Superfine graphite gray cast iron: high ultimate tensile strength combined with low hardness

Nowadays there are many alloys in the market that can satisfy the required physical and mechanical properties for a given application. Gray Iron (GI) is still among the cheapest of ferrous materials available for engineers.

Its high fluidity and ability to be cast into complex shapes together with the excellent machining qualities, damping capacity against vibrations, wear and fatigue resistances, thermal conductivity, and the possibility to use it in as-cast conditions are the main reasons why this alloy is traditionally chosen in many industrial applications, as cylinder blocks and heads, gears, flywheels, brake discs and drums, clutch plates, machine bases and beds, pipe fittings, flanges, etc.

The possibility to improve the mechanical properties of GI is of great interest, as when high UTS (Ultimate Tensile Strength) is needed, designers prefer Compact Graphite Iron (CGI) or even ductile Iron (DI), which are higher in price.

Physical and mechanical properties of GI depend on the length and distribution of the graphite flakes, on the ferrite/pearlite ratio and on pearlite fineness. The usual procedure to increase UTS is by lowering the carbon content or by alloying element addition, such as Cu, Mn, Cr, Sn, Mo, Nb, etc. The resulting chemical composition is associated to relatively high hardness and high shrinkage propensity, both due to less amount of precipitated graphite.

IK4-Azterlan has patented a quite high Ti (0.3-0.4%) and very low S (<0.010%) content gray iron which allows to achieve tensile strengths corresponding to low C content (about 3.0%) with 3.4% of C.

Notwithstanding that Ti is frequently limited to 0.03%, as it forms complex compounds that are usually related to a reduction in the tool lifetime during machining, when it is used in a quite high level, it promotes a great reduction of graphite length combined with primary dendrite nucleation and secondary arm spacing refinement. Those effects favor a good distribution of the Ti containing compounds, reducing their harmful effect during machining.

Sulfur modifies the length and graphite distribution, promoting type A graphite, as it forms sulfide particles which act as graphite precipitation sites. Decreasing S content, interdendritic graphite distribution type D is favored.

High Ti together with low S contents result in a higher amount of primary austenite and very fine (10-20 µm) and highly branched fibrous type graphite, rather than short and stubby of graphite distribution type D (see picture), even at moderate cooling rates (0.8-1.2ºC). For this reason it has been called superfine graphite.

This structure increases the UTS up to more than 40% in low alloyed gray irons with medium CE content (about 4.0%), meanwhile the hardness is maintained at values corresponding to its CE (<215 HB). The thermal conductivity is slightly diminished, being anyway higher than the one for Compact Graphite Iron. The machining of this new alloy does not show any harmful behavior or higher tooling wear, compared to a gray iron with similar UTS.

This new alloy shows great potential, as it can compete with some compact graphite or ductile irons in some applications where no ductility is needed and besides the shrinkage problems associated to high resistance gray irons are greatly diminished.