The Complete Guide to Multi-Pass Welding

Welding is a process used in many global industries, from construction and energy to aerospace and maritime. It’s a vital and nuanced skill, making it a highly respected profession that requires years of training. 

One particular welding nuance is multi-pass welding. Often used on heavier materials and when a superior weld is necessary, this technique allows welders to fuse materials that might otherwise not be fit for welding. 

This complete guide by Meritus Gas Partners explores multi-pass welding, its uses and its benefits.

What Is Multi-Pass Welding?

Multi-pass welding is a process that deposits several weld metal layers to completely fill a joint and create a stronger weld than a single-pass weld could create. Multi-pass welding requires carefully controlled heat input to minimize distortion, which becomes more likely with multiple passes. Proper heat management also ensures that each layer properly fuses.

There are different types of passes in multi-pass welding. The first pass is called the root pass, which fuses the root of the joint. The fill passes follow the root pass to build up the weld body until it reaches its required thickness and strength. The final passes are the cap passes. These are used to complete the joint and ensure the weld meets the required visual standards.

Every pass adds more heat to the joint, so heat control is vital. Improper heat management can lead to distortion and hydrogen cracking. It could also compromise the base metals’ mechanical properties.

What Is Multi-Pass Welding Used For?

Multi-pass welding is used on joints that need a strong, durable weld. It’s often used on thick metals and is essential for any materials thicker than 3/8 inch (9 ½ millimeters). It can be used to repair thicker materials or build up more material to fill an area. It’s also commonly used on materials that are more sensitive to heat, as multiple passes can allow for better temperature control than a single pass can. 

Industries that commonly require multi-pass welding include:

• Construction: Multi-pass welding is used in the construction industry to weld heavy beams and structural components.
• Oil and gas: Pipelines that have to be exceedingly strong, some of which are underwater, may require multi-pass welding.
• Power generation: Many parts in power generation facilities require the strength and flexibility that multi-pass welding delivers.
• Aerospace: Multi-pass welding is ideal for light materials that must withstand strong forces. 
• Maritime: Thick, heavy materials used in maritime operations like shipbuilding can benefit from the additional strength that multi-pass welding provides.

Benefits of Multi-Pass Welding

The key advantages of this welding technique are:

• Compatibility with thicker materials: The main benefit of multi-pass welding is that it facilitates the joining of thicker materials that would otherwise be unsuited to welding. It does this by creating stronger welds that improve a joint’s durability and integrity. It even improves the joint on a microscopic level, reducing grain size and enhancing the microstructure. Each new layer normalizes the grain structure of the previous layer, which improves its mechanical qualities, durability and strength.
• Lower risk of defects: Multi-pass welding also reduces the risk of defects, as the welder has more control over heat input and other parameters. With the ability to evaluate and address any minor issues between each pass, there’s much less pressure for the welder to deliver a perfect single pass.
• Advanced designs: Multi-pass welding also allows for more complex shapes. Without this technique, many structures would be impossible to build.

Challenges of Multi-Pass Welding

Multi-pass welding poses several challenges, such as:

• Distortion: The additional heat required in multi-pass welding can cause the metal to contract and expand, leading to warping. Distortion is more likely in thinner or larger pieces.
• Cracking: Incorrect heat management, improper preheating or high residual stresses can cause the weld metal to crack. Certain materials are at additional risk due to hydrogen-induced cracking. 
• Undercut: Multiple passes increase the risk of undercut, where the base metal is melted but not sufficiently filled by the weld metal, resulting in a weakened material.
• Trapped porosity and slag: Insufficient cleaning between passes, incorrect travel speed or improper shielding can cause porosity or slag to become trapped.
• Lack of fusion: Caused by an incorrect torch angle, improper travel speed or ineffective heat management, the weld metal may fail to fuse to the previous layers or the sidewall, weakening the entire weld. 
• Time and cost: Multiple passes require more time and resources than single-pass welding.

Different Multi-Pass Welding Techniques

Multi-pass welding can use one of several techniques.

MIG Welding (GMAW)

Metal inert gas welding, also called gas metal arc welding, uses a wide electrode to heat the two metals to be joined. The electrode is fed through the welding gun at a continuous speed. As it’s fed through, it melts to form the weld metal. An inert gas is also used to create a shield around the weld. Argon, helium and carbon dioxide are commonly used. 

MIG is one of the fastest welding techniques with a high deposition rate, helping reduce the additional time that multi-pass welding demands. It’s also an efficient technique for filling larger joints and is commonly used on steel and aluminum. For the best results, MIG welding requires consistent travel speeds.

TIG Welding (GTAW)

Tungsten inert gas, or gas tungsten arc welding, uses gases to protect the weld material like MIG welding does, although MIG and TIG processes vary from there. Rather than a consumable electrode, TIG welding uses a non-consumable tungsten electrode to create the arc that heats the metal. A separate weld material is then added. 

This technique requires excellent control and skill, and although TIG welding is slower than MIG welding, it delivers superior results. That’s why it’s often used for thin materials or applications where a superior weld is vital. Common uses of this technique include pressure vessels, airplanes and maritime applications. It’s also regularly used on steel or titanium.

Stick Welding (SMAW)

Shielded metal arc welding, commonly referred to as stick welding or arc welding, is a versatile technique. It doesn’t require a welding gun and instead uses a consumable electrode rod, which creates the arc and heat for the filler material to weld the materials. The electrode rod is coated in flux that produces carbon dioxide to prevent oxidation or contamination. Specifically, either E6010 or E6011 electrodes are used for the root passes, while E7018 electrodes are used for fill and cap passes.

Since stick welding doesn’t require a welding gun, it’s seen as the most accessible welding technique and often chosen for outdoor welding. It’s also a simple — and usually less expensive — approach that doesn’t require as much skill as some other welding techniques.

