Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: To meet the requirements of pipeline consumers abroad,in this study,an automatic gas metal arc welding type of girth welding was performed with a heat input of no more than 0.25 kJ/mm and high-strength solid-wire matching for an X70M large-diameter UOE pipeline.An integral evaluation of the X70M pipeline girth weldability was performed with respect to the girth-weld cold-cracking sensitivity during the welding process and the strengthening,hardening,and embrittlement of the girth weld joint after girth welding in especially rigorous welding conditions with accelerated failure characteristics.The performance of the above girth weld joint with joints welded using the main construction-field girth welding procedure was compared to ensure the good girth weldability of the X70M pipeline.The results of this study have important supervisory significance for field-construction girth welding.Finally,the girth weldability of the X70M pipeline was found to satisfy the field construction requirements even when the welding conditions are especially rigorous.

Key words: pipeline; X70M; special condition; automatic welding; girth weld joint; weldability

1 Introduction

Rapid industrial development has resulted in an urgent global energy requirement.Currently,a low-carbon economy and green industry practices are attracting increasing attention.Correspondingly,clean energy such as natural gas is gradually being widely applied and has broad development prospects.However,most natural gas is located in remote areas far from consumers,which means the critical problem is how to deliver this gas from where it is produced to where the consumers live.The large-diameter long-distance pipeline is an economical,safe,continuous,and highly efficient gas transportation method,and these are unique advan-tages in the transportation of natural gas,petroleum,and slurry.As such,the use of long-distance oil and gas transportation pipelines has surged rapidly around the world[1-2].

For the owner of a pipeline construction project,field girth welding is a key technology in long-distance pipeline construction,and its quality,efficiency,and cost are integral factors.Long-distance pipeline girth welding involves a systematic engineering approach in which the welding quality,efficiency,and cost are determined by many internal and external factors,including the pipeline standard,transportation pressure,medium properties,field construction conditions,welding method selection,equipment capability and experience of the construc-tion group,and the cultural environment.As the pipeline supplier,the pipeline girth weldability merits close attention.First,the pipeline must be able to form an integral welding joint without defects in certain welding conditions,which is called workmanship weldability.Second,the service provided by the weld joint must be safe and reliable in certain environmental conditions.Currently,for the most commonly used low-carbon macro alloying pipeline,the cold-cracking sensitivity during welding and the strengthening,hardening,and embrittlement of the welding joint must be determined[1-3].

There are many methods available for deter-mining the weldability of different materials,in-cluding direct evaluation methods such as the slanting Y bevel joint cracking resistance test,implant test,rigid restraint cracking test,variable restraint hot cracking test,FISCO welding cracking test,and Z direction tensile test.Indirect evaluation methods include the carbon equivalent and cold-cracking sensitivity index method,maximum hardness method,continuous cooling transmission curve method,and heat and stress simulation method.Despite the popularity of these methods,they are known to be fairly conservative with respect to many project cases and analyses.As such,the supervisory approach is inadequate for field construction[1].

In a weldability test,the same welding procedure as that in the construction field is used to evaluate the materials in special conditions and guide field-construction girth welding.However,this weldabil-ity test method cannot reflect the safety margin of the materials.For the pipeline project owner,the greater the weldability safety margin,the wider the application range of the field girth welding procedure,which decreases cost and increases efficiency.Therefore,most consumers around the world refer to the weldability test for especially severe conditions,from which a safe margin can be guaranteed for the convenience of field construc-tion[4].

To satisfy the weldability evaluation requirements in especially severe conditions for pipeline con-sumers abroad,no more than 0.25 kJ/mm super low heat input gas metal arc welding(GMAW) is the type of girth welding conducted with high-strength consumable matching for the X70M large-diameter UOE pipeline.The cold-cracking sensitiv-ity of the girth weld joint was evaluated through macro inspection,X ray testing (XT) and ultrasonic testing(UT).The girth weldability of the X70M pipeline was comprehensively evaluated based on the strength-ening,hardening,and embrittlement of the girth weld joint in especially severe welding conditions with accelerated failure characteristics.The X70M pipeline was confirmed to have good girth weld-ability based on a comparison of the girth weld joint performance in both especially severe and general field-construction welding conditions.The obtained girth weldability evaluation results with respect to especially severe welding conditions provide significant guidance for field-construction girth welding.

