简介概要

Formation mechanism of titanium and niobium carbides in hardfacing alloy

来源期刊：Rare Metals2017年第8期

论文作者：Ke Yang Kè Yang Ye-Feng Bao Yong-Feng Jiang

文章页码：640 - 644

摘要：The most effective carbide-forming elements titanium and niobium were added into hardfacing alloy.Formation and composition of carbides in the hardfacing alloy were investigated by means of optical microscope（OM）,scanning electron microscope（SEM）,X-ray diffraction（XRD）and energy-dispersive spectrometer（EDS）.Hardness and impact toughness of the hardfacing alloy were measured.The thermodynamics and formation mechanism of carbides were also discussed.It is found that the carbides consist of TiC and NbC which are able to form directly from welding pool during the welding process.The formation mechanism of carbides involves nucleation of TiC followed by epitaxial precipitation of NbC on the surface of TiC.The formation of titanium and niobium carbides can obviously refine the microstructure and deplete the carbon in the matrix.The micros tructure transforms to well-distributed carbides and a tough martensite matrix,contributing to a good combination of high hardness and high toughness in the hardfacing alloy.

稀有金属(英文版) 2017,36(08),640-644

Formation mechanism of titanium and niobium carbides in hardfacing alloy

Ke Yang Kè Yang Ye-Feng Bao Yong-Feng Jiang

College of Mechanical and Electrical Engineering, Hohai University

收稿日期：25 May 2015

基金：financially supported by the National Natural Science Foundation of China (No.51101050);theFundamental Research Funds for the Central Universities (Nos. 2013B18114 and 2015B22614);the Natural Science Foundation of Jiangsu Province of China (No.BK20141156);

Formation mechanism of titanium and niobium carbides in hardfacing alloy

Ke Yang Kè Yang Ye-Feng Bao Yong-Feng Jiang

College of Mechanical and Electrical Engineering, Hohai University

Abstract：

The most effective carbide-forming elements titanium and niobium were added into hardfacing alloy.Formation and composition of carbides in the hardfacing alloy were investigated by means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD)and energy-dispersive spectrometer(EDS).Hardness and impact toughness of the hardfacing alloy were measured.The thermodynamics and formation mechanism of carbides were also discussed.It is found that the carbides consist of TiC and NbC which are able to form directly from welding pool during the welding process.The formation mechanism of carbides involves nucleation of TiC followed by epitaxial precipitation of NbC on the surface of TiC.The formation of titanium and niobium carbides can obviously refine the microstructure and deplete the carbon in the matrix.The micros tructure transforms to well-distributed carbides and a tough martensite matrix,contributing to a good combination of high hardness and high toughness in the hardfacing alloy.

Keyword：

Titanium; Niobium; Hardfacing alloy; Carbides; Microstructure;

Author： Ke Yang,e-mail:yangke_hhuc@126.com;

Received： 25 May 2015

1 Introduction

Weld hardfacing technique is a kind of surface treatment used to extend or improve the service life of engineering components and reduce their cost [ 1] .Many kinds of hardfacing alloys are being used for the weld hardfacing technique.The Fe-Cr13-C hardfacing alloy has high hardness and high resistance against corrosion and wear and is often deposited on the surfaces of engineering components subjected to wear [ 2, 3, 4] .In order to increase the wear resistance,hard carbides are expected to be embedded in the hardfacing alloy to protect the matrix from wearing.Additionally,a tough matrix is necessary for the hardfacing alloy to prevent the falling off of carbides and create a resistance to cracking.Titanium and niobium are strong carbide-forming elements and can combine with carbon in steels to form granular carbides with high hardness [ 5, 6, 7] .Few qualitative studies had been published [ 8, 9, 10] on the microstructure and wear resistance of hardfacing alloys used Ti and Nb as carbide-forming elements.Furthermore,there is little experimental work on the formation mechanism of carbides in the Fe-Cr13-C hardfacing alloy.

In this study,a Fe-Cr13-C hardfacing alloy containing titanium and niobium elements was developed.Investigation was aimed at understanding the formation mechanism of carbides in the hardfacing alloy.

