Welding defects happen to even the best and most experienced welders. However, there are best practices you can follow to help you avoid them, and preheating is one of them.
What are the benefits of preheating before welding? It helps you achieve stronger welds by minimizing the likelihood of cracking, slowing down the cooling of the welded materials, as well as reducing the chance of shrinkage stress. It ensures that the weld has the mechanical properties that you want to achieve.
How does preheating before welding help you avoid defects and distortions?
Before we dive into the benefits, here’s a great video that explains why certain materials benefit from pre-heating (in this case steel) and why it is good practice.
The Benefits of Preheating Before Welding
The reason why you get these benefits are that you are bringing up the temperature of the base material. As such, preheating will help lessen the difference in temperatures between the base materials and the welding arc. This will result in:
1. Less Cracking and Distortion
Preheating the base material before you weld will lessen the shrinkage stress.
Weld shrinkage can happen to anybody, including experienced welders. This problem occurs when the base material expands and then contracts as you weld it. When you don’t preheat the metal you are going to weld, the big difference in temperatures will result in internal stress.
The weldment will attempt to normalize the temperatures between the arc weld and the metal. This will increase the likelihood of cracking and distortion.
Preheating will solve this problem because there is not much difference between your base material and your weld.
2. Slows Down the Cooling Rate of the Weld
When you preheat the base material properly, the finished weld will not cool as fast as when you weld a cold base material. When the weld cools slowly, more hydrogen escapes from the weld puddle, thus preventing cracking.
What’s more, preheating will make moisture evaporate from the weld area and help get rid of other contaminants. These contaminants are sources of hydrogen.
You also lessen the hardness in the heat-affected zone, which makes the weld more brittle. Additionally, your weld metal has more ductility, which refers to its ability to be bent or stretched without breaking.
Most materials you weld will have trace amounts of hydrogen, a substance that can cause your weld to fail. The worst part is that the cracking does not happen immediately after your welding job.
Hydrogen-induced cracking is the result of martensite formation. Martensite is brittle and very hard, and it usually forms when the steel parts you have just welded together cool very quickly.
3. Heating Preps the Base Material
When you preheat, you are effectively prepping the weld area, guaranteeing proper penetration. As such, thicker materials, or those that conduct heat fast will benefit from preheating.
Even when you are not using as much heat in the welding arc, your weld will still be strong because of the optimal penetration you get from preheating your material.
Furthermore, preheating also removes the moisture in the joint and surrounding areas. According to Alcotec, moisture is one of the sources of hydrogen.
The arc temperatures will break the water down and release hydrogen into your weld. In turn, the hydrogen makes your welded area more porous than how you prefer it to be.
When Should You Preheat?
Preheating is a type of thermal treatment that you may need to do in order to guarantee the right weld integrity. It can also remove moisture and unwanted elements from the base materials that you are working on.
While preheating is recommended for some welding jobs because of the benefits outlined above, there are instances when it is an absolute must.
There are times when heat treatments like preheating are required, such as when you are working with heavy sections of low-alloy steel. Preheating is a necessary precaution when you work certain metals such as cast iron, high carbon steel, and other materials that are more brittle. Preheating these metals will help them become more malleable as you weld them.
You should also preheat when you have highly restrained weld joints, which are more prone to shrinkage stresses.
Not Sure if Preheating is Needed?
Preheating adds to your operating costs, and you can save some money by skipping this process. However, there are some instances when preheating might be necessary. Such instances include:
1. If you have a base material that is at least one-inch thick.
2. If the material you are using has been stored outside in the colder months of the year.
3. If you are welding in an outdoor setting and the temperatures are below freezing point.
4. If the base material you are using contains at least 30 points of carbon.
If these conditions apply to you or to the base material that you are working with, consult the welding engineer, steel supplier, or the production manager to make sure that a preheating is necessary.
How Do You Preheat Your Base Materials Before Welding?
