Effect of Nitrogen on the Microstructure and Mechanical Properties of Gray Cast Iron

The use of gray cast iron in the manufacture of engine cylinder heads is limited because of its poor mechanical properties. Herein, the effect of nitrogen on the microstructure and mechanical properties of gray cast iron was examined. The results showed that with increasing nitrogen content, the graphite in gray cast iron gradually became shorter and thicker, and the ends were passivated, while the curvature of the graphite layers increased. Furthermore, with increasing nitrogen content, pearlite was refined, while its quantity and lamellar spacing increased and decreased, respectively. The hardness also increased with nitrogen content. When the nitrogen content was < 0.0119 wt.%, the tensile strength increased with nitrogen content; however, when the nitrogen content was 0.0119 wt.%, the maximum tensile strength was 332 MPa, and when the nitrogen content was 0.0122 wt.%, dendrite fissure nitrogen porosity defects occurred, leading to a decrease in the tensile strength.