Preheating in Welding: When and Why Is It Necessary?

Preheating is familiar to anyone who cooks or bakes, and it’s rare that a recipe doesn’t start with “preheat your oven to X temperature.” In welding, though? Preheating may seem like a more alien concept. Some welding operators go their whole careers never having needed to preheat a weld, while others preheat everything they handle.

So what is preheating, how does it work, why is it necessary, and when should you use it? Let’s answer these questions and more.

What is Preheating in Welding?

Preheating is exactly what it sounds like: heating your workpieces to a warmer than ambient temperature before actually welding them. There’s a bit more nuance to it than just that, though.

Preheating can be done on a specific area around a joint, or it can be done to the whole part. This is usually determined by the size of the part; you can’t exactly preheat an entire ship when you’re making a repair in the field, right? But smaller parts like joints in pipework and certain parts of construction can be preheated in their entirety.

What Does Preheating Do and Why is it Used?

One of the biggest challenges with welding is ensuring full penetration in a joint, without going overboard and melting through the workpiece. For most standard and thin materials, this isn’t terribly difficult, and every modern welding machine is perfectly capable of achieving a full penetration weld with little or no preparation.

What about for thicker materials, though? You often see welders that have limitations, and say they can’t weld more than a ¾” thick material, or something of that nature.

This is because of how welding works. Arc welding concentrates an electric arc on a small area where it short-circuits, and that imparts an immense amount of energy into a workpiece. That energy rapidly melts the material and, combined with a filler, fuses it all together as it cools.

There are two primary challenges when welding thicker materials. One of them is moisture. Moisture present in a workpiece will be blasted apart by the energy of welding. This can leave behind hydrogen, which can create voids in a weld and weaken it.

More commonly, though, the bigger problem is the temperature differential. Because of how physics works, adding heat generally causes a material to expand, and cooling it contracts it. Well, applying a large amount of heat to a small, concentrated area of a relatively inflexible material like steel – even mild steel, if it’s thick enough – expands the metal melted in the area. But, since the material is large, it works as a significant heat sink, cooling the material more rapidly than thinner materials cool. This causes the workpieces to contract, often away from the still-hotter joint, leaving cracks in the material around the joint and compromising the weld.

Preheating – and post-heating, in many cases – helps in two ways. It increases the overall base temperature of the entire workpiece (or a larger area of a massive piece) so the temperature differential isn’t as extreme between the baseline temperature and the peak welding temperature. And, since the overall piece is hotter, it can’t sink that heat as effectively, so it cools slower and has less of a chance of cracking.

Removing the possibility of flaws in the weld is the core goal of preheating. Flaws can be an aggravation in many kinds of fabrication, but for industrial pipes and pipelines, shipbuilding, construction, and other industries, flaws aren’t just an annoyance; they can be fatal. Therefore, any step that can be taken to minimize the chances of those flaws should be taken.

When Should You Use Preheating?

If you’re looking at a workpiece and you’re wondering whether or not you should preheat it, the simplest answer to the question is “check the WPS.” Your welding specifications should tell you if a part needs preheating, or if it’s not necessary.

If not WPS is available, it comes down to judgment.

Generally, certain factors mean you’re more likely to need a preheat.

• Usually, if a material is over half an inch thick at the joint (and is not otherwise prepared with beveling) it can benefit from preheating, and anything over 3/4th an inch almost always needs preheating for the best possible weld.
• If you’re welding a brittle material like cast iron, preheating is necessary to help avoid cracking.
• If there are code requirements that stipulate preheating when working with specific materials or equipment.
• Certain materials have metal chemistry that makes them react to heat or being melted in specific ways that preheating can minimize.
• If experience tells you that welding without preheating will leave you with cracks, preheating should be used.

You may also need to pay attention to ambient temperature. Welding basic steel can behave differently if you’re doing it when it’s a 100-degree day in the peak of summer, versus a -10-degree day in the depths of winter. That hundred degrees of difference makes a significant impact on the performance of a weld.

A common application of preheating is in repairs to high-carbon working parts on tools. Stamp dies, excavator shovel teeth, and other parts need an immense amount of durability to survive their work, but that also means they are susceptible to cracking when welded using normal procedures. Making repairs on these kinds of parts requires preheating.

Are There Different Methods of Preheating?

Nothing in life is ever simple, so of course there are multiple preheating methods.

First is open flame preheating. Open flame preheating uses something like an oxyfuel torch to apply a flame directly to the section of a workpiece you’re working on and the area around it. This can be used to preheat an entire small part, or just the section of a seam you’re welding. This is a simple and easy method that can be done with materials you already have in your shop.

