Preheating in Welding: What Is It, When Should it Be Done, and How Is It Done?

All professional welders know that preheating metals is critical for industrial projects. Does the same rule apply to humble at-home DIY enthusiasts?

The choice to preheat your materials will depend on several factors. Remember to keep in mind the welded design and the quality of your metals.

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What is Preheating in Welding?

Much like preheating your kitchen oven to bake a fresh batch of cookies, preheating your metals before welding is vital to a successful project!

Preheating is done in many ways. A welder can choose to apply a specified amount of heat only around a weld joint or apply it to the entire area.

Preheating in welding lowers the cooling rate of the joint and the metal to prevent moisture from entering. Moisture and other contaminants can eventually lead a weak bond prone to cracking.

Why preheating is done in welding?

Effectively reducing the area’s cooling rate before welding will ensure a high-quality and robust joint that resists cracking. This also prevents the material from shrinking quickly after heat is applied.

When is preheating necessary?

Preheating in welding is usually applied in the industrial and construction industry. It ensures a strong weld in shop and field welding, power plants, mining, structural construction, pipelines, and other heavy equipment.

You will need to consider a few factors to determine if your material needs to be preheated. Two of the most important factors to consider are:

Base metal type
Base metal thickness

A welding code will specify the preheat requirements for welding based on your project specifications. The preheat requirements include:

the min. and max. preheat temperatur
how long preheating should be done

Before welding begins, operators need to monitor the metal’s temperature closely. The weld must remain within the temperature range.
Temperature can be measured using various tools such as thermal imaging, infrared thermometers, crayons, and thermocouples.

General rules for preheating metal

All welders should be familiar with the general rules of preheating metals.
The factors to consider are: metal grades and thickness.

The weld thickness rule of thumb for preheating is that low-grade steel that is an inch or less does not need to be preheated. These metals should at least be at room temperature.

You might need to consider preheating if you are working outdoors with low temperatures. You will also need to let metals adjust to the room when transferring sites.

For metals with higher carbon content and is more than 1 inch, preheating should be considered.

Tables are easily available online to compute for standard preheating measures.

Generally, high-grade carbon steel should be preheated at max. 400°F.
Alloy steels should be preheated at 250°F.

What are the methods of preheating?

Preheating in welding can be done in multiple ways. The best method for your specific welding application will depend on:

quality and thickness of the material
Project scale
Project deadline and budget
Personnel skill level

Here are the most common preheating methods:

1. Open Flame

This method involves directly applying heat to the metal. A flame is produced by a compressed air torch that uses a gas fuel.

The most common preheating method in welding shops is open flame preheating using a propane torch.

Advantages

This method is budget-friendly and very easy to use. The only equipment needed is the torch, which requires minimal training.

Disadvantage

A disadvantage of open flame preheating is difficulty controlling the temperature. Having an open flame will involve inconsistent temperature application.

This temperature inconsistency definitely poses an issue with efficiency. This will mean a much slower preheating time.

The lack of control in the torch also poses several safety concerns. The process also produces a considerable amount of smoke and soot, which may damage respiratory health.

Extra expenses for open flame preheating will include the cost for gas fuel (about 40 to 60 USD worth of gas an hour), storage, and distribution to welding cells. It will also be best to hire personnel that can handle any fire-related accidents and emergencies.

2. Induction

This method uses a magnetic field to generate currents within a base metal. This means that heat is created internally.

Tools for induction preheating include blankets and cables that are placed directly on the base metal to form a magnetic field.

Advantages

This method is incredibly hassle-free and produces fast results. If you choose to go with an induction blanket, simply cover the metal, and the area will be heated in a few minutes!

Induction preheating also ensures a uniform distribution of heat. This gives the operator considerable control of the temperature.

The availability of different induction tools also provides welders with the flexibility to work on various material sizes and parts. It is also extremely easy to localize the heating area.

The process is generally safe and does not require any safety personnel on standby. Output cords and blankets generally do not create an uncomfortable and hot environment for operators.

Disadvantage

One of the disadvantages of induction preheating is the initial cost. Induction tools may be expensive. To fully maximize your induction equipment, some operator training may be required.

3. Ovens

This method works similarly to your regular kitchen oven.

The entire metal to be preheated is put inside the oven. This method uses convection heating.

Advantages

Since the whole metal is placed in the oven, this method can guarantee uniform heat distribution.

This method is the most efficient if you need to heat the entire part or when you need to heat smaller parts in batches.

Disadvantage

Ovens are incredibly bulky and need a power outlet. You can usually find ovens installed in a permanent station because they offer little mobility.

Since this is the case, welders will need to bring the metal parts to the oven. This can become a problem when massive parts are to be welded.

A transport system will be required to move large metal pieces. These will also require some fuel or an electrical outlet, which will hike up your project expenses.

Using an oven is not very time-efficient because you will also need to preheat the oven for a few hours before it can generate consistent heat. This also raises the ambient temperature considerably, affecting worker discomfort.

Most welding projects will outsource ovens for preheating purposes. This means more expenses and less control over project schedules and deadlines.

4. Resistance heating

This method uses ceramic pads that are powered by electricity.

The heated pads are placed directly on the metal. Radiant and conductive heat is effectively transferred to the metal in contact.

Advantages

This method produces a uniform and consistent temperature distribution. Operators should be careful to ensure that the heating pad is fully-functional.

Resistance heating is also ideal for industrial projects and large metal parts.

Disadvantage

Preheating through resistance heating can be costly. Aside from the resistance heating pads, operators will also need to invest in insulation equipment to retain heat.

Projects should also expect high electrical charges for resistance preheating. It will be best to keep a professional electrical on-duty to keep an eye on the power source and additional maintenance supervision.

Since this preheating method is commonly used in industrial settings, operators will usually hire a third-party company to take charge of the schedule and timeline.

If work environments are not appropriately managed, highly sensitive resistance heating and insulation equipment can easily be damaged. Broken heating pads will result in an inconsistently heated metal.

Aside from this, resistance heating poses safety hazards because the equipment can get dangerously hot. It also affects ambient temperature, which can be uncomfortable for the workers.

This preheating method is also time-consuming because of all the set-up preparations and removal involved.