Railway Track Castings Quality: Defect Analysis & Comprehensive Prevention Guide

1. Introduction: The Importance of Quality and Failure Analysis of Track Castings

Track castings are important components in the railway transportation system, responsible for supporting train operation and transmitting loads. The quality of castings directly affects the safety and service life of the railway system. In actual production, the casting process is complex, and various defects often occur in castings. These defects, after being subjected to alternating loads for a long time, may become the source of cracks and eventually lead to the fracture of the casting. Therefore, analyzing the failure causes of track castings, understanding how defects are formed, and formulating preventive measures are very necessary for improving the quality of castings and ensuring railway safety. This article will analyze the common failure forms and causes of track castings from the perspective of casting technology, providing theoretical support for quality control.

2. Analysis and Control of Porosity Defects

2.1 Formation and Manifestation of Porosity Defects

Porosity is a common problem in the production of track castings, mainly manifested as spherical or elliptical cavities inside or on the surface of the castings. Pores mainly come from three aspects: First, gases dissolved during metal smelting (such as hydrogen and oxygen) are released during solidification; The second is the air drawn in during the pouring process; The third type is the bubbles formed by the evaporation of water in the molding sand. If these gases cannot be discharged, gas holes will form in the castings. The inner surface of pores is usually smooth, with sizes ranging from a few micrometers to several millimeters, and may be locally concentrated or scattered. There are often component segregation and microstructure abnormalities around pores, and these areas are prone to stress concentration, which reduces the fatigue performance of the material.

2.2 Prevention and Control of Porosity Defects

A comprehensive prevention and control system needs to be established to address the defect of stomata. During smelting, it is necessary to control the gas content of the molten metal, such as protecting the smelting with inert gas, rotating degassing treatment, and strictly controlling the quality of the charge. When molding, it is necessary to optimize the performance of the molding sand, keep the moisture content below 4%, and ensure air permeability. New breathable coatings can be used to enhance the exhaust capacity. In the design of the gating system, the bottom injection type is recommended. The pouring speed and temperature should be controlled to avoid turbulence and splashing of the molten metal. Large key castings can be treated with vacuum casting technology to eliminate gas entrainment. In addition, it is also very important to establish a complete process monitoring system, such as detecting the gas content during the smelting process and recording the pouring parameters, etc.

3. Analysis and control of shrinkage porosity and shrinkage cavity defects

3.1 Formation and Influencing factors of shrinkage porosity and cavities

Shrinkage porosity and shrinkage cavities are caused by insufficient feeding during the solidification of castings. When molten metal cools and solidifies, its volume shrinks. If there is not enough liquid metal to replenish it, holes will form at the final solidified area. Shrinkage cavities are large, concentrated cavities that often occur in the hot spots of castings. Shrinkage porosity is a small, dispersed pore with a wide distribution range. The main factors influencing shrinkage porosity and shrinkage cavities include the shrinkage characteristics of metallic materials, the structural design of castings, the feeding capacity of the gating system, and cooling conditions, etc. These defects will reduce the effective bearing area of the casting, cause stress concentration, and affect the mechanical properties and service life of the casting.

3.2 Optimization of the feeding system and process control

The key to preventing shrinkage porosity and cavities is to establish a complete feeding system. Riser design is the core and should follow several principles: The riser position should be above the hot spots of the casting; The riser volume should meet the feeding requirements, usually 1.2 to 1.5 times the volume of the feeding part. The riser shape should be conducive to directional solidification. The feeding time can be extended and the feeding efficiency improved by using insulated risers or heating risers. The application of chill irons is also very important. By arranging chill irons in the thick and large parts of the casting, the local cooling rate can be adjusted and the solidification sequence can be controlled. The control of pouring temperature is also crucial. If the temperature is too high, it will increase the tendency of shrinkage; if it is too low, it will affect the fluidity of the feeding metal liquid. In modern casting processes, computer solidification simulation technology can optimize the feeding system, predict the position of shrinkage cavities, and guide process improvement.

4. The causes and prevention of cold barrier defects

4.1 Manifestations and Formation conditions of cold lap defects

Cold lap is a join-like defect formed when two streams of molten metal fail to fully fuse during the filling of the mold due to a drop in temperature or flow obstruction. Cold shuts are usually manifested as concave patterns on the surface of castings or discontinuous interfaces inside, and in severe cases, they can lead to cracking of castings. The formation of cold shuts requires the satisfaction of three conditions: First, the temperature of the molten metal drops below the critical value, and its fluidity decreases; The second is that an oxide film appears at the front of the molten metal flow, hindering the fusion. The third issue is that the contact pressure between the two metal liquid flows is insufficient to break through the oxide film and achieve complete bonding. Cold lap defects are most likely to occur in thin-walled castings, structurally complex parts and areas far from the gate. Such defects can reduce the mechanical strength and air tightness of castings and are one of the important reasons for early failure.

4.2 Optimization and improvement of filling process

To prevent cold lap defects, efforts should be made from two aspects: improving the fluidity of the molten metal and optimizing the filling process. In terms of the quality of the molten metal, it is necessary to ensure sufficient superheat. Different materials require corresponding pouring temperature specifications. For instance, cast iron parts should be controlled at 1350-1450℃, and cast steel parts at 1550-1650℃. Meanwhile, the chemical composition of the molten metal should be reasonable, and appropriately increasing the carbon equivalent can improve fluidity. In the design of the gating system, the principle of “short, flat and fast” should be followed to shorten the filling distance and reduce the flow resistance. Multiple inner gates can be designed to enable the molten metal to fill the cavity simultaneously from different directions. For complex thin-walled parts, the stepped gating system can effectively improve the filling condition. The control of the pouring speed is also crucial. It is necessary to ensure a sufficient filling speed, but it should not be too fast to avoid air entrainment. In practical operation, the quick-pouring process can be adopted, combined with an appropriate pouring temperature, to ensure that the molten metal maintains good fluidity in the cavity. In addition, special processes such as inclined pouring and vacuum-assisted pouring can also improve the filling quality and reduce the occurrence of cold lap defects. By establishing a database of pouring parameters and analyzing the filling effect under different process conditions, the process plan can be continuously optimized and the stability of casting quality can be improved.

Railway Casting Parts Supplier

Luoyang Fonyo Heavy Industries Co., Ltd, founded in 1998,is a manufacturer in cast railway parts. Our factory covers an area of 72,600㎡, with more than 300 employees, 32 technicians, including 5 senior engineers, 11 assistant engineers, and 16 technicians. Our production capacity is 30,000 tons per year. Currently, we mainly produce casting, machining, and assembly for locomotive, railcar, high-speed trains, mining equipment, wind power, etc. Our products have been exported to Russia, the United States, Germany, Argentina, Japan, France, South Africa, Italy and other countries.
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