The common graphite forms in spheroidal graphite iron contain globular graphite type, quasi-spherical temper graphite type, interdendritic graphite type and worm graphite type and etc.. The most representative one among all of them is globular graphite structure. Its appearance is roughly spherical in low power and is close to a radial-pattern polygon with a clear structure under the optics microscope. The surfaces of spheroidal graphite are unsmooth and uneven, even form something bubbles when those samples are discovered by SEM in low after deep etch tests. Meanwhile, the surfaces of spheroidal graphite from spheroidal graphite iron form annual ring texture, which are divided into several fan-shaped areas by the radial stripes under SEM condition, after being treated with thermal oxygen corrosion and ion bombardment. In addition, the appearance of annual ring dilacerations or dehiscence will be discovered after being treated with stress corrosion.
Spheroidal graphite iron mainly contain hypereutectic compositions. So, there are generally divided into 2 different phases during the growth-up. Firstly, that is proeutectic crystallization phase. It means the phase happens in melted iron and austenite halo under less-carbon and much-ferrum condition after forming cores. Secondly, is eutectic crystallization phase, when an austenite shell forms around spheroidal graphite iron. At this point, the spheroidal graphite iron is being developed in the surroundings of the austenite shell. Though the development rate of spheroidal graphite iron in the eutectic crystallization phase is slower than the one in the liquid phase, most of this material is developed in the eutectic crystallization phase.