How to reasonably choose cemented carbide grades?

The basis for a reasonable selection of cemented carbide is the performance of the workpiece to be processed and the performance of the cemented carbide. There are many factors that determine the performance of cemented carbide, such as composition, particle size, metallographic structure, etc., but the most obvious influence is the composition. Therefore, to reasonably select cemented carbide, one must understand the impact of alloy composition on performance.

 

Cemented Carbide Grade

 

1. The role of several main components in the alloy in use

 

1) The role of WC

 

In WC-Co alloys, the higher the WC content, the better the wear resistance and higher cutting speeds can be used.

 

2) The role of titanium carbide

 

(1) Increase high-temperature hardness

(2) Enhance wear resistance

(3) The bending strength and toughness of the alloy have decreased.

 

3) The role of TaC and NbC

 

(1) Significantly improve the high-temperature properties of the alloy

(2) The addition of TaC can also reduce the thermal sensitivity of the alloy, reduce the thermal cracks and collapse wear of the blade, and extend the life of the tool.

 

4) The role of Co

 

(1) As the Co content increases, the strength and toughness of the alloy increase

(2) When the Co content decreases, the hardness and high-temperature durable strength increase

(3) Low Co alloys are suitable for higher cutting speeds, while high Co and medium Co alloys are suitable for continuous and rough machining tools.

WC-Co type

 

WC-Co (YG) carbide is mainly used for processing cast iron, non-ferrous metals and non-metallic materials. When machining cast iron, the chips are in the form of broken pieces, the tool is greatly impacted, and the cutting force and cutting heat are concentrated near the blade and tip. YG-type alloys have higher bending strength and impact toughness (compared with YT type), which can reduce edge chipping during cutting. At the same time, YG alloy has good thermal conductivity, which helps the cutting heat dissipate from the tooltip, reduces the tool tip temperature, and prevents the tooltip from overheating and softening. When processing non-ferrous metals and their alloys, since non-ferrous metals and their alloys will not dissolve with WC at the melting temperature or the dissolution rate is very slow, and there will be no chemical interaction even at the melting temperature, YG alloys can successfully Processing of non-ferrous metals and their alloys.

YG alloy has good grindability and can grind sharp edges, so it is suitable for processing non-ferrous metals and fiber composite materials. When YG carbide contains more cobalt, its bending strength and impact toughness are better, especially the fatigue strength is improved, so it is suitable for rough machining under conditions of impact and vibration. When the cobalt content is small, its wear resistance and heat resistance are high, and it is suitable for finishing in continuous cutting. When the cobalt content is small, the hardness of the alloy is higher and the wear resistance is better.

 

WC-TiC-Co cemented carbide

 

WC-TiC-Co (YT) carbide is suitable for processing plastic materials such as steel. Due to the large plastic deformation of the steel material during processing, the friction between the steel material and the tool is severe, so the cutting temperature is high. YT alloys have higher hardness, especially higher heat resistance. Their hardness and compressive strength at high temperatures are higher than those of YG alloys, and they have good oxidation resistance. In addition, YT alloys have high wear resistance when processing steel. The thermal conductivity of YT-type cemented carbide is poor, and less heat is transferred to the tool during cutting. Most of the heat is concentrated in the cutting. The cutting will soften after being exposed to strong heat, which is conducive to the smooth progress of the cutting process.

When YT carbide contains more cobalt and less titanium carbide, it has higher bending strength and is better able to withstand impact, making it suitable for rough machining. When it contains less cobalt and more titanium carbide, it has better wear resistance and heat resistance and is suitable for finishing. However, the higher the titanium carbide content, the worse its grinding processability and welding performance, and cracks are prone to occur during sharpening and welding.

 

WC-TaC(NbC)-Co cemented carbide

 

The properties of many WC-Co cemented carbides can be improved by adding a small amount (about 3%) of carbides such as TaC, NbC, Cr3C2, VC, TiC, HfC, etc. The function of these carbides is to refine the grains so that the alloy maintains a uniform fine-grained structure without significant recrystallization. At the same time, the hardness and wear resistance of the alloy can be significantly improved without reducing its toughness. In addition, adding a small amount of carbide can also improve the high-temperature properties of the alloy and produce a tough, self-compensating oxide film that resists adhesion and diffusion wear when cutting certain metals and alloys. Currently, carbide tools with TaC (NbC) and Cr3C2 added are more commonly used, which can smoothly process various cast irons (including extra-hard cast iron and alloy cast iron). Low cobalt cemented carbide containing TaC (NbC) 3-10%, can be used as a general grade.

