Microstructure and properties of laser-arc hybrid welding of high-strength low-alloy steel

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

Abstract: Laser-arc hybrid welding has the characteristics of optimal surface formation and greater penetration;it is extensively used in the welding of plates of medium thickness.However,for hybrid welding of lasers,the welding seam cooling rate is rapid;thus,the welding seam has a higher tendency to significantly harden,which has a negative impact on the weld quality of the high-strength low-alloy (HSLA) steel plates of medium thickness.In this study,laser-arc hybrid welding is performed on the BG890QL HSLA steel produced by Baoshan Iron & Steel Co.,Ltd.,and the quenching tendency of the welded structure is examined.The results demonstrate that the specific growth direction of the columnar crystal structure of the laser-arc hybrid welded joint is obvious.However,at the center and top of the welded seam,there are equiaxed crystals.The impact properties at room temperature and -40 ℃ of the weld area are 58.0 J and 40.0 J,respectively,and those of the heat-affected zone (HAZ) are 147.0 J and 66.5 J,respectively.The impact performance can meet these requirements.Laser-arc hybrid welding of HSLA steel can yield strong and durable welds and the HAZ structure to meet the requirements of engineering applications.

Key words: high strength steel; laser-arc hybrid welding; microstructure; impact properties

1 Introduction

Because of its high strength,high-strength low-alloy (HSLA) steel has been extensively used in constructing infrastructures such as railways,high-ways,airports,and subways.However,when welding HSLA steel,the welding area is not uniformly heated,and the cooling rate is inconsistent,thus resulting in uneven joint structure and mechanical properties.The amount of heat input is directly related to the grain size and width of the coarse-grained zone in the heat-affected zone (HAZ) and determines the performance of the weld seam.A large heat input will cause the HAZ to be soft and the width of the HAZ to be larger,while a small heat input will cause the weld to be brittle[1].The final welding performance reduces the use of high-strength steel.Therefore,the welding of high-strength steel mostly adopts CO2welding,or Ar-rich mixed gas shielded welding with concentrated welding heat input density,high efficiency,good molten pool protection and dehydrogenation effect,and small welding deformation;however,there are manual arc welding,automatic submerged arc welding,and hybrid welding method welding[2- 6]as per the actual production and manufacturing.ZHANG et al.[7]concluded that the welding of Q890 high-strength steel had an obvious tendency to harden;further-more,the preheating temperature and welding heat input had little effect on the cold crack sensitivity.LOU et al.[8]used manual arc welding and sub-merged arc welding to examine the weldability of the 690 MPa grade HSLA steel and finally con-cluded that the pre-weld preheating method with a preheating temperature of 80 ℃ can obtain a welded joint that meets the performance requirements.

Laser welding has the characteristics of high energy density,high welding efficiency,strong controllability,good processing flexibility,relatively low pollution,and is not affected by the environ-ment.However,during laser welding,because of the relatively large depth/width ratio,compared with ordinary welding methods,defects such as pores and voids are prone to occur[9-11].Furthermore,the rapid cooling of the weld leads to the appearance of a martensite structure,which reduces the toughness of the entire weld and induces cold cracks.To improve the toughness of weld metal(WM),ZHANG et al.[12]used a laser-arc hybrid welding method to achieve the welding of high-strength steel,and the weld was well formed,no defects occurred,and the performance met the requirements.

This study aims at the 16-mm-thick BG890QL HSLA steel plate produced by Baoshan Iron & Steel Co.,Ltd.(Baosteel).Because the laser hybrid welding method is adopted to reduce the cooling rate of the weld by introducing the arc,the welding speed increases and the width of the HAZ is reduced by introducing a high-energy laser.Thus,we can solve the welding problem of HSLA steel by the weld formation and mechanical properties con-trol,and provide theoretical guidance and engineering application reference for the engineering application of HSLA steel.

2 Test materials and methods

The BG890QL steel used in this test was produced and provided by Baosteel,with a yield strength of 890 MPa and a tensile strength of ～1 000 MPa.The structure of BG890QL steel is tempered martensite;Table 1 shows the main element content.After smelting and rolling,BG890QL steel is tempered at 640 ℃ after quenching,maintained for 60 min,and then air-cooled to obtain the structure of tempered martensite.

Table 1 The main chemical composition of BG890QL steel %

The laser welding uses fiber laser (IPG,Oxford,MA,USA) with a maximum power of 10 kW.The arc welding adopts the TPS 5000 fully digital weld-ing machine (Fronius,Wels,Austria).In this study,the laser hybrid welding method is used in the weld of 16-mm-thick plate BG890QL steel.Fig.1 shows the specific groove diagram.In this study,the fiber distance between the laser and arc was 4 mm,and the laser-guided arc trailing method was used for the laser-arc hybrid welding.Because the plate thickness is 16 mm and it is difficult to penetrate at one time in direct laser hybrid welding,we initially used laser bottom welding and then laser hybrid welding,thereby ensuring that the weld was formed on both sides.The filler material of the weld is GM120 (Bohler Thyssen,Suzhou,China);Table 2 shows the main element content.The yield strength of the welding wire cladding metal is ≥890 MPa,the tensile strength is 940-1 180 MPa,and the average value of the impact energy at -60 ℃ is >47 J.

