 Cast iron cookware is normally seasoned with oil or grease and heat, which causes the oil or grease to be absorbed into the pores of the cookware, thereby giving a smooth relatively non-stick cooking surface which is stripped off by the combination of alkali based detergent and hot water in a dishwasher.Knives and other cooking tools that are made of carbon steel, semi-stainless steels like D2, or specialized, highly hardened cutlery steels like ZDP189 corrode in the extended moisture bath of dishwashers, compared to briefer baths of hand washing.Cookware is made of austenitic stainless steels, which are more stable.