The Physics of Utensils

Forget about cooking classes--forks, knives, and spoons can provide a rich lesson in physics. The physics of utensils is explained by the University of Virginia's Lou Bloomfield in the May issue of Physics Today, the flagship monthly publication of the American Institute of Physics.

Forks, knives and spoons are generally made of steel--an alloy of iron and carbon with other elements mixed in. A room-temperature iron crystal is soft, as it is susceptible to shear stress. In other words, pushing layers of the iron crystal in opposite directions causes the layers to slip, bending the iron permanently, which, as Bloomfield points out, is "fine in a twist tie, not so good in a knife."

When the iron crystal (known as ferrite) takes in even a small amount of carbon, the situation changes. Dispersed throughout the ferrite (the carbon is generally insoluble in it), the carbon makes it more difficult for crystal impurities known as dislocations to move, thereby frustrating shear forces and hardening the solid. Put even more carbon into the ferrite and it distorts the crystal into the hardest possible steel structure, suitable for the cutting edges of knives.

Making useful steel cutlery generally requires heating an iron-carbon mixture to well over 727 degrees C, in order to facilitate structural transformations throughout the entire steel material. What type of utensil results depends on how quickly the steel is cooled, or "quenched." Slowly quenched steel results in pearlite, a sturdy but relatively soft compound often used for spoons. The most quickly cooled steel leads to martensite, the hard steel used in cutting edges. Reheating the martensite rearranges some crystalline structures and "tempers" it so it becomes less brittle.

Making stainless steel, Bloomfield says, involves adding elements including chromium. When the Cr content exceeds 11.5% by weight, a chromium oxide layer forms on the surface to prevent rusting.

More details can be found in Bloomfield's article, which is freely accessible at http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_60/iss_5/88_1.shtml