Fig.1 Profile of the single groove in laser-arc hybrid welding(mm)

During the welding process,laser is used in backing welding,and then laser hybrid welding is used to finally complete the butt welding of 16 mm plates of medium thickness.In laser backing weld-ing,the laser power is 5 kW,and the welding speed is 1 m/min.In laser hybrid welding,the laser power is 7 kW,the arc current is 240 A,the voltage is 21.6 V,the welding speed is 1 m/min,and the arc is extended by 12 mm during welding.

Table 2 Typical chemical composition of Bohler Thyssen GM120 weld wire %

3 Test results and analysis

3.1 The welding seam forming characteristics of laser-arc hybrid welding

Fig.2 shows the macrostructure of the cross-section of the BG890QL steel welded joint,which is produced from the dual heat source and double-layer welding.The composite welded joint is the primary part of the entire joint,and the thickness (reinforcement and penetration) is ～12 mm.The cross-sectional shape of the weld is similar to the “Y” type,and the upper and lower parts of the “Y” weld have different dominant heat sources.The upper part of the weld is superimposed by two heat sources of the arc and laser;however,the lower part of the weld is primarily affected by a single laser heat source.Hybrid welding “Y”-type joints can be roughly divided into three areas:weld,HAZ,and the base metal(BM).The weld is divided into a columnar crystal zone (CCZ) and a central equiaxed crystal zone (ECZ),and the heat-affected zone is divided into a coarse-grain heat-affected zone (CGZ) and a fine-grain heat-affected zone (FGZ).The lower part of the entire joint is formed by laser welding,and the thickness is ～4 mm.Under the action of the dual heat sources of the laser and arc,the microstructure of the hybrid welding seam and HAZ is more complicated.Fig.3 shows the structure of the laser-arc hybrid welding seam,which is primarily composed of the columnar crystals on both sides and the equiaxed crystals on the top.During the welding process,the heating temperature and cooling rate of each area in the molten pool differ.The solidification of weld first starts from the fusion line and grows in the molten pool in the direction of the maximum temperature gradient,which is perpendicular to the fusion line in the form of columnar crystals.The width of the crystal grain is ～100 μm,whereas the length can exceed 500 μm.The introduction of the arc makes the peak temperature acting on the top of the weld higher than other areas,resulting in a decrease in the tem-perature gradient of the upper part of the molten pool.Moreover,the electromagnetic contraction force generated by the arc will mechanically agitate the upper molten pool with great force and promote the flow of liquid metal,which makes the heat and mass transfer of the molten pool more uniform as well as refines the top structure of the weld.There-fore,the weld reinforcement area will form the obvious equiaxed crystals,and the original austenite grain size is ～50 μm (refer to Fig.3(d)).

Fig.2 Macrostructure of the weld joint and BM in laser-arc hybrid welding

Fig.3 Macrostructure for WM (hybrid welding)

3.2 Microstructure characteristics of laser-arc hybrid welding seam

Figs.3-5 show the structure of the weld and HAZ of the BG890QL steel composite welded joint.Figs.3 and 4 show both the equiaxed crystal region and columnar crystal region of the weld-formed lath martensite and granular bainite.Further-more,a small amount of massive ferrite is etched out and enlarges as the ferrite strip along the grain boundaries of the original austenite during the weld-ing process.A certain amount of carbides are distri-buted in the original austenite grain boundary and the phase boundary of the weld.As per Fig.3(a),the HAZ is divided into the coarse grain zone,fine grain zone,and a two-phase zone,which is called the intercriti-cal heat-affected zone (IC-HAZ),with increasing distance from the fusion line.Fig.5 shows the enlarged tissue morphology of each specific area.In the fine-grained region,while the peak temper-ature is slightly higher thanAc3,the original base metal structure undergoes a complete austenitized phase transformation during the welding process,forming smaller austenite grains and transforming them to martensite during the rapid cooling process.The bulk lath structure and the pre-austenite grain size is <10 μm.It can be observed under the scan-ning electron microscope that,as shown in Fig.5(d),there are fine carbides distributed between the martensite laths in the fine-grained region and on the original austenite grain boundaries.Because the peak temperature of the coarse-grained region is much higher thanAc3,and this region stays above the au-stenitizing temperature for a long time,the austenite grains grow up seriously and form coarse martensite lath after rapid cooling,as shown in Figs.5(e) and (f).The average size of the pre-austenite grains is ～50 μm,and there are even grains with a size of >100 μm near the fusion line.

Fig.4 Microstructure for WM (hybrid welding)

Fig.5 SEM images of the microstructure for HAZ

A large number of massive phases appear in the two-phase zone of the HAZ.They are primarily the lath martensite and carbides after magnification,as shown in Figs.5(a) and (b).Because the temperature in the two-phase zone is betweenAc3andAc1,ferrite is partially dissolved into austenite,while martensite is transformed into austenite and then is quickly cooled to martensite.

