Xiao Hongjun,Tian Zhiling,Cui Bing

1.State Key Laboratory of Advanced Steel Processes and Products,Central Iron &Steel Research Institute,Beijing 100081,China;

2.Anhui University of Technology,College of Materials Science and Engineering,Maanshan 243000,China

Abstract By means of metallographic microscope (OM),scanning electron microscope (SEM),back scattering electron diffraction(EBSD)and transmission electron microscope (TEM),the effect of Cu on microstructure transformation and mechanical properties of deposited metal of 590MPa class steel welding wire was studied.The results show that the microstructure of deposited metal is composed of acicular ferrite,lamellar bainite,granular bainite and residual austenite.With the increase of Cu content,the phase transition temperature of the deposited metal decreases,making the phase transition region of ferrite and pearlite shift to the right,expanding the phase transition region of bainite and shrinking the phase transition region of ferrite and pearlite.The microstructure of deposited metal changed,the content of M-A elements increased but the size decreased,and the ferrite-bainite biphasic microstructure was matched.The reduction of M-A component content in strips and blocks and the reduction of effective grain size will reduce the nucleation probability of microcracks,increase crack growth resistance,and improve the impact toughness of the deposited metal.

Key words 590MPa high strength low alloy,deposited metal,microstructure,mechanical properties

0 Introduction

In the traditional high strength alloy steel,Cu is often seen as harmful impurity elements and strictly control its content,even if is certain to add a small amount of alloy steel,also just in order to improve the corrosion resistance of steel[1-5],rather than the United States put forward to improve the strength of the steel cen,rather than the United States put forward to improve of ship steel,Cu as alloying elements in steel,using its aging reinforcement effect to get good comprehensive performance in the precipitation strengthening steel after the success of the development,in the role of weld metal has been becoming another hot[6-10].

The properties of deposited metals directly reflect the quality of welding materials,so it is very necessary to analyze the structure and properties of self-developed welding wires for hull of 590 MPa class,which is of important engineering significance for the quality identification of welding wires and provide important theoretical basis for the development of high-quality welding materials for hull of 590 MPa class.

1 Test materials and methods

YM751A welding test machine was used in the welding laboratory of iron and steel research general institute.The welding shielding gas was 80%Ar+20%CO2,and the gas flow was 20 L/min.Welding heat input for 20 kJ/cm,for more clear understanding of alloying elements on microstructure and mechanical properties of deposited metal effect,the former should try to reduce the base metal deposited experiment of welding wire composition dilution,first in the groove on both sides of surfacing welding layer of a layer of the same test wire (thickness is not less than 4 mm),then deposited metal welding,three kinds of welding wire composition as shown in Table 1.Deposited metal tensile test shall be carried out in accordance with the GB/T 2652.2008 standard,test instruments as WE -300 hydraulic universal testing machine,the sample size for the M16 mm ×Ф 10 mm ×105 mm.Deposited metal impact test shall be carried out in accordance with the GB/T 2650.2008 standard,test equipment for JBZ -300 automatic impact testing machine,impact test specimen is 55 mm × 10 mm × 10 mm with a v-notch standard specimen,using liquid nitrogen cooling medium and anhydrous ethanol,test temperature is 50 ℃.Deposited metal test plate was showed in the Fig.1.

Table 1 Composition of solid core wires

Fig.1 Weld deposited metal test plate (mm)

2 Microstructure of deposited metal

Fig.2 was OM morphology for the three trials wire microstructure of deposited metal.It can be found in Fig.2 that the microstructure of the deposited metal is mainly acicular ferrite (AF),the width between 1-2 μm,the phase relationship between each other,in the form of a mixed distribution of acicular ferrite grain boundary for high angle grain boundary which can hinder the propagation of the crack,and deposited metal grain fine,the fracture path is very twists and turns so the impact toughness is improved.Therefore,the deposited metal with acicular ferrite structure has the best toughness.In addition,a small amount of island-like white granular bainite (GB)and a small amount of lath bainite (BL)tissues were precipitated between acicular ferrite (AF).The typical SEM morphology of the deposited metal is shown in Fig.3.

