A hardness paradox
Diamond is harder than any steel, so on hardness alone it ought to cut steel with ease. In practice, diamond tools are poorly suited to machining most steels, and a workshop that tried to turn mild steel with a diamond insert would probably see the cutting edge degrade quickly. This surprises many people, since scratch resistance and cutting life are assumed to go together.
The explanation is that cutting is not only a mechanical process. At the tool tip, local temperatures can be high and the metal is freshly exposed and reactive, so chemistry begins to matter as much as hardness.
What happens between carbon and iron
Diamond is a form of carbon with strong bonds that hold it stable at room temperature. Iron and the metals near it in the periodic table, such as nickel and cobalt, have an affinity for carbon. At high temperature they can dissolve carbon, and they can also act as catalysts that help carbon rearrange into graphite, the softer form.
Several processes are usually described:
- Diffusion. Carbon atoms from the diamond surface move into the hot iron and are carried away in the chip. The diamond loses material atom by atom, even if it is never scratched mechanically. This diffusion is one of the recognized tool wear mechanisms, alongside abrasion and adhesion.
- Graphitization. Under heat, and with a metal catalyst present, the diamond surface can convert to graphite, which is soft and wears off rapidly. It is a surface effect, but one that continuously renews itself while cutting continues.
- Chemical bonding and adhesion. Iron can form carbides or adhere to the clean diamond surface, causing material to be pulled away.
- Oxidation. In air, diamond starts to oxidize at temperatures that cutting can approach, which compounds the problem. The oxidation limits of diamond are lower than many people expect.
Which of these dominates depends on temperature, speed, contact pressure and the specific steel. All the processes become faster as heat rises, and steel cutting produces a lot of heat at the tool tip.
What is observed in practice
The effect appears as rapid flank and crater wear, a dulling or smeared edge and a poor surface finish. Machinists and tool makers have long reported that diamond should be avoided on most steels, and the guidance is often summarized as “diamond for non-ferrous, something else for ferrous.” This rule is generally right but not absolute. Some conditions reduce the effect, such as low cutting speeds, very light cuts, good cooling or special materials, and certain high-carbon or alloyed workpieces behave differently from plain low-carbon steel. Results vary, so claims of success on a particular steel should be checked against trials.
Not all ferrous materials behave identically either. Cast iron contains free carbon and often reacts less severely than steel, though diamond is still not the usual choice for it. Austenitic stainless steels and nickel alloys can also be difficult.
Why other metals are fine
Diamond works well on aluminum, copper, brass and many non-ferrous alloys, since these metals do not dissolve or catalyze carbon in the same way. Machining aluminum and similar metals with diamond tools is routine, and diamond turning produces mirror-quality surfaces on certain non-ferrous materials. Diamond also cuts non-metals such as composites, stone, glass and ceramics without that chemical issue.
One caution applies to non-ferrous work: workpiece alloys that include iron, nickel or cobalt in meaningful amounts, or tool material containing a cobalt binder, can bring the problem back in a smaller form.
What is used for steel instead
Cubic boron nitride is the standard superabrasive for ferrous work. It has the same crystal structure type as diamond but is chemically stable against iron to much higher temperatures, so it can cut hardened steel and cast iron without the same degradation. For general steel cutting, cemented carbide, ceramic and coated tools are widely used. Diamond-coated tools face the same chemistry, and coating limitations on ferrous materials are one reason a coating is not always a good idea.
Conventional abrasives such as aluminum oxide are used for much steel grinding, and CBN wheels are used where their longer life repays the cost.
Why it matters when choosing tools
The practical lesson is that selecting a tool material requires matching chemistry as well as hardness. A tool must be stable against the workpiece at the temperatures it will reach. Mistakes here can be expensive: diamond tooling costs more than most alternatives, and a wrong choice destroys it quickly. Reading the workpiece specification, using trial cuts and consulting tool suppliers help avoid it.
This also illustrates a broader theme in materials: no single property decides performance. Hardness, toughness, thermal stability and chemical compatibility all take part, and the best choice depends on the combination.