Causes and Prevention of Spheroidization Fading of Rare Earth Silicon Magnesium Alloy Before Casting
Sep 18, 2026
Rare earth silicon magnesium alloy is the core nodulizer for ductile iron production. With the synergistic effect of magnesium and rare earth elements, it can convert flake graphite in molten iron into spherical graphite, greatly improving the strength, toughness and wear resistance of castings. However, in actual casting production, spheroidization fading often occurs during the standing stage after nodulization treatment and before formal pouring. This problem directly reduces the number of graphite spheres, distorts the graphite morphology, forms vermicular and flake graphite, and eventually leads to unqualified mechanical properties and high scrap rate of castings. This common industry pain point mainly stems from element loss, chemical reaction imbalance and process control deviation of molten iron system.
The core inducement of spheroidization fading is the continuous burning loss and escape of active magnesium. As the key element for spheroidization, magnesium has a low boiling point and strong activity. The residual free magnesium in molten iron after nodulization will continuously react with oxygen in the air and sulfur in molten iron at high temperature. On the one hand, magnesium is directly oxidized to form magnesium oxide slag and floats away; on the other hand, magnesium sulfide formed by the combination of magnesium and sulfur has poor stability and decomposes secondarily at high temperature, releasing sulfur back into molten iron and consuming effective magnesium continuously. With the extension of standing time, the residual magnesium content decreases continuously, and the spheroidization effect fades rapidly when it is lower than the critical threshold.
The attenuation of the auxiliary stabilizing effect of rare earth elements is a key secondary factor exacerbating spheroidization fading. Rare earth elements can effectively purify molten iron, stabilize sulfide and oxide systems, inhibit excessive burning loss of magnesium, and make up for the performance shortcomings of magnesium. However, if the original sulfur and oxygen content of molten iron is high, or the rare earth ratio is insufficient and the purity of rare earth silicon magnesium alloy is substandard, rare earth elements will preferentially participate in impurity reactions and be consumed rapidly, failing to maintain a stable effect continuously. Without the synergistic protection of rare earths, the loss rate of magnesium will increase greatly, and the spheroidization state cannot be maintained, resulting in obvious fading in a short time.
Improper process control is an important artificial inducement of spheroidization fading, mainly focusing on two dimensions: temperature and standing time. Excessively high molten iron temperature will greatly aggravate the gasification escape and oxidation reaction of magnesium, and the burning loss rate of magnesium rises significantly with the increase of temperature. Excessively long standing time of molten iron after nodulization is the most common cause of fading. Industry data shows that the residual magnesium content has a fixed loss with every minute of standing, and long waiting time before pouring will completely exhaust effective spheroidizing elements. Meanwhile, if the slag is not removed in time after nodulization, the sulfides and oxides in the slag layer will reversely pollute the molten iron and further accelerate spheroidization failure.
Improper process control is an important artificial inducement of spheroidization fading, mainly focusing on two dimensions: temperature and standing time. Excessively high molten iron temperature will greatly aggravate the gasification escape and oxidation reaction of magnesium, and the burning loss rate of magnesium rises significantly with the increase of temperature. Excessively long standing time of molten iron after nodulization is the most common cause of fading. Industry data shows that the residual magnesium content has a fixed loss with every minute of standing, and long waiting time before pouring will completely exhaust effective spheroidizing elements. Meanwhile, if the slag is not removed in time after nodulization, the sulfides and oxides in the slag layer will reversely pollute the molten iron and further accelerate spheroidization failure.
