Effect of the Nitriding Process on Impact Resistance of Austempered Cast Irons and Quenched and Tempered Ones

The nitriding process improves in some applications the fatigue resistance of mechanical components. There is a lack of information about the nitriding process effect on the toughness of components.

It is well known that the austempering heat treatment improves the toughness of the nodular cast iron. Some efforts have been made to find a compromise between the toughness of the bainitic structure of nodular cast iron and the high wear resistance undertaken by the nitriding process. Nitriding causes smaller dimensional problems if compared with other termochemical processes in steel and cast iron, but during exposure to the nitriding temperature the stabilized austenite of the nodular cast iron bainite may decompose, and induce component distortion. It is well known that the stabilized austenite is responsible for high toughness of the nodular cast iron.

The toughness of austempered and quenched / tempered nodular cast iron and the effects of the nitriding temperature and the nitriding process were studied by Charpy impact test using non-notched samples.

This paper shows that the higher toughness of austempered nodular cast iron as related to the quenched and tempered material vanishes after the nitriding process. Optical metallographic samples were prepared and the fracture examined by scanning electron microscope.