Research Institute,Baoshan Iron & Steel Co.,Ltd.,Wuhan 430080,China

Abstract: Submerged arc welding (SAW) and gas metal arc welding (GMAW) experiments of Nb-bearing X80 steel were conducted with high-toughness wires.The inclusions in weld metals were analyzed in terms of their types and sizes.In GMAW,the inclusions are primarily Ti,Ca,Si,Al,and Mg compounds with no Nb and are generally less than 0.8 μm in size,whereas,in SAW weld,the inclusions are larger,mostly approximately 2-5 μm in size,and are cored with Ca and Ti,exhibiting obvious oxidation metallurgical features.The SAW joint was hot-deformed,and Nb-bearing nano precipitates were newly found in the weld metal through transmission electron microscopy,and Nb-free core-shell inclusion was found through scanning electron microscopy.The inclusions and precipitates were dispersed in or on the boundaries of acicular ferrite,contributing to acicular ferrite nucleation and grain refinement.

Key words: X80 steel; weld; nanometer precipitate; inclusion

1 Introduction

With the advancement in the steel industry and petroleum & natural gas industry in China,X80 steel has become the main steel grade in pipeline applications and is receiving increased use.Because of the significant length of pipelines and the rigid physiognomy and climate conditions,X80 steel is subjected to stricter requirements.

Gas metal arc welding (GMAW) and submerged arc welding (SAW) are the main methods for pipeline welding.Because of a lack of welding wires with high strength and toughness suitable for X80 steel,Baosteel has developed GMAW and SAW wires for X80 steel,and the actual strength and toughness of both deposited weld and X80 butt weld can well meet the technical requirements:tensile strength of 625 MPa or more,impact energy of 60 J for single and 80 J for average[1-3].These wires produce high-toughness weld metals.Apart from being alloyed with some strengthening elements such as Ni and Cr,both wires take good advantage of microalloying elements to form fine inclusions to improve weld metal toughness.

At present,fast cooling thermo-mechanical control processing (TMCP) technology has been broadly used in green steel production,with reduced alloying elements.By adding Nb and fast cooling,solution,phase,and precipitation strengthening can be improved,and the use of costly alloys such as Mo and Ni can be reduced.There are a lot of reports about Nb behavior in steel and welding heat-affected zone,but none in weld metals.The authors of this article analyzed inclusions in SAW weld metals of Nb-bearing X70 steel in as-welded and reheated samples.Under transmission electron microscopy (TEM) observation,the inclusions are mostly from 0.2 μm to 1.5 μm in size,and some are as small as 80 nm,and they are mainly oxides of Ti,Al,Mg,and Si with no Nb[4],indicating that Nb-containing precipitates and inclusions with sizes larger than dozens of nanometers are difficult to form in an as-welded metal.

To further the research,GMAW and SAW experiments on Nb-bearing X80 steel were con-ducted,and the precipitates and inclusions were analyzed on tough weld metals,especially on addi-tional hot-deformed samples,providing a reference for weld metal toughening associated with micro-alloying.

2 Analyses of inclusions in as-welded metals of X80 steel

2.1 Materials

The main chemical composition of deposited weld metals of GMAW and SAW are presented in Table 1,mainly containing Ni,Mo,Cr,Cu,and Ti,which can produce tiny particles with a high melting point as a nucleus of acicular ferrite.The mechanical properties of deposited metals are presented in Table 2.The thickness of the test X80 steel is 17.5 mm,and its chemical composition and mechanical properties are presented in Tables 3 and 4,respectively.

2.2 Analyses of fracture surfaces and inclusions of GMAW weld metals

All-position welding was used for pipeline girth welds,including horizontal,up/downhill,and overhead positions,so the welding heat input needs to be limited,typically around 15 kJ/cm,which was also used in this article.Fig.1 depicts the welding groove.Welding was completed in several passes for the 17.5-mm thick workpiece and 15-kJ/cm heat input.When heat input is low,the dilated base metal only takes up a small portion of the weld metal,so the weld metal composition is mainly determined by welding con-sumables.The test on the mechanical properties of the welded joint was conducted with qualified strength,ductility,and excellent toughness.The fracture surfaces of weld metal impact samples were analyzed with the Quanta 400 scanning electron microscopy (SEM) and the attached energy dispersive X-ray spectrometry (EDS) system after being cleaned with ethanol and ultrasonic waves.

Table 1 Main chemical composition of deposited weld metals %

Table 2 Mechanical properties of deposited metals

Table 3 Composition of X80 steel %

Table 4 Mechanical properties of X80 steel

Fig.1 GMAW workpiece groove

On one fracture surface of the GMAW weld metal,the micromorphology of the area away from the notch exhibits a tough dimple feature by two-thirds of the surface,whereas that near the notch exhibits a cleavage feature by one-third of the sur-face.Some inclusions are observed in the dimples.SEM morphology and element characteristic spec-tral analysis are shown in Figs.2(a) and (b),respectively.

