The ability of nitrogen atomic absorption in the formation of iron nitride on flake structure and nodule in cast iron

Abstract

The nitriding process can be effectively applied to steel with alloying elements. The higher the alloying element in the steel maximizes the mechanical properties, including hardness. However, the raw material can be costly, therefore using cheap materials (without alloys) is a challenge in increasing surface hardness through iron nitride formation. Furthermore Grey and nodular cast iron have different properties and characteristics in the structure. This research focuses on the ability of the cast-iron structure to affect the nitride layer formation. Gas nitriding was conducted in a fluidized bed reactor with a 550°C in 20 % N2 and 80 % NH3 atmosphere at a flow rate gasses of 0.7 m3/hr process temperature and holding for 2, 4, and 6 hours. Tests are conducted by observing the depth of hardening, SEM, and EDAX. According to the results, the nitriding process increases the surface hardness of cast iron. The highest hardness value is nodular cast iron with a holding time of 6 hours (345 HV) and a hardening depth of up to 20-micron meters. The compounds formed in the nitride layer include FeN, Fe2-3N, Fe4N, and Fe2N. The compound formed is strongly influenced by the treatment time. Furthermore, the comparison of nitriding treatment on gray and nodular cast iron was influenced by the flake and nodule structure. In general, the nodule structure is responsible for maximum hardness. The longer treatment time allows the nitrogen atoms to diffuse more to the surface, while the flake structure limits the absorption of nitrogen atoms into the surface of the cast iron. Characterization of Nodular Cast Iron shows that The hardening depth distribution trend due to the nitriding process in nodular cast iron was not much different from gray cast iron.

Evaluation of Metallurgical Quality of Cast Iron Using Quality Criteria

“…Although some are not intentional, they do have a measurable effect on the gray cast iron. Some promote pearlite, such as tin, while others compact graphite and increase strength, such as nitrogen [11,12].…”

Metallurgical Quality of Cast Iron Made from Steel Scrap and Possibilities of its Improvement

“…Deliberately increasing the N content in cast irons is done using nitrogen alloys and salts in order to increase (tensile strength -Rm and hardness -HB). On the other hand, the increased content of N in the liquid metal is the source of bubbles in the castings [21,22,23,24].…”