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摘要：
1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Thermodynamic calculations▲
5 Conclusion▲
图表▲

Fig.1 Microstructure(SEM images)of samples under as-welded(a)and heat conditions treated at 650°C(b)

Fig.2 Microstructure(SEM images)and energy spectrum figure of samples in as-welded and heat treated conditions.a As-welded,b heat treated at 510°C,c heat treated at 650°C

Rare Metals2013年第1期

Formation of carbonitride precipitates in hardfacing alloy with niobium addition

Ke Yang Qin Yang Ye-Feng Bao

College of Mechanical and Electrical Engineering, Hohai University

摘要：

Niobium, as the most effective second-phase forming element, was added in the Fe-Cr13-C-N hard-facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther-modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb(C, N) precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condition, a large number of secondary carbonitrides can pre-cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter-granular chromium depletion.

作者简介：Ke Yang e-mail:yangke_hhuc@126.com;

收稿日期：22 February 2012

基金：financially supported by the National Natural Science Foundation of China(No.51101050);Natural Science Foundation of Jiangsu Province of China(No.BK2011257);

Formation of carbonitride precipitates in hardfacing alloy with niobium addition

Abstract：

Niobium, as the most effective second-phase forming element, was added in the Fe–Cr13–C–N hard-facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther-modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb(C, N) precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condition, a large number of secondary carbonitrides can pre-cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter-granular chromium depletion.

Keyword：

Niobium; Carbonitride; Hardfacing alloy; Precipitate;

Received： 22 February 2012

1 Introduction

Weld hardfacing technique is chiefly employed to extend or improve the service life of mechanical equipment and to reduce their cost,of which hardfacing alloy that has excellent resistance to wear and oxidation is deposited onto equipment.The second phase material particles that precipitate out have an important effect on the wearing life of metal[1–5].Niobium is a kind of element that has a strong combination with C or N.If Nb is added to the steel hardening phase,which is dispersedly distributed,has regular shape,high melting temperature,and high degree of hardness that will be precipitated from the matrix;this will strengthen the matrix,raise the mechanical property and improve the thermal fatigue property and the abradability[6,7].The researches related to the combination of Nb with C or N in structure steel are frequently reported but the researches about the precipitated shape and behavior of the combination of Nb with C and N in the hardfacing alloy are rarely seen.Therefore,this paper aimed at elucidating the form of carbonitride and assessing the effect of niobium on the formation of carbonitride in Fe–Cr13–C–N hardfacing alloy.

2 Experimental

The test materials consisted of steel strip and alloy powders,such as low-carbon ferrochrome,manganese–iron silicon–iron,niobium–iron,chromium nitride,etc.Alloy powders were mixed homogeneously,and the flux-cored wire was made into diameter 3.2 mm through flux-cored wire manufacturing equipment.The welding was performed using submerged arc welding.Weld flux used in this study was sintered flux prepared,and the alkalinity was1.8–2.0.The welding conditions were carried out as follows:welding voltage 30–35 V,welding current 400–450A,wire extension 15–20 mm,welding speed 18–23 m?h^-1and welding using direct current electrode negative.Not less than six layers of hardfacing were deposited on the low carbon steel plate to ensure that the thickness of surfacing layers metal was above 15 mm.Chemical composition of hardfacing alloy is shown in Table 1.The hardfacing alloy is one of the martensitic stainless 13%Cr-steel series and has adequate Nb which has no porosity in it.

Table 1 Chemical composition of hardfacing alloy(wt%) 下载原图

Table 1 Chemical composition of hardfacing alloy(wt%)

Rectangular-shaped samples of dimension 20 mm 910 mm 9 5 mm were cut from the deposited alloy at room temperature.Twenty-one samples were made and averagely classified into seven groups.Six groups of samples were separately heat treated at different temperatures 420,490,510,530,550,and 650°C for 1 h in an electric box furnace.Samples were polished and eroded with aqua regia to be observed by Axiovert 200 MAT optical microscope.HR-150AL rockwell hardness test(HRC)was used to measure the hardness of samples.Quanta 200 scanning electron microscope(SEM)was used to obtain a comprehensive picture of distribution and morphology of niobium carbonitride,and electron probe microanalyzer(EPMA)was used to determine the chemical composition of carbonitride.

3 Results and discussion

Microstructural feature of sample in as-welded condition is shown in Fig.1a;there are two main phases in the hardfacing alloy,i.e.,the lath martensite,the residual austenite,and some point particles are distributed on the grain boundary and the martensite bulk.Figure 1b is the microstructure of sample after heat treatment at 650°C.It exhibits tempered martensite and residual austenite where more fine precipitates of carbonitride can be formed.Moreover,the tempered martensite maintains initial lath form very well after high temperature temper.

