News

For example: On a certain day, a company used an electric furnace to melt six batches of gray iron HT300 molten iron and cast hydraulic valves such as G03 and G02. After dissecting the internal structure, we found extensive microscopic shrinkage cavities, looseness, and cracks—totaling 830 pieces that were all scrapped (see attached figure). The Brinell hardness test showed HBS 241, and the chemical composition was as follows: C 3.27%, Si 1.78%, Mn 0.83%, S 0.087%, P 0.04%. The microstructure consisted of 98% pearlite and 80% flake graphite (with 20% Type A graphite), and the graphite length was rated at Grade 5. According to research and analysis by relevant personnel, the issue appears to stem from problems with the molten iron material itself.

The results of the chemical composition analysis appear normal for typical thin-walled HT300 castings; however, problems have arisen with hydraulic valve castings (which have thicker walls). Preliminary assessment indicates that the defect—micro-shrinkage, porosity, and cracking in the castings—is caused by an excessively high content of MnS in the molten iron. In other words, the levels of S and Mn in the molten iron exceed the ranges suitable for the specific casting (with varying allowable concentrations depending on the particular casting design).

Since a certain amount of sulfur-increasing agent was added during the melting process, the accumulated sulfur and manganese levels in the molten iron eventually reached a threshold that caused the sulfur content to exceed the normal solidification and crystallization requirements of the casting itself, thereby giving rise to this type of defect. Countermeasure: Stop adding the sulfur-increasing agent, adjust the manganese content to ensure that the five-element composition of HT300 gray iron remains within the normal range. After these adjustments, all defects were completely eliminated.

Adding an S-increasing agent to the molten iron from electric furnaces forms a certain amount of MnS, which serves as a heterogeneous nucleation core and enhances the inoculation effect. This approach is theoretically sound. However, most recent literature suggests that for high-grade gray iron produced in electric furnaces, the sulfur content should be controlled within the range of 0.05% to 0.10%. Yet, practical experience from many foundries has shown that when the manganese content is around 1%, if the sulfur content in the castings, as determined by compositional analysis, exceeds 0.05%, shrinkage defects begin to appear. Moreover, when the sulfur content surpasses 0.07%, batch-wise shrinkage defects become prevalent. How can this phenomenon be explained?

In gray cast iron, sulfur exists in two forms: one is elemental sulfur, and the other is MnS in a compound state. Among these, the sulfur that acts as a crystallization nucleus in gray iron is primarily in the form of MnS compounds. Currently, our analytical methods—whether chemical analysis or spectral analysis—can only detect elemental sulfur in both castings and molten iron; sulfur existing in compound form (MnS) cannot be detected by these methods. When the content of elemental sulfur exceeds 0.05%, the proportion of sulfur in compound form (MnS) becomes relatively high. At this point, in the molten iron:

MnO + FeS = MnS + FeO, FeO + C = Fe + CO, or 2FeO + C = 2Fe + CO₂

At this point, as the molten iron solidifies, it not only releases CO or CO2 but also produces a small amount of brown MnS powder, leading to the formation of gas-shrinkage cavities in the iron slag. Provided certain conditions are met, such gas-shrinkage cavities can occur not only in molten iron from electric furnaces but also in that from cupola furnaces. In fact, during the melting process in an electric furnace, we have already introduced a certain amount of sulfur—this sulfur originates from:

II. Inoculation and Modification Treatment for High-Grade Gray Iron in Electric Furnaces

Regarding the inoculation process for high-grade gray iron (taking HT300 as an example), the traditional inoculation dosage is typically 0.3–0.4% of the molten iron volume (primarily used in cupola furnace production). In recent years, with the increasing prevalence of electric furnaces, the inoculation dosage has gradually risen. The latest data recommend a dosage of 0.5–0.6%. Based on my long-term practical experience, I have found that an inoculation dosage of around 0.8% yields comprehensive improvements in both strength, hardness, and machinability. Moreover, internal defects in castings significantly decrease after machining.

A certain company manufactures high-grade solenoid valves. According to technical requirements, the casting must have a hardness greater than HB200 and a tensile strength exceeding 300 N/mm². The product’s wall thickness typically exceeds 50 mm. After numerous trials, while increasing the initial inoculation dosage, the company also adopted a secondary in-furnace inoculation process. This approach effectively eliminated the defect of coarse microstructure caused by thick walls, improved the density of the castings, and ensured product quality.

Regarding secondary inoculation of molten iron during the pouring process, adding a uniformly distributed inoculant with a particle size of 0.2–0.7 mm before casting is particularly suitable for thick castings. However, when used for small castings, this practice tends to increase the shrinkage tendency of the molten iron.

At one point, some products manufactured by a certain company exhibited white bright spots on their surfaces after processing, with exceptionally high hardness that caused cutting tools to slip. Upon analysis, it was found that the problem stemmed from the oversized size of the inoculant pellets, which were incompatible with the capacity of the molten iron ladle. As a result, the inoculant failed to fully dissolve during the pouring of molten iron, leading to localized enrichment of silicon in the castings and the formation of hardening phases. Similarly, when secondary inoculation was performed during the second flow of molten iron at a relatively low temperature, the same defect would occur.

There is a factory specializing in the production of HT300 gray iron hydraulic components. They are casting KP pump bodies with a wall thickness of around 30 mm. Based on the typical composition for HT300, the molten iron has the following chemical composition: C 3.0–3.1%, Si 1.7–1.8%, Mn 0.95–1.05%, P 0.05%, and S 0.04%. The tensile strength of the castings themselves reaches 300 N/mm². However, in several consecutive batches, shrinkage cavities and cracks have been observed near the inner gating system. No matter how much the gating system is adjusted, the problem persists without any improvement.

There was no other choice but to increase the carbon equivalent and reduce the strength, adjusting the composition to C3.2–3.3% carbon and Si1.8–2.0%. After this adjustment, the defects disappeared. However, once the products were processed and subjected to pressure testing, most of them showed expansion and leakage. Moreover, their tensile strength failed the in-house tests, resulting in a large-scale return of goods by the OEM. This reminded me of a previous batch of similar pump bodies that, following someone else’s advice, had sulfur added via ferrosilicon. When the molten iron contained more than 0.07% sulfur, the castings developed extensive shrinkage cavities, leading to a huge number of defective parts. To deal with this batch of scrap, drawing on the principle of rare-earth-based desulfurization, we added a small amount of rare-earth magnesium silicon iron (about 0.2%) during the inoculation process whenever such scrap was introduced. This effectively reduced the sulfur content and successfully resolved the shrinkage cavity issue.

Regarding the shrinkage and cracking issues present in KP pumps at the time, although the original molten iron did not have a high sulfur content, a small amount of rare-earth magnesium silicon-iron alloy (about 0.2%) was still added during inoculation, yielding ideal results and completely resolving the shrinkage cavity problem. Analyzing the underlying mechanism, the primary cause of shrinkage in cast iron is gases—such as oxygen, nitrogen, and hydrogen—contained within the molten iron. As these gases precipitate during the late stages of solidification, the molten iron cannot replenish them, leading to defects. However, the rare-earth magnesium silicon-iron alloy, being both a spheroidizing agent for gray iron and an inoculant, happens to be exceptionally effective at removing these gases. Consequently, the gas content in the molten iron is significantly reduced, thereby eliminating the defects.

By properly addressing the issues mentioned above, approximately 80% of defects can be avoided. If you have any other questions regarding iron casting and melting, feel free to contact us—Kangda Electric Furnace provides 24-hour technical support and consultation.