piping porosity

Date 08-13-2003 14:08

rackengr,
Welcome to the forum.
Piping porosity is where gasses have traveled through the molten weld metal pool and caused what some refer to as worm holes. Piping porosity will most likely(not always) be parallel to the weld joint.
Porosity(piping or linear, cluster, and randomly scattered) usually comes from moisture, grease contaminates on the surface of the base metal or from the consumables that have been contaminated. Low Hydrogen rods after they are out of the original packaging must be kept in a rod oven at 250F to keep the moisture content low hence the name low-hy. Also welding processes that use an external shielding gas can cause porosity if the gas flow is too little or too great to properly cover the weld pool.
I have seen piping porosity at the root of a weld due to improper fit-up of the joint. If you leave too large a (root) gap on a fillet weld, atmostphere can be drawn in and contaminate the weld pool from behind and the shielding gasses cannot cover the weld pool to prevent this from happening.
Is this the type of info you were in question about?
John Wright

By chall

Date 08-13-2003 14:43

Another cause is excessive voltage when using FCAW. I really don't know why, because I'm not a welder. However, our test shop instructor pointed it out to me.

C Hall.

By rackengr Date 08-13-2003 18:18

Thanks for all the helpful info.
I'm trying to settle a disagreement about the acceptablity of a weld I recently saw. This weld had numerous "pin holes" along the length of the weld (a butt weld of 2 square tubes). Based on your comments this doesn't sound like "piping" porosity ("Piping porosity will most likely(not always) be parallel to the weld joint."), however it's the only type of porosity the code mentions for a visual inspection. And the code has limits on the DIAMETER (and amount) of piping porosity which leads me to think that what I saw was piping porosity since it had a diameter. So is what I saw an example of piping porosity or is there somewhere else in the code that addresses other types of porosity?
Thanks.

By jwright650 (*****)

Date 08-13-2003 18:48

rackengr,
I may be confused on the term "piping porosity". What I had called piping porosity didn't always exit to the surface to where you could visually inspect it. I found it when I would UT the joint and uncover it when the welder was cutting it out. What you are describing is what i call cluster or scattered porosity. Sorry for the confusion in terms(my bad). See Annex B for "correct definition" which states: "Piping porosity
-Elongated porosity whose major dimension lies in a direction approximately normal to the weld surface. Frequently referred to as pin holes when the porosity extends to the weld surface."
I do know that the type of porosity that you are seeing there in those welds have limits to size and frequency as you have seen, noted in Table 6.1 note(8)A,B&C of your D1.1:2002 code book. Also see the Commentary section on table 6.1 Item 8 on page 457 bottom left side of the page.
Hope I didn't cause too much confusion,
John Wright
I always have my guys fix all porosity regardless of size, but that is my choice in my shop, and not because of code. If someone in our shop is experiencing alot of porosity, I try and figure out what we are doing to cause it and make it stop. I think it reflects on our overall quality.

By rackengr Date 08-14-2003 13:14

John,
Thanks again. I'm not very familiar with the code so I appreciate you directing me to the proper sections and giving me some background info on the causes.
rackengr
PS-After a heated argument with the welder, I made him repair the welds.

By TimGary (****)

Date 08-14-2003 14:08

Good for you!
The Porosity should be repaired, even if it is within the allowable tolerance of the code. (Personal opinion)
The reason why is that even if the porosity is within code tolerance, there are secondary determents to consider such as, leaks in fluid or gas handling systems and the fact that spray paint will not fill the holes. I hate to see a brand new carbon steel weldment leave the shop with rust streaks already forming on the fresh paint due to weld porosity.
Also porosity, even within tolerance, is an indicator that there is something is wrong with the weld process, i.e. insufficient shielding gas, contaminates, excessive root opening, etc. The best way to get the welder/fitter to fix the problem causing the porosity is to make him do the weld repairs. Grinding out porosity gets aggravating very quickly.
Stick to your guns!
Tim

By Mwccwi

Date 02-18-2005 00:17

Hello,
I'm new to the forum and was looking through the past posts and I thought it's better late than never, I found some info that conflicts with the info that I've read @ this post
A3.0:2001 piping porosity- A forn of porosityhaving a lenght greater than it,s width that lies approximately perpendicular to the weld face.

By jwright650 (*****)

Date 02-18-2005 12:40

I can't further comment on your definition, my AWS A3.0 is a 94 edition. My edition only gives a definition for "porosity" and doesn't breakout and explain "piping porosity" by itself.

I posted the definition for "piping porosity general" given in AWS D1.1:2004, Annex B, page 318(top left corner of the page).

I see why you have questioned the definition as there is a conflict with what you posted concerning the later edition of AWS A3.0:2001 and AWS D1.1:2004.(parallel vs perpendicular to weld face)
John Wright

By - Date 02-18-2005 16:07

An additional comment:
D1.1 places limits on piping porosity for VT (at Table 6.1). Since what you often can see visually is only the end of a "pipe" or "worm hole", how does one know if it is "piping porosity" or simply "porosity".

This always makes for some fun discussions with welders. One trick I've used in the past is to use a very small paper clip, straightened out. When inserted into the open pore, if it goes in a distance greater than the diameter of the pore, I consider it piping porosity. A good welder will have a hard time arguing that when he can do and see it for himself.