For the samples in Figs.2(a) and (b),the GMAW weld metal has a very high impact energy of 152 J and 192 J at -30 ℃,respectively.Characteristic spectral analyses show that the inclusions are mainly of two types:one mainly consisting of Ti and minor Si and Al,and the other one consisting of Ti,Ca,Si,Al,and Mg,with almost the same size of approximately 0.8 μm.The GMAW weld metal is inspected for approximately 0.025% Ti and minor Ca and Al,which are thought to be beneficial for the formation of Ti-Ca-Si dominant fine particles,improving weld toughness.

Fig.2 Analysis of fracture surface and inclusions of GMAW weld metal

2.3 Analyses of fracture surfaces and inclusions of SAW weld metal

Spiral SAW pipes are generally welded with two passes:first,an inner pass,and then,an outer pass without root removal.To obtain an integrated weld,a high welding current should be used.For the 17.5-mm thick X80 steel,the double-Y groove is shown in Fig.3,welding heat input of 28 kJ/cm for one side and 35 kJ/cm for the other were used.Due to the large root face and the large heat inputs,the dilated base metal made up a large portion of the weld metal,so the composition of the weld metal is mainly determined by both the welding consumables and the base metal.Thus,whether Nb was added to the welding consumables or not,the weld metal contained a considerable amount of Nb from the X80 steel,providing a condition for analyzing Nb in the weld.The mechanical properties of the welded joint were tested,and the strength,ductility and toughness were all qualified.The fracture of the weld metal impact specimen was analyzed by SEM.

Fig.3 SAW workpiece groove and welded joint

Analyses of inclusions on fracture surfaces of SAW weld metal impact samples are shown in Fig.4,including SEM images of the tough dimple area and characteristic spectral analyses of inclusions.

Fig.4 Analyses of fracture surfaces and inclusions of SAW weld metal

Si,Al,and O are present in the inclusion of SAW as-welded metal,and Ti is also present in some parts of the inclusions,but Si and Al are minor in some Ti dominant parts.From SEM images of Fig. 5 for the sample in Fig.4(a),Ca is at the center of the inclusion,indicating the nucleating function of Ca in the formation of this type of inclusion.Other compounds accumulate on the Ca nucleus and grow to form the next Ti,Si,and Al oxide layers of the inclusions.

From the viewpoint of thermodynamics,the elements are in the order of Ca,Al,Ti,Mn,and Si related to their oxide formation potency,so it is imaginable that the inclusions are cored with Ca,Al,and Ti,and Mn and Si are the covering layers,as illustrated in Fig. 6(a)[5].The SEM observation of inclusion of the SAW weld metal is illustrated in Fig.6(b) according to Fig.5.The prediction and observation are similar but slightly different.The results show that the formation mechanism of inclusions is complex and is related to not only the oxide formation potency but also the content of related elements.The formation of inclusions is the balancing results of different elements.

Fig.6 Illustrative elements distribution in inclusions

2.4 Comparison of inclusions in weld metals of SAW and GMAW

The analyses show that the main inclusions in the tough dimples on the fracture surfaces of the high-toughness GMAW weld metal are mainly 0.5-0.8-μm particles,mostly consisting of Ti and Ca.The main inclusions in the tough dimples of fracture surfaces of the high-toughness SAW weld metal are mainly 2-5-μm compound particles,consisting of Si,Al,Ti,and Ca,and Ti oxides were also in sizes less than 1 μm.The majority of the Ti,Ca,Al,and Si compound inclusions in the SAW weld metal are approximately 2-3 μm in size.Among the alloying elements in some inclusions,Al has the highest spectrum,followed by Si and Ti,but Ca is almost non-existent.

The analyses reveal the following:(1) Sites where the tough dimple and cleavage occur and the portion of the tough area to cleavage area indicate that the impact energy is mainly influenced by the fracture feature of the area near the notch.For example,although the cleavage fracture area makes up one-third of the surface,the weld metal still has high impact energy.(2) The inclusions in the GMAW weld metal are smaller than those in the SAW weld metal.(3) The impact energy is greatest when the inclusions in the GMAW weld metal are dominantly fine Ti and Si oxides.Larger Al,Si,and Ti compound inclusions are dominant in the SAW welds,and the toughness decreases to some extent but remains high.(4) The formations and sizes of different inclusions are related to alloying elements.Si has dual functions,and its presence is beneficial.Because the flux contains Al,the presence of Al in the SAW weld metal is inevitable.However,even if the sizes of Al-containing inclusions are as small as 2-3 μm in this study,a high-toughness weld metal can be obtained.(5) Most of the inclusions are compounds formed in the process of melting and solidification,and their sizes are dependent on their types.

