Compare white and nodular cast irons with respect to (a) composition and heat treatment, (b) microstructure, and (c) mechanical characteristics.

White cast iron generally has 1.7% to 4.0% carbon content, along with small percentages of silicon (less than 1%), manganese (0.2% - 2%), and other elements such as sulfur and phosphorous. This type of cast iron is brittle and hard, primarily due to its high carbon content. Heat treatment for white cast iron typically involves heating the material to a temperature a bit below its eutectic temperature and cooling it rapidly to form a martensitic matrix. Nodular cast iron, also known as ductile cast iron or spheroidal graphite cast iron, has a similar carbon content but contains small amounts of magnesium (0.04% - 0.06%) and sometimes other elements such as cerium. The presence of these elements changes the shape and distribution of the graphite inclusions. Heat treatment processes for nodular cast iron often involve annealing, normalizing, and quenching & tempering processes which are used to achieve desired mechanical properties.

The microstructure of white cast iron consists of cementite (Fe3C, a very hard and brittle compound) and pearlite (a lamellar mixture of ferrite and cementite). The matrix of white cast iron may also contain small amounts of ledeburite and phosphide eutectic structure. The primary factor responsible for the hardness and brittleness of white cast iron is the presence of cementite throughout the matrix. In contrast, nodular cast iron has a unique microstructure that features graphite nodules or spheroids dispersed in a matrix of ferrite, pearlite, or both. This microstructure results from the addition of magnesium, which alters the shape and distribution of the graphite inclusions during solidification from molten metal. The spheroidal or nodular form of graphite reduces stress concentrations and improves ductility, giving nodular cast iron higher mechanical strength and impact resistance than white cast iron.

White cast iron is known for its high hardness, wear resistance, and brittleness. It has a relatively low tensile strength (up to 500 MPa) and a very low percentage of elongation (less than 1%). Due to these properties, white cast iron is mainly used for abrasion-resistant applications, such as grinding balls, slurry pumps, and wear plates. Nodular cast iron has significantly higher mechanical properties than white cast iron. It possesses good tensile strength (up to 1200 MPa), superior impact and fatigue resistance, and exhibits high ductility (3% - 18% elongation). Due to these characteristics, nodular cast iron is suitable for making high-strength components, such as crankshafts, axles, and gears which require both strength and toughness.