Repair weld 5

[kslee1000]

What is the detrimental effect on the base metal, for which repair has been repeatively performed over the same, or lengthened, cracks by air arc removal of the defective old weld, and replaced by new passes with minimal, or no, grinding?

 

[metengr]

kslee1000;
If cracks are found in base material, they can be removed by arc gouging or local grinding. If arc gouging is performed, the heat affected material from the arc gouging process must be removed by grinding the base material back for 1/16" to 1/8" to remove heat affected base material. Otherwise, welding on arc gouged material could result in cracking from carbon pick-up or cracking caused by welding on hardened base material.

One step you have missing is to verify defect removal using some type of surface nondestructive testing method. Once defect removal has been verified and heat affected material has been locally removed, weld repair can be completed.

The only detriment to repeated weld repair of base material is possible loss of strength and/or notch toughness if post weld heat treatment is required each time weld repair is performed OR the original base material was heat treated for strength requirements in service. If PWHT is not required and the base material had no original heat treatment for minimal strength, I see no detrimental effects on base material mechanical properties.

 

[kslee1000]

metenr:

Thanks for the informative response.

Yes, we do require the drilling of 1/4" hole at each end to determine end of defective base material. Root thickness is determined by NDT. PWHT is usually not required.

The practice concerns me the most is the frequent/repeative repairs performed over the same defect. How can we tell the metal has reached the metallurgical limit that the original strength can not be restored? If there is a such limit, what test method is suggested?

I have very limit knowledge on welding practice, but happens to oversee a lot of repair works in an urgent manner, mainly dealing with heavy duty cranes (60T-350T) and runway structures. I could have missed out a lot, comments, suggestions on this subject are welcome.

 

[racookpe1978]

Performing weld repairs on heavy cranes and runways?

You NEED to be absolutely, positively, behind-any-doubt CERTAIN you have removed ALL defects before proceeding - and - quite frankly - I doubt your current procedure is doing the job you need for everybody else's safety.

The 1/4 drilled hole at the end of a crack is only a short term, poor-man's mechanical temporary fix to TRY to prevent a crack from growing. NOT to verify that a crack has been removed. It won;t tell you anything because you can't see INTO the end of 1/4" drilled hole to SEE what's at the bottom! You haven'y really verified anything.

Worse, a 1/4" hole ISN'T a weld prep and will be the source of the future crack BECAUSE the welder can't fully melt the base and filler metal into and around the tiny 1/4" hole that you have just put in the (what used to be) base metal.

Arc gouging is only a first step. It - as noted above - MUST be first followed by grinding out the defective steel AROUND the crack. Then you need to verify the crack has actually been (1) completely detected and (2) completely ground out.

SO, an independent NDT (Dye Penetrant (PT) and Visual Test (VT) by an independent person NOT the grinder, and NOT the welder to be sure the crack has been ground out is required.

Then you MUST grind a weld prep - for access into the "V" area and for good penetration back to base metal.

Only then can you begin to weld build up the metal.

Pre-weld heating? Post-weld heat treatment? CHECK WITH A WELDING ENGINEER - the requirements will change with the base metal, the filler metal, the depth of the crack, the length and depth (mass) of the cracked original weldment and casting and steel.

On deep cracks (deep repair welds) you SHOULD check with another PT test after every second (or every third) weld layer to make the repair weld is itself not cracking. That the repair has no inclusions or holes.

After welding - and cooldown - then grind the final surface flat enough to NDE AGAIN = VT and PT.

----

If you don't do that - as a minimum - don't send your crane to my site. I'll leave before letting a load go over my head - or my crew's head - if the welding and checking isn't correct.

 

[kslee1000]

Thanks.

As I have pointed out, I am not an expert on welding process, but required to oversee urgent repairs, even it is not desirable long term solution (as you can see why it is a re-occurring event). The repair I mentioned happens to be in the real world of busy production lines, usually it means to temporarily allow the equipment to continue its normal operation until the next stop point for more controlled repair. However, the latter does not always occur in time, thus, another round of temp repair...on and on.

I think what I want to understand is when and how to identify the base metal has weakened to the point that repair is waste of time, and can causing more damages than do it good. No matter how good the weld process/procedure is, I guess there is a limit the base metal can endure, and it is my true interest.

