Study on the Influence of Silicon and Aluminum on the Microstructure and Properties of Low Cobalt M42 High-Speed Steel

. M42 HSS (W 2 Mo 9 Cr 4 VCo 8 ) is widely used as a high cobalt super-hard high-speed steel (HSS) for various cutting tool materials, but high cobalt HSS does not bring good economic benefits. In this paper, the effects of Si and Al on the organization and mechanical properties of low cobalt M42 HSS were investigated. The microstructure of HSS was analyzed by OM, SEM, EDS, XRD, etc. Rockwell hardness test, impact toughness test and flexural strength test were conducted. The results of the study showed that only increasing the Si content in M42 HSS containing 4.0% Co did not significantly improve the material organization and properties, and caused a sharp deterioration in impact toughness. After adding 1.0% Si and 1.0% Al to M42 HSS containing 4.0% Co, Si and Al can partially replace Co, resulting in a more uniform and diffuse carbide distribution. The hardness of the material is increased to 68.3 HRC and the impact toughness is increased to 13.8 Jꞏcm -2 , which has better overall mechanical properties and can meet the processing and cutting requirements under actual working conditions.


Introduction
With the rapid development of machining and manufacturing, bi-metal band saws have gradually become one of the indispensable cutting tools in metal cutting. As a key component in bi-metal band saws, the selection of bi-metal band saw blade tooth material is crucial. The current mainstream tooth material is M42 high-speed steel (W 2 Mo 9 Cr 4 VCo 8 ), which is extremely hard and easy to grind, and has excellent red hardness and high temperature performance, can well meet the requirements of the bi-metal band saw blade tooth material performance. However, due to the addition of a large number of high-cost elements of cobalt, the price of M42 HSS is high, with a material cost of up to 200 yuan/kg, and the high price limits the popularization of M42 HSS [1][2] . Therefore, by reducing the content of high-cost element cobalt and increasing the content of alternative elements silicon and aluminum, so that silicon and aluminum can partially replace the role of cobalt and reduce the material cost, it is important to study the effect of silicon and aluminum on the organization and properties of low-cobalt M42 high-speed steel to promote the research and application of new low-cobalt M42 high-speed steel for bi-metal band saw blade tooth material [3][4] .
Previous studies have shown that the role of Co has many similarities with Si and Al in HSS, which are reflected in the ability to improve the hardness and strength of the material by refining the grain and carbide size, the ability of Si and Al to partially replace the role of cobalt, and in addition Al helps to improve the impact properties of the material [5][6][7][8][9][10] . The results of Fusheng Pan [11] et al. showed that Si can improve the hardness of matrix steels, low-alloy HSS and molybdenum-based HSS with W content ≤ 6%, and can improve the flexural properties of these steels. Katharina Gammer [12] et al. studied high-performance HSS containing Al and found that the addition of Al to M2 HSS resulted in a more uniform distribution of eutectic carbides and hardness of 67~69 HRC, with better overall mechanical properties and excellent cutting performance. The current research work is mainly focused on studying the effect of single elements on HSS, while there are few reports on the combined effect of silicon and aluminum on HSS by simultaneously enhancing the silicon and aluminum content. In this paper, the effect of silicon and aluminum on the organization and properties of low cobalt M42 HSS is studied, which provides a new idea for the development of new low cobalt M42 HSS for bi-metal band saw blade tooth material.

Experimental materials and process
The experimental materials were based on M42 highspeed steel containing 4.0% cobalt, and then 1.0% Si, 1.0% Si + 1.0% Al were added to obtain three materials, numbered 4Co, 4CoSi, and 4CoSiAl, and the chemical compositions of the specimens are shown in Table 1. The material preparation process: The high-speed steel ingots were obtained by melting in a vacuum induction furnace under an argon gas protected atmosphere. The ingots were forged into 65 mm*35 mm*160 mm steel by triple upsetting and triple drawing at 1180°C. After forging, the steel was promptly isothermally annealed at 870°C to refine the grain size and eliminate the residual stress. Finally, the steel was sampled and quenched at 1180°C, followed by three times tempering at 540°C quickly.

Experimental methods
The specimens were wire cut to obtain suitable size samples, ground and polished to a mirror finish, and then treated with 4% nitric acid alcohol for corrosion, and the microstructure of the specimens was observed by optec MDS400 metallurgical microscope and Gemini SEM500 field emission scanning electron microscope. The carbide types in the HSS were analyzed by the Science Ultima IV X-Ray Diffractometer. In the mechanical property test, SCTMC HR-150A Rockwell hardness tester was used for surface hardness test, JBN-300B pendulum impact tester was used for room temperature impact test, and JGNT600Y universal material testing machine was used for flexural strength test.

Microstructure analysis
The microstructure observation can visually observe the tissue morphology of the material, and the distribution and approximate quantity of the matrix and second phase in the tissue can be observed statistically at lower magnification, and the morphology and size of the matrix and second phase in the tissue can be observed at higher magnification. Figures 1 and 2 show the tempered OM microstructure and SEM microstructure of 4Co, 4CoSi, and 4CoSiAl specimens, respectively. From the figures, it can be found that the tempered organization is mainly composed of tempered martensite and eutectic carbides, and the larger eutectic carbides are distributed in chains at the grain boundaries, and the smaller eutectic carbides are distributed in a large number of particles at the grain boundaries and inside the grains. 4Co steel has smaller grains in the tempered organization, and the carbides are smaller in size and more in number, and the larger massive carbides are distributed in bands at the grain boundaries, and a large number of particles are distributed inside the grains. In the tempered state of 4CoSi steel, the size of the carbides in the grain is smaller and more numerous, and the size of the massive carbides at the grain boundaries is larger, some of them even exceed 10μm, which may significantly reduce the impact performance of the material and seriously affect the material performance. In the tempered state of 4CoSiAl steel, the bulk carbide size is similar to that of 4Co steel, which is distributed in chains at the grain boundaries, with more uniform carbide distribution and less fine granular carbide inside the grains.   Figure 3 and 4 show the EDS surface scan analysis and XRD analysis patterns of the microstructure of the tempered state of the three specimens. From the figures, it can be found that the eutectic carbides have significantly higher Mo, Si, V and W than the matrix tempered martensite organization, and the eutectic carbides are mainly composed of cubic structured M 6 Ctype carbides, MC-type carbides and hexagonal structured M 2 C-type carbides.

