What are the effects of high and low residual magnesium on excessive graphite diameter and graphite blooming defects in ductile iron

The residual magnesium content in ductile iron production needs to be precisely controlled within an "optimal window range" (usually around 0.04% -0.055%, depending on the composition and process). Deviation from this range, whether too high or too low, can cause deterioration of graphite morphology, but the manifestation and fundamental mechanism are completely different.

1、 The impact of low residual magnesium content is that the residual magnesium content is lower than the minimum critical value required for spheroidization (generally about 0.03% -0.035%), which is the most direct and fundamental reason for graphite flowering defects, and the impact on graphite diameter is secondary. The fundamental mechanism of the decisive influence on graphite flowering is that the core role of magnesium element is to adsorb on the crystal surface of graphite growth, suppress its layered growth nature, force its isotropic growth, and thus form a spherical shape. When the residual magnesium content is insufficient, this adsorption and inhibition effect fails in the later stage of graphite growth, especially in the late stage of eutectic solidification. Defect formation: Unconstrained graphite will restore its rapid and unstable growth mode, causing the already formed spherical graphite to rupture and deform, resulting in hollowing inside and bursting or coral like edges, which is a typical "flowering graphite". This indicates that spheroidization has essentially failed. The indirect effect on graphite diameter: In the local areas where residual magnesium is on the verge of insufficient but has not completely failed, the reduction of effective nucleation cores may result in a small number of residual graphite spheres growing larger. However, the more prominent feature in this case is the appearance of a large amount of non spherical graphite (worm like, flower like), and the simple coarseness of graphite is not its main manifestation. ·The common cause of low residual magnesium is the high sulfur content in the original molten iron, which consumes too much magnesium. Insufficient calculation of the amount of spheroidizing agent added or low reaction absorption rate. After spheroidization treatment, the residence time of molten iron is too long, and magnesium is severely degraded. There are strong interfering elements such as lead and bismuth in the molten iron, which neutralize the spheroidization effect of magnesium. Summary: Low residual magnesium leads to loss of spheroidization ability and directly promotes graphite flowering.

2、 The impact of excessive residual magnesium content is significantly higher than the optimal range (such as exceeding 0.06% -0.07%), mainly not leading to flowering, but through a series of indirect effects, becoming an important factor in promoting excessive (coarse) graphite diameter, accompanied by other serious casting defects. The indirect promotion mechanism for graphite diameter that is too large (coarse) is to weaken the incubation effect and reduce the nucleation core. Magnesium is a strong anti graphitization (whitening) element. Excessive residual magnesium will significantly increase the supercooling tendency of molten iron. This makes it difficult for the heterogeneous core provided by conventional ferrosilicon inoculants to function stably, resulting in a deterioration of the "incubation response". The direct consequence is a reduction in the number of graphite spherical nuclei. Under the premise of constant total carbon content, the fewer cores there are, the larger the size that each graphite ball can grow to, thus forming coarse but possibly still relatively round graphite balls. Mechanism 2: Causing inappropriate process adjustments. In order to counteract the white tendency caused by high magnesium, operators may be forced to increase carbon equivalent (especially silicon content) or undergo excessive incubation. Under high carbon equivalent conditions, especially when the cooling of thick and large sections is slow, it provides favorable conditions for the coarsening growth of graphite. Magnesium, which has a high potential impact on the morphology of graphite, may cause a decrease in the roundness of graphite spheres, making it easier to produce clumpy or irregular graphite, but it usually does not directly form typical explosive blooms. The risk of slag inclusion has increased dramatically due to other serious process problems: excess magnesium is prone to react with oxygen and sulfur to generate slag such as MgO and MgS, which can be rolled into castings and form slag inclusion defects. Intensifying shrinkage tendency: High magnesium widens the solidification range of the paste like iron liquid, hinders shrinkage supplementation, significantly increases the micro shrinkage tendency, and seriously affects the density of castings. Decreased liquidity and increased contraction.

Summary: Excessive residual magnesium indirectly leads to graphite coarsening through "inhibiting nucleation and reducing the number of spheres", and brings a series of malignant side effects such as slag inclusion and shrinkage.

3、 The impact of residual magnesium "appropriate but declining" is the most common scenario encountered in actual production, which leads to excessive graphite diameter. It reveals the importance of dynamic changes in "effective magnesium content". Starting point: At the end of spheroidization treatment, residual magnesium is in the optimal range, fully nurtured, and graphite balls are small, round, and abundant. Decline process: From the completion of treatment to the solidification of casting, the molten iron undergoes retention, resulting in "spheroidization decline" (magnesium element burning and floating) and "incubation decline" (nucleation core dissolution or failure). ·Defect formation mechanism: The effective residual magnesium content gradually decreases, and the constraint on graphite growth weakens. The number of effective nucleation cores decreases over time. The superposition effect of the two: Before the residual magnesium reaches the "critical point" that causes flowering, the remaining graphite spheres will continue to grow under conditions of reduced constraints and sufficient carbon sources, ultimately forming graphite with coarse size but still acceptable morphology (such as grade 6 or even coarser). If the decline continues, it will slide towards poor spheroidization and flowering.

The core objective of the final practical guidance summary is not only to control residual magnesium at the target value, but also to ensure its effectiveness and stability throughout the entire pouring process. Preventing flowering (key is to prevent low magnesium): Strictly reduce and stabilize the sulfur content of the original molten iron. Ensure sufficient and accurate addition of spheroidizing agent. Minimize the residence time after spheroidization to achieve rapid pouring. Preventing coarsening (key to maintaining a balance between effective nucleation and magnesium): Using efficient and anti-aging late stage incubation techniques (such as flow inoculation and in mold inoculation) to continuously provide fresh nucleation cores is the most effective way to counteract decay and refine graphite. Avoiding blindly increasing residual magnesium content for the sake of "insurance" is a divergent path towards shrinkage, slag inclusion, and graphite coarsening. For thick and large sections, it is necessary to comprehensively optimize the carbon equivalent design and cooling conditions. In short, "stabilizing sulfur, controlling magnesium (moderate), rapid pouring, and strong post inoculation" are key process criteria for obtaining high-quality ductile iron structure while avoiding graphite flowering and coarsening.