It was observed that the content of acicular ferrite in DM 3 deposited metal increased,and the granular bainite was evenly distributed with a small amount.The microstructure of the deposited metal in DM 2 was evenly distributed,and the grain boundary ferrite content was slightly higher than that in DM 1,but the overall content was less.The microstructure was mainly composed of acicular ferrite and a small amount of granular bainite.Appearance of the main causes of microstructureal change is due to the DM 3 in the deposited metal Cu as the main elements of austenitizing,the increase in the content of austenite stability relative increase,transition temperature is reduced,so the microstructure of a small amount of bainite,weld microstructure brings out the ferrite -bainite duplex microstructure collocation makes the microstructure of the deposited metal further elaboration.The addition of Cu element can improve the stability of austenite,and make it difficult to carry out pearlite phase transition.The CCT curve moves to the right,and with the decrease of phase transition temperature,the bainite transformation is more likely to occur.By comparing the microstructure of the three kinds of welding wires,it can be found that with the increase of Cu content in the deposited metal,the microstructure changes A trend that ferrite is gradually replaced by bainite,and M-A elements also show A trend of dispersion distribution in the matrix phase.

3 Characteristics of M-A components in deposited metals

The M-A is of high hardness and brittleness.Under the impact stress,the matrix structure will undergo plastic deformation,and stress concentration will occur around the M-A element.When the stress exceeds the critical stress,the microcrack will form at the interface between M-A element or M-A element and the matrix[11-13].Lepera etching method (4% picric acid alcohol +1% heavy sodium sulfite at A volume ratio of 1:1)was used to color and corrode the microstructure of deposited with different Cu contents,and the matrix structure after coloring was gray-black,while the M-A component with the flagpole was bright white,as shown in Fig.4.It can be found that with the increase of Cu content in the deposited metal,the number of M-A components presents an obvious increase trend.This is mainly due to the enrichment of carbon in austenite due to the increase of Cu content,which leads to the formation of carbon-rich austenite area and increases the stability of austenite.In the process of continuous cooling,the carbon-rich austenite part gradually turns into martensite,thus promoting the formation of M-A.In addition,the increase of Cu content improves the stability of austenite,and the increase of supercooled austenite content leads to the increase of the number of M-A elements formed in the subsequent cooling process.

Fig.2 Microstructure of deposited metal (a)DM1×200 (b)DM1×500 (c)DM2×200 (d)DM2×500 (e)DM3×200 (f)DM3×500

Image.Pro Plus 6.0 software was used to conduct statistics on the number,area and maximum length of M-A components in the three deposited metals.The results are shown in Table 2.As can be seen from Table 2,the maximum lengths of M-A elements in the three deposited metals are 5.8,5.3 and 4.8 μm,respectively.The average lengths were 1.5,1.3 and 0.9 μm,respectively.The surface densities were 1.45 mm-2,1.56 mm-2and 1.86 mm-2,respectively.It can be seen that with the increase of Cu content,the size of M-A components in the deposited metal decreases at the same time.

4 Characteristics of inclusion in deposited metals

Nonmetallic inclusions are common impurities in deposited metals.Table 3 was energy spectrum analysis results of inclusions of deposited metals with different Cu contents.As can be seen from Table 3,the inclusions of deposited metal with different Cu contents contain alloying elements such as C,O,Si,Mn,Ti and Al.It can be seen that the inclusions in deposited metal are composite oxide inclusions composed of SiO2-MnS-Al2O3-TiO2.

Fig.3 Microstructure of deposited metal for SEM (a)DM1 (b)DM1(high magnification image)

Fig.4 Distribtion of M-A constituent of the deposited metals (a)DM1 (b)DM2 (c)DM3

Table 2 Character of M-A constituent of the deposited metals

Fig.5 is the inclusion distribution diagram in the of different Cu elements content deposited metal.The Pro Plus6.0 software for three kinds of inclusions in the deposited metal for statistical geometric features,the results as shown inTable 4,from Table 4 can be found in the three different Cu content of inclusion in the amount of deposited metal,similar size distribution are approximately obey the lognormal distribution,as shown in Fig.6.The size distribution is mainly concentrated in 0.2 -0.8 m,and the proportion of large-size inclusions larger than 2 m in diameter is 1:0.28%in DM,2:0.19% in DM,and 3:0.20% in DM.Combined with Table 4 and Fig.6,it can be seen that the alloy element Cu has little influence on the composition,size and morphology of inclusion in the deposited metal.

