Welding a steel hull

You are right in that a stray arc dissipates heat just like a weld. The reason a stray arc can become martensitic is because your arc strike only heats up a small volume of material. The large volume of material surrounding your arc strike sucks the heat right out of it at a rapid rate -it quenches it.

If you were to put a weld on a large piece of one inch steel you would have to preheat the area to be welded to prevent the surrounding material from sucking out all the heat -quenching it.

Understand?

 

Hmm - not really, I have never heard of anyone preheating big steel bits before welding - what would you do it with - an oxy torch perhaps? You would also have to heat it to the right temparature to fit within the range, and that would vary with the carbon content etc etc.

Is that common practice?

I know that big flanges for pipes, couplings, they dont pre-heat them, just start welding, and they would be prime risk types of jobs.

 

Mastadon

Like you, i understand this very well. I've also tried to explain, but realised i was flogging a dead horse. Not everyone understands metallurgy nor its affects. I don't understand how a cell "decides" to split into two or how DNA just appears, for example, but so what?..does this mean it doesn't happen?

Just because i can read, does not mean i understand everything that is written. I've read some seriously complex books, read and re-read, however, just because i do not understand it, does not make it untrue or impossible. It just means my comprehension is poor or lacking.....comprehension is so under rated these days, in the cut and paste world in which we now live!

We are all wired differently and think and understand differently. The assumption today is because it is on a computer it must be easier to understand...er...ummm...why??

Oddly enough I'm just doing some background reading on Guinier-Preston zones and their relationship with age-hardening of ally alloys. I suspect the result would be the same in trying to explain this, not everyone will understand it.

 

There are none so blind as those who will not see.

 

yes, it isnt a simple subject, and thats why I am asking all the questions. I appreciate you sharing what you know, and you have been very informative - thank you.

Of course, if something is 'true', there must be empirical evidence - and evidence with real examples is the only real test.

All my experience, and that of every other welder I have been able to speak to, suggests that the 'brittle spot' can occur, but no-one can place an actual example of it being a significant problem in general production, and certainly no-one has said they have encountered it in boat hull production.

Indeed, steel boat hull production seems to be a very forgiving and reliable method, and surveyors have said its amazing what inexperienced builders can get away with.

All the worry over the odd accidental strike still seems very misplaced in general boat hull construction to me still. I look forward to any account of actual examples anyone may have experienced.

 

So do you pre-heat any things you weld?

 

rwatson

"...Of course, if something is 'true', there must be empirical evidence - and evidence with real examples is the only real test..."

This is my point. There is evidence of many things that occur in this world which i do not understand and others do. Every "thing" that is beyond our normal 'instant' comprehension that we attempt to understand becomes more complex, requires more of a wider understanding of other aspects to comprehend further. This is a good case in point....arc-strike, welding, metallurgy, fatigue, slip bands, dislocations etc etc, it becomes like an onion, peel away one layer of understanding oh...ops, bugger, another appears. But constantly repeating them in the vein hope that the spark will ignite and allow me or anyone to comprehend such a subject which was previously "impossible", suggests otherwise. I do not mean this in any patronising sense at all.

My wife often does things i shall never understand, even though she tells me in advance what she is going to do.

 

Preheating depends upon the type, grade, quality and in-service conditions of the metal, beit steel or ally. But it is done.

As a good exmaple, in your part of the world, the Kings bridge failed 10th July 1962. Summary of findings below:

"..The 365N/mm2 yield strength steel (to BS 968:1941) used in construction was known to have restricted weldability and the specification for the bridge required precautions to be taken in welding. The investigation into the failure found that the combination of improper welding procedures, inadequate preheat and the failure to dry the low-hydrogen electrodes properly in addition to local stress elevation due to design, led to the formation of hydrogen cracks at the toes of the transverse welds at the end of the cover plates in over 50% of the girders fabricated for the bridge. In the span which failed, the hydrogen cracks had penetrated the flange in three of the girders. During the winter of 1961 one of the inner girders failed but this had remained undetected. The crack in the second inner girder extended by fatigue along the web until, in the winter of 1962, the conditions of crack length, load and temperature for rapid brittle fracture were met. The outer two girders of the span failed at the same time in a brittle manner.

The Royal Commission into the failure of the Kings Bridge found that the inexperience of the fabricator in welding low-alloy steel and the highly variable quality of the steel used were major factors in the failure. The steel supplied was not adequately tested and the toe cracks were not detected by either the fabricator or the Country Roads Board inspectors. The lack of clear and precise specifications for manufacture, testing and inspection was contributory as was the failure to fully investigate the proposed type of steel before construction was undertaken... The need for proper supervision and inspection during fabrication must not be underestimated.."

 

Aha - juts as I suspected. It stands to logic that there would be situations where pre-heating was required based on the previous articles.

The thing is - is it ever prescribed for steel boat hulls ?

