basic material knowledge for Mechanical engineer : Carbon Steel

Carbon steel

Callister and Rethwisch (2010) classified iron alloys based on carbon composition into 3 groups: iron with carbon content <0.008%, steel with a carbon content of 0.008% C to 2.14% C and cast iron with carbon content> 2.14%. The steels were subdivided into 3 groups: low carbon steel, medium carbon steel, and high carbon steel.

low carbon steel

Low carbon steels generally contain less than about 0.25% C and are unresponsive to heat treatment intended to form martensite. Reinforcement can be achieved by cold work. The microstructure consists of ferrite and pearlite constituents so that the alloy is relatively soft and weak but has excellent ductility and toughness. Low carbon steels are easy to work with (machinable) machines, easily welded (weldable) and least expensive to produce.

Steel classification
Steel classification

medium carbon steel

Medium carbon steels have a carbon concentration between 0.25 to 0.60%. This alloy can be heated by austenitizing, quenching, and then tempering to improve its mechanical properties. This group is most often used in tempered conditions, so it has a tempered martensite microstructure. Medium carbon steel has a low hardness and can be successfully performed heat treatment only in very thin parts and with a very fast cooling. The addition of chromium, nickel, and molybdenum enhances the ability of these alloys in the heat treatment process, resulting in various strength-duct combinations. The heat-treated alloy is stronger than the low carbon steel, but sacrifices toughness and toughness. Medium carbon steels are used in applications that require a combination of high strength, wear resistance and toughness.

High carbon steels usually have a carbon content of between 0.60 and 1.4%. This group is the hardest, strongest, and least resilient carbon steel. This steel is almost always used in hardened and tempered conditions that produce wear-resistant properties.

Steel is a metal alloy, in which iron metal as a basic element with several other elements, including carbon. Carbon element content in steel ranges from 0.2% to 2.1% by weight of its grade. The following elements are always present in steel: carbon, manganese, phosphorus, sulfur, silicon, and a small portion of oxygen, nitrogen, and aluminum. In addition, there are other elements that are added to distinguish characteristics between several types of steel such as manganese, nickel, chrome, molybdenum, boron, titanium, vanadium and niobium. By varying the carbon content and other alloying elements, different types of steel qualities can be obtained. The function of carbon in steel is as a hardener by preventing the dislocation from shifting to the crystal lattice of the iron atoms.

FE-Fe3C phase diagram
Fe-Fe3C phase diagram

Addition of carbon content in steel can increase hardness and tensile strength, but on the other hand, makes it brittle and ductility. Some materials are also added to the iron/carbon mixture to obtain the steel with the desired characteristics. Nickel and manganese are added to add strength, chromium added to increase hardness and boiling point, and the addition of vanadium also adds to the hardness as well as reduces the impact of metal fatigue.

Chromium is added at least 11 wt% to form a tough oxide on the steel surface to prevent corrosion, known as stainless steel. Tungsten is added to the formation of cementite so that at lower quench speed will form martensite. On the other hand, sulfur, nitrogen, and phosphorus make steel brittle, so this element must be separated while processing.

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