Welding Preheating Explained: Why, When, and How to Preheat Metal

Heat control is crucial in welding, especially in heat-sensitive, crack-prone, and thick pieces. Arc welding methods use the high heat of an arc to melt and fuse the pieces. However, that heat can sometimes cause thermal stress in pieces.

To reduce cooling rates, dry out excess hydrogen, or reduce the risk of cracking, welders preheat the pieces before welding. Welding preheating is a simple process of raising the temperature of the base metal before welding to improve its weldability and achieve the desired penetration.

In this article, we will thoroughly explain welding preheating, including why, when, and how to do it. So, stay tuned to learn more.

What is Preheat in Welding?

Preheating in welding is the process of heating the base metal, or the region around the joint, to a desired temperature before welding. The necessary heat is called the preheat temperature.

In multi-pass welding, welders can preheat the pieces after each pass, but the heat from the welding is often sufficient. However, if the welding specification calls for a specific interpass temperature (minimum temperature between passes), welders must maintain it.

Preheat Control in Multi-Pass Welding
Source: https://www.youtube.com/shorts/_-JIuUJ7-Bw

Heating the pieces before welding reduces the thermal stress caused by rapid cooling. It also dries out moisture and hydrogen, reducing the overall risk of cracking and aiding in penetration on extremely thick pieces.

As a result, preheating is a common practice in industries that involve heavy equipment manufacturing, maintenance, and repair. Common examples include oil and gas, structural construction, mining, and shipbuilding applications.

2500 DIA Industrial Pipe Pre-Heating Process
Source: https://www.youtube.com/shorts/3zmFZJNKkcQ

In general, the role of welding preheat is to reduce the risk of cracks and other defects while improving weld quality. However, as straightforward as it sounds, preheating takes knowledge and understanding. Unless you know why, when, and how to use it, you could be wasting time, money, or ruining the integrity of the welds.

Why Preheat the Welds?

Raising the temperature of the base metal before welding brings up several benefits, with the crucial ones being:

• Lowering the cooling rate: Rapid heating and cooling can create a brittle, hard microstructure prone to cracking. Slow cooling improves ductility, especially in steels with higher carbon content, reducing the risk of cracking.

• Limiting the hydrogen diffusion: Heating the base metal dries out moisture and hydrogen. Slow cooling allows harmless hydrogen diffusion out of the metal, preventing hydrogen-induced or hydrogen-assisted cracking.

• Reducing the shrinkage stress: When heated metals expand and contract. Rapid cooling can cause excessive shrinkage stress, especially in highly restrained joints. Slowing the cooling and reducing the heat difference between the joint and the pieces help address excessive shrinkage.

• Improving weldability: Raising the temperature of some steels can yield specific desired properties or exceed the temperature at which brittle fracture occurs.

When to Preheat the Pieces?

Preheating offers various benefits that improve weldability and weld quality, but only when used in the right applications. Successful preheating involves knowing when to use it, and these are the crucial factors:

• Welding code requirements (Welding Procedure Specification - WPS)
• Metal section thickness (including combined thickness)
• Base metal chemistry (particularly carbon content)
• Diffusable hydrogen (environment and filler metal)
• Internal and external joint restraint
• Ambient temperature and conditions
• Cracking problems in the past

Of all the mentioned, welding codes and WPS are a no-brainer. If the procedure specifications call for preheating, you will have to do it. In that case, the codes state the minimum preheat temperature you must achieve, regardless of other factors. However, you can increase the preheat temperature, but never go below the stated values.

If there are no codes, you will have to determine whether preheating is needed and the minimum preheat temperature. To determine the need, you will have to consider all other factors, including thickness, chemistry, restraint, hydrogen, and other conditions.

Base Metal Chemistry

Not all metals require preheating, and the primary reason is the base metal chemistry. The composition of the metal, along with its thermal properties, plays a crucial role, making it a nightmare to weld without preheating.

Welders commonly preheat the following metals:

• Medium and high carbon steel: As the content of carbon increases, so does the hardness. However, added carbon creates a hard martensite microstructure, which is brittle and prone to cracking. Preheating slows the cooling and improves ductility, making it crucial for steels with more than 0.40% carbon.

• Low alloy steels: A group of ferrous metals with low amounts of alloying elements (typically 1-5%, or less than 12%), designed for superior mechanical properties. Low-alloy, high-strength steels that contain chromium, molybdenum, or nickel often require preheating to improve weldability.

• Cast iron: With a carbon content of 2-4%, it cannot be welded without preheating. Low ductility and high carbon content make it prone to cracking during rapid heating and cooling, so preheating is paramount. However, even with proper heat control, cast iron welding can be hit-or-miss due to its properties.

• Metals with high thermal conductivity: Copper and aluminum are known for their high thermal conductivity. They quickly dissipate heat from the joint, so welding requires precise heat control. Preheating can help reduce distortion and achieve better results, especially in thicker sections.

Metal Thickness

In general, preheating is not required for mild steel (low carbon steel). However, as the metal thickness increases, the overall mass (the combined thickness of both pieces and the weld metal) increases as well.

