grinding welds

[RayThackeray]

Yep, confirmed. As I said - pedantry.

[Ad Hoc]

Originally Posted by RayThackeray

My understanding is that ....

And here in lies the problem. You understanding is somewhat lacking, yet you steadfastly refuse to accept an answer/explanation that does not align with your 'understanding'.

Demonstrated by you regurgitated mantra:

Originally Posted by RayThackeray

As I said - pedantry.

If you wish to cite why, you should do so; with more than just ill-conceived opinions. Since just repeating yourself and with no rationale and supporting facts, all you’re doing is showing your ignorance on the subject. It does not bother me one bit if you don’t agree or cannot grasp the difference between elastic or elasticity. But it seems to bother you that you have shown to be ignorant on the subject where you feel your understanding is greater than already well established and known engineering facts.. Not sure why…

[RayThackeray]

So you choose to launch into a lecture (multiple times) about a term that was used (har) in an elastic fashion. I hope you don't do that with your wife.

[MikeJohns]

Originally Posted by RayThackeray

This still doesn't make sense. My understanding is that its steel's combination of elasicity, strength and other properties make it a very good boatbuilding material (the point that Poida has tried to make).

Do you dispute that the property of elasticicy (as relatively small as it is in steel) helps in this respect?
If not, then steel would be even better if it had all the existing properties but one change - as elastic as ceramics. We all know where that would lead. Again, I charge the offence of pedantry.

Every load resisting material develops its ability to resist the load by deflecting.
Understand that all materials distort under load, there is no exception. In otherwords all materials have an elastic response, whether it’s diamond or a wooden batten. The amount it deflects is related to its stiffness.

If materials didn’t have an elastic response they couldn’t resist the load that's fundamental physics. Parts of the material are pushed closer together and others are pushed further apart. Depending on the material you are either affecting atomic or molecular bonds or both.
Physics defines elasticity (and inversely stiffness) as the relationship between the applied load and the amount of distortion in the recoverable portion of the response.

A highly elastic material distorts a lot. At the other extreme a very stiff material distorts only a little.

Steel like all materials gets its strength from deflecting. Its necessary that it deflect to resist a load. But steel doesn’t distort much at all before it permanently deforms. It’s not very elastic, but it does have quite a high strength. So for the small amount of distortion it can absorb quite a lot of energy. That’s why steel springs are useful.

Steels small deflection under load up to yield makes it a very useful as a structural material. Alloyed steels are even stiffer and stronger.

So yes like all materials its the strain in the material than provides the internal forces to resist the applied load.

And as for the 'elasticity' of ceramics they are usually more elastic than higher carbon steels and tungsten. When it comes to glass or concrete would it surprise you to find out that they are far more elastic than steel ? And we do make small ships out of glass as the structural material.

[Poida]

All the guy wanted to know is, should he grind his ******* welds.

He's still standing there with his grinder in his hands waiting for a conclusion.

[MikeJohns]

Poida
I thought that was clearly and well enough answered n the first few posts ?

I can't see why people who are apparently confused about material properties should take umbrage at an effort to help understand those same properties.

You apparently don't understand stress strain elasticity or strength. If you simply make up your own definitions that's your prerogative, but it's not helpful to other readers to see statement like " a material stresses if you exceed it's elasticity" and other such misleading statements. It's in every ones interests including your own to get a good grasp of material properties if you want to discuss the structural side of boat design. Isn't it?

[petereng]

Hi All,
The main reason to "Dress" welds is to improve their fatigue resistance. As long as the weld is full penetration and the back bead is good or "Dressed" and the parent metal is not removed in the dressing the weld is statically as strong with or without a "crown" or reinforcement as we call it here in Australia. The toe of the weld is a stress concentrator due to its sharp geometry and under cycling stress will crack easily. In fact the toe of a weld due to it being a "cold" area of the weld can be full of micro cracks (this is why an apparently good looking weld can fail a coupon test). Cold cracking or hot cracking are terms to investigate. These problems occur due to incorrect rods been selected or incorrect or no preheat being used on various alloys. Since its a boat hull the steel is probably mild steel and hot or cold cracking is very unlikely with this. So grind away happy in the knowledge that you are improving the welds performance as long as you have 100% penetration. If grinding always use sharp, clean wheels and cut freely. If the metal is getting hot it does not matter with mild steel but means you are working the grinder too hard and and not removing material efficiently. If you have a TIG welder the quick and very good way is to "Torch Dress" the weld vs grind. This improves the fatigue performance considerably as the waves in the weld surface, the weld toe and even the metallurgy are improved. All of these techniques are mentioned in many documents on welding so do some research if needed. Cheers Peter S Mechanical Engineer/Ex weld trainer (manual and robotic) and inspector.

[MikeJohns]

Peter yes I agree

This is what I posted earlier in this thread

Originally Posted by MikeJohns

....

From a metallurgical perspective grinding improves weld geometry and consequently reduces fatigue failure. Significant to this thread the heat from grinding can actually have a beneficial effect on a weld similar to peening. Other methods of flattening a weld such as Plasma dressing are also beneficial to weld metallurgy.

[Poida]

Yes Mike I realise the issue has been adressed, I didn't think you would take that seriously.

Sorry Mike it dawned on me yesterday when there was a short episode on TV showing how a ship designer, designs ships. It was all calculated on a computer program.

I realise my mistake in talking engineering to a designer.

So the calculations from the moment of inertia, point loads, or equally distributed loads together with any twisting moments are already done for you.

