Using a metal material when creating a new product or structure comes with a host of benefits: high strength, ductility, toughness, ability to withstand the environments, and many others. One of the most important benefits of metal is its ability to withstand heat. However, all metals can be heated to the point of degradation. Overheating and burning are common occurrences that can happen to metals at high temperatures.
What is the Difference Between Overheating and Burning a Metal?
In order to understand the difference, one must first gain an understanding of each of overheating and burning, which we explain below.
What is Overheating?
Overheating a metal occurs when its previous properties are impaired due to elevated temperatures. Perhaps the most concerning ramification of overheating is the effect on the mechanical properties of a metal. The alteration of mechanical properties as a result of overheating can lead to property damage, harm to the environment, injury, and loss of life depending on the situation.
Overheating can cause a variety of changes to the mechanical properties of a metal. The type of change is very dependent on the metal being overheated. A hardened steel that is subjected to overheating may have a serious loss of hardness and strength. This is because overheating changes the microstructure of steel. Overheating can cause changes to the mechanical properties of other metals as well. Precipitation-hardened materials such as 6061-T6 aluminum can undergo a process known as overaging when overheated. The precipitates in the aluminum alloy can grow from overheating, which in turn results in them being too large to block dislocations. Without the precipitates in a precipitation-hardened metal to block dislocations, strength is greatly decreased.
Overheating can change other properties in a metal as well. Aesthetics can be negatively affected. Many metals will change colors as a result of overheating. Also, metal coatings can be damaged or destroyed as a result. Electrical properties are typically altered when metals are overheated, although they usually return to normal once the metal returns to regular temperatures. The dimensions of a metal can also be altered by localized overheating that causes distortion.
Many of the effects of overheating can be lessened or eliminated completely by subsequent heat-treatment. For instance, a precipitation-hardened aluminum alloy that has undergone a large reduction in strength as a result of overheating can have its strength restored by additional heat-treatment that resizes its precipitates to block dislocations. A hardened steel that has been weakened by overheating can have its mechanical properties restored by heating and quenching, or through some other method.
What is Burning?
Burning occurs when a metal is overheated to the point where it cannot be repaired. While many instances of the negative effects of overheating can be reversed entirely or at least to some degree, the effects of burning are much more permanent. This often results in metal structures or components that must be scrapped.
There are two common degradative processes that can happen when a metal undergoes burning. One common phenomenon that occurs is intergranular oxidation. Intergranular oxidation is corrosion that occurs at the grain boundaries of a metal. This commonly occurs when oxygen no longer bonds to corrosion-resistant elements, and instead bonds to a more readily corroded element. An example of this could be stainless steel that has its chromium depleted because it has been burned. The result of burning is that iron oxide is formed instead of chromium oxide, and the stainless steel begins to degrade. The other occurrence that can happen when a metal is burned is incipient melting. Incipient melting happens when melting of some of the alloying elements occur at the grain boundaries of a metal alloy, but not all of the elements. This can result in porosity and other discontinuities that cannot be repaired by further heat treatment.
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