Abstract: In traditional pouring systems, molten steel flowing through a sand mold easily undergoes sand washing, leading to sand inclusions, surface water marks, and other defects when the liquid material is unstable. In traditional exhaust systems, floating sand easily forms in the airway after the air hole is drilled, resulting in sand inclusion in the casting. When cutting the air hole of the casting body, it is easy to cut the casting, forming a lack of material defect. To continuously improve the internal and external quality of knuckle castings for railway freight cars, a number of measures were implemented, such as adopting double-pass double-injection pouring cups, improving pouring methods, designing integrated splicing refractory runners, testing grid exhaust valves, testing multiple types of sand, and optimizing special sand box tooling. These measures significantly improved the quality of the knuckle castings, increasing the qualified rate from 82% to over 96%.
1. Introduction
The coupler is a critical component connecting railway freight cars, directly related to the operational safety and reliability of railway freight cars. The knuckle is an important product that comprises the coupler assembly. As China’s railway transportation continues to develop towards high speed and heavy haul, the longitudinal impact force of freight cars increases dramatically, leading to severe friction and wear on the contact surfaces between knuckles. The performance requirements for knuckles are increasingly high.
Our company has traditionally produced knuckle castings for railway freight cars using a “traditional” casting process. The “traditional” process primarily involves single-pass single-injection pouring, a sand mold runner system, and drilling air holes in the sand mold body, which can result in air entrapment during pouring, sand inclusions caused by molten steel overheating and eroding the sand mold runner surface, and porosity defects caused by high gas generation from the core.
To improve product qualification rates and production efficiency, various research tests were conducted on current process methods and material applications, exploring casting processes such as double-pass double-injection pouring methods, integrated bottom-pouring systems, and fixed grid exhaust valves in exhaust holes. These significantly enhanced product quality and addressed the drawbacks caused by fast pouring speeds and high molten steel erosion from 25-tonne LF refining ladles. After process trials and validation, the adoption of new processes, methods, and materials yielded positive results, with certain promotion value.
2. Product and Original Process Introduction
2.1 Product Problem Analysis
The molding process utilizes an automated production line with environmentally friendly and renewable ester-hardened water glass sand. Pouring is done using a 25-tonne LF bottom-pouring large refining ladle. The knuckle product dimensions are 265mm × 156mm × 321mm. The high impact force of molten steel during pouring results in numerous defects in critical areas such as the upper traction platform and impact platform. The current casting qualification rate is only 82%.
2.2 Original Process Analysis
The original knuckle process was set up with 8 pieces per box, using a central pouring system and ester-hardened water glass mixed sand (30% new sand + 70% reclaimed sand by mass fraction). The internal cavity uses coated sand with high gas generation, which can easily cause internal porosity. The runner in the pouring system, formed by sand, is prone to sand erosion and sand inclusion. The vent holes in the risers are drilled using extended drills, which can cause sand to fall and form sand inclusions, and there is a risk of cutting damage during cutting.
During the initial stage of molten steel pouring, due to the large static head of the 25-tonne LF bottom-pouring large refining ladle, the molten steel flows rapidly at high temperatures and erodes the ordinary sand runner for a long time, easily forming turbulence. Additionally, the traditional pouring method for ordinary sand mold casting is single-cup single-injection, and the sand box is a general conventional sand box. During pouring, the molten steel forms a gravitational negative pressure in the pouring cup due to the lateral force, drawing gas into the mold cavity. When the local sand or gas in the knuckle cannot float up to the riser in time, sand inclusions and gas shrinkage pores are formed inside the knuckle.
3. Process Optimization
3.1 Pouring Process Optimization
Based on the issue of gas being drawn into the mold cavity when pouring using the traditional single-cup single-injection method, a double-pass double-injection pouring method was designed. The pouring cup material is refractory, designed as double-injection with dimensions of 316mm × 235mm × 200mm. The double pouring gates can effectively reduce the molten steel head, smoothly fill the mold cavity with molten steel, achieve stable mold filling, and smoothly float the sand up to the riser, ensuring a dense internal structure of the knuckle product.
When pouring molten steel using the original process, it directly flows from the pouring cup into the sprue, bringing air into the mold cavity until the end of pouring. By adopting a large flow rate to fill the double-injection pouring cup to 2/3, followed by a small flow rate for pouring, a certain amount of liquid is always maintained in the pouring cup, preventing air from being drawn in when the molten steel leaks from the pouring cup. This pouring method reduces the problem of bring-in porosity defects formed by air being drawn in due to the gravitational negative pressure formed by the lateral force of the molten steel in a single-pouring cup.
Additionally, an integrated spliced refractory material runner was designed and adopted. The runner is pre-embedded in the positioning location on the mold plate. During molding, the refractory material for the ingate is placed according to the positioning, followed by sand filling and molding. This ensures that the molten steel flows more smoothly in the integrated runner at high temperatures without contacting the sand mold, avoiding turbulence caused by molten steel eroding the sand mold.
3.2 Exhaust Valve Design
A high-temperature resistant porous grid exhaust valve was explored and adopted to replace manually drilling through the vent holes using an extended drill. This porous grid exhaust valve has functions such as slag retention, preventing floating sand from falling into the mold cavity, and automatically exhausting gas during high pouring pressure. It can also reduce molten steel waste and eliminate the quality hazard of cutting off the exhaust rod, which can cause casting defects.
3.3 Core Sand Testing for Internal Cavity Lightening Holes
To explore various types of core sands, multiple batches of tests were conducted using six types of internal cavity core sands (high-strength high-collapse water glass sand, two types of furan resin sands, and three types of coated sands) in the same mold. Five refining heats were produced using coated sand, and it was found that the main parameter causing casting defects in coated sand is gas generation. As shown in Table 2, coated sand with a gas release of below 16 mL/g yields better results. Based on the surface condition and internal dissection of the castings, a coated sand with low gas generation and good collapsibility was selected. Through small-batch trial production and verification, it meets the requirements for the internal tissue density and performance conditions of the knuckle.
Table 2: Parameters of coated sand
| Heat number | Gas release /(mL · g^-1) | Ultimate tensile strength at room temperature / MPa | Bending strength at room temperature / MPa | Hot bending strength / MPa |
|---|---|---|---|---|
| 1 | 12.6 | 3.8 | 8.3 | 4.5 |
| 2 | 15.7 | 4.6 | 10.0 | 1.4 |
| 3 | 15.6 | 3.5 | 8.1 | 4.3 |
| 4 | 16.6 | 4.4 | 9.3 | 1.3 |
| 5 | 16.7 | 4.5 | 9.6 | 1.6 |
3.4 Special Tooling Design
Considering the structural characteristics of the knuckle and the actual production situation of our company’s ester-hardened water glass sand automated production line, which produces a variety of products and uses large, general-purpose sand boxes for producing knuckles, resulting in a large amount of sand filling and subsequently increased gas emission and gas generation. To address this, we have designed a special sand box tooling for knuckles.
The highlight of this special sand box tooling design is that its top is completely designed to avoid all exhaust hole positions and double-injection pouring cup positions. This design not only further optimizes the sand box structure but also effectively reduces the height of the sand box, thereby decreasing the amount of sand filling. The reduction in sand filling directly decreases the total gas emission during the pouring process of the sand mold, contributing to improved casting quality.
By adopting this special sand box tooling, we can significantly enhance the casting quality of knuckle castings while ensuring production efficiency. This innovative design not only demonstrates refined management of the production process but also showcases our continuous exploration and advancements in the field of casting technology.