2 Welding procedure

2.1 Pipeline

Tables 1 and 2 show the composition and mechanical performance of the X70M pipeline prior to testing,which satisfy the corresponding require-ments of the American Petroleum Institute’s(API) standard 5L.The X70M pipeline steel is produced using a thermal,mechanical cooling processing technology with a fairly low-carbon equivalent and cold-cracking sensitivity,which results in good weldability.The strength,ductility,and toughness of the pipeline are all fairly good.

Table 1 X70M pipeline composition prior to the test

Table 2 X70M pipeline mechanical performance prior to the test

2.2 Consumable matching

Table 3 shows the automatic-girth-welding con-sumable matching plan for special conditions and the main welding procedure used in the construction field.Here,automatic inside root welding uses the same consumables,but filler and cap welding principally use high-strength and equal-strength matching,respectively.In welding conditions with no more than 0.25 kJ/mm super low heat input and high-strength consumable matching,the cold-cracking sensitivity of the weld joint increases while also promoting hardening and brittleness,which accelerate failure in weldability tests.Equal-strength consumable matching is the main welding procedure used in the construction field,so by comparing the evaluation results for these two conditions,two objectives can be met.First,the pipeline weldability can be confirmed,and second,the pipeline weld-ability margin in the construction field can be deter-mined.The above research will benefit consumers in their establishment of welding procedure plans.

Table 3 Automatic-girth-welding consumable matching plan for special conditions and the main welding procedure used in the construction field

2.3 Weld joint type and dimensions

Fig.1 shows the automatic-girth-welding groove type and dimensions of the test pipeline for special conditions and the main welding procedure used in the construction field.The formation of root welds using an automatic inside welding machine is confirmed for the small V-type groove.The small-angle double-V compound bevel can decrease the amount of filler metal required and increase the welding efficiency.The root-face dimensions are related to the welding procedure parameters and guarantee the welding penetration depth.

2.4 Welding procedure specification

Table 4 shows the details used in the automatic-girth-welding procedure of the test pipeline for special conditions and the main welding procedure used in the construction field.Here,root welding is performed by automatic inside welding,which is the same as that used in the construction field.Filler and cap welding use special rigid welding condi-tions and the main construction-field welding procedure,respectively.In general,the automatic GMAW welding heat input range in the construc-tion field ranges between 0.6-1.0 kJ/mm,which varies according to the groove type and bead sequence.The no more than 0.25 kJ/mm heat input welding procedure is applied to special and severe GMAW conditions,which result in a high cooling rate,high hardening sensitivity,and a heterogeneous microstructure,so the cold-cracking sensitivity and brittleness of the weld joint increase after welding.Compared with the construction-field welding procedure,this special super-low heat input welding procedure accelerates the failure characteristics.

Fig.1 Automatic-girth-welding groove type and dimensions of the test pipeline for special conditions and the main welding procedure used in the construction field

First,the pipeline end must be mechanically polished within the range of 25 mm to prevent welding defects.To eliminate the water on the pipeline surface,the pipeline is preheated to no more than 60 ℃,which has no obvious effect on the cooling rate or hardening sensitivity after welding and is not the same as the high-temperature preheating applied to prevent cold cracking.The interpass temperature is no more than 150 ℃.To guarantee the quality of the girth welding,a straight draw is used in root welding,whereas weaving is used in filler and cap welding.The CPP900-IW56 automatic inside welding machine with an assembly clamp and the CPP900-W2 automatic outside welding machine are used for root welding and filler and cap welding,respectively.

Table 4 Automatic-girth-welding procedure for the test pipeline in special conditions and the main welding procedure used in the construction field

3 Results and discussion

3.1 Cold-cracking sensitivity of girth weld joint

The girth weld joints inspected just after welding show that the transition is continuous and uniform,with no obvious welding defects.After 24 h of welding,X-ray photography and ultrasonic inspec-tion were performed,which showed that the girth welding quality satisfied API 1104-2016 without any cold cracking.The X70M pipeline was deter-mined to have a good weldability margin without cold-cracking sensitivity even in especially severe welding conditions.Fig.2 shows micrographs of the girth weld joints in the above two welding condi-tions.

