Distorted deck plating

I am left in suspenders, what is the rest of this sentence, please?

 

Spaceboy, whatever is done, you'll have to watch the hatches so you don't melt them. There seem to be a large number of hatches.

 

Again MR Tansl, you made a comment, I asked your for an explanation for it. You said that it is a fact because some shipbuilders think that this happens. I was MERELY asking you to
supply some scientific validity to your comment.

If you were able to supply a supporting argument, I would have expected something like this
The alloy of the steel is A36 or Aisi xyz or some other HSLA material designation, it contains X% carbon and with the application of heat the carbon leaves the crystal structure of the metal and imparts a brittle condition to the metal. Attached is a document that explains this process.

I have looked on the internet for anything to support your guess that carbon will leave metal and leave the metal brittle and could not find anything in the normal steel that would be used for a boat

Perhaps you could guess at the material that the OPs boat is made of?
My humble 40 year experience with mild steel .25% C equivalent is that heating the metal will not make the metal more brittle.

Adhoc, are you around, you have a lot of experience with steel vessels, what type of steel would be used in say 40 - 50 boats?
Again, just a mere query.

sorry I have edited within the post

 

Tansl loses a lot in translation and you may not be talking about the same subject he thinks he’s posted about.

Changes are in Microstructure, not Carbon content. It’s a function of cooling rate and where in the phase change is locked in with Pearlite and Martensite that’s the issue.

Unless you melt metals you can only achieve surface changes in alloy composition. Line heating is to a red heat only. Ductility reduction really isn’t going to be an issue .

But it’s not going to shrink the plating enough anyway, and alone it’s simply not practical for the distortion shown. If the distorted material is not cut out and replaced then the only practical method is to slit, clamp it down and weld. Line heating would be better utilised for smaller corrections but it needs practiced skill .

 

Im currently in transit between several countries, travelling...so i'll try and contribute when i'm not so busy and also have a decent wifi access!

However, I do applaud your efforts to engage with tanls, but as you have also discovered, not worth it. It becomes polemic and pointless diarrhoea of words with zero validation! I'll take your huge experience and knowledge over almost anyone else on this forum with respect to fabrication/ally too, btw!

You are correct on many counts. The flame, is just heat. It wont impact/effect the metallurgy in the way heat serious effects aluminium. It may, only may, influence localised areas, but marginal if at all. Since the carbon % is not alerted merely by heat it remains the same, so any change in its metallurgy would only be at the microscopic level where the possibility of a change in the carbon % has occurred owing to extreme extenuating circumstances; not 'plate wide' to minimise distortion, per se.

As MikeJ also notes above it microstructure changes, not the %'age of carbon. But not knowing the grade/temper of the steel hard to say 100% but, since basic steel for shipbuilding it will most likley be a Grade A mild steel of 235MPa yield, which has around 0.21% carbon. I cannot image an expensive high tensile steel being used for such a vessel.

The pictures show extensive distortion. The sequence of any welding must be with the hull/deck plate fully welded prior to any frames/stringers being welded. So i surmise that it was owing to classic poor alignment, i.e gaps, and then pulled down onto the structure and welded. Which promotes the beginning of distortion across the deck.

Only real way is to cut all the frames to deck welds, let the deck take its own natural shape, and then go from there, before rewelding the frames to the deck. Big job and requires skill/knowledge of this simple but complex sequence.

 

Ad Hoc, admired friend (LOL), when you stop traveling around the world, could you, please, validate some of your affirmations ?, only one, the one you want. Thank you.

 

When heating steel, the type of flame used is important. That is the ratio of oxygen to gas. If the mix is oxygen rich, it will combine with the carbon in the steel and remove some of it. On the other hand a gas rich mix will carburize the steel and make it harder/brittle.

 

without a quench??

 

When heating steel to what temperature is the issue. Just the term Heat is too subjective.

Steel would need to be heated close to 1000 degrees C before you could carborize a thin surface layer that way. There would need to be a low temperature sooty flame too. The very first thing any welder learns with gas is to adjust for a suitable flame.

But this is not about welding or surface treating but line heating which uses lower temperatures. Class societies say line heating does not adversely affect normal ship building steels and it is an approved fabrication method. But it requires skill to execute and experience to know what can be achieved.

 

Even without a quench, increasing the carbon content of steel will change its structure. Line heating uses heating and cooling to deform the plates.

 

Carburizing is done at about 850 to 900 C. Line heating is done at the same temperature when air cooled and about 600 C for water cooled.
http://www.dtic.mil/dtic/tr/fulltext/u2/a443648.pdf

 

This thread has turned into the metallurgist equivalent of an "oil debate"....

 

Only if the temperature of the steel is above its transition temperature. Which for any type of Line heating is not permissible.

Carburisation as such, can only occur by the addition of carbon to the surface....which of course is not the objective of the line heating. This is also basic 'case hardening'. The objective to harden the outer layers, while keeping the ductile core. It is what make the Katana sword so hard too - basic carburising.

 

Not really, ABS approval limits temperatures for line heating to those levels, but that doesn't mean that's the temperature used for the process.

Working with shipbuilding plate, a duller red heat is usually used. Fabricators are trained to stay around 650 C and to never heat over cherry red. There's no advantage going hotter it just costs more.

I think the flame is usually set on the oxygen rich side of a neutral flame but I need to check that.

 

Cherry red is about 750 C. Dark cherry is about 640 C.