2 Experimental

Two kinds of hardfacing electrodes were produced by a core with 4 mm in diameter and coating consisting of alloy powders such as manganese iron,silicon iron,titanium iron,potassium feldspar,and zircon sand,etc.Alloy powders were mixed homogeneously,and the electrodes were made into 6.5 mm in diameter.After drying in the furnace,the electrodes were weld-deposited on low-carbon steel by using manual shielded metal arc welding (SMAW)under direct current with a reverse polarity.The welding process parameters were as follows:welding current of100-130 A,welding voltage of 24-26 V and welding speed of 9-13 m·h^-1.Six layers of welds were deposited on the substrate so that the undiluted hardfacing alloy could be obtained.The main chemical compositions of the two hardfacing alloys (El and E2) are shown in Table 1.It can be clearly found that the two hardfacing alloys belong to one type of the Fe-Cr13-C martensitic stainless steels.

Rectangular-shaped samples were cut from the electrode coatings using a wire cutting machine at room temperature.The samples were polished and then etched with a 4 vol%nitric acid and 4 vol%hydrofluoric acid aqueous solution.The microstructures were observed with optical microscope (OM,XJG-05) and scanning electron microscope(SEM,S-4800).Carbide precipitates were analyzed by X-ray diffractometer (XRD,ApexⅡ) and energy-dispersive spectrometer (EDS) equipped by SEM.The hardness of the samples was measured by a HR-150AL Rockwell hardness testing machine (HRC).The impact toughness was tested by a Charpy impact testing machine.The size of the specimens was 5 mm×5 mm×50 mm,and an average of three observations was reported.

3 Results and discussion

The microstructures of Samples El and E2 are shown in Fig.1.The microstructure of Sample El is mainly composed of lath martensite,as shown in Fig.la,b.The carbides do not precipitate due to rapid solidification of welding pool,in which Nb element remains in solution within the matrix of hardfacing alloy.When adding Ti and Nb into the hardfacing alloy (Sample E2),the microstructure contains lath martensite and lots of carbide particles.These carbide particles range from 0.5 to 2.0μμm in size and are uniformly distributed in the matrix of hardfacing alloy,as shown in Fig.lc,d.

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Table 1 Main chemical compositions of two hardfacing alloys (wt%)

Fig.1 Microstructures of Samples El and E2:a OM image of Sample El,b SEM image of Sample El,c OM image of Sample E2,and d SEM image of Sample E2

Fig.2 XRD patterns of Samples El and E2

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Table 2 Comparison of FWHM ofα'-Fe peak (°)

Figure 2 shows XRD patterns of Samples El and E2.It is found that the precipitated particles of Sample E2include TiC and NbC.The full width at half maximum(FWHM) of matrix phase (α'-Fe) is listed in Table 2.It can be seen from Table 2 that the FWHM of Sample E2 is larger than that of Sample El,which indicates that the grain is refined.So Sample E2 has a finer grain with refined lath martensite structure than Sample El.The micros trueture and elemental distribution of carbide particles in Sample E2 are shown in Fig.3.These carbide particles are characteristic cubical,as shown in Fig.3a.The EDS line scan spectra (Fig.3b,c) show that the peak values of Ti and Nb correspond to the sites of carbide particles.The average size of carbides is about 0.5-2.0|μm,and the EDS result can provide elemental information of the specimen surface with about 1μm in depth.According to XRD result,it can be clearly confirmed that these carbide particles homogeneously distributed in the hardfacing alloy are a type of complex carbides which consist of TiC and NbC [ 11, 12] .The Ti peak has a similar shape and is obviously higher than the Nb peak on the center area of the two carbides,which means that the core of these carbides is based on TiC.Meanwhile,the Nb peak on the edge of the two carbides is higher than the Ti peak,as marked by arrows in Fig.3b,c,which means that NbC is the epitaxial precipitate.Therefore,it is clear that TiC is the first phase and NbC grows from previously formed TiC.

Fig.4 Hardness and impact toughness of Samples El and E2

Figure 4 presents hardness and impact toughness of Samples El and E2.Compared with Sample El,Sample E2 has a little decrease in hardness value and a significant increase in impact toughness.Therefore,the formation of carbides plays an important role in the improvement of mechanical properties of the hardfacing alloy.