Preheating is the process of applying heat to the base material to achieve a specific temperature which is recommended for the particular metal that you are welding.
Welders need to understand that preheating is not just making the joint hotter and then letting the arc weld do the rest. You should attain the required preheat temperature within three inches of the joint. The entire area should have a minimum temperature that is equal to or more than the desired preheat temperature.
There are three categories of preheating methods: torch, induction, and electrical resistance heating.
Which one is the best method? The answer depends on the thickness of the material you are working with, the weldment size, and the heating equipment that is available to you.
For instance, if you have smaller pieces to weld together, using the torch method is preferable. Bigger pieces, however, will do better with radiant or induction heaters, strip heaters, or electric torches.
Here is a video of a crew torch pre-heating steel.
You can use torch heating if you need something portable. It is like using a large barbecue lighter to heat up your base material. Some examples of materials used for this method are gas burners, furnaces, and oxy-gas flames.
This method is affordable and easy to do. However, it is not accurate when used as a preheating method. You can easily overshoot and make the steel hotter than you need it to be.
Likewise, you can quickly lose heat to the surrounding air. What’s more, you will need to move the torch a lot, and it may not even heat a thicker metal all the way through.
More than being inefficient, torch heating may be dangerous because you run a higher risk of suffering from burns.
If you need a more controlled preheat temperature, then you can try induction heating or electrical resistance heating.
In induction heating, you will need more expensive equipment than what is required in electrical resistance heating, but you will get to your preheat temperature a whole lot faster.
With induction heating, you can use an air-cooled blanket or a liquid-cooled braided hose. Wrap either of these around your base material, and it will create a magnetic field that can heat the metal from the center to the surface. Induction heating can handle thicker metals better than torch heating.
This method of heating is increasingly becoming more popular because of the lower cost of the consumables you use. It also offers better heat quality and uniform heating. It’s also reliable and easy to do.
Electrical Resistance Heating
If you have a big welding job and would like a more controlled preheating like what you get in induction heating, then you should definitely use electrical resistance heating.
This method can heat up to three welding joints simultaneously, as opposed to induction heating, which only allows you to work on one welding joint. Instead of using a magnetic field, heat is conducted all throughout the metal, making it hot.
What’s more, the equipment you use for electrical resistance heating is much more straightforward and cheaper than the one you use for induction heating.
More Tips When Preheating Before Welding
There are other things to remember when you are preheating your base material. For some metals, such as carbon steel, you don’t need to be accurate with your preheating as you only need to reach the minimum temperature. You can exceed the recommended preheat temperature by 100 degrees Fahrenheit.
In a few cases, such as for quenched and tempered steels, however, you will need to follow the recommended minimum and maximum preheat temperatures. If you overheat these types of steel, you may end up with a more cracking than desired.
When you are heating a joint that you are going to weld, you should bring the entire area to the recommended preheat temperatures. This area is the equivalent of the thickest member, but it should be three inches or more in all directions from the end of the joints.
Heat the side opposite of the one that you will be welding and take temperature measurements next to the joint. You do this to ensure that the base material is sufficiently heated all throughout its entire volume.
You should also check the temperature of the base material just before you start welding to make sure that the preheat temperature has been achieved.
In addition, remember that heat should be applied uniformly to the required area. Uneven preheating will not effectively slow down the cooling process and can even lead to more residual stress and distortion. To learn more about this, check out the article by Alcotec here.
Recommended Preheating Temperatures for Various Metals
There are several factors that you should consider in order to know how hot you should preheat a particular metal, such as the kind of base material you have, the hydrogen content, the heat inputs, and the plate thickness.
There are several methods that you can use to get the preheat temperatures for your metals, including:
- The manufacturer’s recommendations
- The American Welding Society’s recommendations
- Slide rule preheat calculator
Most manufacturers of base materials publish their preheat temperature recommendations. However, some companies tend to play on the safe side and put a much higher preheat temperature than necessary.