Fig.2 Micrographs of the girth weld joints in the above two welding conditions

3.2 Analysis of the strengthening,hardening,and embrittlement of girth weld joint

Fig.3 shows the mechanical performance of the girth weld joint specimen at certain positions for the two welding procedures.Mechanical testing was used to evaluate the strengthening,hardening,and embrittlement of the girth weld joint.

Fig.3 Mechanical performance of the girth weld joint specimen at certain positions for the two welding procedures

Fig.4 shows the transverse tensile performance of the girth weld joints in the two welding conditions.The results show that the tensile strength of the pipeline base metal of all the specimens greatly exceeds the X70M specified minimum tensile strength to fracture.

Fig.4 Transverse tensile performance of the girth weld joints in the two welding conditions

Table 5 shows the longitudinal tensile perfor-mance of the girth weld for the two welding procedures.Super low welding heat input together with high-strength consumable matching results in excessive strengthening of the girth weld metal while decreasing the ductility of the girth weld.When the main construction-field welding pro-cedure is used,the strength of the girth weld metal decreases and its ductility increases.

Fig.5 shows the Vickers hardness testing method used on the girth weld joint for the two welding procedures,with the test results shown in Fig.6.In the girth welding condition of a super low welding heat input and high-strength consumable matching,the Vickers hardness of the girth weld metal is gene-rally high with the maximum value of 389(HV10),which indicates a serious hardening tendency.The coarse-grained heat-affected zone(HAZ) near the fusion line also shows an obvious hardening tendency from the rapid cooling after super low heat input welding,whereby the weld joint strength increases acutely and the ductility decreases.When the main construction-field welding procedure specifica-tion is used,there is very little hardening of the girth weld joint.

Fig.6 Vickers hardness results for girth weld joints for the two welding procedures

Fig.7 shows the impact toughness of the girth weld joints in different locations for the two welding procedures.In the girth welding condition of super-low welding heat input and high-strength consum-able matching,when the girth-weld strength and hardness increase,the impact toughness decreases significantly,but it is still higher than the accep-tance index 50/60 J specified by the consumer.Otherwise,the impact toughness of the girth weld metal is very good in construction-field welding conditions.In all welding conditions,the impact toughness of the fusion line and HAZ is very good.

Fig.7 Impact toughness of girth weld joints in different locations for the two welding procedures at a test temperature of -10 ℃

The ductile-to-brittle transition behavior in a certain temperature range was investigated through a series of temperature impact toughness tests in construction-field welding conditions.Fig.8 shows the impact toughness of the GW190 girth weld joints at a series of temperatures in different locations.Within the test temperature range,at three of the locations,the impact toughness of the GW190 was found to be fairly high.Even at -40 ℃,the impact toughness did not obvi-ously decrease,although some of the impact energy values are unstable.Therefore,the ductile-to-brittle transition temperature of the automatic girth weld joint is lower than -40 ℃ in construction-field welding con-ditions,which means that it is safe for most pipeline design and service temperatures.Based on these results,the UOE pipeline from Baosteel has a certain weldability margin for use in the construction field.

Fig.8 Impact toughness of the GW190 girth weld joints at a series of temperatures in different locations

Fracture toughness tests were conducted on a single-edged notch bending specimen for girth weld joints produced by two different welding procedures.The specimen dimensions areB·B,whereBrepresents the pipeline wall thickness.The notch direction is the through thickness,which is recorded as NP,and the test temperature was -10 ℃.Fig.9 shows the characteristic crack tip opening displacement(CTOD) values of the girth weld joints in different locations for the two welding procedures.In the girth welding condition of a super low welding heat input and high-strength consumable matching,with increases in the strength and hardness of the girth weld metal,the CTOD fracture toughness obviously decreases,which decreases the characteristic CTOD value in the fusion line and HAZ.However,even in the girth welding procedure for especially severe conditions,the CTOD fracture toughness values of the fusion line and HAZ are more than 0.2 mm,which is recognized as the acceptance criteria in most engineering projects.Thus,the UOE pipeline from Baosteel has a great weldability margin for use in the construction field.Similarly,every notch position of the girth weld joint has excellent CTOD fracture toughness in the main construction-field girth welding procedure.In addi-tion,compared with the impact toughness,there is more discretion in the CTOD characteristic values,which is related to the fact that the influence of the heterogeneous microstructure of the girth weld joint on the CTOD characteristic value is greater than the influence of impact toughness.In addition,the impact toughness and CTOD fracture toughness do not have a completely positive correlation.The influencing factors and mechanisms are more complicated with respect to CTOD fracture toughness,which means even more uncertainty.