Table 3 Composition of inclusions of the different deposited metal(wt%)

Fig.5 Distribtion of inclusion for the deposited metals (a)DM1 (b)DM2 (c)DM3

Fig.6 Statistical distributions of inclusions in deposited metal (a)DM1 (b)DM2 (c)DM3

5 Mechanical test of deposited metal

The results of tensile test and impact test at room temperature for three kinds of wire deposited metals are shown in Table 5.It can be seen from Table 5 that the strength and plasticity indexes of the deposited metal basically meet the technical index requirements specified in the contract specification.However,it can be seen from Table 5 that the mechanical properties of Cu vary with its composition.

Table 5 Tensile test and Charpy V test results of deposited metal

Solid solution strengthening is the most popular and economical way of strengthening.Solid solution strengthening is divided into interstitial solid solution strengthening and replacement solid solution strengthening,which is to use solid solution atoms to produce lattice distortion with matrix metal to cause point defects and interact with dislocation to strengthen metal matrix.The solid solution atoms with small radius,such as carbon and nitrogen atoms,were embedded in the octahedral gap of the -Fe lattice to improve the strength by solid solution gap.The solid solution atoms with a large radius,such as Si,Mn,Ni,etc.,will replace the iron atoms in the -Fe lattice to strengthen in the form of replacing the solid solution.The effect of interstitial solution strengthening on matrix is much greater than that of replacement solution strengthening,but the weakening of toughness and plasticity is also significant,while replacement solution strengthening has no effect on plasticity.In this paper,the Cu content in the electrode is high,and no precipitate is found in DM3 deposited metal.It can be seen that Cu is completely soluble in the matrix.With the increase of Cu content in the deposited metal,the content of solid solution in ferrite increases.The remaining elements of deposited metals are solidly dissolved in the matrix,and the contribution of solid solution strengthening to the yield strength of deposited metals in this paper cannot be ignored.Moreover,there is a linear relationship between the yield strength of deposited metals and the carbon equivalent,so it is believed that solid solution strengthening is the main reason for the difference in the strength of the three deposited metals.It can be seen that the increase of Cu content plays an obvious role in strengthening and toughening the weld.

In three kinds of deposited metal,analysis of the impact fracture was observed under 50 ℃.The impact fracture is composed of fiber zone,radial zone and shear lip zone.Three kinds of deposited metal -50 ℃ impact fracture rate of fiber in the order:55%,63% and 70%.By observing the fiber area of each fracture under SEM (Fig.7),it can be seen that there are densely distributed round or oval dimples of different sizes,and most of them have small second phase particles at the bottom.The typical morphology of quasi-cleavage fracture such as dissection plane,river pattern and tearing edge is shown in the radioactive area (Fig.8).Some ductile ridges can be seen locally.These ductile ridges are composed of dimples,which can delay crack growth and improve toughness in the process of thrust.By comparing the microstructure of the extension zone in the three figures,it can be found that there are significantly more ductile ridges in DM3 than that of in the other three figures,which also explains why the toughness of DM3 is higher than that of other samples.

