolling direction.Though with some dispersion,the average data of HAZ results can reach 0.485 mm,and also has good toughness.The order of CTOD values ranges from high to low is:base metal,weld metal and HAZ.It indicates that base metal possesses the best resistance to cracking,weld metal possesses slightly worse,and HAZ possesses the worst.

Fig.9 Distribution of δ values for X65,X70 grade pipeline steels

And through observing the distribution of δ values,it is not difficult to discover that compared with X70 grade pipeline steel,the condition of regional toughness in grade X65 pipeline steel material has no obvious law.Moreover,the CTOD values of base metal are even lower than the values of weld joint HAZ.In Fig.7(a),the macroscopic fractures of the base metal specimens with lower-values,X65P3 and X65P7,have delamination crack appeared on them.The delamination crack is a kind of crack usually appeared in the base metal.It appears mostly because of the rolling defects such as inclusion,segregation and banded microstructure exist in the material.In the hot rolled coils,all these defects are parallel to the rolling direction of the steel plates.Under the stress perpendicular to the defects,the tip part of these defects will produce high stress concentrations,thus make delamination cracks appeared at the root of the notch[8].Through observing the microstructure of base metal by using scanning electron microscopy(SEM)(see Fig.10(a)),it is also found that the base metal of X65 pipeline steel contains a certain amount of pores and inclusions.These rolling defects have some influence on the initiation and extension of cracks.That is also the causes of the appearance of delamination cracks on X65P3and X65P7 fracture surfaces,and the worse values of the CTOD toughness evaluation for base metal.

Fig.10(b)and 10(c)represent the microstructure morphology respectively for weld metal and CGHAZ of X65 pipeline steels.In Fig.10(b),it can be observed that the X65 pipeline steel weld is mainly composed of different multiphase microstructures such as irregular block and lath proeutectoid ferrite,and M-A constituents which distribute dispersedly in grain interior and grain boundary.M-A constituent can be also called martensite-austenite constituent or island structure,usually appear in the weld metal or HAZ of the low carbon high stress quenched and tempered steels.It has great influence to the toughness,and received enormous attention from the domestic and foreign researchers[9].M-A island structure looks bright white under the SEM and has convex sense.Form the Fig.10(b),it can be known that the microstructure of weld metal is the mixed crystal structure composed of polygonal ferrite(PF),lath ferrite(LF)and M-A constituent.The distribution of the grains is quite asymmetrical.The M-A constituent on the ferrite matrix is unevenly distributed as blocky shapes.Some of the M-A islands present as large sizes and cuspate shapes.While in Fig.10(c),it can be seen that X65 pipeline steel CGHAZ microstructure mainly consists of granular bainite(GB).There are many quasi-polygonal ferrite(QF)grains in the CGHAZ which has retained some of austenite grain boundaries.The distribution of grains has no obvious orientation and is more homogeneous.There have less M-A constituents in the CGHAZ,and most of them are dispersedly distributed in a granular form.

Fig.10 SEM microscopic morphology in different regions of X65 steel

As is known to all,M-A constituent is a kind of brittle microstructures,the number,size,shape and distribution of the islands will have some impact to the pipeline steel toughness.The ideal situation for the M-A inlands should be less quantity,small size,uniform distribution and as granular shape.The M-A constituent with long strip shape or cuspate shape is negative for the properties of the material[10].

Based on the above analysis,it is easy to see that as being the mischcrystal structure,the grains of X65 pipeline steel weld metal distribute unevenly.While the mischcrystal structure can increase the hardness,that will have an adverse effect on toughness.In addition,these M-A inlands compared with the M-A in CGHAZ,have the characteristics such as large quantity,uneven distribution,large size and in the shape of stripes and cusps.Thus lead directly to the weld metal toughness worse than CGHAZ.The grain morphologies of CGHAZ microstructure are polygon block shaped.The grain boundaries are mutually occluding together,making the structure able to resist the crack expansion effectively.This is the reason that the NQHAZ specimens could possess high toughness in the CTOD test evaluation.

5 Conclusions

(1)According to Canadian standard CSA Z662-07[5],and combined with British standard BS7448[3-4],the pipeline steel specimens for base metal,weld metal and HAZ have been prepared respectively.Through the Vickers hardness test method to research the material properties of various areas on HAZ,the appropriate type of specimens has been chosen to conduct CTOD toughness evaluation.CSA Z662-07 standard[5]has put forward new preparation requirements of pipeline steel CTOD test.That will promote the research of HAZ toughness,and provide important basis for welded pipeline system safety assessment.