 

WC-TiC-TaC(NbC)-Co cemented carbide

 

Adding appropriate TaC to WC-TiC-Co cemented carbide can improve its bending strength (significantly increasing blade strength), fatigue strength and impact toughness, improve heat resistance, high-temperature hardness, high-temperature strength and oxidation resistance, and improve Its wear resistance increases the resistance to crater wear and flank wear. This type of alloy combines most of the best properties of WC-TiC-Co and WC-TaC-Co alloys. It can be used to process steel (main purpose), cast iron and non-ferrous metals, so it is often called universal Alloy (YW). This type of alloy is usually used to process various high-alloy steels, heat-resistant alloys, various alloy cast irons, extra-hard cast iron and other difficult-to-machine materials. If the cobalt content is appropriately increased, this type of cemented carbide will have higher strength and toughness and can be used for rough machining and interrupted cutting of various difficult-to-machine materials.

 

2. The effect of surface coating on coated blades on tool performance

 

1) Reasons for surface treatment-coating

 

(1) Improve the performance of carbide cutting tools

(2) Needs for high-speed cutting (HSC) and high-speed machining (HSM)

(3) The need for machining of difficult-to-machine materials such as aviation heat-resistant alloys, composite materials, and hard materials.

2) Advantages and functions of coating

 

(1) The coating can improve the high-temperature hardness (red hardness) and improve the wear resistance of the tool. Can significantly improve tool life.

(2) The friction coefficient of the coating is small, and the friction coefficient between the coating and the workpiece is small. Most of the cutting heat is transferred to the workpiece and chips, which can reduce cutting resistance and smooth chip removal, which is beneficial to improving product surface quality and extending tool life.

(3) Many coated tools can be used for dry cutting on many occasions. The elimination of cutting coolant can reduce consumption and thus reduce product production costs. At the same time, it has far-reaching practical significance for protecting the environment and the health of employees at the production site.

Research on the tool wear mechanism shows that during high-speed cutting, the wear of the tool is not only mechanical friction wear (the main form of wear behind the tool), but also adhesive wear, diffusion wear, and oxidation wear (tool edge wear and crater wear). Main forms of wear)

The CVD coatings of carbide inserts mainly include the following categories: TiN/TiCN/TiN, TiN/TiCN/Al2O3 and TiN/TiCN/Al2O3/TiN.

PVD coatings mainly include: TiN, TiCN, TiAlN, TiN/TiCN, TiN/TiAlN

 

CVD coating

 

(1) Titanium nitride (TiN) has a slightly lower hardness, but has higher chemical stability and can greatly reduce the friction coefficient between the tool and the workpiece being processed.

(2) From the perspective of coating technology, both TiC and TiN are ideal coating materials. However, regardless of titanium carbide or titanium nitride, a single coating is difficult to meet the comprehensive requirements for tool coating in high-speed cutting.

(3) Titanium carbonitride (TiCN) has the comprehensive properties of TiC and TiN, and its hardness (especially high-temperature hardness) is higher than TiC and TiN, so it is an ideal tool coating material.

(4) No material can compare with aluminium oxide (Al2O3) in terms of its resistance to oxidation wear and diffusion wear. However, due to the large difference in physical and chemical properties between alumina and the base material, a single alumina coating cannot make an ideal coated tool. The emergence of multi-coatings and related technologies enables coatings to not only improve the bonding strength with base materials but also have the comprehensive properties of multiple materials.

 

PVD coated alloy

 

(1) PVD is a process that uses certain physical processes, such as the thermal evaporation of a substance or the sputtering of atoms on the surface of a substance when bombarded by particles, to achieve a controlled transfer of substance atoms from the source substance to the thin film.

(2) The deposition temperature of PVD coating is approximately 300-500°C. PVD has been successfully used in integral rotating tools and precision indexable inserts

(3) PVD-coated inserts are mainly suitable for solid cutting tools (milling, drill bits), milling, threading, hole processing, cutting off, grooving, and turning of stainless steel and heat-resistant alloys.

PVD coating properties

 

(1) TiN/TiCN, TiC/TiCN/TiN, TiN/ZrN and other multi-layer coatings have been used in carbide cutting tools and some high-speed steel cutting tool coatings through the PVD process, and their service life is longer than that of a single TiN PVD coating. More than doubled. Among them, ZrN-coated tools are particularly suitable for processing materials such as stainless steel.

(2) TiAlN is the only PVD coating containing aluminium. Al2O3 is formed during the oxidation of aluminium during the cutting process, which plays an anti-oxidation and anti-diffusion wear role. However, its anti-oxidation performance is slightly worse than that of a single Al2O3 coating because The Al2O3 formed in TiAlN is generated and worn away during the cutting process. But in high-speed cutting, its effect is better than aluminum-free TiCN coating.

Ti-Al-N coating

 

The ternary (Ti, Al) N coating is mainly composed of the (Ti, Al) N (fcc) phase, in addition to (Ti2Al) N (hcp), and (Ti3Al) (CuTiO3 structure).

The reason for the improved oxidation resistance of (Ti, Al)N coating is that a dense Al2O3 protective layer is formed on the surface of the coating to prevent further oxidation inside the coating, and the high-temperature oxidation resistance can reach 800°C. Since some Al atoms in the film replace Ti atoms, the internal stress of the (Ti, Al) N film is reduced, the toughness of the film is greatly increased, and the bonding force of the film is improved.

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