Abstract:
In the manufacturing process of large steel castings, which are indispensable components in machinery, chemicals, energy, and other fields, various defects that affect casting performance frequently emerge. This paper focuses on porosity defects encountered during the production of large steel castings. By analyzing the causes and proposing preventive measures such as optimizing the molding process and improving the melting process, the occurrence of porosity can be effectively reduced. These measures aim to solve the problem of porosity defects in large steel castings and enhance their quality and mechanical properties.
1. Introduction
Large steel castings play a crucial role in various industries. However, quality issues such as porosity and cracks are common during production, leading to delays in delivery cycles, shortened service life of castings, and even safety accidents. The occurrence of porosity defects often stems from unreasonable molding processes, improper melting processes, and varying quality of casting materials. This paper investigates the causes of porosity defects in large steel castings and proposes preventive measures to improve casting quality and performance.
2. Problem Description
2.1 Casting Overview
The castings in question are of two types. One type has an overall cylindrical shape without a wheel flange, with a diameter of 1170mm, a height of 780mm, a central axial hole with a diameter of 390mm, and a weight of 4.8 tons. The material is ZG40CrNiMoA. The other type also has a cylindrical shape but with a wheel flange of 90mm thickness and 50mm width at both ends of the outer circle. The other dimensions are similar to the first type, but the weight is slightly heavier, at approximately 5.1 tons, and the material is also ZG40CrNiMoA.
2.2 Process Introduction
The molding process uses furan resin sand, with a bottom gating system. The sand core adopts core design, and the riser size is Φ720mm×700mm. Alcohol-based coating is applied, with 10 coats achieving a coating thickness of >1.5mm. There is no internal or external chill design.
Table 1: Chemical Composition of ZG40CrNiMoA Alloy Steel (%)
| Element | C | Si | Mn | P | S | Cr | Ni | Mo | Cu |
|---|---|---|---|---|---|---|---|---|---|
| Content | 0.38 | 0.28 | 0.68 | 0.024 | 0.022 | 0.70 | 1.29 | 0.16 | 0.180 |
2.3 Defect Description
After opening the molds, dense and obvious porosity defects were found on the outer surface of the castings. The inner walls of the pores were smooth, with an iron oxide color. The defect depth was approximately 20mm, and the length and width ranged from 80 to 150mm.
3. Defect Analysis
3.1 Macroscopic Analysis
By observing the characteristics of the porosity on the casting surface, it was determined that these pores belonged to intrusive porosity. The formation of such porosity is mainly due to excessive sand ingestion during the casting process and poor gas permeability of the sand mold. Since the casting wall is relatively thick, the time for the surface to form a crust is relatively long. During this process, if the gas generated in the sand mold cannot escape from the sand mold or the interface between the sand mold and the molten steel before crust formation, it will form a gas trapping phenomenon in certain locations. Especially near the outer edge of the riser, due to gas compression, the shell, which is still at a high temperature, is prone to plastic deformation. As the temperature gradually decreases, these plastic deformations are permanently retained, forming porosity.
3.2 Microscopic Analysis
When manufacturing such castings using furan resin self-hardening sand, a large amount of gas is generated during the pouring process, especially at the interface between the sand mold and the molten steel, where a series of intense chemical reactions occur. Furan resin, as a synthetic resin containing a furan ring, burns or incompletely burns under the action of high-temperature molten steel, generating a large amount of CO2 gas. These reactions include complete and incomplete combustion of carbon, as well as further oxidation of CO. The specific reactions are as follows:
C + O2 → CO2
2C + O2 → 2CO (incomplete combustion under oxygen-deficient conditions)
2CO + O2 → 2CO2
In addition to these basic combustion reactions, polymers and sulfonic acids near the interface also undergo decomposition and oxidation under the action of high-temperature molten steel, generating more gaseous components such as H2 and SO2. The specific reactions are as follows:
R-SO3H → CO + SO2 + H2O
CH4 → C + 2H2
CnH2n+2 → nC + (n+1)H2
C + O2 → CO2
2C + O2 → 2CO (under oxygen-deficient conditions)
2CO + O2 → 2CO2
2H2 + O2 → 2H2O
S + O2 → SO2
Under the action of temperature and pressure differences within the sand mold, although most of these gases can escape through permeable sand molds, risers, vent ropes, and air vents, a small portion of the gases will accumulate in certain areas to form porosity. Sometimes, these gases are even adsorbed by the molten steel, spreading into the interior of the casting to form internal porosity. However, from the final processed castings, no needle-like porosity was observed internally. Therefore, the possibility of gas invading the interior of the casting is excluded.
4. Preventive Measures
4.1 Optimizing the Pouring System
To address the porosity issue, the original bottom gating process for Type II castings was upgraded to a stepped gating process. An additional ingate leading to 150mm above the root of the riser was specifically added. This design significantly reduced the static pressure of the molten steel at the bottom and effectively introduced high-temperature molten steel from the ladle into the open riser, greatly enhancing the riser’s feeding ability and effectively avoiding situations where the riser cools while the casting remains hot.
4.2 Adding External Chill Iron
To further improve casting quality, external chill iron was added to the outer circumference, ensuring a sand ingestion distance of approximately 40mm between the chill iron and the casting. This measure aimed to reduce the total amount of gas generated by the resin curing agent under high-temperature conditions. At the same time, the chill iron thickness was designed to be 200mm, which rapidly cooled the outer surface of the casting, ensuring its quick crust formation and effectively preventing gas from entering the casting interior or accumulating locally to form porosity.
4.3 Improving the Sand Core
The traditional core was abandoned for the sand core, and a through-type sand core was adopted. This design reserved a hole of approximately 20mm in the center of the sand core, ensuring that the gas in the axial hole sand core could smoothly escape from the vent hole, preventing gas from entering the casting interior to form porosity.
4.4 Optimizing the Smelting Process
During steelmaking, electric arc furnace smelting technology was adopted, and the purity of the molten steel was improved through redox reactions. Additionally, a steel ladle with a bottom argon blowing function was selected, and the argon blowing time was ensured to exceed 3 minutes after pouring the molten steel into the ladle. This measure not only purified the molten steel but also significantly reduced the H2 and N2 content in the molten steel.
5. Case Study
By optimizing the pouring system, adding external chill iron, improving the sand core, and other measures, the feeding efficiency of the riser was ensured, gas generation was reduced, ventilation was facilitated, and the gas content in the molten steel was reduced through optimized smelting processes. This series of process improvements and optimizations successfully addressed the porosity issue during the production of large steel castings. Taking two castings produced in October 2023 as examples, the casting surfaces were smooth and free of visible porosity defects after opening the molds. After heat treatment and machining, the casting surfaces had further improved smoothness, with no porosity defects. Moreover, the processed castings did not require welding repairs or rework, and the porosity defects in the castings were significantly improved compared to previous quality.
6. Conclusion
Through analysis and research on porosity defects in heavy steel castings, a series of preventive measures have been proposed, mainly including optimizing the gating system, adding external chillers, improving sand cores, and optimizing smelting processes. Practices have proven that these measures have achieved remarkable results in improving porosity defects in castings and enhancing casting quality.