What Is Preheat? It is heating the metal before welding it. When you are preheating metal, you can either heat the entire piece or just the region where you are working. Once you get the metal heated, the temperature from welding will typically maintain the heat. However, you can continue to apply heat while welding, if needed.
Welders are a resourceful group of people. When faced with an impossible task, the wrong equipment, and demanding bosses, they always find a way to push the envelope of their technology and get the job done.
Preheat enables a welder to “punch above his weight class” and to get a better weld in demanding scenarios that are requiring thicker metals.
When welding thick metals, 2 to 3 passes are commonly required. The metal must maintain at or above the preheat temperature from the time you begin welding to the time the last weld is completed (interpass temperature).
A maximum and minimum interpass temperature are generally recommended to ensure that each subsequent welding pass is as near the first one as possible. You want to minimize hydrogen cracking that can occur if the temps are too low and, of course, a maximum interpass temperature protects the microstructure of the metal.
Often, the preheat needs to continue for a while following the completion of the weld to allow hydrogen to escape. Preventing the escape of hydrogen would mean maintaining the preheat temperature throughout the welding process and then using postheat following the last bead.
Why Use Preheat?
Preheat minimizes the difference in temperature between the base material and the welding arc. The minimal difference translates into several benefits.
- Preheat is used to protect the metal’s integrity and to prevent brittleness.
- Slowing the cooling rate (postheat) allows the hydrogen to escape to prevent cracking.
- Reduce the risk of cracking and distortion from shrinkage as the weld cools.
- Raises certain metals above the point where brittle fracture would occur.
When Should Preheat Be Used?
There are a wide variety of scenarios where Preheat should be used. It is going to take some time and calculations (and possible referencing some cool charts) to get the most accurate answer.
If you are working with low-carbon steel that is under an inch thick, you probably won’t need to preheat.
There may also be code requirements for whether preheat is used. The thickness of the metal affects the requirements for buildings as well as meeting DOT standards for transportation projects.
The American Welding Society often sets these codes in their list of standards (https://www.aws.org/standards/). In these scenarios, you are going to match the ASTM metal type with a particular electrode rod, and the proper preheat and postheat.
How Much Preheat Do You Need?
1. Manufacturer’s recommendation
The first and easiest way to get a preheat recommendation for your job is to work off of the manufacturer’s recommendation. There are a couple of problems with this. The first is that the manufacturer’s recommendation may not be good enough in certain structural engineering settings.
Additionally, they may play it on the safe side and recommend a higher preheat than necessary. So you might be preheating to 300 degrees when 200 degrees would have been sufficient. Using unnecessary and extra heat adds costs to your project.
However, for the at-home mechanic who needs to get their job done, the manufacturer’s recommendation is the easiest way to figure out what temperature to preheat to.
2. Slide Rule Calculators
The good news about metallurgy is that it doesn’t change much. Sure, new allows are invented, but the bulk of our civilization still uses the same tried-and-true metals over and over.
Our tendency to stick to what we know makes welding a perfect candidate for slide rule calculators. These calculators take the standards that are in the AWS standards book and converts into an easy-to-use format for the shop.
Now, anyone can easily select their metal and welder type and get the information they need on preheat and postheat requirements.
One word of caution is that many folks are trying to offer online calculators and apps to fill the role of these old slide rules. The downside of modern online calculators and apps is that they don’t show the calculations used to figure out preheat suggestions.
There is also very little liability for them if they provide inaccurate calculations, and it results in project failure.
3. AWS D1.1 Methodology
There are two methods available in AWS D1.1 for determining your preheat. One is the heat-affected zone control method, and the other is the hydrogen control method.
The Heat Affected Zone Control method is great if you are doing fillet welding and know the carbon density of your steel. This methodology is focused on slowing the cooling process to prevent martensite from forming (which causes cold cracking). Find your carbon equivalent and use the Table in AWS D1.1.
The Hydrogen Control Method is another option. This methodology assumes that if we can keep the hydrogen level in the metal below a certain level, cracking will not occur. The downside of this methodology is that it often requires more preheat than is absolutely necessary.
4. Prequalified Welding Procedures
Similar to the slide rule method above, buying a prequalified welding procedure for your project allows you to copy a proven process and duplicate the results.
For shops who don’t have engineers on staff, this can be a considerable time- and money-saver.
The AWS welding code, section D1.1, clause 3, explains how these prequalified welding procedures can be used.
Keep in mind that this is more than just preheat requirements. It is also going to dictate welding positions, electrode type, bead thickness, joint configurations, amperage size, and more.
In most cases, these are going to be custom-created for each project by an engineer that your business hires.
These procedures are typically the slowest and most cumbersome way to perform a project. However, in some scenarios, this higher level of quality is required.
A Wide Range
With so many options, it only adds to the confusion. Each method may disagree with the next, so it is impossible to form a consensus. For the small weld shop where one-off projects are the norm, using preheat on thicker projects is relatively standard. For more extensive operations, several test pieces are typically done at different heats, and then the finished item is inspected before the process is finalized for the shop.
Best Way To Preheat Before Welding
So, how are you going to incorporate welding into your shop? Here are some easy ideas.
Additionally, use Torch Heating
Torch heating is the method commonly used by smaller shops. If you only need a couple hundred degrees of preheat, a torch preheat can do the job easily. And, once you get the material heated, it will often hold the temperature for the entire time you are welding.
A Propane + Oxygen setup is a swift way to heat the metal for your project. As a bonus, you can also use these same kits for acetylene work as well.
Electric Blanket Heat
Using strips of electric wires or electric blankets can allow you to heat larger pieces of metal and maintain that heat for an extended period. The sustained heat is handy on larger projects where multiple people might be working at once or where you need to heat a lot of metal to maintain integrity.
The BriskHeat blankets are an excellent choice for these types of projects.
Induction and Furnace Heat
Commercial applications may have a furnace that they can place their metal into or will invest the several thousand dollars required to get an induction heating unit (such as the Miller Proheat 35). These systems only make sense in an environment where a high volume of welding fabrication is being done.