Guide: How a Material’s Melting Point Can Affect Your Welds

Controlling heat is one of the most important elements of welding. Whether you’re using a direct heat application technique like oxyfuel welding or the more common and popular arc welding, you need to know how much heat you’re putting into a material and what that heat is doing. 

When heat is applied to a metal, that heat does many things. First, it adds energy to the molecules in the metal, which gets them moving and can break them free of crystalline lattices or other metallic structures at an atomic level. With even more heat, it changes their state from solid to liquid. Heat beyond that can burn or vaporize the material. 

All the while, you have to be aware of the thermal conductivity of the material, and how much the heat spreads, as well as any molecular or chemical changes that can happen when heat is applied. Heat can do many things, including cause warping and distortion, make the material more susceptible to chemical changes, and more. 

So, how does the melting point of a material affect your welding? Let’s dig deeper into the subject and discuss what you need to know to be a talented welding operator.

What is the Melting Point?

In physics, the melting point is the temperature at which a material transforms from a solid into a liquid. This is known as a phase transition, and similar transitions exist for turning from a liquid into a gas and even beyond that point and into plasma for materials capable of becoming plasma. Some materials also don’t have melting points; a common example is carbon dioxide. When solid, CO2 is “dry ice,” but dry ice does not melt into a liquid. Instead, it sublimates directly from solid to gas. Liquid CO2 does exist but can only exist at high pressures, so solid CO2 never melts into a liquid unless it’s already in a very high-pressure environment.

The melting point of a material is an attribute of that material determined by the chemical and molecular structure of the atoms that make it up. It’s also directly related to the material’s ability to conduct thermal energy, and the rate at which it expands when exposed to heat. All of these factors combined determine the appropriate uses of the material.

We’re used to thinking of the melting point of a metal as a very high temperature. Indeed, for many metals, it is. However, other metals are liquid at much lower temperatures. The most obvious example is mercury, which is liquid at room temperature because its melting point is -38 degrees F. 

Mercury also demonstrates the correlation between melting point and thermal expansion. A metal expands when it is heated, but the amount it expands relates to its melting point. Mercury is very sensitive to temperatures because of its low melting point. That’s why, contained in a narrow tube, it can even be used to measure temperature, which is how mercury thermometers work.

What Does the Melting Point Affect?

The melting point in a metal impacts pretty much everything about that metal.

For example, the melting point is a critical component of the alloying process. An alloy is a combination of more than one metal, combined, while the metal or metals are liquid. The addition of a secondary element changes the properties of the base material. For example, tungsten is strong, but the addition of carbon to create tungsten carbine makes it extremely strong and useful in cutting tools. Similarly, carbon added to iron is an extremely common method used to produce varieties of steel, which we’re all familiar with in welding.

The melting point of a material can also substantially impact how it is used and how products made out of it are manufactured. If you need a metal part, do you cast the metal, do you cut and grind it into shape, do you weld pieces together to form it? The properties of the metal – and the desired final structure – define which methods are usable.

Many industries also require materials that can resist extreme heat. If a material has a melting point too low, that heat will melt the material, making it useless to manage or contain that heat. In some cases, however, this can even be an intentional safety feature; a material with a lower melting point can be used as, effectively, an intentional breakpoint to tactically shut down operations in a way that saves much worse failure. Electrical fuses, which conduct electricity at a fixed amperage or below but melt and break when a higher amperage goes through them, are a primary example.

One thing to bear in mind, however, is that the melting point is not also the failure point of a material. Simply applying enough heat to soften a metal can cause catastrophic failure, even if it’s not enough to fully melt the metal. The online meme of “jet fuel can’t melt steel beams” comes from this: in the events of 9/11, burning jet fuel applied heat to the steel structure of the World Trade Center. This heat was not enough to melt the steel – or rather, it can give off enough heat but not in a concentrated enough area – but it was enough to soften the metal to the point of collapse and the ensuing tragedy.

What are Common Melting Points?

Above, we mentioned the metal with the lowest melting point, mercury, but you’re not exactly welding mercury.

What are the melting points of metals you may commonly be welding?

• Aluminum is 660 degrees C
• Bronze is 913 degrees C
• Copper is 1084 degrees C
• Steel ranges from 1371 degrees C up to 1540 degrees C
• Wrought iron melts around 1482 degrees C
• Stainless steel starts at 1400 degrees C and higher
• Titanium melts around 1670 degrees C
• Tungsten melts around 3400 degrees C

When you consider that the arc in arc welding can turn these metals to liquid in a literal flash, you can see how incredible the forces are behind electricity.

