Weld defects

Date 04-12-2017 14:37 Edited 04-12-2017 14:39

Both are the result of the difference in the solubility of gas in liquid versus a solid.

Generally, the solubility of gases such as carbon dioxide, nitrogen, or hydrogen, is much higher in a liquid than in a solid. While in the liquid state the gas is in solution, but as the liquid cools and starts to solidify the gas comes out of solution and attempts to escape into the surrounding environment (air). Any gas that fails to escape before solidification is complete manifests itself as a bubble of gas entrapped in the solid. The shape of the bubble is dependent on the volume and rate of solidification.

In welding the presence of porosity can be seen visually when the gas bubble breaks the surface of the solidifying metal. Since the metal is not pure, it solidifies over a temperature range where multiple states exists, i.e., there is a mixture of both solid and liquids, known as the mushy state. The bubble can leave a "hole" that is either spherical or elongated in the mushy multiphase region.

Often the porosity is located in the last region to solidify, i.e. the centerline of the weld, resulting in aligned porosity along or near the centerline of the weld.

The porosity incubates at the interface between the liquid weld pool and the trailing edge of the weld pool where the temperature drops suddenly to near the solidification temperature. At the interface the weld is in the mushy state where some of the higher temperature constituents have solidified and the lower melting temperature constituents are still in the liquid state. The solubility of the gas drops and it comes out of solution, and makes it way to the surface of the weld.

The porosity that is subsurface is gas doesn’t escape into the air and is frozen in metal. It may be detected by a volumetric examination if it is large enough. However, if the subsurface porosity is small enough, it may escape detection.

Essentially, a gas pore and porosity is one and the same thing. A gas pore is a single "bubble" of gas where as porosity is usually more than one gas "bubble" in the weld.

By mmkmtech Date 04-13-2017 05:08

Thanks for your explanations. It is useful.

I am having few doubts.

Is it need to classify the "gas pore" or "porosity" in the same weldment area? That is, if both are present in the same metal then how do we mention these defects ("gas pore" or "porosity")? Should we classify these defects?

If classification is necessary then I will go further in my research.

Thanks for your support

By welderbrent (*****)

Date 04-13-2017 12:51

In a highly critical situation where exact analyzing of a defect is of importance it might make a difference and the terminology would be important. But, as Al stated, for the most part they are the same thing and in most cases you are perfectly correct to just call the discontinuity 'Porosity'.

Brent

By 803056

Date 04-13-2017 13:28

A couple of examples may be useful to illustrate the difference;

Example 1 - There is a single pore hole in the center of the weld termination. It is called a "crater pore" or sometimes referred to as a "crater pore hole."

Example 2 - There is are several pore holes, all measuring about 0.5 mm in diameter, along the center line of the single pass fillet weld. The "aligned porosity" measures about 30 mm in length.

English is a fickle mistress.

Best regards - Al

By mmkmtech Date 04-15-2017 05:35

Thanks for your valuable suggestions. Therefore, can we classify the defects "porosity" and "gas pore" using some terminology?

By 803056

Date 04-15-2017 13:22

Plural versus singular. Other than that, they describe the same discontinuity.

Al