Why are rails always rusty?
Rail is the abbreviation of railway track. Rail is a customary term. It is made of high-hardness and high-toughness steel. In order to improve the compressive and wear-resistant strength, manganese is usually added. Manganese steel is hard and wear-resistant, has a certain impact toughness, and meets the conditions for manufacturing rails. In addition, manganese rails have a large density. When an oxide rust film is formed on the surface, it will form a protective effect on the inside and will not affect the use.
Iron and steel are distinguished by the amount of carbon content. Iron-carbon alloys with a carbon content of less than 2% are steel, and those with a carbon content of more than 2% are called pig iron.
From a metallurgical point of view, there is no such thing as stainless iron. Stainless steel is also a kind of steel. Steel has toughness, elasticity and rigidity; as for rust, rust means corrosion resistance. The main element that determines the corrosion resistance of stainless steel is chromium.
Steel with a chromium content of more than 10.5% will not rust. At this time, the oxidation of chromium and the corrosive medium forms an oxide film on its surface, which can prevent basic further corrosion.
The cost of iron making is much lower than that of stainless steel. When designing and manufacturing some items with a short service life and iron can meet the material use requirements, iron will of course be used.
The reason why stainless steel cannot be used is that it is not resistant enough and is easily damaged by thermal expansion and contraction. Stainless steel will be easily damaged under daily wind, rain and exposure. Although the high manganese rails used now look rusty, there is only a layer of rust on the surface, and the inside is still intact.
In addition, the density of manganese rails is very high. When an oxide rust film is formed on the surface, it will form a protective effect on the inside and will not affect the use.
Stainless steel material characteristics
Stainless steel has some other characteristics.
First of all, technically speaking, most people think that stainless steel is rust-free. In fact, stainless steel, like other alloys, also rusts. It's just that after stainless steel rusts, the color of the surface oxide film (rust) is the appearance color of stainless steel, and this oxide film is relatively dense, which can prevent further oxidation and rust of the internal organization.
However, this oxide film can still be corroded under certain conditions, especially in a humid acidic environment, where corrosion is obvious.
Therefore, stainless steel cannot be used in many cases. There are many types of stainless steel, and stainless steel (especially austenitic stainless steel) has a low hardness. For example, it is difficult to sharpen a knife when used for making knives, and even if it is sharp, it will easily become blunt. Experienced people know that the reason why bright stainless steel plates are easily scratched is because of their soft texture.
Technically, stainless steel is not rust-free, but the color after rusting is similar to the color before rusting; its performance is not suitable for use in all places, so stainless steel cannot be used to replace other materials in general.
In terms of resources, nickel and chromium used in stainless steel are relatively scarce minerals, especially nickel. Useful and easy-to-use things must always be used in key places and cannot be used and wasted at will.
In terms of cost, the cost factor will be considered when buying materials, making parts and installing machines. The price of stainless steel is much higher than that of other materials such as ordinary steel, aluminum, magnesium-aluminum alloy and plastic, and the price difference is several times.
In addition, even if other materials are rusty and not corrosion-resistant, there are many surface treatment technologies that can protect materials from corrosion, such as painting, powder coating, electroplating, electrophoresis, surface oxidation and surface chemical treatment (nitriding, cyaniding, etc.) can improve the surface corrosion problem of materials. So it is not necessarily that rusty materials cannot be used in places with corrosion.
In general, stainless steel cannot be used to replace all other materials. Only after fully considering the function of the part, material performance, working environment, part cost, processing performance and possibility, can a suitable material be selected for the part.
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