grinding welds

[alanrockwood]

Newbie boat builder wannabe here.

Tell me about grinding welds, generally speaking but especially welds below the water line.

I have read that some people grind their welds. Even some boat yards do it. Others warn against it saying that it can weaken the welds. On the other hand, I think I recall seeing some diagrams in my copy of "The Procedure Handbook of Arc Welding" that indicate that flat top welds are stronger than crowned welds, though my recollection could be mistaken, Anyway, I am rather confused about the whole matter.

I'm also interested in one sided vs. two sided welds, but maybe that should wait for another thread.

[Ad Hoc]

alanrockwood

Grinding welds below the waterline is only worth doing if you boat is running at a high Fn...in other words going fast. Otherwise, not worth worrying about. You can also be selective where you grind too, such as iwo of waterjet inlet ducts, also at the stem.

You don't mention where you're referring to ally or steel.

[gonzo]

The grinding technique is important too. Overheating a weld by grinding can cause chemical and physical changes.

[Ad Hoc]

Originally Posted by gonzo

Overheating a weld by grinding can cause chemical and physical changes.

That is what the welding has already done

[gonzo]

Not exactly. Welding rods or wire have flux or gas shield.

[MikeJohns]

Originally Posted by alanrockwood

............ Tell me about grinding welds, generally speaking but especially welds below the water line...........

Depends on the welder and the bead produced ! But usually on a steel hull you do grind the outside welds, and a smooth surface is kinder to the paint system too. And as Ad Hoc noted I've never heard of grinding the welds causing significant metallurgical changes.

[Ad Hoc]

Originally Posted by gonzo

Not exactly. Welding rods or wire have flux or gas shield.

What has that got to do with, over heating when grinding?

Originally Posted by gonzo

Overheating a weld by grinding can cause chemical and physical changes.

Gas or no gas shielded, rod or wire...welding is achieved via heat, extreme heat. What ever the temp of grinding, it is nowhere near that of welding.

I think you're getting confused with grinding on parent metal compared to grinding on a weld. One already has a HAZ the other does not.

[alanrockwood]

Originally Posted by Ad Hoc

alanrockwood

You don't mention where you're referring to ally or steel.

Steel.

[michael pierzga]

I work with many top class steel yachts and see both stand proud and ground welds. There seems to be no difference in longevity.

One thing to consider if you decide to grind. When you build a small craft, the steel plate will be thin. Grinding the weld also erodes the steel hull plating . After grinding the weld you will sandblast...sandblasting removes steel and furture reduces plate thickness.

Be careful. Be wise.

[welder/fitter]

Originally Posted by MikeJohns

...And as Ad Hoc noted I've never heard of grinding the welds causing significant metallurgical changes.

Yes, it can. In fact, the most common reason for properly welded test coupons braking is concentrating heat in one area too long, even moreso than those who forget to polish all of their edges or grind the face & root flush. A welding inspector who's name came up during earlier discussions with BS, Nat Wingate, used to always rag me - during check-tests - before I headed into the grinding room, because he'd seen so many people mess up decent coupons by over-grinding them. The last time I did a check test I was with a co-worker who got carried away with the grinding & one of the poor guy's coupons snapped like like a cracker. It's pretty funny when it's the other guy, but he had to take a week off of work to do a re-test on the following week, because he'd delayed testing until he only had a day left on his certification. To this day, I don't let him forget it.

But, as has been mentioned by michael pierzga, if you decide to grind, watch your plate! leaving the bead face won't add strength, especially on a groove, but why bother unless the faces are outrageously proud? (I'd grind them, but I'm different that way). If you do grind, don't remain in one area long enough for the metal to start changing colour, because that's when you're screwing the pooch.

Mike

P.S. an acceptable face is 1/8" proud, or less.

[welder/fitter]

Sorry all, haven't slept in 44hrs, so I'm not explaining my point(s) well. When the welder is grinding in one area for too long it is because he/she is trying to remove too much metal at one time. Although the greatest indicator of this is a "bluing" of the bead & surrounding plate area, one doesn't even want color change to that slight yellowish-brown hue. As you all know, a grinding wheel is extremely abrasive and the average 5" grinder will be running in the neighbourhood of 10,000RPM, say 7,000 for a 9". Thats a lot of heat! Not as hot as an arc, by far, but the linear weld speed is much faster & heat more uniform about the plate thickness than when grinding, providing we're not talking about 1" plate(which we're not). I'll dig around my library, later, & see what I can find, but right now I have to zzzzzzzzzz for a while!

