I work for a contractor as a Structural QA Inspector in a ship yard and i have come across thing problem time and time again. But nobody knows what the true answer is.
We have a serious problem with bulkheads (walls) buckling and warping due to welding (FCAW) most of the bulkheads are 1/8''-3/16 mild steel.
after all the welding is done, most of the time they have to hit the bulkhead with severe heat to straighten it. The bulk head will look like polka-dots from all the various places where they imput the heat. for example. they heat one spot up till it gets bright and hot and then spash it with water they may do this 100-200 hundred times on a wall thats 10'X10', depending on how bad the wall is warped. i have seen some that are out of tolerances by 4-5 inches.
its seems to me..once the wall is fit up..one could tack some angle iron diagonally in opposite directions, on each side. ( one going one way, the other side going the other) to restrict the bulk head from deforming, and use the back-step method to weld it, skipping around
i used to be a welder in the navy and still weld on the side now...seems to me that taking this step would prevent it from happening, at least...it always helped me.
in a nutshell, they are saving 5 dollars in welding labor welding it this way, but spending 50 dollars to have it fixed after.
P.S. i have checked the ship specs and have not found anything that pertains to this problem. i just says flame straightening may be used when nessisary, but it does not tell us how much is too much...surely, there must be a limit.???
David, you have some good suggestions there. The solution may be more difficult than it seems, but I can see some pros and cons there.
You probably already know all this, but I'll touch on it anyway.
The practice of heating spots in the panels and cooling with water is common practice (but not on Structural Steel, or highly stressed weldments).
By welding all around the panel, the metal shrinks along the perimeter but not in the center. That causes the puckers because now the metal in the center has to fit into a smaller space than before, but it can't, so it displaces to the outside. The easiest way to correct that is to shrink the metal in the center of the panel which is what all those spot heats do.
Adding angles, or "strongbacks" to the panels will reduce the distortion somewhat, but it can't take care of all of the problem. Once you remove the angles, you will still have some of the buckling and warping. But now you also have to clean up all the tack welds if the angles must be removed.
The thinner the panels are the more difficult it will be to keep the panels flat because thinner is easier to bend. And, unfortunately, you can't prevent welded metal from shrinking; you can only try to compensate for it.
So the questions are: Can the design be changed to leave angle iron strongbacks in place? Will the amount of distortion you will get in spite of the strongbacks be within acceptable limits? Is spot heating the panels really the most cost effective way to solve the problem, or is it being done that way out of habit?
Is it possible or feasible to alternate the welds and stagger some of the heat around to minimize some of the distortion? It will not get rid of all of it but maybe it will make it less time consuming straightening. i assume that it must be welded continuously and possibly on both sides of the plate. Fillet welds can be started and stopped so that the weld after you are done is still continuous. Weld a couple of inches and stop, move around to the other side or further down the same side and weld a couple of inches. By the time you get all the way around, the weld may have cooled, and you can start back with where you started and weld a couple of more inches, etc, etc.... You will still have some straightening to do but if done properly it should help. On the other hand will this save time vs welding continuously and then straightening? Maybe not. Restraining with angles may be you best solution as you have already mentioned. The material changes as heat is applied and minimizing the heat or the effects of heat is all you can do.
Distortion is something we all battle,
Here is some interesting reading- I know you are aware of this, but I'm bringing it up for the new guys. It has some good info on preventing stress warpage.
thanks for all the replies...im guessing what it boils down to is that they are just going to weld it the fastest way possible..and just fix the mistakes later....i gathered a few ideas..ill keep you posted.
Normally, bulkheads have angle stiffeners that run horizontally or vertically spaced 2 or 3 feet apart as part of the ships structure. If you have a 10' x 10' bulkhead without stiffeners, then I'm going to guess that either it's not finished, or something is wrong. However, I've seen some engineers find some pretty strange ways to save weight, so who knows.
Anyway, my point is that the best cost effective method I've seen for installing bulkheads is to weld the stiffeners on the plate prior to installation, while lying flat on the floor. They even make special sub-arc auto welders that travel along the stiffener and place either continuous or stitched fillets on both sides at the same time. As you've already figured out, the stiffeners do help prevent warpage, but they don't eliminate it. They do, however, keep the warpage down below tolerance levels most of the time, especially when the bulkhead is sequence welded in place.
There are a few things, other than weld shrinkage, that cause bulkhead/deck warpage.
One avoidable cause is improper fit up. Sometimes Fitters will force a plate in a hole that's a little too small. When this happens, something has got to give somewhere. Usually it's the weakest section of the part, which is the unsupported plate between stiffeners. I've seen quite a few bulkheads that were warped beyond tolerance before being welded. This also affects bulkheads that have already been welded and checked. Like I said, something has to give somewhere. Often these types of warps cannot be fixed by heat shrinking. Rather they have to be cut along the perimeter weld, or slit in the middle of the warp in order to remove enough metal to allow the plate to relax.
