The brown coloration that appeared on the blade of your kitchen knife after heating it and subsequently quenching it in water is likely due to a process called oxidation. When the hot metal comes into contact with oxygen in the air, it undergoes a chemical reaction known as oxidation. This reaction can lead to the formation of various oxides on the surface of the metal.
In the case of steel, which is commonly used for kitchen knives, the brown coloration is often caused by the formation of iron oxide, specifically a compound called ferric oxide (Fe2O3). Ferric oxide is commonly referred to as rust and is typically associated with the red-brown coloration seen on corroded iron or steel surfaces.
The heating of the knife caused the metal to react with oxygen, forming a thin layer of iron oxide on the surface. Submerging the hot blade in water rapidly cools the metal, which can promote the formation of iron oxide. This is because the rapid cooling causes the metal to contract and microscopically exposes more of its surface area to the surrounding air, allowing for increased oxidation.
It's worth noting that the brown coloration is generally superficial and affects only the outer layer of the blade. It can often be removed through cleaning and polishing the knife's surface. However, it's important to take care when conducting such experiments, as heating metal to high temperatures can be dangerous and may damage the knife.