As the mass of the weldment increases, so does the quenching effect. The metals become harder, but also more brittle when welded. To improve ductility and reduce the risk of cracks, welders preheat pieces thicker than 1 inch, even in weldable metals such as mild steel.

Preheating Before Welding Heavy Thick Steel Plate
Source: https://www.youtube.com/shorts/5F_Gspl3NaA

The preheat temperature will mostly depend on the combined thickness and metal properties. Combined thickness accounts for the thickness of the entire weldment (both pieces).

For example, welding 3/4 inch pieces in the corner, lap, edge, or butt joint means adding the thickness of both pieces (3/4 + 3/4"). In tee joints, configuration involves adding the thickness three times (3/4 + 3/4 + 3/4"), while in cruciform joints, adding the thickness four times.

Preheating pieces thicker than 1" improves penetration and fusion. But it also reduces the cooling rate, removes moisture, and reduces the risk of hydrogen cracking.

Diffusable Hydrogen

Excessive hydrogen in welded joints, especially in hard metals with higher carbon content, is a leading cause of cracks. During rapid cooling, the diffusion of atomic hydrogen into the metal lattice reduces its ductility. It creates pressure that develops cracks.

Hydrogen-assisted cracking is likely to occur once hydrogen levels exceed 15 mL per 100g of deposited weld metal. That's why manufacturers recommend low-hydrogen electrodes, such as H4 or H5 types, which contain ~5 mL/100g, for welding metals at higher risk of cracking.

However, hydrogen doesn't only come from electrodes. Inside the joint, hydrogen can get by:

• Moist fluxes or shielding gas
• Moisture in the surrounding atmosphere (welding in damp conditions)
• Surface contamination (oil, grease, or rust)

Preheating can be particularly helpful in these conditions. Raising the temperature of the pieces helps hydrogen escape the welds, removes condensation, and surface impurities. When paired with low-hydrogen consumables, preheating can be crucial in reducing the risk of hydrogen-assisted cracks, especially in hard metals with higher carbon content.

Low-Hydrogen Electrode Welding on Pipe
Source:  https://www.youtube.com/shorts/0bTd8InYND8

Joint Restraint

Metals expand and contract when heated, and that also applies to welding. However, welding often involves localized heating, especially on thicker parts, which can cause uneven expansion and contraction. Uneven expansion and contraction may cause hot cracking, cold cracking, or distortion.

The issue with joint restraint is commonly present in thicker sections, on high-strength steels, and specific joint configurations. Dealing with it requires a good fit-up, a measured configuration, an even gap across the joint, and preheating.

Preheating reduces the effect of uneven heating in the joint. Increased temperature reduces the thermal stress caused by localized heating. With other measures such as good fit-up and even gaps, preheating can help reduce the risk of joint restraint, cracking, or distortion, especially on hard, thick metal pieces.

Ambient Conditions

Many less experienced welders overlook environmental factors when welding. Temperature and moisture, among other factors, can have significant negative effects on your welding, and you may not realize it until it is too late.

Low temperatures can greatly affect your welding. Low temperatures reduce the metal's ductility and speed up cooling. 

Outdoor Stick Welding in Winter
Photo by @Kirsten Sophie (TikTok)

Each metal has a ductile-to-brittle transition temperature (DBTT). At 32°F (0°C), most steels become brittle, so it is often not recommended to weld below these temperatures. Rapid cooling can cause cracks even in mild steel.

Preheating is the easiest solution when welding in cold temperatures. Increasing the temperature of the pieces will reduce the stress that develops when the high-temperature arc strikes the surface. In addition, heating will dry the moisture as noted above.

Codes even demand preheat if the temperature is below 32°F (0°C). Check out our article to learn more about welding in winter.

Previous Cracking

If you have had issues with cracks in the past and have ruled out other factors, consider light preheating and observe the results. As noted, thermal properties of some hydrogen can get into your welds, and light preheating won't do much harm to joints.

It may solve the crack problems, but be careful not to overdo it. Inadequate preheat, especially on thinner and heat-sensitive pieces, can do more harm than good.

What is the Recommended Preheat Temperature?

Typical preheat temperatures range from 50°F to over 600°F. But the right temperature depends on many factors (metal type, composition, thickness, joint configuration, ambient temperature, etc.).

Codes specify some guidelines and minimum preheat temperatures. Here is a quick reference chart of minimum preheat temperatures for the most common steels:

These same rules apply to ASTM 1010, while ASTM A252 needs 225°F (107°C) when welding with low-hydrogen electrodes. If the base metal temperature is below 32°F (0°C), preheat to at least 70°F.

If you want an alternative calculation method, consult AWS D1.1-96, Annex XI: "Guideline on Alternative Methods for Determining Preheat". There are two methods:

• HAZ hardness control (applicable only to fillet welds)
• Hydrogen control method

These guidelines determine the required preheat temperature to prevent hydrogen cracking in steel welding. They use advanced calculations based on the Carbon Equivalent (CE) to determine the right preheat temperature:

CE = C + ((Mn + Si)/6) + ((Cr + Mo + V)/5) + ((Ni + Cu)/15)

The safest way is to follow the codes and manufacturers' recommendations. In some cases, manufacturers will publish information about preheat temperature. However, if that's unavailable, consult the AWS D1.1 Structural Welding Code tables, or use alternative calculations.

