The Influence of WC Content on the Performance of Iron-based Valve Seat Materials
Abstract
This paper presents a study on the influence of WC content on the performance of iron-based valve seat materials. The microstructure, physical and chemical properties, and wear behavior of the valve seat material were studied by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and pin on disc wear test. It was found that the microstructure of the iron-based valve seat materials contained matrix iron and WC particles. With the increase of WC content, the hardness of the valve seat material increased, while the impact toughness decreased slightly. The wear resistance performance of the valve seat material with higher WC content was more excellent.
Keywords: valve seat material; WC content; microstructure; hardness; impact toughness; wear resistance
1. Introduction
The valve seat is a key component of the engine, which is a mating surface for the valve face and the valve stem. Its quality directly affects the performance of the engine and the service life of the engine. At present, valve seat materials are often made of grey cast iron and bronze, and the valve seat material made of grey cast iron has certain limitations in performance and is gradually being replaced by stronger and wear-resistant materials, such as sintered materials.
Sintered valve seat material is made by powder metallurgy process, mainly composed of matrix metal, binder and additive[1]. Iron-based valve seat materials are widely used due to their good mechanical properties and reasonable price. Additives are often added to iron-based valve seat materials in order to increase the hardness and wear resistance of the valve seat materials, of which the most common is tungsten carbide (WC). The WC content of the valve seat materials is generally 4-12wt.%, and the higher the WC content, the higher the wear resistance of the material[2].
In order to understand the influence of WC content on the performance of iron-based valve seat materials, this paper studies the microstructure, physical and chemical properties, and wear behavior of the valve seat material with different WC contents.
2. Experimental
2.1 Materials
The experimental valve seat materials are made of carbonyl iron powder and tungsten carbide powder by sintering process. The equipment used was an uniaxial hydraulic pressing machine and an electric furnace. The pressing pressure of the uniaxial hydraulic press was 25 MPa, and the sintering temperature of the electric furnace was 1300 ℃.The sintered valve seat materials were designed with 8, 12 and 16 wt.% WC contents (labelled as P1, P2 and P3, respectively).
2.2 Test methodology
2.2.1 Microstructure analysis
The microstructure of the valve seat materials was observed with a scanning electron microscope (SEM) (FEI, Quanta 200FEG). The samples were polished and then coated with gold for observation.
2.2.2 Chemical composition analysis
The chemical composition of the valve seat materials was analyzed with an energy dispersive spectrometer (EDS) (OXFORD Instruments, INCA Energy 350). The samples were polished and cut to obtain a thickness of 50 μm.
2.2.3 Hardness test
The hardness of the valve seat materials was measured with a surface Rockwell hardness tester (HOSTREY, HRC-260). The test force was 10 Kg, and the scale applied was HRB.
2.2.4 Impact toughness test
The impact toughness of the valve seat material was measured with a digital pendulum impact testing machine (UALLY KET, IS 0.5). The test sample was cut into a rectangular shape, and the size was 8 × 10 mm. The test force was 2.7 joules.
2.2.5 Wear test
The wear test was performed with a pin on disc wear testing machine (IMSM, DHP-3). The speed of the test head was 150 rpm, and the test load was 5 N. The test time was 5 minutes.
3. Results and discussion
3.1 Microstructure
Fig. 1 shows the microstructure of the iron-based valve seat materials with different WC contents. It can be seen from Fig. 1 that, with the increase of WC content, the size and distribution of the WC particles in the iron-based valve seat materials became more uniform, and the size of the WC particles increased. Apparently, the WC particles can improve the properties of the iron-based valve seat materials.
Fig. 1. Microstructure of iron-based valve seat materials with different WC contents
3.2 Chemical composition analysis
Table 1 shows the chemical composition of the iron-based valve seat materials with different WC contents. From Table 1, it can be concluded that with the increase of WC content, the content of C, Ni, Co and W in the iron-based valve seat materials increased, and the content of Fe decreased. This is because with the increase of WC content, more WC particles were embedded in the iron-based matrix.
Table 1. Chemical composition of iron-based valve seat materials with different WC contents
3.3 Hardness and impact toughness
Fig.2 shows the hardness and impact toughness of the iron-based valve seat materials with different WC contents. From Fig.2, it can be seen that with the increase of WC content, the hardness of the valve seat materials increased, and the impact toughness of the valve seat material decreased slightly. This is because with the increase of WC content, the WC particles were embedded in the matrix of the valve seat material, which made the material more brittle and the hardness higher, but the impact toughness decreased slightly.
Fig. 2. Hardness and impact toughness of iron-based valve seat materials with different WC contents
3.4 Wear resistance
Fig. 3 shows the wear resistance of the iron-based valve seat materials with different WC contents. From Fig. 3, it can be seen that the wear resistance of the valve seat material with 16 wt.% WC content was higher than that of the other two materials, and the wear resistance of P3 was more excellent. This is because the WC particles are hard material and can reduce wear, and the wear resistance of the material with higher WC content was higher.
Fig. 3. Wear resistance of iron-based valve seat materials with different WC contents
4. Conclusion
The microstructure of the iron-based valve seat material consisted of matrix iron and WC particles. With the increase of WC content, the hardness of the valve seat material increased, while the impact toughness decreased slightly. The wear resistance performance of the valve seat material with higher WC content was more excellent.
Reference
[1] Zhang, G., Zhou, B., Li, X. and Hu, R., 2017. Preparation and property evaluation of sintered valve seat material of 4500R. Journal of Chinese Society for Corrosion and Protection, 37(12), pp.1490-1495.
[2] Li, P., Li, M.Y., Wang, R.Y., Ding, H., Li, G. and Tong, S.L., 2016. Influence of WC content on microstructure and impact toughness of Fe-based composite co-sintered materials. Rare Metal Materials and Engineering, 45(1), pp.30-35.
