Have you heard of the MS Carolinian? She was the first entirely welded U.S. motor ship built in the 1930s. Instead of using 85,000 pounds of rivets like other ships of that time, it took only 8,000 pounds of welding wire and nine welders to assemble its hull. This new welding technology saved one-quarter of the cost and created a strong ship that stayed in service for over 50 years.
Today, we take welding almost for granted. It is an essential process in the manufacturing and construction industries. Preheat welding techniques are one of the best ways to maximize weld strength. Preheating steel before welding means heating the base metal to a specific temperature before fusing the metals.
This article will explore the importance of weld preheat, what it is, and common mistakes to avoid before preheating in welding.
What is Weld Preheat?
Weld preheat is a process that applies heat to the weld area to a specific temperature before welding. The type of base metal, thickness, and welding process determines the preheat temperature.
What is The Purpose of Preheating?
Weld preheating maximizes weld strength, especially when welding high-strength steel. The purpose of preheating includes:
Reduces Thermal Stress – Preheating the base metal before welding helps reduce the temperature between the base metal and the weld metal. It reduces the metal’s cooling rate, called thermal shock. Thermal shock creates hard and brittle areas in the weld joint that can cause the weld to crack or shatter.
Slows Heat Loss – Thick metals like carbon steel, aluminum, and copper have high thermal conductivity. Without preheating, the weld has less time to fuse.
Decreases Hydrogen Cracking – Moisture from coatings or flux increases the chance of hydrogen entering the weld pool. Preheating allows hydrogen to escape.
Reduces Porosity – Preheating evaporates residual moisture. Water molecules contain two atoms of hydrogen which quickly dissolves in metal at welding temperatures. Hydrogen creates pores in the weld, making it weak and brittle.
Eliminates Rapid Weld Cooling – If the weld metal and HAZ cool too quickly, it can cause a crack. Preheating allows them to cool slower, reducing brittleness.
Additionally, preheating can help reduce the risk of cracking and porosity in low alloy steels containing Cr, V, Mo, and Ni.
The best practice is determining the adequate preheat temperature for the base metal you are welding. This way, you will ensure you are not heating excessively and incurring unnecessary costs, damaging the steel’s properties, or running into cracking problems caused by rapid cooling if your preheat is too low.
How to Determine the Right Preheat Temperature for Your Welding Project
There are five ways to determine the correct preheat temperature for specific materials.
- Use Manufacturer’s Recommendations – The steel manufacturer’s website or published information is the fastest and easiest way to get the preheat temperature value. However, some manufacturers may play it safe by increasing preheat welding temperatures… and your costs.
- Refer to Structural Welding Code – The American Welding Society (AWS) D1.1 Structural Welding Code publishes the required preheat and interpass temperature charts for various steel types and Prequalified Welding Procedure Specifications.
- Use Standard Manual Calculators/Slide Rules – They are a handy tool if you can locate a manual slide rule calculator, a.k.a. welding preheat and interpass temperature calculator. Alternatively, you can try this online calculator or this preheat calculator.
- Apply Heat Affected Zone (HAZ) Control Method – This method applies only to fillet welds. Rapid cooling can cause martensite to form, which is susceptible to cold cracking. Using this method requires knowing the chemical composition of the steel to calculate the CE.
- Hydrogen Control Method – The hydrogen control method works well for high-strength, low-alloy steels with high hardenability. The preheat temperature value calculated may be too high for low-carbon steels because it assumes the steel will reach maximum hardenability.
You can find more information about the last two methods in “Annex B – Guideline on Alternative Methods for Determining Preheat” in the AWS D1.1/D1.1M:2020 Structural Welding Code – Steel guide.
What Temperature Do You Preheat Steel Before Welding?
The preheat temperature you use for your welding project will depend on a few factors, including the type of metal, its thickness, and the welding process you use. Look for the recommended preheat temperatures for each metal in welding guides or manuals.
Generally, a preheat temperature between 200°F to 400°F (93.3°C to 204.4°C) is typical for most low-carbon steels. However, for high-carbon steels or thicker sections, the preheat temperature is 500°F to 800°F (260°C to 426.6°C).
If you are unsure of the correct temperature, start with a lower temperature and increase it as needed, using a pyrometer to measure the metal’s temperature. Adjust the temperature if you find it is not providing the desired results.
Preheat Techniques – Different Ways to Preheat
There are several preheat techniques to preheat the base metal before welding. The most common are:
Torch heating is a common preheat technique for small welding projects that uses a flame from a torch to heat the base metal before welding.