The real world is imperfect, unfortunately I am live in one, just want to learn to be more smarter, but not too rigid.

Both of the responses have something would benefit me enormously, more to come?

 

[kslee1000]

Also, as repair involves crane components and runway girder structures, all suject to dynamic repeative loads and instantenous stress reversal, thus, what is fatigue's role in all of these?

 

[metengr]

kslee1000;

Also, as repair involves crane components and runway girder structures, all suject to dynamic repeative loads and instantenous stress reversal, thus, what is fatigue's role in all of these?

These repaired components will be subject to the same fatigue loads as originally designed. If weld repairs are not performed correctly, they will crack again and possible result in damage to the overall structure and to personnel.

Obviously from your original post you have omitted critical information – you are dealing with cranes and runway structures. As I stated in my original response, IF base material is originally heat treated for strength as is normally the case with crane components, repeated attempts at repair welding will weaken this steel.


Why are you drilling holes at the ends of the crack? This makes no sense unless you are using this as a short term measure to attempt to arrest crack growth. You verify the length of the defect either visually or with some type of surface NDT followed by immediate removal of the defect. I would not drill holes to mark anything.

As for removing the defect, it sounds like all you are doing is welding over existing cracks.

Most crane manufacturers have very specific requirements for weld repairs to steel components in their crane. Some of these steel components are manufactured from high strength low alloy steel that have very specific welding requirements to avoid altering the original properties of the steel. Preheat and interpass temperature are two of the most critical variables for welding that need to followed (depending on material)

If you are in charge of overseeing these repairs, you are going down a very dangerous path with limited knowledge of welding. I would review any information you have on the type of cranes you are weld repairing to determine if welding requirements are provided. Since you have limited knowledge of weld repair, you need to subcontract a Certified Weld Inspector or welding engineer and have them review your bill of materials and develop a consistent weld repair program. Do you even have any qualified welding procedures for this work?

 

[kslee1000]

metngr:

I sincerely appreciate your inputs, yes I am hanging by a thin thread, and scared from time to time.

We do have a so called precedure. I will put it up on Monday for comments. At the meantime, enjoy the remaining weekend. Thanks again.

 

[kslee1000]

metengr:

Sorry has mis-spelled your name each time.

 

[kslee1000]

Here is a tentative repair procedure for comment.

 

[metengr]

kslee1000;
I reviewed the attached procedure. I would revise it by changing the following;

Step 2 to read, Determine the length of the crack by visual and NDT, if necessary. Delete the 1/4" stop holes they are not needed and confusing.

Step 3 Remove the defect by local grinding or air arc gouging. If air arc gouging is used, the edges of the excavation for weld repair shall be ground back 1/16" minimum to remove heat affected material.

Step 4 (new) Verify defect removal using NDT prior to welding.

 

[stanweld]

kslee1000,
Assuming that you have confirmed removal of cracks prior to repair welding, the additional comments of metengr and racookpe1978 apply. Based on recent problems with crane repair, I would also question the original design or whether the original welds were made as designed. I have seen welds made on cranes that did not confom to the original design requirements; e.g., fillet welds made instead of full penetration groove welds, undersized (1/2 size) fillet welds, partial penetration groove welds in lieu of full penetration, as welded surface condition instead of contour ground, etc. These may be the cause of repetitive cracking in service.

Another problem with multiple repairs is a tendency to relocate the toe of the weld relative to the original geometry of the joint. Stress may be increased at the new location resulting in decreased time to cracking.

 

[racookpe1978]

Dropping back to the focus of your original question - before we so rudely interrupted with a critique of the repair process itself 8<) - IF


- big IF there -

If the grinding (crack removal) process is clean and gives you "clean" metal; and if the weld-pre-heat, weld temperature, post-weld-heat-treatment are held properly for the steel alloy in question; and if the post-weld NDE allow you to very no internal flaws or cracks were induced by the welding itself, then there is no realistic limit to the number of times that a single piece of metal may be re-welded.

If, on the other hand, you fail to use:

the correct filler metal,
the correct pre-weld heat treatment,
the correct weld heating and arc current,
the correct post-weld heat treatment,
or try to weld a "dirty" crack that has not been ground out -

then that ONE bad weld has destroyed (more accurately - could have destroyed) the structural integrity of your crane and crane support beams.