Flux Core Welding (FCAW)

Fast and highly portable, flux core welding, also known as flux-cored arc welding, can operate at various voltage levels and angles. This is one of the most efficient welding techniques, with the joined metals cooling quickly. It is also commonly used outdoors, making it perfect for construction projects, particularly those operating in windy conditions.

How to Multi-Pass Weld on Different Base Materials

While the process doesn’t inherently change, you may have to adapt your technique to ensure you’re working with your base material’s unique properties in mind.

Stainless Steel

Stainless steel is particularly sensitive to heat and can become sensitized if it gets too hot. Multi-pass welding can help reduce the heat used. Welding with a faster travel speed helps keep the temperature down, and multiple passes give the welder more control over the temperature. Clean the metal between passes to prevent contamination, and if necessary, allow the metal to cool.

Carbon Steel

Steel that has at least 0.3% carbon should be preheated before welding, and the temperature should be carefully monitored between each pass. For higher-strength grades of steel, use low-hydrogen welding techniques, such as TIG. Beware of hardening and distortion after each pass.

High-Strength Alloys

High-strength alloys can quickly lose their properties when they become too hot, but can fail to fuse effectively if they’re too cold. That’s why it’s vital to stay within the narrow heat range specified for each base material.

How to Plan and Execute Your Multi-Pass Weld

A successful multi-pass weld requires proper preparation and careful execution:

1. Select the welding technique: Decide which technique is best based on the location and application of the weld.
2. Determine the number of passes: Evaluate the joint thickness and design, as well as the diameter of your electrode or wire, to estimate how many passes will be required.
3. Consider preheat requirements: Certain materials, such as high-carbon steels or thicker metals, will require preheating. 
4. Create an effective sequence: Use back-stepping to minimize the risks of distortion or cracking. Apply balanced welding to maintain structural integrity and reduce warping.
5. Prepare the joint: Create a smooth and clean bevel of the correct angle, ensuring that it’s free of any contaminants, such as oil, paint or rust.
6. Root pass: Apply a weaving motion for open root joints using a smaller electrode or wire. Ensure proper penetration for the best results.
7. Fill passes: Cover half of the previous pass with the next pass while maintaining good fusion with previous layers and sidewalls.
8. Cap passes: Create a smooth appearance that meets the required visual standards by carefully controlling the weld. Maintain proper reinforcement, around 1/16-1/8 inch above base metal.

Multi-Pass Welding Best Practices

While you may have to adapt your technique at times, some best practices can be applied to any multi-pass welding process.

Choose the Right Welding Technique

Some welding techniques are better-suited to certain situations than others, so choosing the best technique can deliver better results. To find the right technique for your application, consider:

• What materials you’re welding.
• How thick the materials are.
• Whether speed or quality is a higher priority.
• Which weld properties are required.

Adjust Parameters When Necessary

To ensure the best results, remember that you may need to adjust your parameters from one pass to the next. After each pass, consider whether you need to adjust the voltage, current or travel speed of the next pass.

Clean Between Passes

For a strong and smooth weld, clean the joint between each pass. Remove any slag with a wire brush and chipping hammer, paying special attention to the toe areas of the previous pass. Clean until bright metal is visible.

Manage the Temperature

Managing the temperature is one of the most important steps to prevent distortion. Monitor the temperature of the joint after each pass with infrared thermometers or other temperature indicators. Typically, the temperature shouldn’t exceed 350-400° Fahrenheit, although this may vary from one application to the next. If the temperature is higher than desired, allow the metal to cool until it’s below the limit.

Inspect Your Work

Between each pass, check for undercut, porosity or incomplete fusion. It’s much easier to repair these defects immediately after they occur, rather than after you’ve made several more passes, so resolve any issues before moving to the next pass.

Multi-Pass Welding FAQs

Since multi-pass welding differs from single-pass welding in many ways, there are often questions on how to achieve the best results with this technique. 

How Many Passes Are Required in Multi-Pass Welding?

The number of passes needed in multi-pass welding varies from one application to the next. A V-groove joint in a ½-inch plate will likely need three to five passes, while the same joint in a 1-inch plate may need 10 or more passes. Other factors such as the nature of the material, the angle and the travel speed will affect how many passes are needed.

What Causes Defects in Multi-Pass Welding?

Defects in multi-pass welding can be caused by many things, such as:

• Improper slag or contaminant removal between passes.
• Incorrect heat and parameter management.
• Incorrect method choice. 
• Poor technique.
• Incorrect electrode or wire selection.

Can All Welding Processes Be Used for Multi-Pass Welding?

All common welding processes can be applied to multi-pass welding, although not every process can be applied to every application, just like with single-pass welding.

What Is Porosity?

Porosity occurs when gas pockets form in the molten weld metal. As the metal cools and solidifies, these gas pockets become trapped and create cavities in the weld. This weakens the weld and can compromise its integrity.

These gas pockets can either be visible on the surface or hidden underneath it.

How Do Multiple Passes Improve Weld Quality?

Welding with multiple passes can improve the overall weld quality by giving the welder the opportunity to remove or fix defects after each pass. With single-pass welding, the welder can’t fix the defects or further strengthen the weld with additional metal layers. The welder can also control the temperature and other parameters more closely with multiple passes, allowing them to deliver a better final weld.

Achieving Superior Welds With Multi-Pass Welding

Multi-pass welding is a vital process in many applications. With various techniques that can be applied to different scenarios to deliver the best results, multi-pass welding requires in-depth knowledge and skill to perform well. However, if applied correctly, multi-pass welding can offer many benefits and superior results, and help fuse materials that might otherwise not be suitable for welding.