Last Updated on February 2021
Metals are known for their high tolerance in various conditions. Think of caustic environments, heavy loads, high impact, continuous cycling, and even high temperatures.
Yet, combustion engines, furnaces, high-speed machinery, ignition nozzles, exhaust systems, and jet engines are consistently subjected to high temperatures that can melt some metals.
As such, it’s crucial to consider the melting point of a given metal (among other temperature points) for high-temperature applications.
Melting Temperature of Metals
The melting point or melting temperature of a metal is the temperature at which the metal begins to transform from a solid-state into a liquid state, essential in the line of metal casting work.
At the melting point, the solid and liquid states of the metal exist in equilibrium.
On reaching this point, the temperature of the metal remains constant even with continued heating. In short, the change in free energy is zero.
Once the metal is completely molten, any additional heat will again raise the temperature of the metal.
Impurities and the Melting Point of Metals
Rarely do metals exist in a pure state. They contain impurities that can significantly affect their melting temperatures.
The melting point is dependent on the changes in the enthalpy and entropy of a metal. The enthalpy changes are the same when melting a pure or impure metal because similar molecular bonds are broken. However, the changes in entropy vary greatly when melting a pure metal and an impure metal. The difference is even more significant when the metals are in the liquid state. Generally, an impure metal has greater entropy.
See, an impure metal (in the solid-state) is heterogeneous on a microscopic level. As such, the pure regions are distributed across the metal like granite. When the metal is heated, melting occurs in these pure regions beginning with components that have the lowest melting points. This microscopic melting is invisible to the eye.
The preliminary melting of the pure regions form pools of molten liquid that begins to dissolve the surrounding minor components/solid. This scenario makes the metal even more impure, and as such, its melting point is depressed.
The impurities are dissolved until the preliminary melts are saturated (otherwise described as a eutectic composition where the melting point is lowest), and the system continues to melt at the same composition until all the impurity is dissolved.
Once the impurity (minor component) is dissolved, further melting occurs. With it comes an increased purity of the melt, which, in turn, increases the associated melting temperature. As such, the melting process of impure metal is a series of depressions and hikes – where the melting point changes as the major component increases its concentration in the melt.
A visible droplet of liquefied metal occurs when 10-20% of the impure metal has undergone microscopic melting. However, this percentage varies depending on the amounts of impurities present in the metal. The final value of the range is at the melting point of the pure metal; however, this value is often lower to account for the depressed melting point.
In summary, impurities in metals will generally lower and broaden the melting temperature range of metal. And the more the impurities, the larger the effects on the melting temperature.
Changing the Melting Point of a Metal
One of the most common methods used to lower the melting point of a metal is alloying. For instance, lead-based alloys and tin-based alloys have relatively lower melting points than the respective metals.
Alloys contain more than one element, as such; their melting point is a range that depends on their composition.
Besides lowering the melting point, alloys also have a high ductility than individual metals – which help bend the metal without breaking it. That’s key, as friction can be hard to control when it comes to metalwork.
The Importance of the Metal Melting Temperature
When heating a metal, it goes through various changes and achieves different yet important temperature points. The melting temperature of a metal is one of the most important of these temperatures.
The melting point is important because of the component failure (and the associated hazards) that can happen once a metal reaches its melting point. Note, some failure can occur before the metal reaches its melting temperature. For instance, creep-induced fractures can occur before the melting point is reached.
But when the metal begins to turn into liquid, it can cause even more catastrophic failures – let alone failing to serve its intended purpose. For example, if a furnace metal element begins to melt, the furnace won’t function as intended. Similarly, if the nozzle in the jet engine melts, the orifices will clog and sometimes render the engine useless.
Given that these failures can have catastrophic effects, it’s important to conduct thorough research on the effects of various temperature levels on a metal. Such research is particularly impotent for metals used in high-temperature applications.
The melting temperature is also used to tell pure from impure metals, as the latter have a range of melting temperatures instead of a defined melting temperature.
In addition, the melting temperature of a metal is important because most metals are formable when in the molten state. As such, metals are heated to their melting points for a myriad of manufacturing processes. For instance, smelting, casting, and fusion welding can only be performed when the metal is in a liquid state.
To that end, it’s important to know the melting point of the metal in question to prepare the appropriate materials and equipment.
For instance, when welding, the welding gun/rod selected must withstand the ambient heat of the molten metal and the electrical arc. Similarly, the casting equipment must have a higher melting point than those of the metals to be cast.
How to Melt the Metal
You can melt the metals through:
Brazing and welding
Welding involves melting different metal pieces together. Here, different metals are heated to their melting temperatures and joined together. As such, their molecules pool and mix completely. Welding is ideal for metals with similar or the same melting points.
Brazing is handy when joining metals with varying melting points. The process involves the use of a third-party metal (usually a brass-based alloy) to bond the base metals together. The third metal should have a lower melting temperature than the two base metals – otherwise, you may risk melting the base metals in the process.
Melting Points of Various Metals
To that end, here’s a tabulation of the melting points of the most common metals and alloys used by fabricators and welders.
|Metal||Melting Point (T(oC) = 5/9[T(oF) – 32]|
|Admiralty Brass||900 – 940||1650 – 1720|
|Aluminum Alloy||463 – 671||865 – 1240|
|Aluminum Bronze||600 – 655||1190 – 1215|
|Beryllium Copper||865 – 955||1587 – 1750|
|Gold, 24K Pure||1063||1945|
|Hastelloy C||1320 – 1350||2410 – 2460|
|Inconel||1390 – 1425||2540 – 2600|
|Incoloy||1390 – 1425||2540 – 2600|
|Iron, Wrought||1482 – 1593||2700 – 2900|
|Iron, Gray Cast||1127 – 1204||2060 – 2200|
|Magnesium Alloy||349 – 649||660 – 1200|
|Manganese bronze||865 – 890||1590 – 1630|
|Monel||1300 – 1350||2370 – 2460|
|Red Brass||990 – 1025||1810 – 1880|
|Carbon Steel||1425 – 1540||2600 – 2800|
|Yellow Brass||905 – 932||1660 – 1710|