What causes porosity in welding?

Welding /

In the welding industry, ‘porosity’ is a term that’s met with concern. 

Welders understand that their trade involves the manufacturing of materials that can be crucial to various products, machines, and infrastructures.  

They know that defects can lead to major consequences – parts can break and people can get hurt. 

For welders just starting, you may encounter porosity in your welds and should be on the lookout for ways to reduce it.

Here, we’re going into what porosity is, what causes it, and how to prevent or fix it. 

Table of Contents

What is porosity in welding? 

First, let’s define what porosity in welding is.  

Porosity is a kind of weld metal defect that can be characterized as trapped gas. 

It occurs when there are cavities within the weld metal, brought about by the freezing in of gas from the weld pool as it solidifies.

If we dig deeper, there are different reasons behind this occurrence which result in different kinds of porosity. 

Here are some examples of the various types of porosity and their causes.

Kinds of porosity and its causes

Distributed porosity and surface breaks 

When inspecting a metal weld for distributed porosity, you will notice fine pores throughout the weld bead. 

Distributed porosity becomes surface breaks when there is an increased number of distributed pores and when it’s already visible in the weld.

When nitrogen, oxygen, and hydrogen are absorbed and released upon solidification, these pockets are formed.  

Because of poor gas shielding, nitrogen and oxygen can seep into the weld pool and even just 1% of these gases can result in distributed porosity. 

Once it bumps up to 1.5%, the metal welds will have surface-breaking pores. 

Hydrogen, on the other hand, finds its way into the meld through moisture from inadequately dried electrodes, fluxes, or the workpiece surface. 

Other substances like grease and oil found on surfaces of workpieces of filler wire can also be a source of hydrogen. 

Fumes may also accumulate during welding when there are surface coatings and treatments, increasing the risk of trapped gas. 

Elongated porosity

Wormholes or piping are pores that are elongated, meaning that a large amount of gas has been trapped in the weld metal. 

In T joint type welds, elongated porosity can be found in the crevices between. 

It is important to ensure that coating does not exceed the recommended maximum, which is typically 20µm, during the welding of T joints.

Crater pipes 

Crater pipes are another type of elongated cavity formed during the weld finish.  

At the end of the weld run, crater pipes may form because of the shrinkage during solidification.

As the liquid to solid volume changes in the weld pool, there is an increased likelihood of this type of porosity.

Other causes of porosity

  • Arc length – When the arc length or the distance of the tip of the welding electrode to the surface of the weld pool is too high, gas shielding is reduced.  
  • Inappropriate flux – Low-activity flux can result in surface breaks.  
  • Worn down equipment – The usual wear and tear of equipment such as hoses with leaks or exposed pipes may affect the outcome of your weld. 

How to Prevent Porosity

Porosity has a 90% prevention rate, meaning that welders stand a good chance at avoiding this defect as long as they have the right precautions in place. 

The key is to stay on top of your welding procedures and ensure that there are no problems across the different steps.

Preventing distributed porosity and surface breaks

To prevent these two types of porosity, welders should be more vigilant when it comes to gas and moisture contamination. Prevention can be done by:

  • Eliminating gas sources such as air leaks and draughts
  • Ensuring that fillers being used have a sufficient amount of deoxidants
  • Reducing the intensity of gas flow 
  • Drying electrodes and fluxes
  • Cleaning and degreasing workpiece surfaces
  • Cleaning joint edges before welding 
  • Keeping weldable primer at the correct thickness

Preventing wormholes

Wormholes and distributed porosity have similar causes and these elongated cavities can be avoided by following these procedures:

  • Cleaning workpiece surfaces and surrounding locations where the meld will be manufactured
  • Wiping down any surface contamination such as rust, oil, grease 
  • Removing surface coatings from joint areas of the weld
  • Ensuring primer thickness does not reach the manufacturer’s maximum
  • Avoiding joint geometry welds

Preventing crater pipes

When it comes to preventing crater pipes, the focus should be on the welding process and techniques. 

This type of porosity is dependent on how quickly a welding arc is extinguished and how the welding arc and the welding wire is manipulated.

So, here’s how to get ahead of the formation of crater pipes:

  • Tack run-off tabs, base metals with the same grooves as the workpiece, to the end of your workpiece to allow you to extinguish the welding arc outside the joint
  • Progressively reducing the welding current to reduce the weld pool size
  • Adding fillers to make up for the shrinkage in the weld pool

Detection of porosity

Surface breaking pores are easily detectable with a penetrant or a magnetic particle inspection technique.

However, for pores underneath the surface, welders will need radiography or ultrasonic inspection. 

Repairing welds with porosity

The first step is detection, as mentioned in the section above. Once you’ve seen the pores in your weld you have to assess how bad the damage is. 

A major misstep would be to cover up the porosity by welding over it. Instead of fixing the weld, this would just be disguising the defect. 

If the porosity is found in a localized section, you can fix the weld by removing the gouging or grinding it out, using a cut-off disk to slice a bit of the weld. 

When porosity covers a large area of the weld, you may cut out a bigger circle of the affected area with a step drill and weld a sheet metal plug in its place. 

Oftentimes, however, many welders choose a complete do-over. 

For safety reasons, if porosity is widespread, it’s much better to have the entire weld redone. You have to prepare the joint again and weld with much more caution against the usual causes.

In a nutshell

While there are ways to remedy porosity in welding, an ounce of prevention is worth a pound of cure. Stay ahead of it by keeping to best practices and hopefully you won’t have to see those pesky defects in your weld. 

We’ll summarize all the preventive measures you can take with these four points:

  • Optimize workspace conditions
  • Keep material surfaces clean
  • Check your equipment
  • Check your gas flow

📌 Why should porosity be avoided?

Porosity weakens a weld and makes it unstable because of the air entrapped within it. This risks the integrity of the material and could have dangerous repercussions. 

In terms of production, reworking welds also consume time and money so it’s best to get it right the very first time.

📌 How much porosity is acceptable in a weld?

There are different welding construction codes for each country.

To find out how much porosity is allowed in a weld, you can refer to these construction codes. 

According to the American Welding Society, it is stated that the diameters of visible porosity should only be 9.4 mm or less in a linear inch of the weld or 1 mm in a 12-inch length of the weld. 

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