Induction heating uses an electromagnetic field to heat the base metal before welding. It’s a preheating technique for large welding projects.
Electrically heated ceramic pads accurately raise the workpiece temperature up to 1,850°F (1,010°C) for recommended preheat, interpass, and PWHT practices.
Furnace heating involves heating the base metal in a furnace before welding. It is used primarily for large welding projects.
Post-Weld Heat Treatment (PWHT) – What is it and When is it Necessary?
Post-weld heat treatment (PWHT) is a heat treatment process performed after welding to improve the weld joint’s strength, toughness, and flexibility. PWHT involves heating the weld joint to a specific temperature and holding it for a predetermined period before cooling it slowly. PWHT is necessary when welding high-strength steels, which are prone to cracking.
Hydrogen in the weld or HAZ causes cold cracking, and a slower cooling rate helps hydrogen escape the weld. Additionally, any surface moisture evaporates before welding, helping eliminate the source of hydrogen.
Heat Affected Zone (HAZ) – Understanding the Effects of Preheat on HAZ
Heat affected zone (HAZ) is the area of the base metal affected by the welding process’s heat. The preheat temperature, welding process, and cooling rate influence the HAZ.
Preheat techniques minimize distortion, and most welders preheat everything for high-carbon material. Some say mild steel less than 1 in. thick does not require preheating. However, metallurgical charts show that the heat-affected zone (HAZ) starts at about ¾ in.
Weld Preheat Safety Precautions
Weld preheat can be dangerous without proper safety precautions. Some of the safety measures include:
- Wearing proper personal protective equipment (PPE) such as heat-resistant gloves, aprons, and face shields.
- Ensuring proper ventilation to avoid the buildup of toxic fumes and gases.
- Keeping a fire extinguisher nearby in case of a fire.
- Ensuring proper grounding of electrical equipment to prevent electric shock.
- Following the manufacturer’s instructions when using preheating equipment.
Common Mistakes to Avoid When Preheating
If the preheating process is incorrect, it can lead to several welding defects. Here are five mistakes to avoid when preheating welding.
- Not heating the metal to the right temperature produces poor weld penetration and weaker welds.
- Failure to maintain the preheat temperature allows the metal to cool too quickly, causing cracks and other defects in the weld.
- Insufficient time for the preheating. Preheating the metal must be done slowly and evenly to reduce the risk of thermal shock.
- Choosing the wrong preheat method can result in poor-quality welds or damage to the metal.
- Failing to preheat welds altogether can result in weak welds, cracking, and other defects.
The biggest mistake you can make is overlooking your safety. Always wear protective clothing, gloves, and a face shield.
Tips for Properly Heating Metal Before Beginning the Welding Process
Some tips for properly heating metal before beginning the welding process include:
- Clean the metal thoroughly. Contaminants such as dirt, rust, or oil can cause defects in the weld.
- Heat the metal slowly and evenly so it can expand uniformly. Rapid heating can cause the metal to warp or crack, making inconsistent welds.
- Use a temperature gauge to monitor the heat and ensure that the metal reaches the correct temperature needed for welding.
- Preheating the metal before welding helps remove any residual stresses, ensuring a stronger weld.
- Use the correct heating method for the type of metal.
Now that we have covered preheating the metal, we need to consider preheating the gas in tanks for better welding. For improved performance of gas welding tanks in cold environments, consider better temperature control with gas welding blankets.
The Benefits of Using a Preheating Blanket for Welding Projects
Preheating every weld is unnecessary because it adds to the cost, especially if you use gas. Whether you are using Oxy-acetylene, Oxy-gasoline, MAPP gas, butane, or propane, the storage temperature of the tank significantly affects the cylinder’s pressure. In freezing temperatures, the tank will flow at a reduced rate, even if it’s full.
Preheating blankets are a cost effective way to preheat the gas before welding. Custom Welding Blankets fit unique shapes and deliver uniform heat directly to the tank quickly and efficiently. Preheating blankets are available in various sizes for small and large welding projects. Preheating blankets can help improve the weld joint’s quality and strength while reducing the risk of cracking, porosity, and other defects.
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Final Thoughts on Maximizing Weld Strength with Preheat Techniques
Preheating steel before welding is essential for ensuring the weld’s integrity and durability. The technique improves the quality and strength of weld joints in high-stress applications such as bridges, cranes, and pipelines.
Remember to follow all safety precautions and avoid common mistakes when preheating the base metal.
Welding blankets keep you productive in cold weather by delivering heat directly to your gas tank.