Abstract:
In the field of mechanical engineering, chemicals, and energy, thick and large steel castings serve as indispensable components, and their quality stability is crucial. However, during production, these castings often suffer from quality issues such as porosity and cracks, leading to delivery delays, shortened service life, and even safety accidents. This article analyzes the causes of porosity defects in thick and large steel castings and proposes preventive measures to improve casting quality and performance. By optimizing the molding process, improving melting processes, and incorporating various technical enhancements, the occurrence of porosity defects can be effectively reduced.
1. Introduction
Thick and large steel castings play a pivotal role in various industries, including machinery, chemicals, and energy. The quality stability of these castings is crucial for ensuring the overall performance and safety of the equipment they are used in. However, during the production process, issues such as porosity and cracks frequently arise, impacting the service life of the castings and potentially leading to safety hazards.
The formation of porosity defects in steel castings is often attributed to various factors, including unreasonable molding processes, improper melting processes, and variable material quality. By conducting a thorough analysis of the causes of porosity defects in thick and large steel castings and proposing targeted preventive measures, this article aims to improve the quality and mechanical properties of these castings, ensuring their reliability and durability in various applications.
2. Description of the Casting and Issue
2.1 Casting Overview
The thick and large steel castings discussed in this article are of two types. One type has a cylindrical structure without a wheel flange, with a diameter of 1170mm, a height of 780mm, a central shaft hole with a diameter of 390mm, and a weight of 4.8 tons. The material used is ZG40CrNiMoA. The other type is also cylindrical but has wheel flanges with a thickness of 90mm and a width of 50mm on both outer ends. The other dimensions are similar to the first type, but the weight is slightly heavier, approximately 5.1 tons, and the material is also ZG40CrNiMoA.
2.2 Process Introduction
The molding process employs furan resin sand, with a bottom gating system used for pouring. The sand cores are of the bun-shaped type, and the riser has dimensions of Φ720mm×700mm. An alcohol-based coating is applied, with 10 coats ensuring a coating thickness of over 1.5mm. There is no internal or external chilled iron design. The specific molding process.
2.3 Chemical Composition
ZG40CrNiMoA is a high-toughness, high-strength alloy steel with an optimal chemical composition that provides excellent mechanical properties. The chemical composition of ZG40CrNiMoA is presented in Table 1.
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 | 0.18 |
2.4 Issue Description
After opening the casting molds, dense and visible porosity defects were observed on the outer surface of the castings. The inner walls of the pores were smooth, with an iron oxide appearance. The defects had a depth of approximately 20mm and dimensions of 80 to 150mm in length and width.
3. Defect Analysis
3.1 Macroscopic Analysis
By observing the characteristics of the porosity on the surface of the steel castings, it is evident that these porosity defects belong to the category of invasion porosity. The formation of such porosity primarily stems from excessive sand ingestion during the casting process, coupled with inadequate ventilation within the sand mold. Given the relatively thick walls of the castings, the time required for the outer layer to form a crust is relatively long. During this process, if the gases generated within the sand mold cannot escape before the crust forms, they will create a gas trapping phenomenon in certain locations. Especially near the outer edge of the riser, due to gas compression, the outer shell, which is still at a high temperature, is prone to plastic deformation. As the temperature gradually decreases, these plastic deformations are permanently retained, ultimately forming porosity.
The porosity defects observed on the castings exhibit smooth inner walls and an iron oxide coloration on the surface. The defects have a depth of approximately 20mm, with lengths and widths ranging from 80 to 150mm. This is illustrated in Figures 2 and 3, which depict the porosity defects after processing and immediately after opening the mold, respectively.
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 | 0.18 |
3.2 Microscopic Analysis
When manufacturing such steel castings using the furan resin self-hardening sand process, a significant amount of gas is generated during the pouring process, particularly at the interface between the sand mold and the molten steel. Furan resin, as a synthetic resin containing a furan ring, undergoes combustion or incomplete combustion under the action of high-temperature molten steel, producing a large amount of CO2 gas. These reactions include the complete and incomplete combustion of carbon, as well as the further oxidation of CO. The specific reactions are as follows:
C + O2 → CO2 (Complete combustion)
2C + O2 → 2CO (Incomplete combustion under oxygen-deficient conditions)
2CO + O2 → 2CO2 (Further oxidation of CO)
In addition to these basic combustion reactions, high-molecular-weight materials and sulfonic acids near the interface 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 (Decomposition of sulfonic acid)
CH4 → C + 2H2 (Decomposition of methane)
CnH2n+2 → nC + (n+1)H2 (Decomposition of alkanes)
C + O2 → CO2 (Combustion of carbon)
2C + O2 → 2CO (Incomplete combustion under oxygen-deficient conditions)
2CO + O2 → 2CO2 (Further oxidation of CO)
2H2 + O2 → 2H2O (Combustion of hydrogen)
S + O2 → SO2 (Combustion of sulfur)
These gases, driven by temperature differences and pressure gradients within the sand mold, are mostly able to escape through permeable sand mold, risers, vent ropes, and air vents. However, a small portion of these gases may accumulate in certain areas, forming porosity. Sometimes, these gases can even be 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 found within. Therefore, the possibility of gas invading the interior of the casting is excluded.
4. Preventive Measures
To address the porosity issues in thick and large steel castings, a series of preventive measures have been implemented, including optimizing the pouring system, adding external chills, improving the sand core, and optimizing the smelting process.
4.1 Optimization of Pouring System
To solve the porosity problem, the original bottom-pouring process for Type II castings was upgraded to a stepped pouring process. An additional internal gating system was specifically added 150mm above the root of the riser. This design significantly reduced the static pressure of the steel water at the bottom and effectively introduced the high-temperature steel water from the ladle into the open riser, thereby greatly enhancing the riser’s feeding ability and effectively avoiding the situation where the riser is cold while the casting is hot.
The specific process details are shown in Figure 4.
4.2 Addition of External Chills
To further enhance the quality of the castings, external chills were added to the outer circular part, ensuring a sand thickness of approximately 40mm between the external chill and the casting. This measure aimed to reduce the total amount of gas generated by the resin curing agent under high-temperature conditions. Simultaneously, the chill thickness was designed to be 200mm, which could rapidly cool the outer surface of the casting, ensuring it quickly formed a skin, thereby effectively preventing gas from entering the interior of the casting or accumulating locally to form porosity.
4.3 Improvement of Sand Core
The traditional (bun-shaped core) was abandoned for the sand core, and instead, 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 pore, avoiding the formation of porosity due to gas entering the interior of the casting.
4.4 Optimization of Smelting Process
During the steel smelting process, electric arc furnace smelting technology was employed, and the purity of the steel water 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 steel water into the ladle. This measure not only purified the steel water but also significantly reduced the H2 and N2 content in the steel water.
By implementing these comprehensive preventive measures, including optimizing the pouring system, adding external chillers, improving the sand core design, and refining the smelting process, the porosity issues in thick and large steel castings were successfully addressed. These measures not only improved the quality of the castings but also enhanced their mechanical properties and service life, thereby meeting the rigorous requirements of various industrial applications.
In conclusion, addressing porosity defects in thick and large steel castings requires a multifaceted approach that involves optimizing various aspects of the casting process. Through careful analysis, precise implementation of preventive measures, and continuous improvement, the quality of thick and large steel castings can be significantly enhanced, ensuring their reliability and performance in diverse industrial settings.