How To Test For Quality Welds


One day a gentleman came into our shop looking for a fillet gage. He needed it for his course on how to examine welds. His company built a product that required some components to be made by a local welding shop. Unfortunately, there was inconsistent quality in these “farmed out” welded parts. When a welded part failed, it created extra time and expense. So, his solution was to inspect the welded components before they landed on his shop floor, but first he had to learn how to inspect.


Q: Why Do You Want to Check Your Welds?

A: Safety. Even if the object that’s welded is for your own use, you’ll want to examine your welds. Welds usually give out suddenly; if you’re lucky, it’s just a scary moment.

– Quality assurance. You want to make sure that the welds are meeting your expectations.

– Reducing the cost of re work. The longer an error goes undetected, especially during line assembly, the more costly it becomes to correct the error. Catching the problem early is always the best solution.

– In some cases, you don’t have a choice: it’s the law. The welds must be inspected before they’re put into service.


Q: How Are Welds Tested?

A: There are 2 broad categories, Destructive Testing (DT) and Non Destructive Testing (NDT)


Q: What is Destructive Testing (DT)?

A: This is when a completed weld is physically destructed to evaluate its strength and characteristics.

Example is a fillet weld break test: A load is applied until the weld breaks. The piece is then inspected to look for lack of fusion, internal porosity and slag inclusion.


Q: What is Non Destructive Testing (NDT)?

A: As the name implies the weld is tested without destroying the weld.

There are 5 different types of NDT

1. VT – Visual Testing

2. PT – Penetrant Liquid Testing

3. MT – Magnetic Testing

4. UT – Ultrasonic Testing

5. RT – Radiographic Testing


Q: What is the Easiest Way to Check Your Weld Using NDT?

A: VT – this is where you’re inspecting only what’s visible.

For VT to work, you need to know what you’re looking for: porosity, cracks, unfilled craters, variances, warpage and flaws in appearance. The things every new welding student learns to look for before handing in their projects for marking.

Q: How Do You Do VT?

A: To start you need good eyesight, good lighting, a filet gage and a magnifying glass.

Your VT gets better with experience – you know what to look for and where to look. This is one of the characteristics of a good welding inspector.


Q: Is There Another level of Testing That’s Better Then VT But Not Expensive?

A: PT – it’s simple and economical. Something you can do yourself and add another level of testing on your welds. It does require the ability to read the results. Seeing something that’s not actually there or not seeing something that’s actually there can lead to problems.

Q: How Do I Do PT? And Where Do I Get The Stuff? What Do I Ask For?

A: You will need 3 cans: cleaner, penetrant, and developer. Your local welding supplier will have the stuff (or be able to get it for you).

Q: How Do You Do PT?


1. Clean the surface of scale, slag, etc. Then use the cleaner to remove the oil, grease, dirt and anti spatter around the material that will be tested.

2. Apply the penetrant around the area to be tested.

3. Allow the penetrant to enter into the indications that are exposed to the surface. Wait about 5 minutes.

4. Remove the excess penetrant with the cleaner. Being careful not to remove the penetrant in the area that’s being tested.

5. Apply the developer around the area to be tested. You want the penetrant to “bleed thru” the white developer.

6. Examine and inspect the areas. Mark the areas that need more inspection or need to be reworked.

7. Use the cleaner to remove the penetrant. You want to prevent any “bleed thru” on any process that’s done after the testing (eg painting over the area).


Q: What are the other methods of NDT?

A: The other 3 methods (MT, UT, RT) involve more sophisticated equipment.

RT and UT require the welding inspector to be trained in these areas. He has to know how to prepare and read the results.

Q: Who Uses RT and UT Methods?

A: The customer. The gas and oil industry use this type of testing. Failed welds are too costly. They need to catch the problem before it leaves the shop floor.

-Certain codes will demand RT testing be done before the equipment is put into working use: e.g. A.S.M.E. code

– Usually the manufacturer doesn’t do this kind of testing. The customer will hire a 3rd party to do the testing. (Customers don’t want the manufacturer to “grade their own exams”)

– And manufacturers who have a very rigorous quality control program.


Next time you’re asked about the level of quality in your welding, you can now explain what kind of testing you do. If you’re having welding done for you, you’ll now know the difference between Penetrant Testing and Visual Testing.

If we can help you with testing your welds, please give us a call.




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