I want to anodize steel using heat. some steel turns gray instead of coloring when I heat it up using a torch.
Steel doesn't anodize in the sense that aluminum and some other metals do. However, it can be heat-colored. The trick is to clean the surface first (it must be oxide free), then heat gently until the colors appear. These are called temper colors in steel. They are due to a thin adherent layer of oxide that forms and thickens as temperature is increased. They are quite temperature dependent. As the steel is heated, the first color to appear is pale yellow. This will progress through darker yellows, browns, purples, and blues as the temperature rises. Above blue, the oxide becomes the gray/black color you are apparently getting - this is the result of heating too fast and too hot. See the chart at the site below for colors in plain carbon steel. Note that the temperatures are pretty low - It all starts around 400 F and if you go above 600 F the show's all over.