For instance, they might recommend preheating at 300 degrees Fahrenheit, when only 200 degrees would do. This will increase your operating costs unnecessarily.
AWS D1.1 Recommendations
The American Welding Society has a list of preheat temperatures and other temperature charts for steel. However, not all steels are listed in the book, which you will have to buy (check the latest price on Amazon.com below).
Slide Rule Preheat Calculators
Another easy way to determine preheat temperatures is the slide rule preheat calculators, such as the one here.
One problem is that you will need to know the chemical makeup of the metal that you are working on and that they are sometimes very specific to the kind of process you are using. For instance, the Lincoln Electric Welding Preheat Temperature Calculator only works with low hydrogen processes.
As these methods often give you a conservative preheat temperature, you will need to use it in conjunction with other calculations if you want to save on your welding costs.
The AWS gives you two other ways: the hydrogen control method and the heat-affected zone control method. Both of these methods are explained in detail in the AWS D1.1 codebook (check the latest price on Amazon.com HERE).
Furthermore, there are online calculators, such as these, that you can use to determine the preheat temperatures of low alloy and non-alloy steels. For beginners out there who do not have the AWS D1.1 codebook, or have no other way to calculate the recommended preheat temperatures, this table might help.
Monitoring the Preheating Temperatures
You should keep track of the temperature of your base materials when you are preheating. There are several tools that you can use to do this.
Temperature indicating crayons can tell you whether you have reached a specific minimum temperature. Each of these crayons has a rated temperature from 100 degrees to 2,500 degrees Fahrenheit.
You just draw a line using the appropriate crayon that is rated for the temperature you want to achieve. For instance, if you’re going to preheat your metal to 149 degrees, you just select the crayon that is rated for 149 degrees and draw a line on the metal. The crayon will melt when the base material gets to your desired preheat temperature.
You can also use these crayons for determining if you are within a certain temperature range. These products are a quick way to evaluate temperature and can be used for other heating treatments. Aside from crayons, some variations work on the same principles, including indicator cards, pellets, and paints.
If you are worried about contaminants, then you should know that these products are carefully formulated so that they do not use elements such as bismuth, lead, tin, or other metals that have a low melting point that could form alloys on the material.
Another way to measure temperature is to use a contact thermometer, which you can touch to the metal to know how hot its surface is. This type of thermometer usually provides a reading that is accurate within one degree.
You can also use infrared thermometers for the job. You can get temperature readings without the thermometer touching the hot metal. You only point and pull the trigger.
However, compared to contact thermometers, infrared thermometers are less accurate. You also have to make sure that you measure from the correct distance.
Precautions and Tips When Preheating Your Base Materials
There are some things that you should remember when you are preheating.
- When preheating, be sure to follow the guidelines that the metal manufacturer provides you. These guidelines will save you from any potential problem later on.
- When you are using a torch for preheating, you should be careful to avoid getting burned. You should also make sure that you are holding the torch from within the right distance of the joint, which varies depending on the base material and welding procedures that you are using.
- Opt to preheat a larger area to make sure that the desired temperature is kept throughout the welding process. This tactic will also help ensure that the preheated area will not cool quickly.
- Verify the preheat temperature with one of the heat-measuring tools mentioned above right before you start welding.
- Sometimes preheating may be complemented with the use of low-hydrogen filler metals. This helps to control the amount of hydrogen in the weld and help you further reduce the risk of cracking and distortion.
Preheating Before Welding: Summing It All Up
For some welding jobs, preheating is a crucial preparation that can help you avoid defects, cracks, and distortions. It is essential for a variety of reasons, including allowing more moisture to dissipate from the material thereby lessening the chances of hydrogen-induced cracking. It also forces the material to cool down slower and to reduce shrinking stresses.
It is a rather straightforward process, but one that will require several pieces of equipment to heat the base material to the tools you use to monitor the temperature. But it is well worth it if it means a welding job that is closer to perfect.