For cemented carbide tools, used for machining metals, there are three important
ID: 2087200 • Letter: F
Question
For cemented carbide tools, used for machining metals, there are three important properties of toughness, hardness and wear resistance to be optimized to achieve better performance These properties are controlled by the volume fraction of the hard phase, e.g. tungsten carbide (WC), and the softer binder phase, e.g. Co (Cobalt), in case of WC-Co cutting nserts Three carbide inserts (WC-Co) were examined by optical and electron microscopy to confirm that the ratios of the binder layer (Co) thickness (t) to the WC size (r) are 0.069, 0.12 and 0.035 respectively. By calculating the volume fraction of the two phases of WC and Co, discuss the properties of these three carbide inserts as; a) Which one has the highest hardness and why? b) Which one has the highest toughness and why? c) Which one has the highest resistance to wear and why? [3 marks]Explanation / Answer
Cemented carbide is a hard material used extensively as cutting tool material, as well as other industrial applications. It consists of fine particles of carbide cemented into a composite by a binder metal. Cemented carbides commonly use tungsten carbide (WC), titanium carbide(TiC), or tantalum carbide (TaC) as the aggregate. Mentions of "carbide" or "tungsten carbide" in industrial contexts usually refer to these cemented composites.
The three key qualities of a tool material are Hardness, Toughness and Wear resistance.
Toughness is the ability of the material to withstand interrupted cuts. A tool requires higher hardness to cut a harder work piece material, but requires higher toughness to withstand interrupted cuts. Unfortunately, some of the materials with the highest Mohs hardness are also the ones with least toughness. Tungsten Carbide, Ceramic and Diamond are all very hard but have very low toughness.
A tungsten carbide insert actually consists of hard tungsten carbide particles bound together by a soft cobalt binder.
Want more toughness ? Increase the binder and reduce the carbide.
Want more hardness ? Decrease the binder and increase the carbide. Insert material design is actually a balancing act between hardness and toughness, and a cutting tool manufacturer has a different grade of carbide for each application.
And hardness also hardness increases with decreasing binder content decreasing grain size.
Smaller grains give better wear and larger grains give better impact resistance. Very fine grain tungsten carbides give very high hardness while extra coarse grains are best in extremely severe wear and impact applications such as rock drilling and mining applications.
In the very early days of carbide you made carbide tougher or harder by changing the amount of Cobalt in the binder. Cobalt is metal and softer than carbide grains so more cobalt made it tougher and less made it harder. Then people learned how to change the grain size.
Bigger grains made carbide tougher and smaller grains made it harder. By varying grain size and cobalt % you can make carbide a lot tougher or a lot harder.
If you add more Cobalt to large grains then you get even more toughness. However there is a limit to how tough you can make carbide or want to make carbide. If you get it too “tough” then it is too soft. Remember we are using the term ‘tough’ here as the opposite of hard.
If the grains are too large and there is too much Cobalt then the carbide will move and deform under pressure. One of the major strengths of carbide is its ability to handle pressure or compressive force. If it is too soft it loses that ability.
With increase in Co content, a higher specimen friction coefficient may result from the lower hardness. WC–Cocemented carbides with 0.8 wt. % Co exhibited the smallest average grain size, which causes higher applied-stress propagating dislocations through the material and results in the highest friction coefficient.
The mean values of the volume wear rate of WC–Co-cemented carbides with different Co contents .
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