Unfortunately, open flame heating suffers from several major drawbacks. In particular, it’s an uneven and inconsistent form of heating, and exposes operators to the fumes, soot, and gasses of the torch for long periods of time. This means it’s really only suitable for small welds or on small pieces.

Second is convection preheating. Convection preheating brings us back to our comment in the intro: you put a part in an oven and use the oven’s convection/air circulation to evenly heat the workpiece. This method has a benefit over torch heating in that it evenly heats the whole part, and an oven can fit many smaller parts in at once for batch heating.

There are several downsides to oven preheating. The first and most obvious is that you need an oven large enough and capable of reaching and sustaining the temperatures necessary. Not all shops have the space for this, and while personal use can make use of a basic residential oven, most industrial uses require more specialized – and more expensive – ovens. Ovens aren’t mobile, so this method won’t work in the field, and it can be quite time-consuming to preheat parts in an oven as well.

The third method is resistance preheating. Resistance heating works in a manner similar to something like a hotplate; ceramic pads with embedded resistance heating coils (similar to an electric oven’s heating elements) provide heat. The pads are placed or wrapped on a workpiece, so the heat is applied directly via conduction to the material being heated. Resistance preheating is faster than torch heating or oven heating, but can be expensive to purchase and configure. It’s best for large pieces where the whole workpiece – like ship panels – can’t be moved or placed in another device.

Unfortunately, resistance preheating can be slow to set up. It also doesn’t well contain the heat it puts out, so it can heat up an entire room or workspace, which can be both a comfort and a safety hazard. Resistance heaters are also fragile and can break, but can break in ways that aren’t noticeable until you realize an individual pad isn’t heating and leaves cool spots on your workpieces.

Finally, the fourth method is induction preheating. Induction uses a coil of conductive wire wrapped around a workpiece. Electricity flowing through that coil creates a magnetic field that heats the material within. It’s very fast to set up, extremely fast to heat, and very even with heating. And, since the induction tool itself doesn’t generate the heat, it’s not a hot safety hazard when it’s removed.

The biggest downside to induction heating is that it’s more expensive to purchase an induction rig and keep it maintained, as well as a slightly higher burden on operator training than just “put a part in an oven” would be.

As always, it’s also worth mentioning that there are variations with most of these methods. A residential oven might only get up to 400-500 degrees consistently, while an industrial oven can maintain higher temperatures for longer and will be better insulated for the workspace. Induction heating comes in a wide range of form factors, from small portable wraps to large in-place mechanisms. Finding the right method is just part one.

Which Method of Preheating is the Best?

There’s no right answer. Deciding between the methods depends on individual factors about your workshop, your resources, and your needs.

For example:

• Do you weld small parts in batches that need preheating, or require complete, thorough, even heating? Oven heating is probably the best.
• Are you on the move? Smaller induction systems can sometimes be portable, but torch heating is the most reliable in the field.
• Are you concerned with speed and reliability? Induction heating is excellent on virtually every front except whole-part heating and setup costs.
• Do you have specific use cases where other systems don’t work? Resistance heating can be valuable in ways the others cannot.

In fact, many workshops and larger facilities have multiple kinds of preheating apparatuses on hand. Knowing which one to use is almost as important as knowing whether or not you need to use it in the first place.

How Hot Should You Preheat Parts?

Much like everything else, the target heat you need for preheating will vary. For most standard mild steel applications, you want to preheat to around 300-400 degrees F, with a bare minimum of 225 and an upper bound of 450. Too cold or too hot at your preheating may not function properly.

Generally, preheating also needs to be sustained for a specific amount of time, otherwise it can cause the same issues that welding does, only slower. There are a variety of codes and conventions for calculating what your preheat temperature should be.

There’s also some discussion over whether you should measure the specific temperatures at all. Most of the time, the answer is yes. Some small-scale projects that can repeat welds can wing it, but that won’t fly for industrial applications.

Temperatures can be measured using laser and infrared thermometers, or products like Templistiks, which are consumable precision-engineered materials that only melt when a specific temperature is achieved, so you can test to know when you reach the right temperature.

Adding Preheating Gear to Your Shop

If you’re curious if preheating will benefit your work processes, or you know you need preheating and want to try out different methods, you have a lot of options. They can also be daunting, and it’s never a good idea to spend a bunch of money on a system you might not get much use out of.

That’s why we offer our welding equipment rentals. In addition to welders and torches, we also offer a variety of options for induction and resistance heating systems. That way, you can try before you buy! Rent a piece of equipment from us, try it out, and if you don’t like it, return it. If you do, our used equipment sales can sell it to you directly, and you’re good to go.

While some welding operators and workshops never need heating, being able to use preheating opens up a whole new world of more intensive, useful, and valuable fabrication and repair operations. Why not add that capacity to your shop today?