Development of Iron, nickel cemented carbide replace of cobalt cemented carbide

Carbide is widely used in cutting tools, mining tools, wear-resistant parts, mould industries, etc. due to its high hardness, high wear resistance, and high bending strength. Cemented carbide is an alloy made of refractory metal hard compounds and binders through a powder metallurgy process. The binder of traditional cemented carbide is mainly metal cobalt. However, with the development of modern industrial technology, traditional cemented carbide has shown shortcomings in some aspects.

(1) Traditional cemented carbide is insufficient in terms of oxidation resistance, corrosion resistance and other properties, and it is difficult to meet the use needs in some specific environments.

(2) As an expensive and scarce metal, Co has extremely limited global reserves and its price is rising year by year. The demand for Co powder in China is also increasing.

Based on the above factors, it is urgent for cemented carbide researchers to find other metal elements to replace Co as the binder of cemented carbide. In the periodic table of elements, Fe, Co, and Ni are elements of the same period, and their masses, atomic radii, melting points, and physical and chemical properties are relatively close. Like Co, Fe and Ni can wet and wrap the WC hard phase very well, and in comparison, Fe and Ni have more storage capacity, wider sources, and cheaper prices. Therefore, using Fe or Ni as the binder of cemented carbide can greatly reduce the production cost of the alloy.

 

1. Characteristics of iron-nickel-based cemented carbide

 

When using pure Fe powder instead of Co as the binder of cemented carbide, there are many difficulties that need to be solved. For example, the wettability of Fe powder to WC is relatively poor, the two-phase zone is narrow, the Fe powder is highly active, and is easy to oxidize. The fluctuation of oxygen content makes it difficult to control the carbon content in the alloy, and it is easy to form FeWxCy type brittle carbides, which increases It reduces the brittleness of the alloy, and Fe and C easily form stable Fe3C during the sintering process, which hinders the sintering and bonding of the alloy.

Therefore, there are relatively few studies on using Fe powder to replace Co as a binder. Although the mechanical properties of cemented carbide using Ni powder as a binder are not as good as traditional cemented carbide, it has better oxidation resistance and corrosion resistance than traditional cemented carbide, which makes up for the disadvantages of traditional cemented carbide in some fields. It shows the shortcomings of poor oxidation resistance and corrosion resistance when applied. For deficiencies in mechanical properties, methods such as adding other metal elements for solid solution strengthening can be used to improve the mechanical properties of the alloy. Therefore, if Ni can be used to partially or completely replace Co as the cemented carbide binder, the production of “Ni instead of Co cemented carbide” will greatly reduce the production and use costs of cemented carbide, giving it a broader market prospect. and significant economic benefits.

 

2. Factors affecting the performance of iron-nickel-based cemented carbide

 

1) Influence of WC grain size

 

The grain size of WC is mainly affected by factors such as the particle size of the WC raw material, the sintering process, and whether grain inhibitors or rare earth elements are added. Fine WC particles help achieve densification, on the one hand, because the densification speed is faster in this case, and secondly, because a relatively high sintering density can be achieved under a fixed sintering cycle. Research shows that the structure of WC-TiC-Ni-Fe alloy sintered at a sintering temperature of 1400~1480°C is normal. Within the study temperature range, the hardness and flexural strength of the alloy first increased and then slowly decreased. The reason is that as the sintering temperature increases, the WC grains grow excessively. Therefore, in the actual sintering process, in order to obtain fine WC grains, low-temperature sintering should be used as much as possible under the conditions that meet the sintering temperature.

The study shows that when other conditions are the same, as the carbon content in the composition increases, the eutectic temperature of the alloy decreases, the relative sintering temperature increases, and the grains grow more easily. Research shows that adding an appropriate amount of elements or compounds such as VC, Cr3C2, TaC, TiC, ZrC, HfC, etc. to cemented carbide can effectively inhibit the growth of WC grains. Among them, VC and Cr3C2 are the most effective inhibitors, followed by TaC, TiC, and ZrC. In addition, adding rare earth element oxides to cemented carbide can also play a role in refining the grains. For example, adding an appropriate amount of Sm, Y, Ce, Cm, Pr, La and other rare earth metals or compounds can increase the grain size to varying degrees. The binder has the ability to wet WC grains, refine the grains, control abnormal grain growth and improve the performance of the alloy.