6 Analysis and discussion

The content of elongated M-A components (aspect ratio 4)has a great influence on the toughness of the heat-affected zone[14-15].A small increase in the content of elongated M-A components will lead to a sharp decrease in toughness.The morphological changes of M-A components in the coarse grain of different welding heat affected zone can be clearly seen by image analysis technology.Each MA component can be characterized by its maximum lengthLmaxand aspect ratioLmax/Lmin.Lmaxgreater than 2 m is defined as large granular M-A components.Lmax/Lmingreater than 4 is defined as slender M-A components.Image-pro Plus software was used to conduct statistics on aspect ratio of M-A components in the deposited metal,and the results were shown in Fig.9.As can be seen from Fig.9,in the three deposited metals,with the deposited metals,the M-A elements are mostly distributed in the original austenite body in A spot-like manner.Therefore,with the increase of Cu content,the shape of M-A components also changes from small block and strip to granular.The probability of the occurrence of massive M-A components is inversely proportional to the toughness value.The higher the probability of massive M-A components is,the lower the impact toughness of the deposited metal will be.The increase of massive M-A components can seriously deteriorate the impact toughness.The probability of the dotted M-A component is positively proportional to the toughness value.The higher the probability of the dotted M-A component is,the lower the impact toughness of the deposited metal will be.The increase of the dotted M-A component can improve the impact toughness of the deposited metal.This is mainly because of increased with the content of Cu make the deposited metal microstructure present the ferrite and bainite duplex microstructure and further refine microstructure,which give rise to the distribution of the M-A size reduced,so the impact toughness of DM3 is significantly higher than the other two kinds of impact toughness of deposited.

Fig.7 SEM morphology of fracture fiber zone for deposited metal (a)DM1 (b)DM2 (c)DM3

Fig.10 is the difference between the EBSD scanning in the deposited metal grain orientation distribution,different color represents different orientation,the black said greater than 15° large angle grain boundary[16],the cutting line method is adopted to measure effective grain size,the result is shown in Fig.11.It is obvious that the grain sizes of 0-1 m in the three deposited metals maintain a high frequency of 45%,57% and 62%,respectively.The average effective grain sizes of the three deposited metals were 1.35,0.89 and 0.75 m,respectively.

Fig.8 SEM morphology of fracture radical zone for deposited metals (a)DM1 (b)DM2 (c)DM3

Naylor[17]gives the relationship between the effective grain D and crack growth resistance (σ),that is

Fig.9 Distribution of M-A constituents in deposited metals(a)DM1 (b)DM2 (c)DM3

whereEstands for elastic modulus,acrepresents the critical crack size,Wrepresents the deflection plastic work at the slat boundary,Dstands for effective grain size,Dis the width of the lath (normally there is no significant change in size).It is found that the crack growth resistance has a linear relationship with the effective grain size d-1/2σ.The smaller the effective grain is,the greater the crack resistance is,the greater the hindrance to the crack is,the more complex the crack growth path is,and the higher the impact toughness is.This result can be explained by taking amount on effective grain size of the different deposited metals.Small effective grain size can effectively arrest cracks propagation,improving impact toughness of the different deposited metals.Therefore,with the increase of Cu content in the deposited metal,the effective grain size also gradually decreases,which is one of the reasons for the increase of the toughness of the deposited metal.

Fig.10 Crystallographic characteristics for deposited metals (a)DM1 (b)DM2 (c)DM3

Fig.11 Distribution of effective grain size for deposited metals (a)DM1 (b)DM2 (c)DM3

7 Conclusions

(1)The deposited metal structure is generally composed of acicular ferrite,granular bainite and a small amount of lath bainite.There are high density dislocations in acicular ferrite and bainitic ferrite laths in the three kinds of wire deposited metals under TEM.The width of bainite ferrite slats is 1 -1.5 m.With the increase of Cu content,the dispersion distribution of M-A phase is in the matrix phase.

(2)In the selected wire composition,the deposited metal can not only obtain high strength,but also good plasticity and low temperature toughness.

(3)With the increase of Cu content,the phase transition temperature of the deposited metal is reduced,so that the phase transition region of ferrite and pearlite moves to the right,the bainite phase transition region is expanded,and the ferrite and pearlite phase transition region is reduced.The microstructure of deposited metal changed,the content of M-A elements increased but the size decreased,and the ferrite-bainite biphasic microstructure was matched.The strength of deposited metals is produced by the solid solution strengthening effect of Cu.The reduction of M-A component content in strips and blocks and the reduction of effective grain size will reduce the nucleation probability of microcracks,increase crack growth resistance,and improve the impact toughness of the deposited metal.