(By the time I return, someone will have explained why I'm all wet)

[Ad Hoc]

Originally Posted by welder/fitter

A welding inspector who's name came up during earlier discussions with BS, Nat Wingate, used to always rag me - during check-tests - before I headed into the grinding room, because he'd seen so many people mess up decent coupons by over-grinding them. .

Mike,

I think you're getting a bit confused with passing a coupon test to establish whether the weld is "sound" and passes mechanical testing for class compliance and that of general poor quality.

The coupon test the plate must be as "flush" as possible. But not so much that parent metal is taken away. The test is to simply check the mechanical properties against that of the class requirements for the metal and grade. This establishes a baseline of the procedure/gas/speed/thickness etc for the welders cert. If the pass requirements is say 275MPa (as with 5083 ally for example), and each time the coupon test specimen only produces say 250MPa, it matters not what one has done with the weld per se, the procedure is is at fault. Thus the speed, or preparations prior to welding, the gas, the filler wire etc etc..all these are reviewed to ensure there are no quality issues causing the lower mechanical properties. Once this has been done, and the weld still yields just 250MPa, then more NDT will follow. It could be simply LOP/LOF etc owing to the lack of skill of the welder. This is ostensibly the reason for such coupon tests and to obtain your class certs.

With over grinding welds on a boats shell, this is to prevent stress concentrations, localised. If the weld bead is too large, it indicates a problem. Either the plate gap was too large, and the welder has simply "gap filled", in doing so has created a nice blob. The other, inexperience and/or too much heat (for starters), thus depositing far too much weldment for the thickness/plate size and to the now already approved procedure via the coupon test.

An oversized weld bead, and i mean an obvious one, causes localised stress contractions. These stress contractions lead to failure of the weld at the HAZ/weld bead intersection. Again I discussed this on one of those other welding threads, here:
Butt Joints For Hull Plating
and here:
Butt Joints For Hull Plating

The stress contraction caused by over sides weld beads can be significant. It is also related to the angle of the weld bead to the parent plate, which can exacerbate the SCF. The reduction in allowable design stress can be as much as 50% or more depending if another factors come into play, such as porosity, LOP etc. \Typical stress distribution of through a weld is shown here:



Thus the reason for grinding a coupon test, is to ensure the plate being tested, on appearance is as close to the original parent metal as possible, dimensionally that is. Since a weld bead has more "area" than the parent plate, more area means greater load carrying capacity when tested.

The reasons for grinding post fabrication on a boat, is to prevent SCF of localised weld beads. Thus grinding will not affect the HAZ that has already been created by the heat of weld.

[gonzo]

Another reason to grind welds is cosmetic.

[Ad Hoc]

Originally Posted by gonzo

Another reason to grind welds is cosmetic.

Cosmetic is superficial and subject to each owner/surveyors opinion etc of the boat. As such is "open season".

The technical reasons why one grinds down or reduced over sized weld beads is given above. Nothing to do with "image" or flux etc.

[tazmann]

Being a welder myself I would agree with Gonzo and Mike that getting carried away with grinding sure doesn't help the weld strength but when talking over grinding it is when you realy burry in in there and don't move the grinder around, then the metal starts changing colors then to red hot, grinding wheel starts getting pluged up with metal and starts smearing the surface. that is when you could say it has changed the metal properties because it will brake the weld in a test nomatter how well it was welded to begin with.
On a side note I weld used well casing pipe a lot and from time to time you run into some that you simply cannot get a weld to stick, Years ago the fisrt time I had trouble with this I was talking to a freind of mine that had welded on ships during the war, He explained that what had happened was the metal had turned in to somewhat a pot metal from the pounding and vibration, told me to take my grinder and lightly polish the metal with it, that it would bring the molocules of the metal back to the surface, then it will weld ok. Well I thought to myself that aint gona work but I went back out to the job and tried it. worked slick and welded just as good as new metal.
Tom