What do Carbide Cutting Tools have in common with Rice Krispie Treats?
You are probably wondering how anyone in their right mind would draw a comparison between carbide cutting tool materials and rice krispy treats. Well I have never been accused of being right minded so here we go. Carbide cutting tools are made from 3 basic elements Tungsten (W), Carbon (C), and Cobalt (Co) which are found on the periodic chart of elements. These elements are known as refractory metals, which are extraordinarily resistant to heat and are very wear resistant. Rice Krispy treats are made from Rice Krispies and marshmallow cream. Rice Krispies are a breakfast cereal and marshmallow cream is a confection used in baking. So how can there possibly be a connection?
It’s easy. Remember there are two kinds of carbide; hard and tough. Just like there are two kinds of cookies; crunchy and chewy. If you recall, hard carbide can take a lot of heat therefore it can operate at higher speeds but, it can’t take a lot of pressure, impacts or interrupted cutting. Tough carbide can take a lot of pressure, impacts and interrupted cutting, but it can’t take a lot of heat therefore it must run slower. Just like a baker changes the recipes ingredients in a cookie to make it either soft and chewy or crisp and crunchy; material scientists modify the composition of carbides main elements to make it hard and wear resistant or tough and impact resistant.
So, let’s take hard carbide and crispy cookies. In the process of making carbide cutting tools Tungsten (W) and Carbon (C) are combined to form hard wear resistant Tungsten Carbide molecules or (WC). I compare these hard wear resistant WC molecules to the Rice krispie in a rice krispie treat. Both Tungsten and Carbon have very high melting temperatures both over to
6,000 degrees Fahrenheit. I am sure you would agree that the rice krispy cereal is harder than marshmallow cream. That said, let’s look at Cobalt (Co) and how it related to marshmallow. Cobalt (Co) is referred to as a binder and is often used when mixing other metals, it helps them stick together. Think of binder and marshmallow like glue. The Cobalt (Co) in carbide and the marshmallow in the rice krispy treat helps hold the hole everything together. Cobalt (Co) like marshmallow has a much lower melting point than the WC molecules it is mixed with at less than 3000 Fahrenheit. The more cobalt or marshmallow the slower you must go. So, how do we take this information and relate to the multitude of carbide cutting tool grades on the market today.
Actually, it’s pretty easy if we use the Rice Krispie treat comparison. Hard carbide grades have smaller grains of WC material which are tightly packed together with a minimal amount of Cobalt binder holding them together. Think about crushing the rice krispies, and putting them into the baking pan, the smaller pieces fit tightly together with very little space for the marshmallow. This type of recipe makes for a crispy cookie. If we take small grains of WC tightly there too is very little room for Cobalt. The closely packed WC grains help dissipate the heat away from the shear zone which allows it to run at faster speeds. Unfortunately, the limited amount of marshmallow or Cobalt tends to make it brittle and less susceptible to impacts or interrupted cutting. So small grains and smaller amounts of binder make for a hard wear resistant tool good for higher speeds, harder materials with normal un-interrupted cutting applications.
Tough carbide on the other had has larger grains of WC or rice krispies. These larger grains are less tightly packed which allows for more cobalt or marshmallow binder to fill in the spaces in between. The more binder or glue the more tough the tool or chewier the cookie. The bigger grains are easier for the Cobalt or marshmallow to hold. Unfortunately; this added cobalt or marshmallow breaks down at higher temperatures. It can’t take high heat therefore slower temperatures must be used when machining. So big grains of Tungsten Carbide and higher amounts of Cobalt make for a tough impact resistant tool that can take a lot of interruptions but must be run at slower speeds to reduce the heat at the shear zone.
Another way to look at is like this. Once a crack in the surface of the cutting tool starts it follows the path of least resistance. If I have small grains of WC or rice krispies tightly packed together it will have a lot if pathways to follow in the spaces between the grains. The more grains it has the more pathways it can choose from and the more likely it is to run all the way through the cutting edge and break the tool. The more tightly packed together the grains are the less Cobalt or marshmallow you have in the gaps. Less Cobalt to hold the small Tungsten Carbide grains together, thus resulting in a chipped or broken tool. On the other hand; the bigger the grains or rice krispies and less tightly packed together they are the fewer pathways the crack has to choose from. The larger the gaps between the grains of Tungsten Carbide and the larger the pools of Cobalt or marshmallow the more likely the grains will be held together and the crack will stop propagating through the tool.
The illustration below summarize in extreme terms the difference in basic characteristics of tough impact resistant and hard wear resistant carbide.
Manufacturers (ISO) Grade Designation Chart
Most carbide cutting tool manufacturers use the color chart and numbering scale below in their grade designations or names to communicate to the user the relative hardness verses toughness of the cutting tool material. The smaller the number the smaller the WC grain size, the smaller amount of Cobalt, which would run at higher speeds , but is fragile. The larger the number, the larger the WC grains, more Cobalt, the greater impact resistance but the slower it must go.