Unlike other uses of iron and steel, railway rails are subject to very high stresses and have to be made of very high quality steel
Unlike other uses of iron and steel, railway rails are subject to very high stresses and have to be made of very high quality steel.
The rails represent a substantial fraction of the cost of a railway line. Only a small number of rail sizes are made by the steelworks at the one time, so a railway must choose the nearest suitable size.
Rails are made in a large number of different sizes. Some common European rail sizes include:
40 kg/m (81 lb/yd)
50 kg/m (101 lb/yd)
60 kg/m (121 lb/yd)
Some common North American rail sizes include:
115 lb/yd (57 kg/m)
133 lb/yd (66 kg/m)
136 lb/yd (67 kg/m)
140 lb/yd (69 kg/m)
Considerations when selecting a method of casting
The type of casting process most suitable for a particular application is dictated by a number of factors. These include:
- The number of castings
- The cost per casting
- The material being cast
- The surface finish and tolerances of the finished casting
- The size/shape of the casting
Casting Methods
There are several types or processes used in producing castings. The most used casting methods in today's industry are: sand casting, investment casting, die casting and continuous casting.
Continuous casting
Introduction
Continuous casting is the process whereby molten metal is solidified into a "semi finished" billet, bloom, slab or beam blank. Prior to the introduction of continuous casting in the 1950s, steel was poured into stationary moulds to form "ingots". Since then, continuous casting has evolved to achieve improved yield, quality, productivity and cost efficiency. Nowadays, continuous casting is the predominant way by which steel is produced in the world. Continuous casting is used to solidify most of the 750 million tons of steel, 20 million tons of aluminum, and many tons of other alloys produced in the world every year
The Basic Principle of Continuous Casting
In the continuous casting process, illustrated in Figure 1, molten metal is poured from the ladle into the tundish and then through a submerged entry nozzle into a mould cavity. The mould is water-cooled so that enough heat is extracted to solidify a shell of sufficient thickness. The shell is withdrawn from the bottom of the mould at a "casting speed" that matches the inflow of metal, so that the process ideally operates at steady state. Below the mould, water is sprayed to further extract heat from the strand surface, and the strand eventually becomes fully solid when it reaches the ''metallurgical length''.
Figure 1: Schematic representation of the continuous casting process []
Solidification begins in the mould, and continues through the different zones of cooling while the strand is continuously withdrawn at the casting speed. Finally, the solidified strand is straightened, cut, and then discharged for intermediate storage or hot charged for finished rolling.
Continuous casting provides better quality product as it allows finer control over the casting process, along with the obvious advantages inherent in a continuous forming process.
Advantages of continuous casting compared to individual moulds are: More consistent composition and dimension. ·Better surface and internal quality. ·Higher yield. Energy savings. Less labour intensive. Highly automated, Reduces product cost
Analysis of casting methods
To choice the most suitable means of manufacturing rail, the size, function and finishing of rail has to be considered. The length of rail and the demand for consistent as well as surface finishing makes continuous casting the most suitable means.
Rolling
The next phase in producing rail is Rolling. Rolling is the main method used to shape steel into different products after it has been cast. There are two types of rolling - hot and cold.
The rolling process for both hot and cold consists of passing the steel between two rolls revolving at the same speed but in opposite directions. The gap between the rolls is smaller than the steel being rolled, so that the steel is reduced in thickness and at the same time lengthened.
One set of rollers is called a stand, and in any one mill there can be a number of stands. One length of steel can pass through a stand a number of times so that it is gradually reduced in size and progressively rolled to the desired shape. A slab 230mm thick can end up only 1.5mm thick, but many times longer, after the hot rolling process.
Cast steel is a relatively weak mass of coarse, uneven metal crystals, or 'grains'. Rolling causes this coarse grain structure to re-crystallize into a much finer grain structure, giving greater toughness, shock resistance and tensile (stress) strength. Figure 2 below shows the cross section of rolled steel. The far end of the material shows small and uniform grain which is wanted.
Figure 2 Changes in grain structure during hot-rolling (6)
Elements of casting and rolling to be controlled
Cooling rate
The rate at which a casting cools affects its microstructure, quality, and properties.
The products of sand casting and slurry-mold processes, often large with thick walls, generally cool slowly. This increases the metal's grain size, creating a coarse microstructure that lowers the strength of the casting. Coarse grains can allow elements of an alloy to separate, which also weakens the casting. But slower cooling keeps the casting metal liquid longer, which allows more gases and waste metal to escape, reducing the voids and inclusions that can weaken a casting.
