Surface blowholes, or most of the time, referred to as “Pinholes”, can sometimes be seen on your weld and today we are going to learn about the reasons for pinholes to appear in welding. These are considered defects so don’t be proud if you do see some in your work. Pinholes can be reduced and saved, but it’s hard to prevent them from happening. It’s due to the simple reason that welders cannot control every little thing that’s happening. In most cases today, the pinholes can only be seen at the last minute… Let’s say right after mechanical processing. Pinholes should not be taken lightly, because as I said above, they are a defect, and the human eye will be able to see them.
Pinholes can appear in any area of your weld, so you have to be careful and to check every nook and cranny of your work. You don’t want that your client would be the first one to spot them, right?
The cause for pinholes are a little complicated, but that hard to understand for first time welders. The main cause of pinholes can be found in the nature of the metal you are welding and also with the mold system. First the nature of the metal. There are different types of pinholes. We have the hydrogen pinholes and hydrogen-nitrogen pinholes. This first type of pinhole ca be formed because of the reaction of water vapor with the said elements. Metal will be oxygenated and atomic hydrogen will form… This will then be diffused into liquid metal. From this, micro gas bubbles will be form because of the chemical reaction, and these bubbles become the holes in your weld. Second, due to the mold materials. Remember that too much nitrogen in the molding sand will result to excessive moisture content.
Other reasons would be because the slag is not skimmed completely, which can then lead to slag inclusions… And then these slags may be the reason why gas bubbles will form. When it comes to cast iron, it may b a bit hard to prevent them since cast iron with nodular graphite contains numerous oxides that slowly contribute to pinhole formation. Even slags that develop from magnesium treatment can also make an impact.
You can prevent pinholes by keeping an eye on the gating system and maintain it with anon-turbulent flow and short casting runs that will be able to reduce pinhole tendency. If you really want to get rid of them, then you can fill them in with epoxy. You can buy from Amazon or Bestbuy, or even just from your local shop.
So those are some of the reasons for pinholes in welding! I hope you learned something! Have a nice day!