The only thing close to engineering you need to know is calculating the kinetic energy required for your finger to depress the start button on your computer.

Where abouts in Australia are you Mike? I hope it's not too close.

By the way, you can take this seriously too if you like.

[MikeJohns]

Poida
You are apparently totally confused about material properties. For example:

Originally Posted by Poida

....... Steel has to work within it's allowable deflection because after that it will stress.......

Rather than getting a large chip on your shoulder when corrected

Originally Posted by Poida

...Where abouts in Australia are you Mike? I hope it's not too close...

Why not just learn ? Both Ad Hoc and I were trying to help you understand, we weren't trying to put you down. I'm really sorry you choose to take offense that was not the intent.
But I'm curious, do you really think statements that are misleading should be left unchallenged, on a technical forum, simply for the sake of the feelings of the poster? Try going on any engineering forum and making the same comments and see how you get treated. I'm sure you will be fairly treated, but you will be corrected.

Professional technical design has a very healthy culture of encouraging criticism of your thoughts designs and proposals. It's a really good process, it doesn't amount to a personal attack and the culture amongst us is to educate elucidate correct misinformation and always to seek knowledge and peer input to better our own understanding. That's the way the engineers will post too.

I'm more than happy to discuss any technical issue with you now or any time in the future.

[Poida]

OK Mike, Cheers.

Poida

[glasser]

as most of you no i beleve thear is lot to take in to acount for dealing with welds on both steel and aluminin builds of builts unlike in things like back hoes that use a much heaver steel that is not put under the same stres as a boat is becus of the thicknes of it thears lots difrent rules of thumb it seams some old school some newer thouts on it
i workd in the boat shops some 30 years and yes have workd on prity much every thing that flot frum raceing boats doing 200 mph plus to fishing boats doing 12 nots in all forms of of steel aluminin fiberglass one thing i do like to stress is the fisics is not the same for the 3 difrent tipes of build thay all ride in a different way and need to be accounted for in most of the shops i worked we did grind all welds below the water line of the boats on the other hulls only in my shops as a rule we grind frum 1 foot above the water lines for a bunch of different reasons looks is just a small part of this the Resistance is the mane thing smoother you make it better for speed and gas cost grinding a weld shooed only be done by ppl that no what there doing over heating a good weld can in most case make it weaker if a weld is properly done its as strong or stronger then the steel your welding together
we did this in 3 stage the first was use a rely gorse disk to remove the top of it fast with out making heat then stepping down to finer grades if the welder is a good one you shooed not need to do much fearing after as this in its self is a lot of finicky work i have done way to much of that in the 30 years mostly fixing others sloppy work i loved the work building new boats and working on some them nice old mucks and cricecrafts forum the 40s and 50s take your time on the weld to make them deep full penetration the worst thing you can do is over grind one this makes a all new problem as you can not just re weld over it you need to grind it out and start over hope this helps some of you forgot to say lots my work was on wood boats to

[aranda1984]

As far as structural strength is involved, sheets welded to a hull must be bevelled sufficiently, prior to welding, to give a full weld penetration across the total thickness of the plates.

The welding process, will defenetly alter the material. We are talking mostly about austenitic mild steel or some aluminum alloys here.

(Go back to the Onassis shipping era. many of the stretched freighters have broken up in the cold winter, killing everybody on board.)

Welding adds stress concentration along the lines. Also the maximum allowable design stresses of weld lines are lower then the base metal!
Welding structural steel will burn some of the carbon in the immediate area, changing the strength and carbon content. (The carbon is an alloy in the steel also!)
Using different material from the base metal as welding wire or rod will enhance galvanic corrosion.
Plus there are many more fine points what suggests that welding a boat hull should be done by professionals.

As a rule of thumb, a weld is best left as is and should not be ground off, except for looks. (Exemption: in certain situations, uniform material thickness is preferred due to cyclic loading.)
Grinding the protruding crown off might weaken the weld strength, if the weld is not a total penetration and if it has some voids and cracks!
(Believe you me even pro welders with all the tickets, have to grind the weld out to get rid of voids!)
A good welder will lay down a bead that it is barely over the plate surface.
If you grind off the weld, (and if you do not know how to do it properly) you can add to this problem by using too hard grinding stones and overheat the weld bead with the grinder. (It is possible to turn the base metal to red hot with a grinder.)

you can improve all the nagative effects of welding, by various means, as mentioned heat stress relief, by blasting the surface with steel balls etc.)

Discoloration of metal due to heat on austenitic mild steel matters not, it only matters on hardened tool steel!
Once you have passed blue color and gone into straw color on hardened tool steel (due to grinding action) you are drawing the material back. reducing it's hardness. But you don't normally weld hardened material, (you do not use it for hull anyway.)
...Welding hardened materials will destroy the martensitic structure.
The weld might crack, part of the piece will be soft.. But this is another business altogether!

Aluminum is a different matter.
Cold rolled aluminum is small grained and hard. Welding heat (melting the material) will increase the grain size and reduce it's strength.
The less the weld the stronger the hull.
You should stagger the plates if it's possible, so that you do not have a weld line going all around at one place!!!
In the good old days of 4 to 5 knot freighters, steel structures were riveted. We didn't have decent welding technology in those days! ..and we had to find out about cold fracture the hard way!

My apologies to professionals, this is an incredibly simplistic and abbreviated explanation for the people who do not know otherwise!

Regards,

Stephen I. M.