Analysis of the effect of Si and Al on the hardness of HSS
Hardness is the most important mechanical property of HSS. Figure 5 shows the hardness of three specimens of 4Co, 4CoSi and 4CoSiAl. The hardness measurement results show that the hardness of 4Co steel is 68.1 HRC, 4CoSi steel is 67.6 HRC, and 4CoSiAl steel is 68.3 HRC. It can be seen that the 4CoSi steel obtained by increasing the Si content in 4Co steel to 1.0%, the large carbide aggregation distribution in 4CoSi steel makes the carbide cannot be fully dissolved during the heating process, which makes the The degree of matrix alloying is low, the material hardness is reduced by 0.5 HRC, and the hardness measurement error is large, the hardness stability is not as good as 4Co steel, and the hardness value cannot meet the processing and cutting requirements. The 4CoSiAl steel obtained by adding 1.0% Al to 4CoSi steel, the addition of Al dissolves more alloying elements in the solid solution, strengthens the matrix, and the carbide distribution inside the material is more uniform, which makes the material hardness increase by 0.2 HRC compared with 4Co steel, and the hardness stability is better, and the hardness can always be maintained above 68 HRC, which can meet the requirements for bi-metal band saw blades. It can meet the processing and cutting requirements of HSS for bimetal band saw blade tooth under actual working conditions.

Analysis of the effect of Si and Al on the impact performance of HSS
Due to the diversity of the working environment of HSS, the material is required to have high hardness and wear resistance in addition to being able to withstand certain pressure and impact, which requires the material to have good impact properties. Figure 6 shows the impact toughness measurement results of three materials, 4Co, 4CoSi, and 4CoSiAl. The results show that the impact toughness of 4Co steel is 13.07 Jꞏcm -2 , 4CoSi steel is 8.93 Jꞏcm -2 , 4CoSi steel is 13.80 Jꞏcm -2 . It can be seen that the impact toughness of 4CoSi steel deteriorates sharply, decreasing by 4.14 Jꞏcm -2 compared with 4Co steel. The large carbides precipitated along the grain boundaries in 4CoSi steel gather and distribute at the martensite grain boundaries, making microcracks easily expand at the grain boundaries, which makes the material easy to fracture at the grain boundaries in the impact performance test and greatly affects the impact toughness of the material. The impact toughness of 4Co steel and 4CoSiAl steel is better and can meet the requirements of impact toughness of HSS under actual working conditions, which is above 12 Jꞏcm -2 , mainly because the C and alloying elements in the HSS are fully dissolved and distributed in the martensite grain boundaries. The addition of Al slows down the diffusion coefficient of alloying elements in the matrix, refines the tissue grain, and shifts the solid-phase line upward, so that higher quenching temperatures can be used to dissolve more alloying elements into the matrix and strengthen the matrix, which makes impact toughness enhanced.

Analysis of the effect of Si and Al on the flexural strength of HSS
Flexural strength is one of the most important indicators of the strength of HSS. Figure 7 shows the measured flexural strengths of the three materials 4Co, 4CoSi, and 4CoSiAl. The results show that the flexural strength of 4Co steel is 3656.2 MPa, 4CoSi steel is 3584.8 MPa, and 4CoSiAl steel is 3228.4 MPa. It can be seen that the flexural strength of the materials slightly decreases after adding Si and Al to 4Co steel, but the flexural strength of the three materials can maintain a high level of more than 3000 MPa, which can They can meet the requirements of the actual working conditions.

Conclusion
(1) The tempering organization of the low cobalt M42 HSS after upgrading Si and Al content is composed of tempered martensite and eutectic carbides, and the eutectic carbides in the material are divided into agglomerated massive carbides and diffusely distributed granular carbides, which are mainly composed of M 6 C carbides, MC carbides and M 2 C carbides.
(2) As a strong ferrite forming element, the excessive Si content does not significantly improve the material organization and properties of low cobalt M42 high speed steel, and it will promote the formation of large size of massive carbides, which makes microcracks easily expand at grain boundaries, resulting in a sharp deterioration of impact toughness to 8.93 Jꞏcm -2 , which cannot meet the needs under actual working conditions.
(3) After adding Si and Al to the low cobalt M42 HSS, Si and Al can partially replace Co and make the carbide distribution more uniform and diffuse, which increases the hardness of the material to 68.3 HRC and impact toughness to 13.8 Jꞏcm -2 . It has good comprehensive mechanical properties and can meet the requirements of machining and cutting, providing a new idea for the development of a new type of low cobalt M42 high-speed steel for bi-metal band saw blade tooth material.
(4) Further study is needed to investigate the effect of only increasing Al content on the microstructure and mechanical properties of low cobalt M42 high-speed steel.