Stainless steel material: what difference between stainless steel and carbon steel?

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Selecting an appropriate type of steel for a project is one of the most important decisions to make. However, before deciding between individual grades, you first have to decide what type of steel to use, such as carbon steel or stainless steel. In general, steel is an alloy with Iron being the main element. An alloy is a material made of a mixture of metals or a metal with other elements. These types of alloys are often cheaper and have improved properties. In the case of steel, iron is primarily mixed with carbon and few other elements. Mixing with carbon makes steel stronger, improving its tensile strength while keeping the cost low. To help, this article will discuss the difference between carbon and stainless steel.

Iron when mixed with Chromium, is known as Stainless Steel. When Carbon is the main alloying element with Iron, where all other elements are at a very low percentage, it is called Carbon Steel. Therefore, the main difference between carbon steel and stainless steel is that carbon steel is primarily made of Carbonand Iron while stainless steel is mainly made of Iron and Chromium.

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Carbon steel contains at least 95% iron with up to 2% carbon. The higher the carbon content, the stronger the steel. Stainless steel also contains iron, but in addition it must contain at least 10.5% chromium and the carbon content is very low, usually 0.08% maximum. Stainless steel gets it strength from the metallurgical structure, rather than from the amount of carbon present. Carbon steel can be strengthened by heat treatment whereas the 300 series stainless steels cannot.

Stainless steel can be strengthened by workhardening the structure. The 300 series stainless steel grades (304 and 16) contain nickel from 8 to 14% in addition to the chromium that must be present. 316 contains an additional element, molybdenum, from 2 to 3%. It is these alloying elements added to the iron base that makes stainless steel very different from carbon steel.

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Compare them now:

– Corrosion Resistance:

The difference between carbon steels and stainless steels is the ability to resist corrosion. Stainless steels, are generally the more corrosion resistant of the two steels. Both carbon steels and stainless steels contain iron which oxidizes when exposed to the environment, creating rust. The added chromium in stainless steel makes it more corrosion resistant than carbon steels. The chromium will attach itself to oxygen more readily than iron. When the chromium attaches to the oxygen, it creates a chromium oxide layer which protects the rest of the material from degradation and corrosion. Carbon steel does not typically have enough chromium to form this chromium oxide layer, allowing oxygen to bond with the iron which results in iron oxide, or rust. So if corrosion resistance is a key factor, stainless steel is the way to go.

– Mechanical Properties:

It is difficult to make statements about the differences in mechanical properties between carbon steels and stainless steels because of the many different types and grades of each. Stainless steels can be more ductile than carbon steels because they usually have higher amounts of nickel. However, there are very brittle grades of stainless steel as well, such as the martensitic grades. Carbon steels with very low amounts of carbon may not match tensile strengths of some stainless steels due to the alloying elements that many stainless steel grades contain which increase its strength. However, if there is enough carbon (typically at least 0.30% by weight) in the carbon steel, it is more readily heat treated than an austenitic stainless steel.

– Appearance:

The appearance of the metal must be considered if is required. Stainless steels with particular finishes are generally preferred when cosmetic appearance is a factor. Although both can be sanded and polished to have a bright, shiny look, carbon steel requires a clear coat or paint rather quickly after the polishing process. If it is not applied, the carbon steel will begin to tarnish and eventually rust. Also, if stainless steel is scratched, it will retain its luster in the scratched area, while a painted piece of carbon steel would need to be repainted or it will be subject to corrosion.

  • How is carbon steel corrodes?

          – It is the iron in carbon steel that reacts with the oxygen in the atmosphere to produce “iron-oxides” which we can see as “red rust” on the steel surface. Rusting creates a layer  of oxide on the surface that is several times thicker than the original iron present and often results in a spalling or flaking of the surface, reducing the steel thickness.

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  • How is stainless steel resists corrosion?

          –  Because stainless steel contains at least 10.5% chromium, the oxidation of the iron is changed to produce a complex oxide that resists further oxidation and forms a passive layer on the surface. This is a very thin layer (microns in thickness) but very tenacious and will reform if it is removed by scratching or machining. The addition of nickel to the structure (8% minimum in 304 and 10% minimum in 316) broadens the range of passivity established by the chromium. The further addition of molybdenum (2% minimum in 316) further expands the passivity range and improves corrosion resistance, notable in acetic, sulfuric, and sulfurous acids and in neutral chloride solutions including sea water.
–  If stainless steel is properly selected and maintained it should not suffer any corrosion. Stainless steel will, however, corrode under certain conditions. It is not the same type of corrosion as experienced by carbon steel. There is no wholesale “rusting” of
the surface and subsequent reduction of thickness. If stainless steel corrodes, the most likely form of corrosion is “pitting.” Pitting occurs when the environment overwhelms the stainless steel’s passive film
and it cannot heal the interruption. It usually occurs in very tiny dark brown pits on the surface (hence the name pitting), and does not interfere with the mechanical properties of the stainless steel. Stainless steel can also become subject to crevice corrosion when the deposits or other material creates a “crevice” on the surface. It is similar to pitting but over a larger area where again the environment has overwhelmed the ability of the passive layer to heal itself when deprived of oxygen. It is not attractive, but in most cases it should not affect the mechanical properties of the stainless steel. Good design to eliminate sharp corners or seal them will minimize this type of corrosion.

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Summary:

Choosing between carbon steel and stainless steel for a particular job will involve weighing all of these factors and careful consideration. If the metal is going to be hidden from sight, there is no sense in spending extra money on stainless steel strictly for its appearance. However, if it is going to be hidden from sight but subject to a corrosive environment, stainless steel may actually be the best option. In the end, the choice will depend on the specifics of the job or project.

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