On the effectiveness of using carbamide-based salt baths for low-temperature cyanidation of improved carbon and alloy steels

Abstract

   Purpose. A study of cyanidation of improved carbon and alloy steels of grades 30, 30HGT, 30H13 at a temperature of 550–580 °C was carried out in baths based on carbamide and sodium carbonate. Recommendations on the use of these baths for surface hardening of carbon and alloyed improved steels used for the manufacture of machine parts are formulated.   Methods. The microstructure of the diffusion layer of steels on transverse sections was studied using a Quanta FEG – 650 electron scanning microscope with an EBCD focused ion beam microanalysis system. Cyanide wear tests were performed under friction conditions analogous to those of die tools. The wear resistance of the carbonitrided samples was investigated on a SMTs-2 friction machine. The tests were performed under both dry friction and boundary friction conditions.   Results. The high efficiency of the proposed bath composition at a given temperature has been experimentally shown. As a result of cyanidation, modified layers saturated with solid inclusions (ε-phases) are formed on the surface of steels, characterized by high hardness, low coefficient of friction and high wear resistance.   Conclusion. According to the results of the conducted research, it can be concluded that low-temperature cyanidation of improved steels of various grades can be carried out in cyanide-free salt baths. The optimal composition of the cyanide bath is 40% carbamide (NH2)2CO; 40 % soda ash Na2CO3 and 20 % NaCl table salt. The proposed bath does not contain toxic and scarce components, therefore it is environmentally safe and very cheap and can be used with great effect in machine-building, repair and tool industries.