3.3 Impact performance of laser-arc hybrid welding seam

Fig.6 shows the impact test results of the weld,HAZ,and the base metal at room temperature and -40 ℃.At room temperature,the average impact energy of the BM is ～117 J,the higher impact energy of the HAZ is 147 J,and the lower impact energy of the WM is ～58 J.At -40 ℃,the impact energy of the HAZ is higher than that of the BM,at ～66.5 J;however,the impact energy of the WM is lower,at ～40 J.The impact performance of the HAZ is better than that of the WM and BM,and the impact energy of the WM is the lowest.The impact toughness of the welded joint is lesser under the action of a low-temperature environment of -40 ℃.

Fig.6 Impact property for weld joint of hybrid welding at room temperature and -40 ℃

Fig.7 shows the macroscopic morphology of the crack propagation path of the HAZ and weld at room temperature and -40 ℃.The crack propagation process of the HAZ will pass through the BM,the HAZ,and the weld joint.As shown in Figs.7(a) and (b),the cracks of the HAZ samples at room temperature and -40 ℃ propagate to the base material;furthermore,the crack propagation path is tortuous.At room temperature,when the extended cracks in the HAZ sample gradually approach the two-phase zone and over-tempered zone,a significant deflection occurred at pointA.The deflection crack propagates in the BM again,whereas deflection occurs at pointB.At -40 ℃,the cracks change the direction of propagation at pointsCandDclose to the two-phase and over-tempered zones.The more crack deflection times,the larger the surface area of the crack,the greater the surface energy required for propagation,and the greater the difficulty of crack propagation.The characteristics of crack growth are that the lower hardness of the two-phase zone and the over-tempered zone results in higher impact toughness of the HAZ samples.The two-phase zone is softened during the thermal welding cycle,and its hardness is low,and the toughness is high.When the crack propagates to the two-phase zone and the over-tempered zone,it can undergo greater plastic deformation,which makes the cracks restrained,thus resulting in the reduction of crack growth ability.The cracks of the weld specimens primarily propagate in the columnar crystal region of the weld and pass via multiple columnar crystal grains simultaneously to observe the crack propagation path of the weld.The crack propagation of the weld at room temperature and -40 ℃ is relatively straight,and the crack propa-gation process is less obstructed;it can be continuously penetrated in the weld matrix in Figs.7(c) and (d).In the laser arc hybrid welding process,the formation of coarse columnar crystals in the weld seam and the resistance to crack propagation are low because of the addition of the arc heat source.

Fig.7 Fracture path of impact test for the weld joint of hybrid welding

The fractures of the welded joints become trans-granular at room temperature and low temperature,and there is little crack deflection,as shown in Figs.8(a) and (c).The weld matrix on the crack propagation path has a strain,as shown in Figs.8(b) and (d).The capacity of the material to crack growth is reflected in the degree of strain in the matrix near the fracture propagation path.The greater the degree of strain,the larger the material’s plastic deformation,which can absorb the energy of crack propagation and restrict the fracture’s ability to grow.The HAZ has a larger strain degree than the weld at different temperatures,indicating that the HAZ has more structures that may be deformed well.In terms of grain size and crystal orientation,the HAZ has a greater grain size and a substantially single crystal orientation.The grain orientation of the weld,however,is relatively complex,and the degree of anisotropy is high,resulting in a more problematic macroscopic structural strain concentration.The strain concentration area,once subjected to the force in the impact process,is likely to enhance crack propagation.The cracks in the HAZ show transgranular growth at room temperature and low temperature in the inverse pole diagrams of Figs.9(a) and (c),and there is a clear deflection behavior.The strain distributions on the fracture route corresponding to the two temperatures are shown in Figs.9(b) and (d).At both temperatures,there is some strain on the HAZ’s fracture propagation path.At room temperature,the area occupied by the peak strain of the HAZ is smaller than the area occupied at -40 ℃.

Fig.8 EBSD analysis for fracture path of impact test in WM

Fig.9 EBSD analysis for fracture path of impact test in HAZ

4 Conclusions

(1) Welding BG890QL steel was performed using a laser-arc composite process,and the weld seam was well-formed and free of flaws.The weld’s top has a columnar crystal structure with an equiaxed crystal structure on both sides.Lath martensite and granular bainite dominate the microstructure.The thermal influence is split into three zones:coarse-grain,fine-grain,and two-phase zones with lath martensite as the predominant microstructure.

(2) At room temperature,the average impact energy of BM is ～117 J,the higher impact energy of HAZ is 147 J,and the lower impact energy of the WM is ～58 J.At -40 ℃,the impact energy of HAZ is ～66.5 J,the BM impact energy is 65 J,and the WM impact energy is ～40 J.Both room tem-perature and normal temperature impact performance can meet engineering application requirements.

(3) There is a certain strain on the crack propagation path of the weld and the HAZ.The strain concentration in the weld area is larger,which is beneficial for promoting crack propagation.