Microwave ovens convert electric power into radio waves, which are transmitted at a high frequency by the oven's magnetron. These microwaves bounce off the metal cavity walls of the microwave oven, and the resulting vibrations and high temperatures cook the food.
Most metals, including stainless steel, reflect microwave energy, making it unsafe to use them in the microwave.
When stainless steel is placed inside a microwave oven, it can disrupt the microwaves' paths, causing arcing. Arcing is like a miniature lightening bolt. If it occurs repeatedly, it can damage the involved surfaces or even burn a hole through the wall of your microwave.
If the microwave is turned off immediately, the arcing should not cause any damage. Remove the stainless steel immediately and either replace it with a microwave-safe container or cook the food in a conventional oven.
The U.S. Food and Drug Administration recommends using paper, glass, ceramic or plastic containers in microwave ovens instead of metals such as stainless steel because the microwaves can pass through them.
According to the United States Department of Agriculture Food Safety and Inspection Service, there are microwave ovens in more than 90 percent of American homes.