Method for thermally treating metal components

In a method for thermal treatment such as annealing wherein metal components are subjected in relatively large numbers and in a relatively short time to the effects of a mineral oil bath which is at an elevated temperature, the steps of immersing the metal components in a bath of mineral oil which is at a temperature of between 200.degree. and 300.degree. C, continuously moving the thus-immersed metal components through the bath while the metal components remain immersed therein, and then removing the metal components from the bath, wherein the improvement comprises maintaining the bath of mineral oil closed off from the outer air, and introducing the components into the bath and removing them from the bath through seals having oil in contact with the outer atmosphere and at a temperature which is less than 200.degree. C, whereby the bath can be maintained in an effective operating condition over a long period of time.