posted On Tuesday, October 29, 2019 in Blog
Steel has been around for ages, even the Romans used it back in 223 B.C. Ancient civilizations were able to find a way to make steel, as almost 5% of the earth’s crust consists of iron, making it the second most abundant metal on earth.[1]
Steel is made through the addition of carbon into iron (up to 2%) which happens with extremely high heat. One part of steel production is heat treatment. Heat treatment is a method used to make metals stronger, harder and more durable. This method is very important to many steel and metallic parts.
Heat Treating of steel and other metals can lead to:
When common metals, such as steels, are heated to high temperatures, there is a significant change at the atomic level. Iron atoms are originally arranged into crystal structures that change shape when heated; of which, there are two common structures. Depicted in Figure 1 is a body-centered cubic (BCC) crystal structure, which is common in steels at room temperature. Notice that nine total iron atoms make up the unit cell for this arrangement of atoms. Figure 2 depicts a face-centered cubic (FCC) crystal structure. There are 14 total atoms that make up the unit cell for this arrangement of atoms. The FCC transformation occurs when steel is heated above its critical temperature.
The bonds between iron atoms are relaxed from their BCC state, and transformed into the FCC structure. The important thing to note is the effect of the increased atoms in the lattice. With more atoms, there are more interstitial sites that allow alloying elements to bond with iron and move into these lattices. One such element is carbon, a primary element for hardening steel. Because of the increased amount of interstitial sites that fit carbon, carbon atoms move more freely around iron at elevated temperatures. With greater chance to interrupt geometry of the crystals, steel becomes less ductile, resulting in an increase in strength. To increase the amount of carbon in iron (carburizing), the metal is typically placed in an atmosphere with an elevated carbon level to diffuse additional carbon into the surface.
Simply heating these steels with an increased carbon atmosphere is not enough to keep it trapped in these lattices to increase hardness. Slow cooling will allow the carbon to diffuse back out, as the structure slowly changes back from FCC to BCC. To counteract this, several different quenchants can be used to cool the material quickly. The quenching allows a quick change of environment for a steel, from high to low temperatures, undergoing heat treatment. It acts to trap carbon and other elements in the middle as there is not enough time for diffusion out of the steel before a change in crystal structure. With these trapped carbon atoms in the crystal structure, we have an altered BCC structure known as martensite.
Not every steel reacts the same. Chemical composition can vary greatly between the different grades of steel. Certain alloying elements can greatly increase the hardenability of steels such as nickel (Ni), chromium (Cr) and molybdenum (Mo). Hardenability is not how hard a material is. Hardenability directly relates to the ability of a metal to form martensite and martensistic structure upon quenching, which points to how well hardness can be achieved. Ni, Cr and Mo additions, as well as higher carbon, allow more martensite to form, thus the metal is more “hardenable.” High hardenability is the ability of a metal to transform into the martensite throughout the whole part, not just high hardness at the surface.
Sources
[1] http://www.gsa.org.au/resources/factites/factitesIron.pdf
[2] http://www.ce.berkeley.edu/~paulmont/CE60New/review1.pdf
Various properties of metals experience change when they go through a heat treatment. Some changes make the metals more resilient or resistant while others allow them to be reshaped. Even though modern technology has created new methods for this type of treatment, blacksmiths many years ago used to accomplish similar goals by heating and cooling metals for horseshoes, wagon parts and more. To learn additional facts about how heat treatments affect the characteristics of metals, read the following details.
The Effects of Heating Metals
1. Thermal Expansion
As metals are heated, their volume, surface and length will expand. The term for these actions is thermal expansion. Each metal will have a different rate of expansion when exposed to the heat.
2. Structural Alterations
Another effect that heat treatments have on metals is that the structure of them will go through a transformation. This is due to the fact that heat displaces the allotrope atoms in metals and causes them to reform in a different configuration. For this reason, this action is called the allotropic phase transformation. It not only can change the structural shape of the metal, but it also can alter its strength, ductility and hardness of it.
3. Makes the Metals Resistant to Electrical Current
A heat treatment can effectively make a metal have a certain level of electrical resistance. The reason that this happens is that when metals are heated, their electrons can absorb addition energy and makes them move faster than normal.
4. Reduces a Metal’s Magnetism
Magnetic metals such as nickel, cobalt and iron can lose some of their magnetism by undergoing a heat treatment. In some cases, they are no longer magnetic at all.
Types of Heat Treatments
All heat treatments involve heating and cooling metals to change them in some fashion. The most popular reasons for performing these treatments is to increase a metal’s toughness, hardness, strength, corrosion or electrical resistance, and ductility. The following are the most common methods for performing these treatments:
• Annealing softens the metal through heating to make it workable and to increase its ductility. The metal is heated to the appropriate temperature to alter its microstructure and then, it is slow-cooled. It also increases the metal’s electrical conductivity.
• Hardening improves the mechanical properties of steel and other alloys. During this process the metal is heated to a high enough temperature to dissolve a portion of the carbon in it, prior to the appropriate quenching medium being applied. Hardening can increase wear resistance and strength but can also increase brittleness at times, so it is not recommended for some engineering applications.
• Normalising is used on alloys to provide them with a uniform composition and grain.
• Tempering is used on steel to improve its ductility. Steel that does not undergo this process is extremely hard but too brittle to use in many applications.
While there are many other details to learn about how heat treatments affect the properties of metals, the above information gives you a start on your education about this topic. Ensure that your metals receive the appropriate heat method to achieve your purposes.
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