Cutting tool manufacturing is a tricky business. Not only is it expensive to start, but the tools and their applications are becoming highly technical, and one needs to monitor them carefully.
The consumer trend of demanding high-quality, reliable, and long-life cutting tools is rising. One of the most critical factors determining a tool’s quality is its longevity, primarily determined by how well it holds up to wear and tear. Longevity also depends on how well a tool can withstand corrosion and other forms of damage.
This article explores the phenomenon of cobalt leaching and how it affects the longevity of cutting tools.
Cobalt Leaching Explained
Cemented tungsten carbide is a hard material; toolmakers extensively use it to make cutting tools. It consists of fine particles of carbide cemented into a composite by a binder metal. Metallic cobalt is the most widely used binder or ‘cement’ in cemented tungsten carbide cutting tools. Its popularity is because it effectively holds the tungsten carbide grains during liquid phase sintering.
However, the failure of cemented carbides is generally due to the chemical reaction of cobalt with corrosive agents. The corrosion process involves the dissolution of the cobalt binder at exposed surfaces. Thus leaving loosely knit tungsten carbide grains with little structural integrity- this is called cobalt leaching. Cobalt leaching typically leads to the flaking off of unsupported grains in the affected areas.
An example to best understand cobalt leaching is pothole formation on roads. While laying the road, road builders use tar/asphalt as a binder to hold the stones together. Over time environmental conditions like extreme heat, pressure, corrosive fluids, and water (all conditions present in an industrial setting) wash off the binder-tar. The cobalt that holds the carbide grains in place is like this tar. The phenomenon of cobalt leaching is analogous to the tar washing off from roads and forming potholes.
The cobalt leaching processes
The spontaneous release of cobalt happens through constant exposure to corrosive agents in the oil. Leaching describes the dissolution of the cobalt binder matrix at the surface brought on by chemical or electrochemical reactions. These reactions do not affect tungsten carbide grains. However, with the loss of the binder metal, the structure collapses, providing more corrosive fluid to attack new surfaces, thus causing a chain reaction in the leaching process.
Why is cobalt used in cemented carbide-cutting tools?
Cobalt is a complex, ferromagnetic, silver-white, lustrous, brittle element. It is the best choice as a binder for the following reasons-
- It has a very high melting point (1493 deg C); hence it can withstand extreme heat generated during metal-cutting processes.
- Has a high-temperature strength; hence it not only withstands heat but also does not deform readily in high-temperature applications.
- Cobalt has a very effective liquid phase that pulls together the adjoining grains during the sintering process, which avoids voids.
- It is a weakly reducing metal that is protected from oxidation.
Factors and reasons for cobalt leaching?
Aggressive acidic liquids such as contaminated grinding oil attack and dissolve a carbide tool’s cobalt binder. The higher the oil’s acidity or, the longer the tool’s exposure to acidic oil, the worse the leaching will be. Heat is another factor. The rate of cobalt leaching increases dramatically in high-temperature atmospheres.
Leaching often affects surfaces machined early in the tool manufacturing process more than surfaces machined later. Longer and more frequent exposures to corrosive grinding oil make leaching worse. E.g., A grinding oil with a high acid number will leach much more cobalt out of the flutes than it will out of the end teeth. A coating over such a tool will bond poorly in the flutes but coat the end teeth well.
How to avoid cobalt leaching in grinding oil?
Toolmakers can avoid much of this cobalt leaching damage by taking proper precautions during the tool-making process. Cutting tool manufacturers should consider the following measures to ensure cobalt leaching does not affect their operation:-
Choice of grinding oil
Grinding oils are the primary tool to counter the cobalt leaching problem. The grinding oil provides a conducive leaching environment. The cutting tool grinding process dislodges the tungsten carbide grains and the cobalt binder. The cobalt contaminates the grinding oil. If such oil circulates in the system, it causes a chain reaction, thus exaggerating the leaching phenomenon.
Effective Filtration system
The ground carbide and cobalt dust accumulate in the machine oil tank. The longer the cobalt remains in contact with the grinding oil, the chances of leaching increase. Hence, a filtration system handling minute particles helps avoid cobalt leaching.
Transor India manufactures a filtration system specifically to avoid the cobalt leaching phenomenon. The filter elements do not allow carbide or cobalt dust to accumulate in the grinding oil, which avoids increasing acidity.
Grinding oil temperature
As discussed earlier, cobalt leaching accelerates with higher temperatures like any other corrosion process. Experts estimate that the rate of cobalt leaching roughly doubles with each 7° C increase in temperature.
The problem of cobalt leaching is significant for carbide-cutting tool manufacturers, and they cannot ignore it. With relatively low investment backed by proper research, cutting tool manufacturers can control the structural integrity of their tools.
Manufacturers can ensure process stability with simple steps like using an excellent grinding oil, an effective filtration system, and a correctly designed chilling unit. By adopting good manufacturing practices, cutting tool manufacturers will achieve quality control leading to higher customer satisfaction and profits.
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