(2)For HAZ CTOD test,post-test metallographic analysis of the grains near the fracture surface is very important.When the structures that the crack tips locate in correspond to the characteristics of CGHAZ organizations,the specimens are valid.Mainly based on NDV-OSF101[6],this research has used the post-test metallography method in standard BS7448[4]to do the metallographic analysis for the microstructure near the crack tip.From the result,all the CTOD values of valid CGHAZ specimens are greater than 0.15 mm (conform to NDV-OSF101[6]),showing good toughness.

(3)All the CTOD values are generally high in the various regions of X70 pipeline steel longitudinal weld joint.That presents this kind of welding joint possessing good fracture toughness,and having great resistance to cracking.The CTOD values of various regions range from high to low is:base metal,weld metal and HAZ.Namely that the base metal possesses the best toughness,weld metal possesses slightly worse,and HAZ possesses the worst.In the point of view of data distribution,CTOD value distribution of the base metal and weld metal are more homogeneous,having certain regularity.The data distribution of HAZ is more dispersive than the former two.

(4)The base metal microstructure of X65 pipeline steel contains a certain amount of pores and inclusions which have some effect on the crack initiation and expansion.These defects are easy to produce delamination cracks on the fracture surface.All of these lead to the outcome that the CTOD test results are highly disperse and even lower than the result of HAZ.

(5)The CGHAZ microstructure of X65 pipeline steel mainly consists of GB and dispersedly distributed M-A constituents.The amount of M-A islands is small in CGHAZ,and usually appears as granular shape.On the contrary,as the mischcrystal structure composed of PF and LF,the distribution of weld metal microstructure is uneven.The M-A islands as brittle structure often appear as large size and cuspate shape in the weld metal.And its large amount and uneven distribution directly lead to the decline of fracture toughness.It is also the reason why the cracking resistance capacity of weld metal is poor in the CTOD toughness evaluation.

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UOE鋼管焊接接頭CTOD斷裂韌性評定方法

苗 婷1，苗張木1，劉 碩2，曹 能2，馬朝暉2

（1武漢理工大學(xué) 交通學(xué)院，武漢 430063；2寶山鋼鐵股份有限公司研究院（技術(shù)中心），上海 201900）

根據(jù)英國標準BS7448，對UOE直縫埋弧焊管不同鋼級管線鋼焊接接頭進行裂紋尖端張開位移(CTOD)斷裂韌度測試。結(jié)合加拿大標準CSA Z662-07及挪威船級社規(guī)范NDV-OS-F101，在試樣形式選擇、缺口取樣方向、后期金相檢驗等方面對斷裂韌性評定方法加以完善。試驗結(jié)果表明X70、X65級管線鋼均具有良好韌性，符合規(guī)范NDV-OS-F101的要求。X70級管線鋼焊接接頭各區(qū)域韌性分布規(guī)律為：母材性能最好，焊縫次之，熱影響區(qū)（HAZ）相對較差。X65級管線鋼其HAZ的韌性優(yōu)于母材和焊縫。其原因是：X65級鋼母材含有一定氣孔、夾雜，斷口出現(xiàn)分層裂紋，導(dǎo)致CTOD結(jié)果分散性大。另外，X65焊縫組織為混晶組織分布不均，含有大量M-A脆性組織物，導(dǎo)致韌性下降。該評定結(jié)果為海底管線鋼制造工藝改進提供了依據(jù)，也為海洋工程結(jié)構(gòu)安全性評估奠定了基礎(chǔ)。

管線鋼；CTOD；斷裂韌性；焊接接頭

TG142.1

A

苗 婷（1987-），女，武漢理工大學(xué)交通學(xué)院博士研究生；

馬朝暉（1968-），男，寶鋼股份研究院（技術(shù)中心）結(jié)構(gòu)鋼研究所所長。

TG142.1

A

1007-7294（2012）12-1468-11

date:2012-10-22

Biography:MIAO Ting(1987-),female,Ph.D.student of Wuhan University of Technology,E-mail:miaoting1615@gmail.com;MIAO Zhang-mu(1957-),male,professor/tutor,Email:zmmiao1957@yahoo.com.cn.

苗張木（1957-），男，武漢理工大學(xué)交通學(xué)院教授/博士生導(dǎo)師；

劉 碩（1978-），男，寶鋼股份研究院(技術(shù)中心)焊接與表面技術(shù)研究所工程師；

曹 能（1964-），男，寶鋼股份研究院（技術(shù)中心）焊接與表面技術(shù)研究所工程師；