Veritasium - Why Don’t Railroads Need Expansion Joints?
The video explains how thermite reactions, discovered by Hans Goldschmidt, are used to weld rails together, eliminating the gaps that once caused the distinctive 'ta-tak' sound of trains. Initially, railroads were hesitant to adopt this method due to concerns about thermal expansion causing buckling. However, tram rails in cities, which experience less force, were the first to adopt thermite welding to reduce noise. The video details the eight-step process of thermite welding, including cutting the rail, aligning it, preheating, and pouring the thermite steel. It emphasizes the importance of proper alignment and preheating to ensure a strong weld. The video also discusses the testing of welds for strength and the role of mechanical stress in preventing rail buckling. Thermite welding is now widely used, with millions of welds performed annually, contributing to smoother and more durable railroads.
Key Points:
- Thermite reactions are used to weld rails, eliminating gaps and reducing noise.
- Proper alignment and preheating are crucial for a strong thermite weld.
- Thermite welding is widely used, with millions of welds performed annually.
- Mechanical stress is used to prevent rail buckling due to thermal expansion.
- Thermite welds are tested for strength, ensuring durability and safety.
Details:
1. 🔥 The Fascinating World of Thermite Reactions
1.1. Introduction to Thermite Reactions
1.2. Historical Use and Challenges
1.3. Railroad Resistance to Thermite Welding
1.4. Adoption by Tram Systems
2. 🔧 Step-by-Step: Welding Rails with Thermite
- Welding rails is done at the neutral temperature of the rail to ensure no stress, with welding ideally done at night after the rail has cooled.
- The first step involves creating a gap by cutting approximately 2.5 cm off one rail to pour liquid thermite steel.
- Rails must be vertically and horizontally aligned, which is complex as adjustments in one direction can affect the other.
- A maximum tolerance of 2mm is allowed for rail twisting; exceeding this requires untwisting and realigning.
- The mold setup must be watertight since liquid steel is as fluid as water but eight times denser.
- Preheating the rail is crucial to eliminate moisture and ensure even heat distribution, preventing rapid cooling of molten steel.
3. 🌡️ The Critical Role of Preheating in Welding
- Preheating affects the hardness of steel during welding, as demonstrated by cooling two pieces of heated rail at different rates.
- Rapid cooling in water causes steel to become very hard and brittle due to the formation of martensite, a needle-like microstructure.
- Martensite, while hard, is brittle and prone to fracturing because the steel's lattice structure cannot absorb energy effectively.
- Slow cooling in sand results in a less brittle, more ductile steel structure due to a different microstructural formation.
4. 💥 The Thrilling Ignition of Thermite
4.1. Ignition Process
4.2. Filming Challenges and Connectivity Solutions
4.3. Thermite Reaction Details
4.4. Molten Material Separation
4.5. Cooling Process
5. 🛠️ Perfecting the Weld: Cleaning and Finishing Touches
- Remove slag pans and mold shoes after welding to clean the weld area.
- Single-use molds are destroyed during the process, indicating a need for new molds for each weld.
- A weld shear capable of 20 tons of force is used to remove excess steel from the top of the weld, ensuring a clean finish.
- Hammering is used to remove any remaining excess parts, making the process enjoyable for operators.
- Grinding the weld to the same level as the rail is critical to avoid bumps or indents that can affect train operation.
- The grinding process involves two phases: rough grinding followed by fine grinding, with the aim of leaving no more than 1 millimeter of material above the weld level.
- Grinding is focused only on the top surface and not the running flank to ensure proper alignment and finish.
- The process is physically demanding, akin to a serious workout, and requires skilled operators for efficiency.
- Thermite welds on rails are widespread, with almost 100% likelihood that rail passengers have traveled over them globally.
6. 🔍 Analyzing the Weld's Microstructure
- Acid etching reveals distinct microstructural zones in welds: the heat-affected zone (HAZ), fusion zone, and normal rail areas, each with unique properties.
- The heat-affected zone (HAZ) is identified as the weakest part of the weld, showing reduced hardness and yield strength, which can lead to failure.
- German construction standards mandate melting approximately 3mm off each rail to ensure a robust weld bond, highlighting the importance of precise welding techniques.
- Thermite welds display microscopic pores where crystals meet, forming a distinct darker line in the microstructure, which can impact the weld's structural integrity.
- The transition of thermite steel into the rail is not sharp due to partial melting, aiding the formation of a strong bond with the rail.
- Field conditions are less controlled than laboratory settings, necessitating robust welding practices to maintain weld integrity in practical applications.
7. 🚂 Testing the Strength of Thermite Welds
- Goldschmidt conducts regular testing of thermite welds by creating a test weld for every 200 standard welds, focusing on chemistry, hardness, and bending properties.
- Welds are subjected to bending tests until failure, with measurements of force and speed of failure indicating performance.
- A notable test result showed a weld withstanding up to 150 tons of force before catastrophic failure, highlighting the strength of the welds.
- The area of failure is typically not the weld itself, but the zone of heated rail material, indicating a potential area of weakness.
- Goldschmidt's thermite is used in about 50% of the 2 million thermite welds performed annually, contributing 2.5 centimeters per weld and cumulatively creating 50 kilometers of railroad annually.
- Maintaining consistent properties in bending, tension, and hardness is critical given the millions of replications of the process each year.
8. 🌍 Thermite Welding's Global Impact and Future Directions
- Continuous welded rail doesn't buckle in summer due to mechanical stress compensating for thermal expansion.
- Rails can be shortened or lengthened using mechanical stress, which allows for a balance between thermal and mechanical changes.
- The fractional change in length of the rail is referred to as 'strain', which has a linear relationship with stress during elastic deformation.
- Mechanical forces are used to compress the rail when it thermally expands, supported by sleepers and ballast.
- Railroads use a high neutral temperature to avoid buckling in summer, as cracking in winter is less problematic and easier to detect.
- Thermal expansion and contraction counteracted by mechanical stress eliminate the need for expansion joints, allowing faster trains with less maintenance.
- Future content will explore working with thermite in less controlled environments.