JOURNAL OF RARE EARTHS, Vol. 30, No. 1, Jan. 2012, P. 69
Effect of cerium on abrasive wear behaviour of hardfacing alloy XING Shule (㸠㟦Ф), YU Shengfu (ԭ⫿), DENG Yu (䙧ᅛ), DAI Minghui (᠈ᯢ䕝), YU Lu (ԭ䴆) (State Key Laboratory of Material Forming and Mould & Die Designing, Huazhong University of Science and Technology, Wuhan 430074, China) Received 26 August 2011; revised 13 October 2011
Abstract: Hardfacing alloys with different amounts of ceria were prepared by self-shielded flux cored arc welding. The abrasion tests were carried out using the dry sand-rubber wheel machine according to JB/T 7705-1995 standard. The hardness of hardfacing deposits was measured by means of HR-150AL Rockwell hardness test and the fracture toughness was measured by the indentation method. Microstructure characterization and surface analysis were made using optical microscopy, scanning electron microscopy (SEM) and energy spectrum analysis. The results showed that the wear resistance was determined by the size and distribution of the carbides, as well as by the matrix microstructure. The main wear mechanisms observed at the surfaces included micro-cutting and micro-ploughing of the matrix. The addition of ceria improved the hardness and fracture toughness of hardfacing deposits, which would increase the resistance to plastic deformation and scratch, thus the wear resistance of hardfacing alloys was improved. Keywords: abrasive wear; carbides; rare earth oxides
Many machine components are applied to severe conditions for a long time, for instance, the hot roller and coal shearer, whose failure mode is mainly abrasion. Hardfacing is a commonly employed treatment to increase hardness and abrasive wear resistance of mining, oil, steel and many other industries. The hardfacing alloys having excellent resistance to wear and oxidation are deposited onto base metalsˈ and there are lots of welding techniques to acquire hardfacing layers, such as shield manual arc welding (SMAW), submerged arc welding (SAW), plasma arc welding (PAW), oxyacetylene welding, etc.. Self-shielded flux cored arc welding is one of the most popular hardfacing process because of its simple operation. The abrasive wear behaviour of a welding alloy depends on its chemical composition, the microstructure obtained after welding and the welding parameters. The hardfacing deposit formed by different welding processes and welding parameters presents different abrasive wear resistance. Carbonitride alloying elements, such as Nb, V and Ti, can produce much harder precipitation of carbonitride, and these carbonitride with high hardness and melting point can strengthen the base metals and improve the wear resistance. Rare earth (RE) elements can refine and spheroidize the carbides[6–8], thus it is possible to further improve the wear resistance of welding alloy by adding proper amount of RE elements. Some researches[9–11] also showed that the RE elements have great influence on the fracture toughness and hardness of welding alloy. According to literature, improving the fracture toughness and hardness can greatly enhance the wear resistance.
In this paper, the Fe-Cr13-Mn-Nb hardfacing alloy of self-shielded hardfacing flux-cored wire was made and different amounts of ceria were added to the flux-cored wire, the investigation is aimed at understanding the effect of ceria on the abrasive wear behaviour of the hardfacing alloy.
1 Experimental 1.1 Materials The base material for the deposition of the hardfacing is Q235 steel. The flux core powder is composed with mineral powder and alloy powder. The mineral powder contains gas and slag formers, the arc stabilizer, and the deoxidizers. And the alloy powder contains ferrochromium, ferromanganese, ferrosilicon, ferroniobium and nickle power. Table 1 gives Table 1 Flux compositions Core powder
Oxides (TiO2, ZrO2, MgO, SiO2)
Potash feldspar (K2O·Al2O3·6SiO2)
Deoxidizer (Si, Mn)
Ceria 1# flux cored wire
2# flux cored wire
3# flux cored wire
Foundation item: Project supported by the Doctoral Foundation of Ministry of Education of China (20090499) Corresponding author: YU Shengfu (E-mail: [email protected]
) DOI: 10.1016/S1002-0721(10)60641-2
JOURNAL OF RARE EARTHS, Vol. 30, No. 1, Jan. 2012
the main compositions of the flux. The mass fractions of ceria added into the flux cored wire were 0 wt.% (1#), 5 wt.% (2#), and 10 wt.% (3#), respectively. And the chemical compositions of deposited metal are shown in Table 2. The outer shell of the wire was made of low carbon steel strip of H08A. The content of flux in the wire was 35%–40%, and the diameter of the wire was 2.4 mm. 1.2 Methods Three hardfacing flux cored wire with different amounts of ceria were deposited on the base metal for five layers, respectively. The welding parameters of the hardfacing alloys are shown in Table 3. The final samples were cut out of the deposition by linear cutting machine to avoid any heat effect on the final overlay. The cross-section of the deposited layer was polished and was etched using aqua regia. The microstructure of surfacing layers metal was studied by means of Axiovert 200 MAT optical microscopy and scanning electron microscopy (JSM-5510LV). The hardness of deposited metal was measured by means of HR-150AL Rockwell hardness test (HRC). Five points were measured and averaged, and their locations were randomly selected on a standard surface (20 mm×10 mm) of each sample. The fracture toughness was measured by the indentation method with Vickers hardness tester. According to the indentation crack length caused by Vickers hardness tester, the fracture toughness (KIC) could be determined by the following equation: E) 0.4 L –0.5 H D )( ) ( ) KIC=0.129 ( H a ) where H is hardness (HV), E is elastic modulus (GPa), is half length of indentation diagonal (m), L is the indentation crack length (m) and is the limiting factor (3). Table 2 Chemical compositions of deposited metal Elements
Table 3 Welding technology parameters Welding
The abrasive wear test was carried out using dry sand/ rubber wheel abrasion test apparatus, as displayed in Fig. 1. The wear tests were performed with a standard JB/T 7705-1995. Rounded quartz particles were with mean diameter between 212–425 m. The sample was fixed with a specimen holder against the abrasive and the deposition surface (25 mm×75 mm) of sample was the wear surface. The normal loads, rate of revolution in the tests and wear distance for each sample were 130 N, 240 r/min and 1400 m, respectively. A microbalance was used to weigh the specimens before and after wear tests. The abrasive wear resistance was determined by the mass loss results, which were measured with 0.01 mg resolution. The abrasive pattern of wear samples was observed by SEM.
