1. The carburizing time in a fluidized bed (employing propane-butane) is one-third to one-fifth that in gas carburizing in a muffle-free apparatus. 2. Carburizing in a fluidized bed ensures even saturation of the outer and inner surfaces, which makes it possible to obtain high-quality parts, and it can be recommended for small parts. However, the high rate of saturation and ease of automation make it suitable for large parts also.