What type of crack could this be?

[22west]

Hi,

Below is a picture of a recent re-weld at the base of a lug. This is an old 1950's structure with not a lot of good documentation - both the lug and the shell are thought to be mild steel. The weld was re-done by a certified structural welder after some cracking had been found. The welder ground off the old cracked weld first then applied a fresh new weld. A couple of months later the cracking returned as shown in the photo - the cracks look the same was what was originally found. There is not a lot of axial load on the lug, there is some cyclic bending at the base of the lug which could imply a fatigue crack - but it does not seem to look like a fatigue crack. It is very straight and sharp and transitions seamlessly from the weld metal to the base metal. Any thoughts or ideas?

The interface does not see low temperatures. The structure is located indoors and is maintained between 15 and 25 degrees celcius.

Thanks,
22west

 

[EdStainless]

When the first crack wass ground out what NDT was done to verify that it was in fact gone?
Any preheat?
Did they use low strength, low Hydrogen electrodes?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Tmoose]

In the lone supplied picture is the lug cracked, or the " old 1950's structure? "

More pictures and sketches are needed. They should show the overall arrangement, all welds, and anticipated loads, points of application and direction.

Also pictures of the original "cracked welds."
The description of "old cracked weld" being ground off, the statement "the cracks look the same was what was originally found" , and the appearance of the new cracks at the toes of the new welds (not in the weld proper) do not support each other.

What weld electrode/wire was used?
if low hydrogen SMAW/stick were the electrodes kept warm and dry?

Spark test base material. Multiple Hardness tests with portable hardness tester of heat affected zone close to weld and every 1/8" out to .25" beyond cracks, and the weld bead
Were "repair" welds LP or MP tested after completion ?
Were there any thoughts about preheat, interpass temperature, etc?

Compare your welds to these -

https://app.aws.org/files/199/622219/Weld_Inspection_Checklist.pdf

 

[dik]

Have you taken a coupon of the material to determine if it is weldable?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TugboatEng]

It sure looks like a fatigue crack with the bifurcation.

 

[weldstan]

The Carbon Equivalent of older low carbon steels was much higher than today's steels. Thus the need to use low hydrogen electrodes for the repair. It is akso necessary to confirm the original crack removal with MT or PT. Leaving the repair to the welder only is not a good idea. Was he given a repair welding procedure? The ropiness of the repair weld is not the best profile to resist fatigue failure.

 

[mfgenggear]

all previous good advice.
when cracks appear on welded part as as shown. here are some further tips.
do an xray (radio graphic inspection). this will provide information if it is surface or if it is deep.
after a repair such as others commented, grind the surface to remove surface cracks, do both MPI or LP and xray,
to verify it was removed. High carbon steel that has been heat treated will typically crack as shown if the weld procedure
was incorrect. preheat being an important part of the procedure. was there a weld engineer involved?
cracks can not be easily removed, it has to be drilled out at the ends, if it is sub surface, if it is surface crack it can be hand ground to remove.
then rewelded. the cracks absolutely has to be verified it was removed before rewelding.

 

[DVWE]

Have you take a coupon of the material to determine if it is weldable?

I agree with dik. Start here. If a coupon cannot be removed, your other option would be getting a portable OES analyzer to determine your C and CE numbers.

The devil is in the details; she also wears prada.

 

[dik]

had to look it up... first time I've encountered this term...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[22west]

I am digging up some more details on this.

The image below is the material props - take from a sample of each welded piece. The top row is the shell composition and the bottom row is the lug composition.

I keep digging. I will try to find out what rod was used. . .

 

[weldstan]

If the C and Mn were in the shell component was made with the same analytical equipment, the shell material is equivalent to a AISI 1010. It would have very low strength in the HAZ adjacent to the weld. The hardness would indicate some cold work had been done on the shell material. What is the thickness of the shell and lug? Great differences in thicknesses and strength are not conducive to best design, especially in cyclic service. Also it appears that the original weld was ground while the repair weld was obviously not, again not appropriate.

 

Pay for a failure analysis performed by a lab with some welding engineering expertise.

Whatever it is those cracks were not made by fatigue, base on appearance and the fact the new weld has had zero time in service.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[mfgenggear]

even when every procedure is done correct, parts crack, part of the doing business. I don't recall how many time we had to do MRB repairs.
on some parts and not many. the repair procedure is as important as the weld procedure. most of all NDT to verify no indications.
surface cracks are easy to remove with the proper equipment.
deep or subsurface cracks are a PITA, and it has to be XRAYED to lay out the exact location of drilling out and machining out. to completely removing the crack
before rewelding or one will be chasing this from now on, to my experience, if one tries to reweld a crack with out removing it
it will crack again, and again. nature of the animal. IT MUST BE COMPLETELY REMOVED.

and Trust me it is easier to get the proper welding schedule to prevent it in the first place. and very experienced welders.

 

p.s.,
A far more cost-effective way to deal with this kind of issue is to pay a professional welding engineer for a few hours of his/her expertise. But it's fine if you don't want to; failure analysis pays far better.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[dik]

A spreadsheet in case anyone can use it... has macros... but no 'bad' things... If anyone has a carbon equivalency that I can add to it... let me know...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

even when every procedure is done correct, parts crack, part of the doing business.

Are you saying cracks are inevitable?

If anyone has a carbon equivalency

Not actually a thing.
I say this both as an expert in ferrous welding metallurgy and as a competent English language user.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[mfgenggear]

Quote: ironic metallurgist (Materials)14 Apr 21 18:11
Quote (mfgenggear)
even when every procedure is done correct, parts crack, part of the doing business.
Are you saying cracks are inevitable?:Unquote

I started as a general Helper in fabrication shop, and ended up as a sheet metal mechanic apprentice, welding apprentice, I worked there as a youngster for 5 years.
ended up with level 2 NDT radio graphic, penetrant certifications. and other certifications. Yes I observed many cracks. 99% were able to repair.

at some of the bigger companies I processed the rework from MRB engineering on indications from welding all types of materials.
so yes it happens.
a lot of it was operator error. fatigue, condensed schedules. long hours. you name it

I also witness indication in electron beam welding of low alloy steel, that was amess tough to weld with adjacent carburized surface.
but be advised it was stringent aircraft/aerospace requirements. so sometime no indications allowed.

 

[dik]

by things I meant computer gremmies...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[blacksmith37]

I would say hydrogen underbead cracking in the HAZ ( which is somewhat hard). But you still need a failure analysis.

 

[mfgenggear]

yes contamination is always an issue, should be part of the schedule procedure to prevent and remove. improper cleaning and prepping is a big cause