Properly utilizing titanium can enhance the quality of gray iron castings.

In the production process of gray iron castings, with the widespread use of metallic charge materials such as pig iron and scrap steel, titanium inevitably enters the molten iron to some extent. Extensive research data indicate that after adding titanium to gray cast iron, most of the titanium compounds are distributed within the metal matrix. However, at the interface between ferrite and graphite, residual titanium nitride or titanium carbonitride still remains—compounds that exhibit a hardness of 3200 HV or higher. These high-hardness titanium compounds significantly reduce the machinability of gray iron castings.

Additionally, excessive titanium content can lead to thermal cracking during the cutting of castings. In contrast, D-type graphite cast iron containing titanium exhibits excellent oxidation resistance, anti-growth properties, and thermal fatigue resistance, with its service life increasing up to three times longer than that of conventional chromium-molybdenum-copper cast iron when operating at temperatures between 500°C and 700°C. The use of titanium as an interfering element is strictly prohibited in gray iron castings; however, in certain types of vermicular graphite cast iron, titanium is intentionally added to the inoculant as a beneficial element to broaden the range of effective vermiculation treatment. Under high carbon-equivalent conditions, micro-alloying with titanium can significantly enhance the strength and hardness of gray cast iron while improving the uniformity of the casting’s cross-section. Moreover, trace amounts of titanium help neutralize excess nitrogen in the cast iron, thereby reducing the frequency of crack-induced gas porosity. Titanium, therefore, offers both advantages and disadvantages when incorporated into cast iron. Through experimental studies, researchers have investigated how varying titanium levels influence the graphite morphology and mechanical properties of medium- and low-carbon-equivalent gray cast irons under different casting conditions. Additionally, quantitative analyses were conducted to assess the impact of distinct titanium concentrations on the density and compactness of gray cast iron.

Titanium can increase the austenite nuclei in gray iron castings, refining the primary austenite grains. According to data from the American Foundrymen’s Association, titanium serves as both a graphitizing agent and a reducing agent for cast iron, while also helping to refine grain structure, thereby enhancing both the tensile and flexural strengths of gray iron components. When the residual titanium content ranges from 0.08% to 0.25%, the tensile strength of high-carbon-equivalent cast iron can be improved, whereas the strength of low-carbon-equivalent cast iron tends to decrease. In dry-mold casting with low titanium levels, the molten iron exhibits minimal supercooling tendencies, often resulting in the formation of Type A and Type E graphite. However, as titanium content increases, the supercooling tendency grows significantly, promoting the development of Type D graphite. Once Type D graphite accounts for more than 90% of the microstructure, the mechanical properties of the gray iron casting are markedly enhanced. On the other hand, in wet-sand mold casting, the increased supercooling tendency combined with higher titanium levels leads to an abundance of Type D graphite forming at thick, hot sections of the casting. Titanium elevates the supercooling propensity of the molten gray iron, triggering extensive branching of the graphite network. These numerous short, curved Type D graphite formations reduce the diffusion distance of carbon atoms, causing the austenite surrounding the graphite to transform into ferrite during the cooling process—thus compromising the overall mechanical performance of the casting. Moreover, Type D graphite displays excellent spheroidal characteristics; as a result, D-type graphite cast iron demonstrates superior strength compared to conventional gray iron castings with the same matrix structure.

　A certain amount of titanium increases the supercooling tendency of molten iron, promoting the formation of Type D graphite in gray iron castings. Notably, Type D graphite is significantly more abundant when using wet sand molds compared to dry sand molds. As the titanium content in the molten iron rises, the amount of Type D graphite in gray iron castings also increases. When Type D graphite reaches a relatively high proportion, it tends to appear extensively even in thick, hot sections of the casting. Furthermore, castings with higher carbon equivalent and silicon-to-carbon ratios exhibit markedly improved mechanical properties. However, when the carbon equivalent drops below 3.66%, the strength of the material declines sharply as the titanium content continues to rise.