The hardness of samples is tested in as-welded and heat treated conditions,as shown in Table 2.We can see that the temper has a big effect on the hardness values of the hardfacing alloy.When the tempering temperatures are lower than 510°C,the hardness values increase with the increase of tempering temperatures and reach a peak value(HRC 47.7)at 510°C.While the tempering temperatures are higher than 510°C,the hardness values decrease with increasing temperatures.Moreover,the extent of the decrease is high from 550 to 650°C.

The pattern and chemical composition of carbonitride precipitates are shown in Fig.2.The EDX analysis shows the presence of nitrogen,carbon,and niobium peaks,so these particles are Nb(C,N)precipitates existing on dislocations within the matrix and on twin and grain boundaries[8,9].In the as-welded condition,primary niobium carbonitride precipitates have a low number in the hardfacing alloy and have large size of 1–3 lm,as shown in Fig.2a In heat-treated condition,fine secondary niobium carbonitride precipitates with rod or spheric type(size\1 lm)precipitate out and distribute more homogeneously on grain boundary and dislocation of the hardfacing alloy,as shown in Fig.2b.These secondary precipitates have a good invigoration effect on the matrix where the hardness reaches the maximum value at 510°C.The carbonitride precipitate extensively spreads out of the matrix with a good stability at the high temperature.Figure 2c gives the image of carbonitride at 650°C.Therefore,stabilizers,such as Nb,have a greater affinity for carbon or nitrogen than chromium and form highly stable carbonitride.In this manner,the formation of a continuous network of chromium-rich phase at the grain boundaries is prevented,and the hardfacing alloy has good stability and corrosion resistance.

Fig.1 Microstructure(SEM images)of samples under as-welded(a)and heat conditions treated at 650°C(b)

Table 2 Hardness values of samples(HRC) 下载原图

Table 2 Hardness values of samples(HRC)

Fig.2 Microstructure(SEM images)and energy spectrum figure of samples in as-welded and heat treated conditions.a As-welded,b heat treated at 510°C,c heat treated at 650°C

4 Thermodynamic calculations

Since the binary carbides and nitrides of Nb are mutually soluble owing to their similar crystal structure,this combined addition of metallic elements in steels can lead to the formation of multicomponent carbonitride[5,10].In the hardfacing alloy,Nb is used as the most effective carbonitride alloying element which prevents the formation of chromium-rich phase.In order to simplify the modeling work without influencing the accuracy,thermodynamic calculations for carbonitride formation were conducted assuming binary carbonitride of Nb was formed.

During the welding process,the basic precipitation reaction for the formation of carbonitride in the hardfacing alloy can be written as follows:

where M is a substitute for the metallic(Nb)and X for the nonmetallic(C,N)elements.

The activity of M and X in equilibrium with MX in iron solution is given by

where x_M,x_Xis the activity of M and X,respectively,DG is the standard free energy,R is the gas constant,R=8.314 J?K?mol,and T is the temperature in Kelvin.

The activity coefficient and activity value of every element can be calculated using the following equations:

where j stands for solute element in hardfacing alloy(Nb,Cr,C,N,etc.);and w_jis concentration of j in hardfacing alloy.Moreover,i stands for solute element in hardfacing alloy(Nb,C,and N);w_iis the concentration of i in hardfacing alloy.e_i^jis the first-order interaction parameter of j on i and can be calculated according to the following Eq.[5]:

e_i^j(1873)is the first-order interaction parameter of elements at 1873 K and is valued according to the Ref.[11].The activity of Nb in equilibrium with their carbide and nitride meets the equations as follows[10]:

Using Eqs.3–7,the equilibrium temperature of binary carbonitride in the hardfacing alloy can be calculated:NbN is 1713 K,NbC is 1565 K.

The melting point of hardfacing alloy is about 1,700 K Then,it is obvious that the interstitial elements of N are redistributed so quickly that they can combine with Nb in the melting bath during the process of welding,and the NbN could precipitate from liquid metal thermodynamically.According to the solidification theory,the NbN easily became the nucleating center of precipitation;thereby,the other precipitate,such as NbC,could grow on it.Therefore,the carbonitride of Nb could grow to form coarse bulk carbonitrides([1 lm)[12].After post-welding tempering heat treatment,more carbonitride precipitates could precipitate,owing to the high diffusibility of alloy element,which has a precipitation-hardening effect on the hardfacing alloy.Moreover,the dispersion of fine niobium carbonitride homogeneously distributed in the matrix plays an important role in improving the wear resistance of the hardfacing alloy[13–15].

5 Conclusion

Addition of niobium in the hardfacing alloy can get Nb(C N)carbonitrides distributed on grain boundary and matrix of the hardfacing alloy with different number and size under as-welded and heat-treated conditions.Under as-welded condition,primary niobium carbonitride precipitates are formed with massive shape already during solidification with large size(1–3 lm).Under heat-treated condition,the secondary niobium carbonitride precipitate with very fine size(\1 lm)can precipitate out and distribute more homogeneously in the matrix.Precipitation of niobium carbonitride in the hardfacing alloy can not only make a great secondary hardening effect on the matrix bu also prevent the formation of chromium-rich phase on grain boundaries.