Fig.9 CTOD characteristic values of the girth weld joints in different locations for the two welding procedures

Figs.10(a)-(e) show the typical microstruc-tures of the GW152 girth weld joint in different locations.During rapid cooling after super low heat input girth welding,it is difficult for carbon atoms to spread effectively,thus the accumulated marten-site-austenite (M-A) component produced in the ferrite and bainite matrix,which is the main reason for the brittleness of the girth weld metal.Rapid cooling after super low heat input girth welding increases the quenching tendency of the weld metal,which results in the accumulation of lath martensite,which is the main reason for the strengthening,hardening,and embrittlement of the girth weld metal.When the coarse-grained HAZ undergoes rapid rate cooling,a staggered distribution of lath bainite and martensite occurs in the original austenite grain within a very narrow zone,which improves the toughness of the coarse-grained HAZ to some degree.During multi-layer and multi-pass girth welding,a secondary and repeated heat cir-cle in an α+γ double-phase zone complicates the microstructure because of the heterogeneous and inadequate solid-phase transition,which produces lath bainite/ferrite,martensite,and an uneven high-carbon bulk M-A component from the accumu-lation of carbide and the concentration gradient of car-bon.The above high-carbon bulk M-A component becomes separated from the bainite/ferrite matrix,which then decreases the toughness.These local brittle zones(LBZs) are very narrow and randomly distri-buted,so it is difficult to precisely locate the notch in the LBZs.As a result,either at the fusion line or in the HAZ,the LBZs have little influence on impact tough-ness.However,during the evaluation of the CTOD fracture toughness,if a pre-fatigue crack occurs in the LBZs,a rapid extension of the instability will occur and result in a pop-in or brittle fracture,which leads to a significant decrease in the CTOD value.Given the above information,the characteristic CTOD values at the fusion line and in the HAZ are random[5].

Figs.11(a)-(d) show the typical microstruc-tures of a GW190 girth weld joint in different locations.In construction-field welding conditions,lath ferrite and bainite are mainly produced in the girth weld with a small local bulk M-A component due to the appropriate cooling rate after girth welding,which does not separate the matrix and achieves excellent impact and fracture toughness.The coarse-grained HAZ is narrow with an obvious austenite grain boundary and fine acicular ferrite,bainite,and a small M-A component produced in the original coarse-grained HAZ(CGHAZ).The reheated CGHAZ also has an accumulated M-A component.In this welding condition,local brittle behavior in the reheated CGHAZ has no obvious effect on the integral girth weld joint.

Fig.11 Typical microstructures of the GW190 girth weld joint in different locations

4 Conclusions

(1) In girth welding conditions with a super low welding heat input of no more than 0.25 kJ/mm and high-strength consumable matching,automatic GMAW girth welding was conducted without high-temperature preheating for the typical standard X70M UOE pipeline.Even in these severe welding conditions,the X70M pipeline maintained good weldability without any cold cracking.

(2) In the severe welding conditions described above,the X70M girth weld joint showed a certain degree of strengthening,hardening,and embrittle-ment,but still satisfied the requirements of the relevant standard.By comparing the above X70M girth weld joint with one produced in construction-field welding conditions,the X70M pipeline was found to have a great field weldability margin,which provides important guidance for construction-field girth welding.

(3) The strengthening,hardening,and embrittle-ment of the girth weld joint are highly related to its microstructure.Different welding conditions pro-duce different microstructures in the girth weld joint,which result in different performance.Some dispersion aspects of the girth weld joint are also related to the heterogeneity of its microstructure.