I thought of a way to find out today. I will ring my friend the quality manager at the naval dockyard, and ask what the 'standing orders' are for arc strikes on boat hulls. If they dont know, I would be very surprised.

 

Pre-heating: I have used preheating on sections of components which require it. No, I see no reason for preheating the thin skin of a smaller steel boat hull, the HAZ when welding will include the thickness of the plates, nor have I seen this suggested here.

I can't understand the bulldog tenacity in wishing to reject sound advice from those who know what they are talking about. Mastadon has given a very textbook explanation for why stray arc strikes are frowned upon. The reason that there is no information on the effects of stray strikes in small steel boat construction is simple; the x-ray of welds is an expensive process, prohibitive to most builders. On other threads in these forums you will note that when one presents a required repair on a steel vessel, they ask about methods of repair, rather than reasons for failure. They are not concerned about whether a fault was caused by porosity, slag inclusion, embrittlement, etc., yet, simply wish to know how to repair the darn thing.

There is nothing elitist in following sound practices, nor in improving the way things are built. With the many hours one will spend building a boat, why not avoid any potential for failure? Perhaps, to you, this is an exercise in nit-picking. Let's face it, most builds will include incidences of stray arc strikes, the more the amateur welder, the greater the number of incidences. Suggesting that one drag their rod across the plate to burn down to the flux coating, however, shows a general lack of knowledge of basic welding metallurgy. If your friend at the naval yard is an ISO manager, he may not have the answers, yet, can readily find out from a welding supervisor/foreman/chargehand. I think that you have a good idea in asking him, as you seem to have little faith in the credentials of those who have posted here.

 

Look - its not me being pedantic - I actually want to know if I, or anyone else buys a steel boat, does amateur welding practices constitute a real risk?

From a purely academic level, it is even more interesting to find out if a small weld can ever turn 'brittle', and under what circumstances

Its nothing to do with anyones credentials - the textbook info is all very clear.

Nor am "Rejecting Information", and it is very rude of you to suggest that. I am REQUESTING INFORMATION. Apart from the link *I* found early on in the discussion about a Liberty ship that fell apart, *no-one* has quoted any first hand practical examples of problems, and coming back to quoting recomended practices does not add any real information to the main point.

I dont know why people are taking this so personally - I am not 'taking sides', I am after real information. The fact that real life examples of boat hulls suffering from minor arc strikes are so hard to find might mean that it isnt a real problem at all. Consider it a possibility!

 

Pre-heating requirement is a function of the steel ambient temperature and the thicknesses being joined. Its definately a requirement for steel boatbuilding for good construction but not for every weld. For example where a plate terminates on a thicker section then the thick section is pre-heated.

 

Rwatson

Well, i'm confused.

"..Its nothing to do with anyones credentials - the textbook info is all very clear...

Ok, but then you go on to say

"..I am REQUESTING INFORMATION..."

Well, those two statements conflict with each other, since you have been provided information, yet you still seem to think "it" is not enough? Why? Many examples have been given and explained above by many contributors.

Well, finally for your preheat:
Just go to LR rules for Steel Ships, Pt.3.Chpt.10.2.12.19.

I am not going to quote it, since you seem to think that such information does not constitute "information"..?? Feel free to look it up yourself.

 

Alex had some wierd steel for the leading edges of his keels and had to use specialised rods and pre heat the weld zone to get the strength he needed there. It was a rare exception. The chances of a structural weld on standard mild steel plate failing due to a brief arc drag is virtually zero. As pointed out, it has never happened, to anyones knowledge here , so why bother with it. Bridges are designed to much closer tolerances,and welding is far more critical there. On our small boats are so grossly overstrength that worrying about petty things like arc drag is like losing sleep over the odds of being hit by an asteroid.
Brent

 

rwatson,
Probably the best thing I could suggest would be to contact your regulatory body who certify your welders/inspectors/companies. As far as the welding of thinner plate & guage steel, yes, the quality of welding does matter. sufficient penetration & fusion, porosity, slag inclusion, speed of travel/heat, undercut, are all considerations which can detrimentally effect a weld. The ABS website has quite a bit of info. pertaining to the welding of various marine vessels. While mild steel may be more forgiving of some of these flaws than ferrous & non-ferrous alloys, welds can be easily contaminated when proper procedures are not followed & can fail. In terms of arc strikes, Mastadon has made very clear the consequences of such incidences. What value is there in arguing to what degree such occurances can compromise the plate? Knowing that it is not a good thing should be enough to try to avoid it from happening. Those who so quickly dismiss such poor practices as "no big deal" are attempting to excuse their own half-*** work as acceptable. For a prospective buyer or builder, why would one settle for "less than"? I wasn't being sarcastic when suggesting that talking to your friend was a good idea. If what has been said, here, does not give you the confidence to follow sound procedures & avoid poor ones, you should investigate it further with someone you know & trust.