 

2) Influence of WC grain morphology

 

WC is the main component of cemented carbide, so the shape of WC grains is also one of the factors that affect the performance of cemented carbide. For example, cemented carbide containing plate-like WC grains has unique characteristics such as high toughness, high strength and hardness, strong wear resistance and resistance to plastic deformation, high high-temperature hardness and high-temperature fatigue strength, and good resistance to high-temperature creep and thermal shock. performance. Studies have shown that the WC grain size of the alloy after adding plate-shaped WC seed crystals is larger than that of the alloy without adding crystal seeds and has a certain plate-like morphology. When a small amount of plate-shaped seeds are added, the density of the alloy is not affected, but the hardness, flexural strength and fracture toughness are increased. In particular, the flexural strength is increased by 12.8% and the fracture toughness is increased by 46.9%.

Studies have also shown that when there are Mitsubishi columnar plate-shaped WC grains in the WC-10Ni3Al alloy, the transverse fracture strength of the alloy reaches 2092MPa, the fracture toughness is 21.56MPam1/2, and various properties are equivalent to WC-Co. The cemented carbide with disc-shaped WC grains has better room-temperature mechanical properties and high temperature mechanical properties than ordinary cemented carbide. In practical applications, it also shows higher wear resistance and chipping resistance than ordinary cemented carbide, and its development and application prospects are broad. In particular, the directional distribution of disc-shaped WC grains will cause the alloy to show different properties in different directions, making it easier to meet various usage requirements.

 

3) Effect of adding alloy elements

 

When only pure iron or pure nickel is used as a binder, the properties of the alloy obtained are lower than those of cemented carbide using cobalt as a binder. Therefore, in order to improve the performance of iron-nickel-based cemented carbide, researchers often add some alloying elements or compounds to the binder to improve the performance of the alloy. Commonly used alloying elements are mainly Cr, Cr3C2, Si, Co, and Nb. , HfC, VC, TaC, Ti, TiC rare earth elements, etc. Research shows that after adding metals or metal compounds such as TaC, Cr or Cr3C2 to WC-Ni-based cemented carbide, the grain size of the alloy is refined, with an average grain diameter of 1.03~1.31um, and the strength and hardness of the alloy are improved. All have been significantly improved, the transverse fracture strength can reach more than 2000MPa, and the hardness can reach more than HRA90.

In addition, some studies have shown that the way in which alloying elements are added will also affect the microstructure and properties of the alloy. Experimental results show that when added in the form of metallic Cr or chromium-nickel alloy (Ni-18Cr), new sub-alloys will be produced. The stable phase (W, Cr) C, metastable phase reduces the flexural strength of the alloy. When added in the form of Cr3C2, a carburized structure zone will appear, reducing the flexural strength of the alloy. When added in the form of Cr+Cr3C2 composite powder, no obvious defects were found in the structure, and the alloy had the highest flexural strength.

The study found that when the alloy composition is WC-10 (Ni/Si/C), compared with WC-10Co alloy, the Vickers hardness of the two is equivalent, but the flexural strength and fracture toughness of the former are higher than that of the latter. good. When the rare earth addition amount is 1.2% of the Ni mass fraction, the alloy can reach the highest flexural strength of 2230MPa after vacuum sintering at 1510°C, which is about 15% higher than the WC-8Ni alloy without rare earth additions and reaches the YG8 alloy. Series flexural strength. Research has found that adding rare earth boride (LaB6) can make the bonding phase distribution in the alloy more uniform, reduce the contact between WC grains, reduce the porosity of the alloy, and increase the density of the alloy. When the addition amount of LaB6 reaches 0.0967%, the fracture toughness of the alloy increases from 13.1MPa.m1/2 to 15.6MPa.m1/2, and the compressive strength reaches 3500MPa.