This is also one reason why you can’t weld together disparate materials. If you have two kinds of steel you can generally fuse them together, but the melting point difference between steel and aluminum is so great that by the time your steel is melting, your aluminum is completely burned through.

HAZ and Distortion

Melting point, as mentioned above, is related directly to the thermal conductivity and thermal expansion of a material.

When you apply heat to an area of metal, that heat spreads throughout the metal according to its thermal conductivity. The greater the conductivity, the larger the area around where you’re applying heat will end up heating up. Since the area of application of arc welding is very small – just where the arc is touching the metal – the heat affected zone is often rather small as well.

What this means is that you end up with a higher temperature gradient between where you’re welding and the rest of the material. It’s hot enough to melt right where the torch is working, but inches away, it can be significantly cooler.

One of the biggest challenges in welding, particularly when you’re welding thin materials, is the distortion caused by the heat affected zone and the thermal gradient.

Imagine for a moment that you have a piece of paper flat on a surface. One end of the paper is weighed down. On the other, you slide towards the fixed end. What happens? The paper bows up or even crumples, depending on the force applied. In this metaphor, the force holding one end is fixed by the molecular structure of the metal and the colder temperatures. The force moving the other end is the heat, expanding the metal towards the colder side. The way the paper flexes is distorted.

This is why, when you weld thinner materials with too much heat, the material can twist and warp. This distortion can damage or ruin the project you’re working on, and it’s all a physical consequence of how heat interacts with metal.

How do you handle these situations?

• Apply heat more slowly. Controlling the heat you put in with techniques like pulsed arc welding and spot welding, using lower amperages, or even using a lower-heat technique like oxyfuel welding allows you to control the heat-affected zone and reduce warping.
• Preheat the material. The problem isn’t necessarily the heat itself; it’s the difference in temperature between where you’re applying heat and the rest of the material. If you preheat the entire workpiece to bring it closer to the melting point, applying a bit of heat to bring your joint the rest of the way won’t create as much of a gradient and won’t cause the same distortion.
• Use a backer. When welding materials that are subject to easy distortion, attaching a heat sink backer can help absorb excess heat and prevent the workpiece from distorting.

Learning when and how to use various techniques like these can dramatically reduce the frequency of lost or damaged products when welding.

Heat and Chemical Reactions

One other issue with welding – and a reason why some kind of shielding gas is commonly used in welding – is chemical reactions. We’re used to thinking of metals as largely inert and subject mostly to issues like oxidation causing rust, which can be protected against with coatings and treatments.

However, these kinds of chemical reactions are easier to perform in higher-temperature environments. 

• Nitrogen can react with metals when the metals are molten to create nitrides. Nitrides are extremely hard but brittle; they’re often used in tools but when used in something like steel, can cause catastrophic failure under load. Nitrogen won’t do anything to steel under standard atmospheric temperature and pressure, but under the high heat of welding, it’s more likely to cause a reaction.
• Oxygen causes rust in iron. Iron oxides can form very quickly when oxygen contacts susceptible iron, and form inclusions that reduce the strength of a weld joint. Oxidation is also what causes color changes in metals around the heat affected zone, though this can be largely cosmetic when handled properly.
• Hydrogen is also abundant and, when a metal is hot, can be diffused into the molten metal. The hydrogen doesn’t react with the metal, but it does form pockets and weaken the joint, leading to cracking and brittle welds.

Contrast this with inert gasses like helium and argon, and you can see why those gasses are used to shield a molten weld pool from the atmosphere around it. 

Avoiding Temperature-Related Problems in Welding

There’s no easy rule of thumb you can use to avoid all temperature-related problems in welding. Most of it is simply experience. Over time, you build an awareness of how metals react in what ways and at what temperatures.

One of the best things you can do is make use of modern welding equipment. For example, many modern welders have computerized settings that pulse the electricity they use between a higher and a lower amperage. This applies high energy to the peaks of the pulses and low energy to the troughs of the pulses at a frequency of many times per second. The result is that you still apply enough current to melt and weld properly, but you have more leeway and less risk of going overboard and applying too much heat.

If you’re interested in seeing how these advanced modern welding systems work, we can help you out. Here at Red-D-Arc, we’re always on the cutting edge of welding equipment. Our array of rental welding equipment has many of these modern features! You can rent a welder and give it a try, get a feel for these features, and see if they suit your needs. If they don’t, you can return the welder and try something different. If they do, you can continue to rent it for as long as you need, or you can swing by our used equipment sales and purchase one for yourself.

If you have any questions, please don’t hesitate to reach out. Our trained and experienced staff are more than happy to answer any questions you may have and help guide you to the perfect equipment.