Also, you'll find a lot of new bulkhead/deck warpage during modular ship construction. After a new module has been installed the increased weight has a funny way of warping bulkheads in odd places. Especially around the support blocks that are holding the ship off the dry-dock floor or ground. Normally, the Engineers don't account for extra strength members to be placed around temporary support points. Instead, the entire hull is designed to give and flex when stressed. This does not allow for ridged spots. The ripple effect of problems associated with support area bulges in hulls are things that can make both production foreman and inspectors scratch their heads and say Well.... now what?
Of course there are a lot of other causes of warpages as well such as magical decks that are warped during the day shift, but straight at night (due to sunlight induced heat expansion), hydrotesting, shifting loads from cranes, etc. etc. etc.
The big picture is that you've got to figure out what caused the warp in order to effectively fix it. Otherwise you'll just be wasting time. After a while, you'll begin to see these types of problems before they happen and can take preventative measures. That ability plus the ability to recommend appropriate cures for the defects you reject will allow you to rise above the average inspector and be a valuable employee.
Welcome to the fun world of shipbuilding!
When making "long" welds in thin metals, (the conventional way) you WILL get buckling distortion. There is a ratio of weld length versus material thickness, above which buckling takes place. With the materials you are using, you will find this always happening.
While using the typical distortion control techniques such as back stepping and "controlled wandering", may help, it will not really give you any significant improvements in the case of long welds. The only real solution to this problem that I am aware of, is the use of "Low Stress Non-Distortion" (LSND) welding.
This is a technique where jigs are used to hold the material in place, that "pre-heats" the material in certain places, while cooling it down in other regions, during the welding cycle. This leaves a weldment with negligible distortion. It will however be a costly exercise, because the fixturing will not be cheap.
To read more about this welding technique, get hold of the following paper:
Title: Low stress non-distortion (LSND) welding - a new technique for thin materials.
Authors: Q.Guan, D.L.Guo, C.Q.Li, R.H.Leggatt
Published by: Elsevier Science 1994
Hope this helps
thanks for all the quick replies...i finally got a hold on the ship specs that deal with flame straightening. The specs are rigid for high carbon steels...but not for the bulkheads which are generally a mild carbon steel...so what they are doing is with-in code and specs....i just think they might be wasting too much time straightening everything..but thats there labor issue not mine. ;)
For guidelines on Heat Straightening repairs (HSR's) you might want to review the FHWA Guidelines on Heat Straightening Repairs for Damaged Steel Bridge Girders. It has some good information on how to properly perform HSR's without drastically affecting the metal. It also has some do's and don'ts.
They have done a lot of research on the affect to typical mech properties but they haven't done any research (that I know of) on impact properties.
Is SAW an option? we used SAW process to produce some beautiful 3/16" fillet welds on 3/16" thick shell material in the power generation industry. Prebending on an I beam was also an option for us. The increased travel speed and minimal weld clean up afterwards far outweighed the material handling cost to move the parts to the SAW machine. There are handheld SAW machines that can be put on a Bug-All track. (assuming flat position welding is possible) Just presenting food for thought. Increased travel speed = less heat input = less distortion.
Tim, sorry about that...the 10X10 area was an exageration...they do have the stiffeners there....plus the bowing and distortion..
thanks for everyones replies...we will get this worked out ..ill keep you updated.
FROM DON KUMPUNEN/Some additional information on this subject. There are 2 ship yards building these ships.The other shipyard uses a pulse arc method of welding on the bulk heads,etc. The engineers from NAVSEA and NAVSES have put their stamp of approval on the pulse arc method after thorough examination since 1990 on the first ship of the class. I have seen pictures from the other ship yard[using pulse arc] and those [SUP SHIP] people who made a visit and they said there is very little warping of the bulk heads as well as good quality of welding. From what i gather the pulse arc method is slower but it eliminates a lot of heat straightening.The ship yard where we are at uses FCAW that is hot and fast but causes a lot of warpage. There is a tremendous amount of man hours expended on heat straightening. I wonder if this contractor is charging over and above the contract price to get reimbursed for these man hours. I know that there was a flooding incident a while back that came to a cost of 20 million dollars. Anything over 10,000 dollars is paid for by us the tax payers.The articles i have read on FCAW is that it works well on construction when you have say 3/8 and over metal thickness. I also did some flux core arc welding at the local college did get a feel for it. My big question here is what happens to the metallurgy properties with all the heating and reheating. Is this some thing that will show up as defects as the ship continually goes through heavy seas[waves 25 feet and over].This is a classic example of a confrontation between quality vs. production. I wonder if the NAVY METAL JOINING RESEARCH CENTER in OHIO has made a study of this excessive amount of heat straightening.
Welcome back DONK, haven't seen you on here for a while.
Weld sequence is big deal when dealing w/ boatbuilding (as you know also). Many different opinions and many things tried to reduce it. Look at metal boat society's forum and do a search for weld sequence.
I am a big fan of the back step sequnce. Personally when i welded in the Navy i almost always used this technique. I works outstanding.