If the products and metal mould processes generally cool more quickly, this will result in a fine microstructure with small grain and less alloy segregation but more trapped gases and inclusions.
Shrinkage
Like nearly all materials, metal is less dense as a liquid than a solid, and so a casting shrinks as it cools mostly as it solidifies, but also as the temperature of the solid material drops.
The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. Risers add cost because some of their material must be machined off as waste, but they are often necessary to produce usable parts. Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant alloy.
Pattern makers use special shrink rulers to measure their work. These rulers are 2 - 6 % oversize, depending on the material to be cast. Using such a ruler during pattern making will ensure an oversize pattern.
Solidification
An important characteristic of all solidification processes is segregation. This takes place when crystals grow in a multicomponent melt, because crystals are always purer than the liquid melt from which they solidify. Therefore, as steel solidifies, the levels of carbon, phosphorus, and sulfur grow in the remaining liquid, resulting in an enrichment of these elements in the centre of the ingot.
Segregation can be minimized by keeping segregating elements at low levels or by solidifying at a fast rate—i.e., by not providing the time for separation. It is also impaired by stirring the melt
Air Escape
Defects in Casting and Rolling
Porosity Holes in the cast material
Causes:
- Dissolved or entrained gases in the liquid metal
- Gas generation resulting from a reaction between molten metal and the mould material
Improvement Strategy
- Pour metal at lowest possible temperature
- Design gating system for rapid but uniform filling of the mould, providing an escape path for any gas that is generated
- Select a mould material with higher gas permeability
Surface
Among all the quality concerns, the surface integrity is an extremely important quality characteristic of the hot rolled products. Surface defects remain as a weakness or stress concentration site of the bulk material and hence could cause catastrophic failure when the rolled product is in use. Products with severe surface defects have to be scrapped. Therefore, it is highly desired to detect, reduce, and eventually eliminate the surface defects if possible. Unfortunately, the surface defects remain as the most troubling problems in the hot rolling process. Major challenges in the surface quality control fall into the following two aspects.
(1) Effective surface sensing system to measure the surface condition in real-time during production environments (high temperature, high speed, noise, and dirty conditions) is not available.
(2) The root causes of surface defects in hot rolling processes are very complicated. Surface defects could be originated from multiple sources. For example, the non-metallic impurities in the billet during solidification as well as the mechanical failures in the rolling mills are all important potential sources of surface defects.
Figure 3 shows the defects continuous casting can create (8)
Residual stress
Unseen and often unsuspected, residual stress can be the most damaging defect of all. This is because the stress can be so large, outweighing the effect of all other defects. It is usually never specified to be low. It is also practically impossible to measure in a non-destructive way in the interior of a complicated casting. However, it can be controlled by correct processing.
Possible rail failures if defects are not controlled
Buckling is a failure mode of a structural member characterized by a failure to react to the bending moment generated by a compressive load.
Creep
The gradual deformation of metals at high temperatures while under load takes place in typically in three stages.
1. Primary creep is the rapid early phase that gradually reduces in rate, eventually leading into stage.
2. Secondary creep is the steady-state regime, in which creep rate is constant.
3. Tertiary creep is the final stage in which the rate of strain increases because of the growth of microscopic internal pores and tears that gradually link to cause the fracture of the whole component
Fatigue is the progressive, localized, and permanent structural damage that occurs when a material is subjected to cyclic or fluctuating strains at nominal stresses that have maximum values less than (often much less than) the static yield strength of the material. The resulting stress may be below the ultimate tensile stress, or even the yield stress of the material, yet still cause catastrophic failure.
Ductility
The amount of inelastic deformation before failure, often expressed relative to the amount of elastic deformation. Material ductility can be measured by the amount of inelastic strain before failure compared to the amount of elastic strain. It is commonly expressed as a ratio of the maximum strain at failure divided by the yield strain.
Conclusion
Continuous casting offer considerable energy savings, less scrap produced which means improved yield, improved labour productivity, improved quality of steel, reduced pollution reduced capital costs, increased use of purchased scrap when output is maximized. These benefits offer by continuous casting makes it the best way to produce rail.
Reference
1 Campbell, John. Castings. Publisher: Elsevier Butterworth-Heinemann
2 Hodgson WH. Residual stresses in rail. In: Kalker JJ, et al., editors. Rail quality and maintenance for modern railway operation. Dordrecht, The Netherlands: Kluwer Academic
3 http://en.wikipedia.org
4 http://encarta.msn.com
5 http://home.att.net/~africantech
6 www.ucs.louisiana.edu
7 http://urban.arch.virginia.edu
8 https://www.rcnde.ac.uk