The width of acicular ferrite is generally 2 μm,approximately 1/10-1/3 of its length.The inclu-sions observed on the fracture surfaces of impact samples have almost the same size as acicular ferrite,and they can inhibit the grain growth of the weld metal during crystallization and subsequent solid-phase transformation;in the same way,they can suppress grain coarsening of the weld metal when it is reheated by the subsequent welding pass.Such grain refining typically strengthens and tough-ens weld metals.

According to the SEM observation on the fracture surfaces of weld metals,no Nb was found in either GMAW or SAW,which is consistent with the reference[4].

3 Analysis of inclusions and precipitates in X80 hot-deformed SAW weld metal

Fast cooling steel generally contains Nb,Ti,and V.Nb in austenite is in three status: undissolved Nb(C,N) when heated,dissolved Nb,and newly precipitated Nb(C,N) when rolled.The portions of dissolved Nb and precipitated Nb(C,N) vary with Nb content and the rolling procedures of steel[6].Tiny precipitates can inhibit austenite coarsening and suppress austenite recrystallization,resulting in high-density dislocation and a deformed band,thereby increasing strength.Fig.7 shows 10-25-nm Nb(C,N) and Ti(C,N) compound precipitates dispersing in roughly 250-nm wide acicular ferrite in X80 steel made at a 25 K/s cooling rate.A higher cooling rate refined the matrix grain and precipitate,thereby improving the strength and toughness[7].

Fig.7 Nb-bearing nano precipitate in the X80 steel

The precipitate in the coarsening grain heat-affected zone (CGHAZ) is mainly a TixNb1-x(CyN1-y) compound with a size less than 100 nm.These nanoscale particles can effectively inhibit austenite growth and reduce CGHAZ toughness deterioration[8].The size of the precipitate in the CGHAZ is dependant on the Nb content of steel,as shown in Fig.8[9].

Fig.8 Size distribution of precipitates in CGHAZ of X80 steels with various Nb contents

The as-welded metal has a cast microstructure,and compared with the Nb-bearing TMCP base metal,the precipitation of niobide lacks a key con-dition-dislocation under recrystallization temper-ature rolling.To obtain Nb-containing precipitates,the SAW sample was hot-deformed,and mechanical tests were conducted to qualify it,and the TEM film was taken with a wire-electrode cutting from the SAW weld metal and then made with the double spray electrolytic method.The microstructure and precipitates were analyzed with JEM-2100F TEM,as shown in Fig.9,where the three precipitates are dispersed in or on the boundary of acicular ferrite and have similar compositions with Nb and dozens-of-nanometer nanoscale size.Nb-containing nano pre-cipitates are formed during the hot-deforming pro-cess,which is similar to TMCP procedures of the base metal.

Fig.9 TEM observed nanoscale Nb precipitates in X80 hot-deformed weld (w(Nb)=0.035%,Akv2(-20 ℃)=149 J)

Fig.10 shows six inclusions that mainly contain Ti,Al,and other elements with no Nb and are dispersed on the boundaries of acicular ferrite. Some other inclusions mainly contain O,Al,Mn,Si,and other elements,and these elements are roughly evenly distributed in the inclusions.

Fig.10 TEM observed micro-scale inclusions in X80 hot-deformed weld

Interestingly,a translucence oxide was found,and its elements distribution was SEM mapped with Ti on the outer circle,exhibiting a core-shell structure,as shown in Fig.11.

Fig.11 SEM mappings of the inclusion with the core-shell structure

In weld metal hot-deforming,the dominant inclusions are still of micrometer-scale and have the same size level as acicular ferrite.Importantly and expectedly,nanoscale precipitates,especially Nb-containing ones,were newly formed and found.These inclusions and precipitates have a wide range of sizes,and they could have formed because of various processes.They may also contribute to acicular ferrite nucleation and grain refinement.

4 Conclusions

According to the experiment and analyses,the following can be concluded for X80 weld metals.

(1) The inclusions in the GMAW as-welded metal mainly contain Ti,Si,Ca,Al,and Mg with no Nb,and they have small sizes ranging from 0.5 μm to 0.8 μm.

(2) The inclusions in the SAW as-welded metal are compound particles of Si,Ti,Al,Ca,and Mg with no Nb.Their compositions are dependant on welding consumables and the base metal.Their size range is from 2 μm to 5 μm.

(3) In hot-deformed weld,there is a new pre-sence of nanoscale Nb-containing precipitates and a micro-scale core-shell structure inclusion.

(4) In all weld metals tested,the dominant inclu-sions are micro-scale oxides,exhibiting obvious oxidation metallurgical features.The formation of inclusions is affected not only by the oxide for-mation potency of elements but also by the com-positions of the welds.The inclusions and preci-pitates are dispersed in or on the boundaries of acicular ferrite,contributing to the strength and toughness of the weld metals.