 

[stanweld]

kslee1000,
Your procedure uses E7018. Are you sure that the material being welded is A-36? On many cranes, higher stregth steels are used requiring higher strength weld filler metals. Are new cracks located in weld metal or base metal or base metal HAZ? You also indicate that repairs are during initial fabrication; if so, I suspect that your initial weld sequencing for the involved joint(s) may be in error.

 

[kslee1000]

Stan:

- The welding rod and process will change to match base metal and position.
- The re-crack is usually initiated in the weld metal, then grow into the base metal at ends.
- No, repairs are done over equipment built in the 60's. That has been the reason I was woundering if fatigue plays a role in all of this.

Guys, almost like sitting through an excellent lecture in college. Thanks. Please keep chime in, if you have add'l thoughts. I think this would benefit many others also.

 

[unclesyd]

If your are in the US you are treading on very thin ice if the rules haven't changed. OSHA/ANSI did have very specific limits on what can and can't be done in repair of any lifting device. The normal course is the to use a manufacturers repair center for such repairs. There is a way to do the repair procedures yourself if you can get a procedure approved by the manufacturer or let them supply a repair procedure, or get designated as a repair center.


Normally if a weld joint fails in the weld metal you have a strength problem, the weld isn't large enough or the weld metal itself isn't strong enough.

I personally haven't seen any HSLA materials in equipment as old as yours, we had 2 friction cranes built in the fifties that require numerous repairs and some redesign by the manufacturer. All the affected materials was A-36 or equivalent. We either used 7018 or proprietary rod (85,000) for the repairs.

 

[stanweld]

kslee1000
I take it that your firm is in the business of repairing cranes after cracks have been detected in service and you are not having problems with premature cracking of your repair welds prior to placing the crane back in service. If cracking occurs in the same locations, I would susupect design, the original weld quality or lack thereof as a root cause. You also need to carefully sequence the repair and possibly include temper bead techniques or other stress relieving techniques in making the repair. High residual stresses may also contribute to cracking.

unclesyd,
My confidence in crane fabrication and repair is declining exponentially with the problems I have been currently seeing on new and repaired equipment.

 

[racookpe1978]

Well, ALL crane loads (on the underlaying structural and frame pieces at least) comes from either direct loading/overloading or fatigue.

Feel the building shake as an overhead crane rolls down the rails with even a small load: The shaking that moves the building steel members is also moving (shaking!) the carane beams and rails and wheel foundation blocks.

Straight up-and-down loads are simple, slowly applied, and are almost always smoothly applied as the load comes on the cables and hook = little shaking. But rolling, starting, and stopping?

Very significant fatigue movement which makes crack propagation easier to start, easier to move through metal. Starting and stopping (jogging) each load sideways to get it into position is also fatiguing.

Add in very cold weather, rust, outdoor "storage", building settlement, crane movement (truck-mounted cranes) and irregular foundations for field cranes.

 

[kslee1000]

unclesyd:

As noted by you, no, we do not touch any thing from hoist drum down - lifting device, that's another animal.

Though require frequent/repeative repairs, I feel the original design and construction are solid, evident by the fact the equipment has endured so many years of abuses, and/or neglect. What is the average life expectancy of a crane subjects to near capacity loads on 24/7 basis? Just curious.

The ocasion scares me the most is crack on an ugly repair weld - weld pass overlaps passes, it uglier than the scar on the pirate cheeks. At times, there were no choice but to cut out the affected areas then rebuilt. But when the manager anxiously asking: can we put another pass let it stretch through the night, I was scared.

 

[Metalmeister]

I can't say that I would be very concerned about carbon pickup from air arc gouging process. This is a very oxidizing process and you blow away any molten metal that "may" have picked up carbon.

That said, I would still do some light grinding to remove rough edges of kerf that could interfere with depositing a high quality root deposit.

The fact that re-cracking seems to occur mostly in the weld could mean it the weld joint is a highly restrained and/or the component is under a load while it is being repaired. Preheat might help, but if it is under load, I would not add thermal stress.

Ugly welds are usually ugly through and through.