Welding Preheating Methods

Now that you understand why and when, it is time to answer the how. To preheat the pieces, you can use one or a combination of the following preheating methods:

• Open flame (heating the pieces with the torch)
• Induction heating (a magnetic field that causes induction)
• Resistance heating (electrical pads create resistance heat)
• Ovens (convection heating inside large ovens)

Welders choose the method based on applications and available equipment. Preheating can involve heating the entire weldment or just the joints. Let's further explain each method.

Open Flame Preheating

Open-flame preheating involves heating the pieces with a compressed-air torch and gas. As the name suggests, it creates an open flame that directly heats the metal surfaces.

Therefore, this is the simplest method you can likely use at home to preheat the pieces. The equipment is fairly straightforward, though you will need to buy gas.

Although straightforward, open-flame preheating is often inefficient for large-scale projects. It takes time, especially on very thick pieces, to reach the desired temperature. Also, it may cause uneven heating since it is performed manually.

Nonetheless, it remains the easiest way to preheat the pieces at home. With a bit of practice and safety, you can successfully reach the desired preheat temperatures.

Induction Preheating

Induction preheating uses a magnetic field to generate eddy currents in the base metal. Operators use cables or blankets to generate the magnetic field, which heats the pieces pretty quickly.

As a result, induction heating is much faster than open-flame heating and yields more even temperatures. Setup is quick, and it doesn't need personnel to watch it. In addition, it can maintain a strict temperature window.

However, the equipment is more complex compared to traditional open-flame heating. It also requires some operator training to reduce the risk of defects and overheating-induced warping.

Resistance Heating

Resistance preheating uses electrically heated ceramic pads. Heated tiles transfer radiant and conductive heat to the area where the pads contact the base metal.

Like induction, resistance heating is pretty fast, efficient, and precise in preheating. Pads can yield uniform temperature across the pieces, especially on larger parts and weldments, but they take time to arrange and secure.

Due to the complexity of the equipment, resistance heating is often performed by contractors. It adds additional costs, especially for consumables, as broken pads can lead to uneven temperatures.

Oven Heating

Ovens use convection heating to preheat the entire pieces. Welders place the pieces in the oven, and it yields even, desired temperatures across the entire weldment.

Since the oven stores entire parts, it is perfect for full piece preheat. A large oven can store multiple pieces at once, which is ideal for batch production.

However, remember that ovens can be quite large, especially for home welding or fabrication applications. They consume quite a lot of electricity, and can warm up the area around the oven to uncomfortable levels.

How To Check Preheat Temperature?

There are several methods you can use to check if you reached the desired preheat temperatures. The most common ways of checking the preheat temperature include:

• Temperature-indicating crayons (Tempilstik): The easiest method uses crayon sticks that melt at a specific temperature. You rub the surface, and if the temperature is right, they melt.
• Contact thermometers: Digital or analog thermometers that you place directly on the base metal to provide a precise reading.
• Infrared thermometers: Provide quick, non-contact measurements.
  

Common Ways to Check Welding Preheat Temperature
Source: https://www.youtube.com/@technoweldtraining6176/featured

In addition to the method, you must measure the temperature correctly. To measure the preheat or interpass temperature correctly, follow the basic guidelines:

• Measure the temperature at least 3 inches from the weld joint.
• Check the temperature immediately before starting the welding process.
• Make sure the preheat penetrated the entire thickness of the material.
• Maintain the temperature for a sufficient time (2 min per 1 inch of thickness).
  

Final Thoughts

Welding preheating is a straightforward yet fairly useful method. It includes heating the pieces to a specific temperature before welding to reduce the risk of cracks and improve weldability and weld quality.

However, as simple as it sounds, preheating requires a proper understanding of when, why, and how to use it. Crucial factors include metal properties, thickness, hydrogen presence, joint restraint, and environmental conditions.

Once you understand when to preheat the pieces, you will reduce the risk of cracks, improve weldability, and enhance weld quality in through-to-weld metals such as thick steel, medium- and high-carbon steels, cast iron, low-alloy steel, and aluminum and copper.

🧐 Welding Preheating Explained FAQ

1. When is preheating required in welding?

Preheating may be required when welding codes or WPS call for it. It is also commonly needed for thick sections, high-carbon steel, low-alloy steel, cast iron, restrained joints, cold weather, or jobs with previous cracking issues.

2. What temperature should metal be preheated to before welding?

The correct preheat temperature depends on the metal type, thickness, chemistry, joint design, ambient temperature, and welding code requirements. Common preheat temperatures can range from 50°F to over 600°F.

3. What are the common methods of welding preheating?

Common welding preheating methods include open flame heating, induction heating, resistance heating, and oven heating. The best method depends on the part size, job type, available equipment, and temperature control needs.

👏 You may be interested in the following:

• How To Weld Carbon Steel
• Low vs Medium vs High Carbon Steel: Applications and Differences
• Winter Welding Guide