What is heat treatment, and why does it matter?

The quality of a knife’s blade is largely determined by two key factors: edge geometry and heat treatment. Edge geometry influences how well the knife cuts, while heat treatment dictates how well the blade retains its sharpness over time.

Steel is often thought of as inherently ‘hard,’ but without proper heat treatment, it won’t be able to hold an edge. At its core, heat treatment involves heating the blade to a specific temperature—called the "critical temperature"—at which the carbon and iron molecules in the steel can form iron carbides. These carbides are what give the blade its hardness. The finer the grain structure and the more iron carbides present, the harder the blade will be.

Different types of steel respond to heat treatment in distinct ways, requiring various methods to achieve the desired hardness. Some steels are quenched in oil, others in water, and some even require extreme cooling methods, such as liquid nitrogen. With thousands of different steels used in knife making, each type demands a unique approach.

Traditionally, blacksmiths would gauge the steel’s temperature by its color. As the steel heats up, it changes from dull red to bright white (the latter occurring near its melting point). Today, many modern knife makers use high-precision, temperature-controlled ovens that are calibrated to a specific temperature for optimal hardening. While traditional methods can achieve excellent results with enough skill and experience, they take years to perfect.

It’s also important to note that hardness doesn’t always equate to better performance. In simple carbon steels (comprising iron and carbon), harder blades are often more brittle, which can be a problem. For example, while a hard, brittle edge will get extremely sharp and may work well for slicing soft ingredients (like in a sashimi knife), it risks chipping or even breaking when cutting through tougher ingredients, like a butternut squash. Choosing the right hardness, edge geometry, and heat treatment for a specific task is just as crucial as the quality of the steel itself.