It is well known that the mechanical properties of hardfacing alloy are mainly determined by the matrix microstructure and the hard carbide particles [ 13, 14] .In Sample E2,a larger amount of carbon in the hardfacing alloy is consumed with the formation of complex carbides of Ti and Nb,which decreases the C and Nb contents in the matrix solid solution.The exsolution of C and Nb from the matrix leads to a decrease in hardness.Meanwhile,the formation of carbides could cause a precipitation strengthening effect on the hardfacing alloy.Therefore,Sample E2 shows a little decrease in hardness and retains its relatively high hardness as a consequence of the metallurgical reaction among C,Nb and Ti.The depletion of C in the matrix is contributed to the formation of a lowcarbon martensite,which is very beneficial to improving the toughness of the hardfacing alloy (Fig.4).

Fig.3 SEM image a and EDS spectra of carbide precipitates in Sample E2:b Line scan A1 and c Line scan A2

4 Thermodynamic analysis

In the Fe-Cr13-C hardfacing alloy,thermodynamic calculations were used to understand the carbide formation mechanism.During the welding process,the formation reaction of carbides can be written as follows:

where M represents the metallic (Ti,Nb).The activity of M and C in equilibrium with MC in iron solution is given by:

where x_M and xc are the activities of M and C,respectively,ΔG⁰ is the standard free energy,R is the gas constant(8.314 J·K^-1·mol^-1),and T is the temperature in K.The activity coefficient (f_i) and activity (X_j) value of every element can be calculated using the following formulae:

where j is the solute element in the hardfacing alloy (Ti,Nb,Cr,C,etc.);Wj is the concentration of j in the hardfacing alloy;i is the solute element in the hardfacing alloy (Ti,Nb and C);wi is the concentration of i in the hardfacing alloy; is the first-order interaction parameter of j on i and can be calculated according to following equation [ 8] :

where (1873) is the first-order interaction parameter of elements at 1873 K and is valued according to Ref. [ 15] .The activity of Ti and Nb in equilibrium with their carbide meets the equations as below [ 16] :

Fig.5 Formation process of carbides in Sample E2:a TiC forming as the first phase,b NbC growing on surface of TiC,c austenite (γ) forming surrounding complex carbides,and d refined lath martensite structure and dispersive fine carbides forming

Using Eqs.(3)-(7),the equilibrium temperatures of carbides in the hardfacing alloy can be calculated as 2066and 1462 K for TiC and NbC carbides,respectively.

The melting point of the hardfacing alloy is about1700 K,so it is apparent that TiC could precipitate from liquid metal (welding pool) thermodynamically,while NbC does not meet the thermodynamic condition.Consequently,TiC is more likely to be formed in the welding liquid metal than NbC according to the thermodynamic calculations.Based on the solidification theory,TiC can easily become the nucleating center of precipitation,allowing the other precipitates,such as NbC,to grow on it.Therefore,it is proved that Ti can combine with carbon more easily to form carbide particles and act as nucleates of NbC to promote the precipitation of complex carbides of Ti and Nb from the welding liquid metal.A schematic diagram is used to explain the formation of carbides during the welding process,as shown in Fig.5.TiC can be preferentially precipitated from welding pool during the welding process,as shown in Fig.5a.NbC can then grow on the surface of TiC,as indicated in Fig.5b.During the cooling progress of welding pool,the austenite (γ) is formed surrounding the complex carbides,as shown in Fig.5c.Finally,the refined lath martensite structure and dispersive fine carbides are formed,as exhibited in Fig.5d.

5 Conclusion

In this study,Ti and Nb were added into the hardfacing alloy by the SMAW process.With the aid of OM,SEM,XRD,EDS and thermodynamic analysis,Ti is shown to have great affinity for carbon to form TiC in the welding pool,and the prior precipitation of TiC could act as nucleation sites of NbC to form complex carbides of Ti and Nb in the hardfacing alloy.The formation of titanium and niobium carbides can refine the microstructure and cause a depletion of carbon in the matrix.The microstructure transforms to well-distributed carbides and a tough martensite matrix,leading to a good combination of high hardness and high toughness in the hardfacing alloy.

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China (No.51101050),the Fundamental Research Funds for the Central Universities (Nos.2013B18114 and 2015B22614) and the Natural Science Foundation of Jiangsu Province of China (No.BK20141156).The author would also like to thank the Program for Outstanding Innovative Talents in Hohai University.