14 DAY TRIAL // It may be hard to believe, but kitchen utensils are not only ordinary, every-day-life instruments, they can also provide us with important lessons in physics in general, but especially in crystallography and condensed-matter physics.
How do they make a knife so tough and sharp? Which knives are better, smooth or serrated-edged, thick and rigid or flexible and thin? Why don't the teeth of a fork bend when "stabbing" meat or vegetables?
Let's take a look at how they're made.
Generally, the kitchen tools that we know and love are made of steel. You probably noticed the imprint saying "stainless steel" and you must know that this means they don't rust. But getting the steel to behave exactly the way we want it is not exactly a "piece of cake."
Steel is an alloy comprised mostly of iron and carbon, but it also has some other elements in small quantities, such as manganese and tungsten. In fact, the carbon content in steel can range between 0.02 % and 1.7 % by weight, depending on grade. As the most used and effective alloying material for iron, carbon is the main hardening agent, as a room-temperature iron crystal is soft, as it is susceptible to shear stress.
The iron atoms are forming a crystal lattice and carbon is used to prevent dislocations in the lattice from sliding past one another. Pushing layers of the iron crystal in opposite directions causes the layers to slip, bending the iron permanently and the smallest amounts of carbon can disrupt the slipping process, thus hardening the alloy.
It does that by dispersing throughout the iron crystals (known as ferrite), thus making it harder for the dislocations, in fact crystal impurities, to move, which counteracts the shearing forces inside the metal.
The more carbon you add, the harder and stronger iron gets, but it also becomes more brittle, so the 1.7% carbon weight in the alloy the maximum, possible, or else it produces cementite, which reduces the material's strength, resulting in cast iron.
To produce the steel used in cutting tools, such as knives, requires heating an iron-carbon mixture to over 727 degrees C, to allow structural transformations throughout the entire steel material. Varying the cooling temperature of the compound gives of different cutting tools. The slowly cooled 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.
The stainless steel is actually achieved by adding several elements, chromium (Cr) being the most important. When the Cr content exceeds 11.5% by weight, a chromium oxide layer forms on the surface to prevent rusting.
So, there you have it. You now know the processes and applications involved in making some apparently mundane objects, which actually proved their indispensability in our lives.