In simple terms, the melting point of steel for example, is the temperature when it starts to change from a solid state to a liquid. At this specific melting point temperature both phases of the substance exist in equilibrium. Therefore, further heating needs to be passed through the substance to complete the transition from solid to liquid. The melting point is also affected by pressure, acquiring lower values as the latter increases. So, a solid substance (let us consider the melting point of steel again) will liquefy more easily and rapidly when heated under greater pressures. This happens because pressurisation promotes and enhances the molecular movement and subsequent instability that characterises the liquid state of a substance.
To maintain a common ground in technical communication, nominal melting points always correspond to 1 atmosphere of pressure (100 kPa).
Molecular Symmetry
People realised early on that there was a link between melting points and pressure. Thomas Carnelley successfully associated molecular symmetry with a higher melting point after examining many thousands of different chemical compounds back in 1882. This is because symmetrical structures can distribute movement forces to more adjacent knots and feature higher forces of attraction in general. Fast forward to nowadays, we are able to predict the melting point of steel alloys that were never actually tested before. These predictions are based on data sets. These correspond to the molecular structure of the materials and how the amplitude of thermal vibrations affect it as the temperature rises. Another thing that can affect the melting point of steel alloys, for example, and other metals are impurities found in them.
Why is the Melting Point Important?
For steelmakers and metallurgists, the melting point and the range are important figures to consider. They determine the process of forging, annealing (heat treating), and heat forming. For designers and other engineering disciplines the melting point has little value. The structural integrity of the piece will be compromised way before reaching the melting point. This is because tensile strength and rigidness are adversely affected as the temperature rises.
However, all engineers can use the melting point of steel range as a test to figure out whether a steel beam, for example, is pure and up to what point. Because impurities cause molecular structure defects, bad quality steels tend to demonstrate a wide melting range. Pure steels on the other hand feature tighter melting ranges, something that is easy to observe and evaluate.
The Twin Towers
Structural engineers and conspiracy theorists have spent much of their time looking for the melting point of the steel beams used in the twin towers. Many believed that the melting point could also be used in forensics, to prove or disprove conspiracy theories. In this case, it was the ASTM A36 structural steel, which has a melting point of 1510 OC (2750 OF). If you’re looking for the jet fuel flame temperature, it’s about 1000 OC, not enough to melt the steel but enough to weaken it substantially.
Alloy Melting Points
Steel is an alloy of iron and carbon formed as such through the process of smelting. Through experimentation to create something superior/more specialised, we have joined steel with other elements such as:-
- Nickel
- Manganese for strength
- Chromium and tungsten or titanium for hardness
- Vanadium for resistance to fatigue
- Molybdenum and chromium for corrosion resistance.
Mixing these elements affects the various physical and mechanical performance properties of steel alloys – the melting point is no exception to that. Engineers like to use crystalline structure characterisations like austenitic, martensitic, and ferritic, so as to deduce part of the properties of the steel alloy directly from them.
Carbon Content
In general, the higher the carbon content is in a steel alloy, the lower its melting point. This is because the more carbon molecules covalently bonded to those of the iron the more modified the electric fields in the atomic level become. This affects the orientation and thus the molecular structure which gets less symmetrical. As a consequence the inter-molecular forces weaken, resulting in lower melting points. We can assume that the same applies to all alloying elements mentioned above. As reflected by the median melting points of “low” alloy steel which is 1436 OC (2610 OF) and high allow steel which is 1415 OC (2600 OF).
On its own, pure iron (Fe) has a melting point of 1535 OC, so alloying it decreases its melting range as explained above. Chromium and molybdenum are two of the few exceptions, as their presence actually increases the melting temperature of alloy steel. However, this still depends on many other factors, so it’s not definite.
Steel Alloys Melting Range Table
Melting ranges of the most widely used steel alloys in Celsius and Fahrenheit.
| Steel Grade (SAE) | UNS Designation | Alloy Type | Melting Range °C | Melting Range °F |
| 201 | S20100 | Nickel steel | 1400 – 1450 | 2552 – 2642 |
| 254 | S31254 | Nickel steel | 1325 – 1400 | 2417 – 2552 |
| 301 | S30100 | Nickel-Chromium steel | 1400 – 1420 | 2552 – 2588 |
| 304 | S30400 | Nickel-Chromium steel | 1400 – 1450 | 2552 – 2642 |
| 305 | S30500 | Nickel-Chromium steel | 1400 – 1450 | 2552 – 2642 |
| 309 | S30900 | Nickel-Chromium steel | 1400 – 1450 | 2552 – 2642 |
| 310 | S31000 | Nickel-Chromium steel | 1400 – 1450 | 2552 – 2642 |
| 316 | S31600 | Nickel-Chromium steel | 1375 – 1400 | 2507 – 2552 |
| 321 | S32100 | Nickel-Chromium steel | 1400 – 1425 | 2552 – 2597 |
| 330 | N08330 | Nickel-Chromium steel | 1400 – 1425 | 2552 – 2597 |
| 347 | S34700 | Nickel-Chromium steel | 1400 – 1425 | 2552 – 2597 |
| 410 | S41000 | Chromium-Molybdenum steel | 1480 – 1530 | 2696 – 2786 |
| 416 | S41600 | Chromium-Molybdenum steel | 1480 – 1530 | 2696 – 2786 |
| 420 | S42000 | Molybdenum steel | 1450 – 1510 | 2642 – 2750 |
| 430 | S43000 | Nickel-Chromium-Molybdenum steel | 1425 – 1510 | 2597 – 2750 |
| 434 | S43400 | Nickel-Chromium-Molybdenum steel | 1426 – 1510 | 2600 – 2750 |
| 440 | S44000 | Molybdenum steel | 1370 – 1480 | 2498 – 2696 |
| 446 | S44600 | Molybdenum steel | 1425 – 1510 | 2597 – 2750 |