Mankenberg

By jwright650 (*****)

Date 02-18-2005 16:19

I've used that paper clip in the hole method also.

After further thinking about the definition,
...stating that when the length exceeds the width.

I agree that...It really doesn't matter the direction (perpendicular or parallel to weld face)that the piping porosity is travelling,

...only that it is longer than it is wide therefore called "piping porosity"rather than simply "porosity".

John Wright

By - Date 02-18-2005 18:27

Mr. Wright,
It would seem that piping porosity would tend to be parallel to the weld face when the weld is in the vertical position. In D1.1 at paragraph 6.20.3, it recommends RT of electroslag and electrogas welds to detect piping porosity. I don't have any experience with these processes, but this to me would seem to clearly indicate that piping porosity with these processes tends to be subsurface, and I would guess also parallel to the weld face.
What do you think?

Mankenberg

By jwright650 (*****)

Date 02-18-2005 19:04

I've seen it running parallel to the weld face with the joint in the horizontal position too. Visually I would have never found it, the joint was full pen and I UT'd it. I found indications all along the joint intermittantly. The welder carbon arc gouged the weld out and he showed me what I was seeing. There were several long tubular tunnels(1" to 1 1/2" long about an 1/8" in diameter) under the surface where the porosity was trapped in the final pass.
So, I guess it can be oriented in any direction, but I see your thinking about the vertical weld showing porosity parallel to the weld face too. Hot gasses would want to rise to the top, which sorta befuddles me about why I've seen it trapped in the horizontal welds like I did. Would this happen because of high voltage and high travel speed with FCAW? I ask this because I had found this welder running lots of voltage(37-39v) compared to amps(325-350a) and he finishes welding his end waaay before his partner on the other end of the same piece of material.

By dschlotz (***)

Date 02-19-2005 17:40

Hi, thought I would jump in. I have been reading all of your comments and find them enlightening. I have experienced porosity in welding FCAW both as a CWI and as a welder.

First a question: If you have a gap at the root of a weld how can you stop porosity. I have noticed that it is not a fact that it will appear every time. Most of the time there is none.

Second: I have noticed that when I weld multiple pass, FCAW with gas (CO2) welds, that the heat build up causes porosity and that sucessive welds cause more and deeper porosity until I let the joint cool down. What is the proper interpass temp for mild steel, and what is the reason that the hot steel causes pinholing?

By daleloi Date 10-24-2017 19:11

A safe interpass temperature for mild steel (A36 and similar) is anything below 150 degrees in thick sections, unless the WPS lists another provision for the interpass temperature. In other words let the weldment cool down to 150 degrees before welding again. For thinner sections less than ½ inch thick is 100 degrees, before welding again.

Heat in welding does not cause porosity. All porosity is caused by Hydrogen and or a lack of shielding gas covering the molten weld pool. A contributing cause is a dirty substrate, oil, grease, water, grind dust, surface mill scale. Other factors that may affect welding where porosity is a problem are atmospheric conditions like: High humidity, high dew point. Some shops keep their doors open which will cause humidity, and dew to affect welding conditions. The above are the reasons why preheating of the weldment comes into play. Check your WPS to see if a provision is listed for Pre-heating prior to welding. One final mention, is welder technique, i.e. speed of travel, distance of the welding torch from the weld joint, etc.

By Lawrence (*****)

Date 10-24-2017 22:18

daleloi

With all due respect and a little humor

If welders making multi-pass joints waited for A36 base metal to cool to 150 degrees before adding each successive pass... America would have never been built :)

In AWS D1.1 the *minimum* interpass temp for:

A36 greater than 3/4" - 1 1/2" is 150 degrees
A36 greater than 1 1/2" -2 1/2" is 225 degrees
A36 greater than 2 1/2" is 300 degrees

By jwright650 (*****)

Date 10-25-2017 13:47

I'm going to add another caveat....
What Lawrence posted was min preheat/interpass temps for SMAW with "other than low hydrogen rods".

A36 1/8" - 3/4" min. is 32°F
A36 over 3/4" - 1 1/2" min. is 50°F
A36 over 1 1/2" - 2 1/2" min. is 150°F
A36 over 2 1/2" min. is 225°F

^^ for SMAW with low hydrogen rods (also SAW, GMAW, FCAW)

By welderbrent (*****)

Date 10-25-2017 17:17

Now, lets add another dimension to this: you state that "A safe interpass" is below 150° or 100° depending upon thickness.

I would counter that, as Lawrence and John have said, first, that is a safe MINIMUM temp and there is no code interpass safe or MAXIMUM temp in most cases. D1.8 is one of the exceptions to 'most' cases ascribing a maximum temp of 550° which would indicate any interpass temps below 550° would be SAFE while those above are not.

Having said that, it is not about porosity but HAZ, mechanical properties, distortion, and other factors. I can weld multi pass PJP's and/or fillet welds at 700-1000° and not get porosity and still be SAFE as long as my applicable code, approved WPS, and Contract Documents don't forbid it. Been there, done that.

He Is In Control, Have a Great Day, Brent