 

3. Difficulties in iron-nickel-cobalt substitution

 

1) Difficulty in controlling carbon content

 

When the carbon content is incorrect, carbon-deficient tissue or graphite tissue will appear in the structure, seriously affecting the performance of the alloy. Alloys with different compositions have different two-phase zone widths and different carbon content ranges. For a specific brand of alloy, the carbon content range in the two-phase zone can be determined through experiments. As long as the experimental work is careful. Reliable, repeated tests will always yield results. But if there are many grades of alloys and the composition changes greatly, this becomes very troublesome.

 

2) Iron and nickel powder ductile graphite has poor processability

 

Due to the good plasticity and low hardness of Fe powder and Ni powder, the “segregation” phenomenon is prone to occur during the wet grinding and mixing process. Even if very fine iron powder or nickel powder is used as raw material during the production process, the iron powder and nickel powder will thicken into large-sized aggregates during wet grinding, which will cause larger-sized aggregates to be produced in the subsequent sintering process. holes. This is an important factor affecting the performance of WC-Ni cemented carbide, which requires strict control of process conditions during mixing and sintering.

 

3) Oxidation of powder

 

Because of the multi-step nature of the cemented carbide production process and the current inability to produce it under completely sealed conditions, the powder is easily oxidized. And since the raw materials are all granular powders, the contact area with air is large. If the time is too long or the temperature is too high when drying the powder, the powder will easily oxidize. Therefore, preventing powder oxidation is one of the basic conditions to ensure the effectiveness of the process. When iron is used as the binder, the oxidation phenomenon from powder manufacturing to compact sintering is very prominent, and the fluctuation of oxygen content is bound to be attributed to the fluctuation of carbon content in the sintered blank. And because the two-phase zone of iron-based cemented carbide is narrow, the allowed variation range of carbon content is very narrow, which makes the entire sintering process more difficult. Therefore, it is very necessary to prevent the oxidation of iron powder during the whole process.

 

4. Application of iron-nickel-based cemented carbide

 

1) Application of WC-Fe-Co-Ni cemented carbide

 

Carbide with Fe-Co-Ni as a binder is mainly used to make cutting tools for wood and masonry alloys. The relative wear and roughness of the blade are lower than or equivalent to the level of traditional WC-Co cemented carbide. Therefore, in the fields of wood cutting and rock drilling, Fe-Co-Ni alloys as binders can be used to replace traditional WC-Co cemented carbide. Sandvik Asia and others use similar materials with Fe-Ni-Co content up to 50% to make guide wheels for bar rolling, which show excellent performance at a rolling temperature of 900°C and a rolling speed of 80m/s. Toughness and high wear resistance.

 

2) Application of WC-Ni cemented carbide

 

Compared with WC-Co cemented carbide using the same amount of binder, WC-Ni cemented carbide has better impact toughness and meets the usage requirements of composite mining cemented carbide.

In addition, the carbides of traditional cemented carbides are non-magnetic. The Curie points of metallic iron, cobalt, and nickel are 770°C, 1120°C, and 354°C respectively. The Curie points of cobalt and nickel are very high, and the overall alloy is It is magnetic at room temperature, but the Curie point of nickel is relatively low. It can be lowered to room temperature through some methods to obtain non-magnetic cemented carbide. Therefore, WC-Ni-based cemented carbide with Ni as the binder phase is often used in industry to produce non-magnetic cemented carbide.

 

3) Application of WC-Ni-Cr alloy

The traditional WC-Co cemented carbide has excellent wear resistance, but the corrosion resistance of cemented carbide with Co as a binder is relatively poor and is susceptible to chemical media erosion. Research shows that although the flexural strength and hardness of WC-(7~10)Ni-(1~2)Cr alloy is slightly lower than WC-8Co and WC-13Co cemented carbide, the WC-(7~10)Ni- (1~2) Cr alloy has strong corrosion resistance and is suitable for wear-resistant and corrosion-resistant materials in the petrochemical industry and environmental protection fields.

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