Fig. 1 Dry sand/rubber wheel abrasion test apparatus 1-Sand hopper; 2-Sand; 3-Sand nozzle; 4-Steel disc; 5-Rubber rim; 6-Specimen holder; 7-Lever arm; 8-Weight
2 Results 2.1
Microstructure and carbide precipitation of the deposited metal
The microstructure features of two hardfacing alloys are shown in Figs. 2 and 3. Fig. 2(a) shows the microstructures of the hardfacing alloys without ceria, the major microstructure is coarse columnar crystal. Columnar crystal could also be observed in Fig. 2(b), but the grain size of columnar crystal is decreased obviously, which means that ceria has significant effect on the grain refinement of hardfacing alloys. The SEM images and corresponding energy spectrum figure of the hardfacing alloys without ceria are shown in
Fig. 2 Microstructures of hardfacing alloys without (a) and with (b) ceria
XING Shule et al., Effect of ceria on abrasive wear behaviour of hardfacing alloy
Fig. 3 SEM images and corresponding energy spectrum figure of the hardfacing alloys without (a, a’, c) and with (b, b’) ceria
Fig. 3(a, a’). There are two main phases in the hardfacing alloys, lath martensite and the residual austenite, and some white dotted particles are distributed on the grain boundary or the martensite matrix as indicated by the arrow in Fig. 3(c). Fig. 3(b, b’) are the SEM image of deposited metal with ceria and corresponding energy spectrum figure. Comparing with the microstructure of hardfacing alloys without ceria, the size of martensite lath decreases and the dotted particles are refined obviously, and the dotted particles distribute more homogeneously when ceria is added as shown in Table 4. The average size of particles without ceria and with ceria is respectively 6.5 and 3.9 μm. This also indicates that the particles are refined by the ceria. According to the energy spectrum figure, the white dotted particles are carbide of Nb. 2.2 Hardness and fracture toughness of the deposited metal The results of hardness, fracture toughness and mass loss
for deposited metal with different amounts of ceria additions are shown in Table 5. According to the results of section 2.1, ceria can significantly refine the grain size of deposited metal, and the precipitation of carbides becomes more homogeneous and diffuse, thus the hardness of deposited metal is also improved due to the refinement of grain size. The experimental data are basically in accordance with the results of analysis on the microstructures. When the amount of ceria addition is up to 5%, the hardness of deposited metal reaches a peak value, however, no significant change of hardness is observed with the continuous increase of the amount of ceria additions. Microstructure refinement not only improves the hardness, but also is beneficial to the fracture toughness. According to ZumGahr, the fracture toughness and hardness are both important parameters that can greatly enhance the wear resistance. According to Table 5, the wear resistance of 3#
JOURNAL OF RARE EARTHS, Vol. 30, No. 1, Jan. 2012
Table 4 Relationship between size of particles and number of particles in deposited metal (µm) Size of particles
Proportion of particles
with different sizes
Table 5 Hardness, fracture toughness and mass loss for deposited metal with different amounts of ceria additions Hardness (HRC)
sample is improved obviously, but the hardness of 3# sample is equal to 2# sample, thus the increase of fracture toughness for 3# sample should be the main reason for the improvement of hardness in this condition.
3 Abrasive wear behaviour The typical aspect of the worn surfaces is shown in Fig. 4(a) and 4(b). Micro-cutting and micro-ploughing are the main abrasive micro-mechanisms observed in Fig. 4. It is observed that the depth of groove caused by micro-cutting in the direction of sliding on the worn surface in Fig. 4(a) is obviously deeper than that in Fig. 4(b). And the number of abrasive dust of the sample without ceria is more than the
sample with ceria. These indicate that the abrasive particles are more difficult to impress into the worn surface and sliding in Fig. 4(b), which means that the hardness of hardfacing alloys with ceria is higher than that of the samples without ceria. It is the main reason for the situation that the resistance of dislocation slipping becomes bigger with the grain refinement and the precipitation of fine and homogeneous carbides protect the matrix from being cut by the abrasive particles. Grain refinement also improves the fracture toughness, which means the resistance of plastic deformation increased, so that the amount of spalling lips and prows caused by decreased microplugging. Thus the wear resistance of hardfacing alloys is improved by adding ceria.
4 Conclusions (1) The hardfacing alloys microstructure in this research were composed of lath martensite, residual austenite and dotted carbides. With the addition of ceria, the size of columnar crystal was refined and carbides were distributed more homogeneously in the matrix. (2) With the addition of ceria, the hardness and fracture toughness of hardfacing alloys were improved, because the resistance of dislocation slipping became bigger with the refinement of microstructure. (3) The wear resistance of hardfacing alloys with ceria was improved. Micro-cutting and micro-ploughing were the main abrasive micro-mechanisms. The improvement of hardness could decrease the depth of groove caused by micro-cutting, meanwhile, the improvement of fracture toughness could increase the resistance to plastic deformation and scratch, so that the amount of spalling lips and prows caused by microplugging was decreased. thus the wear resistance of hardfacing alloys was improved.
Fig. 4 SEM images of wear surface of the samples ceria oxide (a) and with ceria (b)
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